WO2002086345A1

WO2002086345A1 - Actuator

Info

Publication number: WO2002086345A1
Application number: PCT/AU2002/000489
Authority: WO
Inventors: Nui Wang
Original assignee: Pbr Australia Pty Ltd
Priority date: 2001-04-23
Filing date: 2002-04-19
Publication date: 2002-10-31
Also published as: AUPR453701A0; TW534958B

Abstract

An actuator (24) including first and second members (25 and 31), each of which has a rolling surface (32, 33). The first and second members (25, 31) being arranged so that the respective rolling surfaces (32, 33) are disposed in substantially opposed relationship and for relative rotation. The actuator (24) further including rolling elements (37) disposed between the first and second members (25, 31) and in engagement with the respective rolling surfaces (32, 33) thereof, that engagement causing the rolling elements (37) to roll relative to the rolling surfaces (32, 33) upon relative rotation between the first and second members (25, 31). At least one of the rolling surfaces (32, 33) being at least part conical so as to define a generally curved and tapered surface. Each rolling element (37) being constrained to roll in a spiral path upon relative rotation of the first and second members (25, 31) so as to traverse the gradient of the taperred rolling surface (32, 33) and the other of the rolling surfaces (32, 33) having a profile so that traversal of the tapered rolling surface by the rolling elements (37) causes a shift in the spacing between the first and second members (25, 31). The actuator (24) is suitable for use in a disc brake caliper or a drum brake for actuating the brake friction lining into engagement with the disk rotor or drum brake surface.

Description

ACTUATOR

The present invention relates to an actuator and in particular to an actuator for use in the drum or disc brake of a vehicle. It will be convenient to describe the invention in relation to that particular application, but it is to be appreciated that the actuator of the invention could have wider application in other non-automotive fields.

The present invention has been developed as an actuator for an electric brake, such as an electric caliper or drum brake. In that application, the actuator is required to provide actuation sufficient to cause the brake shoe or shoes to engage the braking surface, typically a rotor or a drum, with such force as to retard a vehicle or hold it stationary. Typically the movement required of an actuator for an unworn brake pad or shoe to engage the braking surface in either a disc or drum brake, is in the order of less than 0.5mm and 2mm respectively. Accordingly, the required actuator movement or stroke is not large. The movement or stroke required can increase as the friction lining of the brake shoes wears and compensation for lining wear normally is required to ensure proper performance of the brake over the life of the friction lining.

While the movement required for brake engagement is small, the load required to be applied by the actuator is relatively large and in the order of 30- 40kN for a disc brake and 10kN for a drum brake. The actuator is therefore required to be robust in construction. The actuator is also required to be reliable, efficient and quick operating.

International patent application WO9902885 discloses an electrically operated brake caliper with a compensation arrangement for lining wear. The electric actuator in this prior art drives a nut which drives and advances a threaded shaft connected to one of the brake shoes, to provide for wear compensation movement. Brake pad actuation is provided by a separate mechanism which employs rollers to roll on ramps provided on opposed rings. Through relative rotation between the rings, the rollers shift on the ramps to move the rings apart. That movement is transmitted to one of the brake shoes, so that a braking force is exerted in the normal manner. The arrangement disclosed in WO9902885 is particularly complicated and it suffers from at least one major drawback. To achieve the required movement for brake actuation, the distance the rollers shift along the ramps is very small. Indeed, it possibly is the case that the rollers shift a distance less than their full circumference. This limited travel means that the actuator is likely to suffer lubrication problems, as the rollers do not tend to roll over the lubricating grease, but instead tend to push the grease to a position along the ramps at the end of the maximum travel of the rollers. As the amount of lubricant diminishes along the length of the ramps, the rollers and/or ramps will wear and the efficiency of the actuator reduces. Limited roller travel also increases the likelihood that the metal bearing surfaces will fret and fatigue prematurely.

Further, because of the concentric arrangement of the rollers, the actuator has limited angular rotation of the threaded shaft in which to achieve the required output displacement. For example, maximum shaft rotation for a 3- roller arrangement is 120 degrees and for a 4 roller arrangement is 90 degrees. This results in a low velocity ratio and hence low mechanical advantage. In this case, either an electric motor of high torque output or an intermediate stage of gearing is required. It is an object of the present invention to provide an actuator which does not suffer from the drawbacks associated with the prior art discussed above. It is a further object of the invention to provide an actuator which can provide relatively small axial movement and withstand relatively high axial loading. It is a still further and preferred object of the invention to provide an actuator which is operable electrically to actuate either a drum or disc brake and which provides compensation for friction lining wear.

According to the present invention there is provided an actuator including first and second members, each of which has a rolling surface, said first and second members being arranged so that the respective rolling surfaces are disposed in substantially opposed relationship and for relative rotation, said actuator further including rolling elements disposed between said first and second members and in engagement with the respective rolling surfaces thereof, that engagement causing said rolling elements to roll relative to said rolling surfaces upon relative rotation between said first and second members, at least one of said rolling surfaces being at least part conical so as to define a generally curved and tapered surface, each said rolling element being constrained to roll in a spiral path upon relative rotation of said first and second members so as to traverse the gradient of said tapered rolling surface, and the other of said rolling surfaces having a profile so that traversal of said tapered rolling surface by said rolling elements causes a shift in the spacing between said first and second members.

In an actuator according to the invention, the rolling elements undergo combined radial and circumferential movement over the rolling surfaces upon relative rotation between the first and second members and by that movement, the spacing between the first and second members can be altered, to shift a brake shoe under the actuation of the actuator, either toward or away from the rotor or drum it is arranged to engage. Conversely, it will be easily appreciated that the actuator could be employed in other machinery or devices that require similar actuation characteristics such as a machine vice.

An actuator according to the invention is highly suitable for use in drum or disc brake arrangements, because it can provide high efficiency and high mechanical advantage, but still provide sufficient lift for brake actuation. High efficiency is achieved by the use of a rolling element to minimise friction losses. High mechanical advantage is achieved by the actuator permitting large relative angular rotation between the first and second members for a given lift or separation of the members. In this respect, the tapered rolling surface can have any suitable angular inclination, and the inclination can be very gentle or slight to provide for only a small lift. A larger lift if desirable, can be achieved through the same relative rotation, by a more acute taper in the rolling surface. Alternatively, a greater relative rotation between the first and second members can be arranged to achieve larger lift. In a further alternative, for a given radial taper and relative rotation, the amount of lift can be varied by tightness of the spiral path on which the rolling elements travel. In other words, the lift can be varied by varying the pitch of the spiral.

The arrangement of the invention attends to at least one of the drawbacks with prior art WO9902885, because the or each rolling element can be made to travel a distance at least several times its rolling diameter during actuation, so that lubricant between the rolling surfaces and the rolling elements will be continually spread over the rolling surfaces. This is despite a lift of 0.5mm or less being required. That level of lift and the need for reasonable travel of the rolling elements can be achieved by a tapered rolling surface profile having only a slight radial gradient, although as discussed above, the radial gradient of the taper and the extent of relative rotation between the first and second members can be arranged to satisfy any, or at least a wide range of lift requirements, including those that require a substantial lift. Only one of the rolling surfaces of the first and second members is required to be curved and tapered in the form described above, although each can be formed with such a profile if desirable. In that latter arrangement each rolling surface preferably is circular in plan view and is aligned coaxially. In the alternative however, the tapered rolling surface may be formed part circularly on one of the first and second members and if provided on the other of the members, part or fully circular on that member. The preference for full circular tapered rolling surfaces on each member is to permit the first and second members to relatively rotate at least through one full revolution to achieve the required lift and by that rotation, to ensure proper lubrication between the members and the rolling elements as discussed earlier.

It is preferred that the first and second members each have a conical or a frustoconical shape and that they be arranged coaxially and with the respective apexes of each facing each other and being arranged as the closest parts of the respective members so that the rolling surfaces taper away from each other. This is termed a "positively" tapered or "convex" tapered arrangement. The gradient of each cone is preferably also equal.

The first and second members may be otherwise shaped or arranged however, and for example, the taper of the rolling surfaces may be opposite to that discussed above, so that respective apexes are spaced apart and the rolling surfaces taper toward each other from each apex - a so-called "negative" or "concave" taper arrangement. This latter arrangement requires the rolling elements to traverse radially outwardly to cause the first and second members to separate. Still a further configuration of the first and second members is that one of the members can define a convex tapered rolling surface, while the other defines a concave tapered rolling surface - a so-called "positive/negative" or "convex/concave" arrangement. The angle of the tapered rolling surfaces will differ and that difference will result in the first and second members shifting relative to each other, when the rolling elements traverse the rolling surfaces. This is a distinction between the same taper configurations of convex/convex or concave/concave, because in those configurations, the first and second members shift as a function of the addition of the respective bearing surface gradients, whereas in a convex/concave arrangement, the shift is a function of the difference in the respective gradients.

Although the following description relates mainly to an actuator construction in which the first and second members have a positively tapered arrangement, it is to be appreciated that the invention includes within its scope alternative constructions, such as above described.

It is also permissible for the first and second members to be disposed non-coaxially and this is particularly the case when one of the members does not have a readily definable axis. For example, one of the members may be of an irregular shape and that member may include a tapered rolling surface, or may for example define a flat planar rolling surface. Such an arrangement is particularly possible in an actuator in which one of the members is stationary or non-rotating. That member could for example, form the rear face of a brake shoe.

To cause the rolling element to traverse the gradient of the tapered rolling surface upon relative rotation between the first and second members, the actuator must be arranged to facilitate that movement. In one arrangement, the tapered rolling surface defines a plurality of concentric grooves of diminishing diameter, or one or more spiral grooves. If concentric grooves are provided, then the rolling element can be of frustoconical form and can define a spiral groove so that the rolling element meshes with the concentric grooves of the tapered rolling surface and by rotation of the rolling element relative to the grooved rolling surface, the rolling element will be caused to traverse the tapered gradient. In the alternative, if the rolling surface includes one or more spiral grooves, then the rolling element may be of frustoconical form and can define a plurality of coaxial grooves extending lengthwise thereof, so that the rolling element meshes with the spiral groove or grooves of the tapered rolling surface. Gradient traversal will also occur in this arrangement upon rotation of the rolling element over the spiral grooved tapered rolling surface.

In one arrangement of the above kind, the tapered rolling surface includes a single-start spiral groove. The tapered rolling surface may however include multi-start spiral grooves, such as two or three starts. The grooves may have any suitable profile to match the type of rolling element employed. In one arrangement, the rolling element is a spherical ball bearing and the groove is shaped to match the curved profile of the outer circumference of the bearing. In this arrangement, the grooves are required to be deep enough to resist release of the bearing in absence of an alternative or additional mechanism or facility to secure the bearing in engagement with the groove. In one arrangement, three spiral grooves are formed in the tapered rolling surface and a ball bearing is disposed in each. The grooves effectively form a separate race for each bearing, to control the position of the bearing relative to the tapered rolling surface upon relative rotation between the first and second members. This arrangement could be varied by any number of spiral grooves and ball bearings as appropriate.

In an alternative arrangement, the rolling element is of generally frustoconical form and includes the coaxial or spiral grooves as discussed earlier. In this arrangement, the mating grooved surface engagement can be such as to maintain the position of the rolling element on the tapered rolling surface without an additional mechanism or facility provided for that purpose.

Grooves may be provided in the rolling surfaces of each of the first and second members, or in only one of those members. In the preferred arrangement, each of the first and second members includes a generally conical or part conical tapered rolling surface, with meshing grooved engagement occurring only between one of the members and the rolling elements. The other member can have non-meshing engagement, so that the peaks of the grooved surface of the rolling member, which preferably are flat, roll against an un-grooved, tapered rolling surface.

In a further alternative arrangement, neither of the first or second members or the rolling element have mating or meshing engagement, but instead, a facility or mechanism is provided to cause traversal of the rolling element relative to the tapered rolling surface upon relative rotation between the first and second members. In one arrangement, the rolling element may be threadably mounted on a radially disposed threaded shaft as well as being in frictional engagement with the rolling surfaces of each of the first and second members. Upon relative rotation between the first and second members, frictional engagement will cause the rolling element to roll and the threaded engagement will cause it to shift axially along the shaft, therefore traversing the tapered rolling surface.

The actuator includes a plurality of rolling elements, ie two or more, and those elements preferably are spaced apart by spacing means suitable to maintain a constant angular separation between them. A suitable cage may be provided for that purpose. Alternatively, each rolling element may be slidably mounted on a shaft that orbits with the rolling element and which permits the required radial movement thereof. One or each end of each shaft may be fixed to a ring or rings, to maintain the shafts and therefore the rolling elements in a fixed angular arrangement. The type of spacing means may be dependent on the form of rolling elements employed. For example, the cage arrangement is particularly suited to rolling elements of the spherical ball bearing kind, while the shaft arrangement is suited to frustoconical shaped elements. Other spacing means may be employed as suited to rolling elements of different kinds.

A rolling element may be formed of several parts instead of having the form of a single piece element. For example, in the above described example of frustoconical rolling elements mounted on shafts, instead of each shaft having a single rolling element mounted thereon, the rolling element may be comprised of 2 or 3 part or lengthwise shorter rolling elements. These rolling elements may be arranged immediately adjacent one another, so as to be actually in contact with each other, or they may be separated, such as by a bearing or bush arrangement. The provision of part rolling elements will reduce slipping relative to the rolling surfaces, that will occur with a longer and single rolling element, particularly at the ends thereof. Thus, by effectively dividing the single rolling element into 2 or more part elements, the amount of slippage is reduced and the efficiency of the actuator is increased. The arrangement will be such that each part rolling element will rotate at a slightly different speed to the part roller element on one or either side of it.

The actuator can be operated by driving one of the first and second members and such driving means may be in any suitable form, but the actuator is particularly suitable for electrical drive. Thus, an electric motor can be attached for direct drive, or for indirect drive through a reduction mechanism, such as a gearbox. As applied to brakes, gearbox reduction is thought to be unnecessary and that is an advantage of the present invention. In this respect, the prior art WO9902885 achieves the required lift for brake actuation with only a small rotation. Thus, if an electric motor is employed to generate the rotation, a reduction mechanism is normally required to reduce the input rotation. This results in extra componentry and therefore extra cost and potential for actuator failure.

When the actuator of the invention is applied to a disc or drum brake, it is preferred to arrange the actuator to provide compensation for friction lining wear over time. For this, an adjuster screw may be provided, which is operable to shift the actuator to take up the clearance between the friction lining and the drum or rotor which comprises a set clearance and any further clearance that occurs due to lining wear (the lost travel) prior to the actuator operating as described above. That is, the arrangement will be such as to provide for a two- stage operation, which first takes up any lost travel and which secondly applies an actuating force on the brake shoes. The first stage can be a normal screw without high efficiency, ie a ball screw. The efficiency is relatively unimportant. What is important is that it takes up lost travel quickly. Accordingly the thread pitch can be coarse. This first stage of movement will not meet significant resistance until the brake shoe engages the braking surface of the drum or the rotor. Upon that engagement, the arrangement will be such as to lock up the threaded engagement and any further axial movement will occur through the actuator, by relative movement between the first and second members.

Friction lining wear can also necessitate a further change or addition to the construction of the actuator, in order to ensure that, despite lining wear occurring, the rolling elements return to the initial or "home" position after completion of an actuating stroke and so maintain the availability of further maximum actuating stroke. Maximum stroke of the actuator is achieved by full traversal of the rolling elements over the tapered rolling surface between the home position and a fully traversed or "end" position. Maximum stroke may not always be required, however it is generally desirable that the rolling elements be returned to the home position after each actuation, so that maximum stroke is available, if required.

For the rolling elements to be returned to the home position, the first member must rotate relative to the second member the required amount each time the actuator releases the brakes. This will occur if the stroke required to apply the brakes is the same as the stroke required to release the brakes. However, if there is lining wear during the brake application, the release or return stroke will be less than the application stroke. To understand why this is so, it is necessary to understand the typical frictional hierarchy of an actuator employed in a drum or disc brake.

In a typical arrangement, friction between the rolling elements and the first and second members is overcome when the friction lining engages the drum or rotor of the drum or disc brake respectively. Thus in operation, the actuator will shift without relative rotation between the first and second members, until engagement of the friction lining with the drum or rotor occurs. This movement is simply the movement required to take up the lost travel clearance between the friction lining and the drum or rotor. Thereafter, the load applied to the actuator exceeds the frictional resistance between the rolling elements and the first and second members, so that the first member rotates relative to the second member and the friction lining is pressed firmly into engagement with the drum or rotor, which constitutes brake application.

During brake release, the load on the actuator is maintained, but diminishes, until the friction lining reaches the point of disengagement with, or actually disengages from the drum or rotor. At that point, the load on the actuator is substantially or fully released, so that the frictional resistance to rotation between the first and second members becomes the dominant load and therefore relative rotation between the members ceases. If the friction lining wears during brake application, the first and second members will rotate a further amount relative to each other (beyond the initial rotation), to maintain the engagement load between the friction lining and the drum or rotor despite the wear. Thus the stroke required for brake application comprises an initial stroke followed by a further wear compensating stroke. However, to disengage the friction lining from the drum or rotor, only a stroke equal to the initial stroke is required and that release stroke is less than the combined initial and wear compensating strokes. Accordingly, relative rotation between the first and second members during brake release is less than during brake application and the rolling elements therefore do not return to the home position. Over time, as lining wear increases, the rolling elements will be returned to positions progressively further away from their home positions. The result is that the available actuator stroke decreases over time as the lining wears, and with sufficient wear, the available stroke may be reduced so far that the brakes cannot be applied, or at least cannot be applied fully. The rolling elements can be returned to the home position consistently and despite friction lining wear, by ensuring that the first member continues to rotate relative to the second member the required amount. To do this, the invention can provide facility to maintain a load on the first member that exceeds the frictional resistance between the rolling elements and the first and second members, to continue rotation of the first member by the driving means relative to the second member until such time as the rolling elements return to the home position, despite the load on the actuator applied during brake application being removed. The arrangement can be such that at the home position, no further relative rotation between the first and second members can take place. This can be achieved by arranging a positive stop between the first and second members at their relative home positions. At that stop, further relative rotation between the first and second members will be terminated (even though the drive means may continue to rotate). The attached drawings show example embodiments of the invention of the foregoing kind. The particularity of those drawings and the associated description does not supersede the generality of the preceding broad description of the invention. Figure 1 is a cross-sectional view of a disc brake caliper according to one embodiment of the invention.

Figure 2 is a cross-sectional view of a drum brake assembly according to another embodiment of the invention.

Figure 3 illustrates a bearing cage suitable for use in an actuator according to the invention.

Figure 4 illustrates a component part of an actuator according to the invention.

Figure 5 shows one form of actuator according to the invention.

Figure 6 is a schematic plan view of the roller bearing arrangement of Figure 5.

Figure 7 is a cross-sectional view of one part of an actuator according to the invention.

Figure 8 is an illustration of the grooves of each of first and second members of an actuator according to the invention superimposed over one another.

Figure 8a is a cross-sectional view of one of the rolling elements of Figure 8 between the tapered rolling surfaces of the first and second members.

Figure 9 illustrates the arrangement of Figure 8, with first and second members relatively rotated through an angle θ. Figure 9a is an enlarged view of an edge section of the Figure 9 view.

Figure 10 is an illustration of the groves of each of the first and second members of Figure 8 superimposed over one another, but showing four rolling elements rather than two.

Figures 10a and 10b are views similar to Figure 8a, but in respect of the Figure 10 arrangement.

Figure 11 illustrates still another actuator embodiment in which four rolling elements are employed and the spiral arrangement is two start rather than single start. Figure 12 is a cross-sectional view similar to Figure 7 of an alternative actuator according to the invention.

Figure 13 is a cross-sectional view showing an arrangement for incorporation in the Figure 1 caliper. Figures 14 to 16 show an alternative arrangement to that of Figure 13, for incorporation in the Figure 1 caliper.

Figure 1 is a cross-sectional view of a disc brake caliper which employs an actuator according to the invention. The caliper 10 includes a housing 11 which accommodates a pair of brake friction pads 12 and 13. The rear of the caliper includes an electrical arrangement including an electric actuator 14 comprising a stator 15 and a rotor assembly 16. The rotor assembly 16 is fixed to a shaft 17 so that rotor rotation drives the shaft. The shaft 17 rotates on spherical seat ball bearing 18 at one end and a spherical thrust bearing 19 at the other end. The electric actuator 14 is confined within a casing 20 which seats one race of the ball bearing 18, while a seal 21 closes the housing recess that accommodates the ball bearing 18. A circlip 22 mounted on the shaft 17 retains the ball bearing 18 in place. A spring washer 22a, such as that known as a Beville washer, preloads the bearing 18 against the thrust bearing 19. The casing 20 is secured to the caliper housing 11 by a plurality of circumferentially spaced screws 23.

The disc brake caliper 10 is operable to apply a braking force to a rotor (not shown) disposed between the friction pads 12 and 13. The arrangement typically is such that the respective pads are shifted a distance of about 0.5mm or less to engage the rotor. An actuator 24 according to the invention is disposed within the housing 11 , between the friction pad 13 and the electric actuator 14, and is operable to apply an axial load to the friction pad 13 to shift that pad into engagement with the rotor. As will be understood by a person skilled in the art, upon engagement of the friction pad 13 with the rotor, the housing 11 will shift to bring the friction pad 12 into engagement with the opposite side of the rotor to thereby apply a braking effect.

The actuator 24 includes a first member 25 which includes a head 26 and a body 27. Each of the head and body are cylindrical and coaxial. The first member 25 is connected to the shaft 17, such as by a screw threaded engagement, and the shaft 17 is arranged to drive the first member 25 to rotate about the axis A. A circlip 28 and a thrust washer 29 define a space within which is disposed a spring 30. The thrust washer 29 could alternatively be a thrust bearing. The spring 30 bears against each of the circlip 28 and the thrust washer 29 to maintain the thrust washer in engagement with the rear face of the head 26. The thrust washer 29 and the spring 30 combine to control actuator hysteresis and hence its operating threshold, while the spring 30 permits axial movement of the actuator. The actuator 24 includes a second member 31 of identical general shape to the head 26 of the first member 25. In that respect, each of the first and second members defines a rolling surface of tapered or frustoconical profile 32, 33 which are symmetrical about the axis A. Each of the rolling surfaces 32, 33 includes grooves which can be cylindrical or spiral grooves and which will be described later in more detail. The second member 31 includes a spherical seat 34 which engages a piston 35, which in turn engages the rear of the friction pad 13. This spherical seat engagement allows for relative tilt or conical misalignment between the second member 31 and the piston 35 that can occur as a result of caliper housing deflection and cocking of the piston within the cylinder 36 of the housing 11. It therefore permits the first and second members 25, 31 to remain properly aligned and spaced.

The actuator 24 further includes two or more rolling elements 37, disposed between and in engagement with each of the tapered rolling surfaces 32, 33. The rolling element 37 is of frustoconical shape and includes a plurality of grooves which are arranged for complementary engagement with the grooves in the tapered rolling surfaces 32, 33. The rolling element 37 is connected to an inner ring 38 by a pin 39 on which the element is mounted. The element 37 can move freely along the length of the pin 39, but the maximum axial travel is defined by the ring 38 and the pin head 40. The position of the rolling element 37 shown in Figure 1 , is the initial or "home" position of that element resting against the pin head 40.

The actuating arrangement includes restraining means which take the form of a pair of mating "pips" designated by the numerals 41 and 42. The pip 41 engages within a slot 43 to prevent rotation between the friction pad 13 and the piston 35, while the pip 42 engages within a slot 44 to prevent rotation of the piston relative to the second member 31.

The first member 25 is mounted on the threaded extension 45 of the shaft 17, which facilitates the take up of lost travel, comprising the set clearance between the friction pads 12 and 13 and the rotor and any further clearance occurring due to friction lining wear. By rotation of the adjuster shaft 17 the entire actuator can be shifted axially. The arrangement operates on a hierarchy of frictional forces. Effectively, in the absence of any load on the actuator 24, the frictional resistance between the first and second members 25, 31 and the rolling elements 37, is greater than that between the member 25 and the threaded extension 45, and thus rotation of the shaft 17 will result in relative movement between the member 25 and the threaded extension 45 only and not between the first and second members 25 and 31. Only when the actuator 24 is subject to load, such as when the pad 13 engages the rotor, will that relative rotation occur. Accordingly, before the actuator 24 meets any load during its forward movement toward the pad, rotation of the shaft 17 will shift the first member 25, and the entire actuator 24, to take up lost travel. Upon engagement between the pad and rotor the hierarchy of frictional forces is such that the frictional resistance between the member 25 and the threaded extension 45 becomes greater than that between the first and second members 25, 31 and the rolling elements 37. Therefore, relative rotation between the member 25 and the threaded extension 45 will cease by the first member 25 locking on the threaded extension 45 and relative rotation between the first and second members will commence. The load at which relative rotation between the first member 25 and the threaded extension 45 will cease or the first member 25 will lock on the threaded extension 45 is termed the "threshold load". The threshold load is governed or set by the force imposed on the member 25 by the washer 29 and spring 30 arrangement. Accordingly, the threshold load can be varied or altered by lowering or increasing that force value.

The thread on the extension 45 preferably, to operate in the above described manner, will experience high friction beyond the load threshold. Such a thread will lock under load above the threshold and by locking, the shaft 17 and the first member 25 will thereafter rotate together. Such further rotation will cause relative rotation between the first and second members 25, 31 for actuator operation. Below the load threshold, the shaft 17 will rotate relative to the first member 25 for lost travel take up. The thread can be such as to shift the actuator 24 very quickly axially, to take up lost travel, before the actuator experiences a load above the load threshold. This quick movement advantageously minimises the delay in brake application.

In the Figure 1 arrangement, when the threshold load has been reached, rotation of the shaft 17 causes rotation of the first member 25. The second member 31 is maintained stationary through the restraining pips 41 , 42 to create relative rotation between the first and second members. That relative rotation and the grooved bearing engagement between the tapered rolling surfaces 32, 33 and the rolling elements 37, is such as to cause the rolling elements 37 to roll between and about the tapered rolling surfaces, and to shift axially along the pin 39 from the home position thereby traversing the taper of each of the surfaces. In Figure 1 , the axial shift of the rolling element will be toward the axis A, causing the first and second members to separate and to therefore shift the piston 35 and the friction pad 13 to the left, or toward the friction pad 12. Accordingly, a braking force can be applied to the rotor between the friction pads 12 and 13.

Reverse rotation of the shaft 17 will rotate the first member 25 also in the reverse direction, so that the rolling elements 37 traverse down the taper of each of the rolling surfaces and the spring 30 and the receding clamp load of the caliper bias the first and second members to move relative to each other until the caliper clamp load is completely removed at which time the threaded extension 45 of shaft 17 will move relative to the first member 25. This reverse rotation operates under a similar friction hierarchy as discussed earlier in relation to brake actuation, to return the actuator to a brake released condition. The maximum stroke of the actuator is achieved when the rolling elements 25, 31 have fully traversed the tapered rolling surfaces and this can be termed the rolling element "end" position. The rolling elements may not always roll fully to the end position, depending on the actuating force required. The rolling elements 37 will traverse fully to the home position shown in Figure 1 , as long as no wear of the friction pads 12 or 13 takes place. No wear would be expected to occur if the brakes are applied when the rotor is stationary, but there would normally be wear if the rotor is rotating. If wear occurs, an increase in the stroke of the actuator 25 a further amount beyond the initial stroke is necessary to maintain the brake pads 12 and 13 in engagement with the rotor despite the wear. Accordingly, the first member 25 will be rotated an amount additional to the initial rotation made for brake application. When the brakes are released however, a return stroke equal to the magnitude of the initial stroke is sufficient to disengage the brake pads 12 and 13 from the rotor. Thereafter, the frictional hierarchy would be such that the first member 25 would stop rotating, and the actuator 24 would shift on the threaded member 45 to the right. Accordingly, because the stroke for applying and maintaining the brake application is greater than the stroke for releasing the brake application (by the "further" amount described above) the rolling element will not return fully to the home position. As discussed earlier, the rolling elements 37 will be returned progressively further from the home position as the brake pads 12 and 13 wear, progressively reducing the available actuator stroke. It therefore is desirable that the caliper 10 includes an arrangement to ensure that the rolling elements 37 return to the home position despite any brake pad wear.

One proposed solution to the above described problem is shown in Figure 13 and 13a, which show the first member 25 and the threaded extension 45 of the shaft 17, all of which were shown and described in Figure 1. Additionally, Figures 13 and 13a illustrates a device which is operable to apply drag to the first member in one direction of rotation and little or no drag in the other direction of rotation. The device comprises a shaped wire 81 which extends about the threaded extension 45 and which extends into an opening 82 formed in the body 27 of the first member 25. The device is operable as follows. As previously described, when a brake application is initiated, the screw 17 is rotated (in an anti-clockwise "A/C" direction for the present purposes) and this shifts the first member 25 and the entire actuator 24 to the left to take up brake pad clearance. When the actuator 24 experiences a load by brake pad engagement with the rotor, the threshold load is exceeded and the first member 25 locks on the threaded extension 45, whereafter continued rotation of the shaft 17 results in a brake application.

Assuming brake pad wear takes place, in absence of the wire device 81 , on movement to release the brakes, the first member 25 will be rotated clockwise "C" so that the rolling elements (not shown) will traverse down the tapered rolling surface 32, but not fully to the home position. The wire device 81 however, imparts a drag load between the first member 25 and the threaded extension 45. That load, plus any inherent friction load between the first member and the threaded extension, is arranged to exceed the friction load between the rolling elements and the first and second members, so that the first member 25 continues to rotate with the shaft 17 and relative to the second member 31 , until the rolling elements 37 reach the home position, at which point they engage the pin head 40. At this point, because the rolling elements 37 can shift no further down the tapered rolling surface 32, the first member 25 ceases rotating because it can only rotate simultaneously with the rolling elements. The resistance to further rotation of the first member relative to the second member therefore increases and the load hierarchy is arranged such that the drag load of the wire device 81 is less than the increased resistance. Accordingly, thereafter the wire device 81 will slip about the shaft 17 and the actuator 24 will shift relative to the threaded extension 45 as required to provide or reset the required clearance between the brake pads and rotor.

It will be appreciated that the wire device 81 imparts drag in both directions of shaft 17 rotation, but the drag is relatively minor in the anticlockwise direction, with the major drag occurring in the clockwise direction. The level of drag can be altered by the amount the wire device wraps around the threaded section 45 and the amount of preload between the wire device 81 and the threaded section 45, which occurs as a result of the interference between them.

An alternative arrangement to that shown in Figures 13 and 13a is shown in Figures 14 to 16. Figure 14, like Figure 13, shows the first member 25 of Figure 1 , as well as the threaded section 45 of the shaft 17. In the Figure 14 embodiment, the body 27 includes an annular recess 83 which accommodates a device 84. The device 84 includes an annular rim 85 and radially inward depending lips 86 and the device is seated within the recess 83 firmly, such as by a friction fit. From the rim 85, a series of fingers 87 is punched, pressed or otherwise formed. These fingers are shown in detail in Figures 15 and 16.

The fingers 87 are pressed radially inward of the rim 85 to engage against the crests of the threaded section 45. The angle of attack of the fingers against the crests is such as to impart a greater drag in the clockwise direction of shaft 17 rotation than in the anti-clockwise direction. The device 84 is operational in substantially the same manner as the wire device 81 although care must be taken to ensure that the angle of attack of the fingers will not lock on the threaded section 45, but rather creates drag only.

Other devices could be employed to achieve the same result as the wire device 81 and the alternative finger device 84. For example, the devices could be fixed to another component of the actuator rather than the first member, and apply a drag to the first member to achieve the same result. Accordingly, such a device would slip relative to the first member in one direction of rotation more readily than the other. In broad terms what is required is a drag force which is less than the frictional resistance between the rolling elements and the first and second members in the brake application direction of shaft rotation, and which is greater than that frictional resistance in the opposite direction of shaft rotation. A further requirement is that the drag is released or overcome when the rolling elements reach the home position.

Returning to Figure 1 , the threaded extension 45 extends into an opening in the second member 31, but it is to be noted that the second member is not mounted on the extension 45. The arrangement facilitates axial space saving while permitting the length of the screw portion to be sufficient to accommodate full lining wear.

In an alternative arrangement to that shown in Figure 1 the first member 25 is mounted in fixed relationship on the shaft 17 and the rotor 16 is in threaded engagement with the shaft. In that arrangement, the take up of lost travel is by the rotor 16 shifting the shaft 17 and the actuator 24 axially before lock-up of the threaded engagement, whereafter the shaft 17 will rotate with the rotor 16, and the first member 25 will rotate relative to the second member 31 , thus resulting in actuating movement of the actuator 24. In this arrangement, the thrust bearing 19 will be shifted to act between the rotor 16 and the motor casing 20.

Figure 2 is a cross-sectional view of a drum brake assembly which employs an actuator according to the invention. The drum brake assembly 50 includes an electric motor 51 disposed within a housing 52 and connected via a shaft 53 to a pinion gear 54. The electric motor 51 is operable to drive an idler gear 55. The idler gear 55 is in toothed engagement with a gear wheel 56 which is fixedly mounted on a shank 57. The shank 57 is disposed for rotation within the bore 58 of a tappet 59 and disposed between the inner end of the bore 58 and the shaft 57, is an anti-friction thrust bearing 60.

Extending from the opposite side of the gear wheel 56 is an adjusting screw 61 which is coaxially aligned with the shaft 57, along the axis BB. The adjusting screw 61 is threadably received by a first member 62 in the form of a nut which includes a body 63 and a head 64. The head 64 defines a tapered rolling surface 65 of a similar shape to the tapered rolling surface 32 of the disc brake caliper of Figure 1.

Disposed in opposed, coaxial alignment with the first member 62, is a second member 66. The second member 66 includes a tapered rolling surface 67 of substantially the same angular taper as that of the opposed rolling surface 65 and is formed on the head 68 of the second member 66. The second member 66 further includes a body 69 depending from the head 68. The screw 61 is operable to shift the first and second members 62, 66 together, to take up lost travel.

Disposed between the bearing surfaces 65 and 67 is a plurality of balls 70 confined within a cage 71 which is shown in more detail in Figure 3. The cage 71 is suitable for three balls 70 spaced apart at 120 degrees.

Each of the heads 64 and 68, the balls 70 and the cage 71 are mounted within a cartridge 72. The cartridge 72 is cylindrical and includes radially inwardly depending lips 73 at each axial end. The lips each form bearing surfaces for bearing against the underside of the head 64 at one end and against a spring 74 at the other end. The spring 74 bears between the lip 73 and the under surface of the head 68, although it could equally bear between the opposite lip 73 and the underside of the head 64. Cooperation between the cartridge 72 and the spring 74 is such as to maintain each of the bearing surfaces 65 and 67 in contact with the balls 70.

The body 69 of the second member 66 extends to form a tappet 75.

Each of the tappets 59 and 75 is arranged to engage the web of a brake shoe of the drum brake assembly. Thus, each tappet includes a shoe location slot 76 of a width suitable to snugly receive the web of a brake shoe and by that engagement, each tappet is prevented from rotating.

The drum brake assembly further includes a drum brake backing plate 77 to which the housing 52 is connected. Between the housing 52 and each of the tappets 59 and 75, a flexible boot 78 is provided to seal against ingress of foreign matter into the housing.

Operation of the assembly 50 is as follows. The electric motor 51 drives the pinion gear 54, which in turn drives the idler gear 55, which drives the gear wheel 56. The mating gears are of such widths so as to allow for relative axial movement of the gears during brake application and to compensate for friction lining wear. Additionally, the idler gear is of a diameter to suit the spacing between the cylinder which accommodates the gear wheel 56, and the electric motor 51. Rotation of the adjusting screw 61 firstly results in a relative shift between the screw 61 and the first member 62 to take up any lost travel due to friction lining wear. Effectively that shift causes a spacing apart of the tappets 59, 75. Upon reaching a predetermined threshold load, the first member 62 will lock on the screw 61 and thereafter, drum brake actuation is achieved by relative rotation between the first and second members 62 and 66. By that relative rotation, the balls 70 are caused to roll in a spiral manner and to traverse the radial gradient of the tapered rolling surfaces 65 and 67 and therefore to cause an axial shift in the spacing between the first and second members. That shift is transmitted directly to the tappets 59 and 75 which in turn, radially expand or contract the drum brake shoes. It is to be noted, that the head 68 is prevented from rotating by its integral connection to the body 69, which includes the shoe location slot 76. Receipt within the slot 76 of the brake shoe web (not shown) prevents rotation of the head 68. However, there is no confinement against rotation imposed on the head 64, because the connection between the first member 62, and the adjusting screw 61 and the shank 57, is such as to permit rotation relative to the tappet 59. A thrust bearing arrangement, similar to that shown in Figure 1 , may be employed in this arrangement to control the threshold load in concert with the spring 74.

An alternative to the arrangement shown in Figure 2 is to form the first member 62 integral or fixed with the shaft 57 and to mount the gear wheel 56 in threaded engagement with shaft 57. The gear wheel 56 would then be axially movable along the shaft 57 and would cooperate with the tappet 59 to shift the tappet and separate it from the opposite tappet 75 for lost travel take up. The gear wheel could cooperate with the tappet 59 by an axial bearing for example and then the bearing 60 would no longer be required. It follows, that it is to be appreciated that the drive arrangements illustrated in Figures 1 and 2 could be interchanged and therefore are not exclusive to the respective disc or drum brakes as shown.

It is to be further appreciated that the arrangements of Figures 13 to 16 could also be employed in the Figure 2 arrangement, or alternative but equivalent arrangements could be employed.

The actuators shown and described in Figures 1 and 2 each comprise only a single pair of first and second members. The invention however extends to additional members such that the actuators may act in parallel or series. In parallel, load can be distributed to more rolling elements, while in series, the total displacement will be equivalent to the sum of individual displacements.

Briefly referring to Figures 3 and 4, these show the cage 71 and the first member 62 in more detail. In particular, Figure 4 shows three grooves or ball races 79 each for receipt of a ball 70. The arrangement is that a ball 70 is received within each ball race 79 and the races are configured to spiral from toward the outer periphery of the head 64, up the tapered rolling surface 65, toward the central axis of the member 62. A similar grooved arrangement is provided on the tapered rolling surface 67 of the second member 66 and the two rolling surfaces cooperate with the balls 70 so that upon relative rotation between the first and second members, the balls 70 are confined to move upwardly or downwardly within their respective grooves along the tapered rolling surfaces. Therefore, rotation of the first member 62 in one direction will cause the balls 70 to move up the tapered rolling surface of each of the first and second members and to cause those members to separate axially. That axial separation is transmitted to each of the tappets 59 and 75 to shift the drum brake shoes (not shown) into braking engagement with the drum. Rotation of the first member 62 in the opposite direction will cause the balls 70 to shift down the tapered rolling surfaces 65 and 67, still within the grooves 79, so that the first and second members can shift axially towards each other under the influence of the spring 74, to release the brake shoes from engagement with the drum braking surface. The cage 71 as shown in Figure 3, is of one-piece construction and is arranged to rotate with the balls 70 upon rotation of the first member 60. The cage 71 maintains the balls 70 disposed approximately at 120° apart.

Figure 5 shows in more detail, one form of actuator of the kind employed in Figure 1. Parts which are common between Figures 1 and 5, are given the same reference numerals. Figure 5 is a perspective view of the actuator 24 of Figure 1 , and from this it can be seen that rolling elements 37 are disposed between the first and second members 25 and 31 and these are maintained through the ring 38, pin 39 connection, at 120° to each other. Figure 6 shows in schematic illustration the three roller bearing arrangement of Figure 5. The rolling elements 37 each have a helix or spiral thread and one or each of the first and second members 25 and 31 have a series of concentric circular grooves, and the thread and the grooves are arranged to mate. The mating arrangements may differ depending, for example, on the number of thread starts. A two start thread may permit the grooves of the first member to overly grooves formed in the second member, so that groove peaks of the first member overly peaks of the second member and grooves troughs overly troughs. A single start thread may require the opposite arrangement, so that peaks overly troughs.

The rolling elements 37 in Figure 6 are shown as single piece elements, but one or more of these, and preferably each, may be provided as 2 or more part rolling elements. That is, the rolling elements 37 may be divided lengthwise to form a plurality of shorter part rolling elements, each rotatable about a respective shaft 39, and this would reduce the amount slipping between the single piece rolling elements and the tapered rolling surfaces of the first and second members, in particular at the radially outer roller element ends.

Disposed between the thrust washer 29 and head 26 of the first member

25, is a bearing 80 which is shown as a ball bearing race, but the bearing could take any suitable form, such as a Teflon bearing. Such a bearing is effective to permit the first member 25 to rotate relative to the thrust washer 29 with a controlled frictional resistance.

The rolling elements 37 are shown schematically to include a plurality of grooves about the rolling surface thereof and in mating engagement with complimentary rolling grooves provided on the tapered rolling surface 33 of the second member 31. Various arrangements of this kind will now be described in relation to the figures that follow.

Figure 7 shows one suitable embodiment of a grooved rolling element actuator arrangement. In Figure 7, parts common to the Figure 1 are given the same reference numerals, plus 100. In Figure 7, the first member 125 includes a tapered rolling surface 132 and while it is not apparent from the cross- sectional nature of Figure 7, the profile formed on the tapered rolling surface 132 is a helically spiralled grooved profile. The angular locating pin 39 of Figure 5 would be employed in the Figure 7 arrangement, but it is not shown in Figure 7.

The rolling element 137 is likewise formed with a series of grooves but these are not formed as a helical spiral, but are formed as a series of three concentric grooves 101.

The tapered rolling surface 133 of the second member 131 is formed as shown, with no grooves formed therein but instead, as a smooth tapered surface. In this arrangement, the spiral grooved profile of the first member 125 constrains the rolling element 137 to rotate toward or away from the axis A100 upon relative rotation between the first and second members 125 and 131. This arrangement having only one of the tapered rolling surfaces grooved has become the preferred actuator arrangement, as it alleviates difficulties associated with the provision of grooves and their phasing on each of the rolling surfaces, but it still provides sufficient mating engagement between the grooved rolling surface 132 and the rolling element 137. An alternative to the Figure 7 arrangement, is to apply a spiral grooved arrangement to the rolling element 137 and to apply a series of concentric grooves to the tapered rolling surface 132. Effectively, that would be the reverse of the grooved arrangement presently described in relation to Figure 7. The outcome, however would be the same, in that rotation of the first member 125 would result in movement of the rolling element 137 toward or away from the axis A100 depending on the direction in which the first member 125 was rotated.

A further alternative arrangement to that shown in Figure 7 is to also apply a grooved profile to the tapered surface 133 of the second member 131. While that arrangement is not the most preferred, it will have the advantage over the Figure 7 arrangement of increasing the amount of cooperating bearing surfaces and thus the load carrying capacity of the actuator.

In an arrangement in which spiral grooves are to be formed in each of the tapered rolling surfaces of the first and second members, it has been found that the construction and alignment of the grooves is important to the proper working of the actuator. Figure 8 is an illustration of the grooves of each of the first and second members superimposed over each other. The dot and chain lines represent the valleys of each spiral groove and in each case, the spirals are formed in the tapered rolling surfaces of the respective first and second members, as a single start spiral. It has been found, that in such an arrangement illustrated in Figure 8, the valleys of each opposed spiral groove intersect along a line XX. As the first and second members relatively rotate, so does the line XX of intersecting valleys but at half the speed. It is apparent from Figure 8 that the two rolling elements 120, 130 have a different construction inasmuch as the grooved arrangement of the respective rolling elements is a half groove out of phase. Otherwise, the respective elements are of identical length and cone dimensions. With this arrangement, the rolling elements 120 and 130 can have complete bearing engagement with each of the first and second members. This arrangement is clearly illustrated in relation to the rolling element 120 in Figure 8a. This full bearing engagement condition will exist throughout the full radial and circumferential travel of the rolling element 120 along the rotating line of intersecting valleys XX. The rolling elements 120,130 rotate at half the angular speed of the relative rotation between the first and second members 125, 131. Likewise, the line XX denoting the line of valley intersection also rotates at that speed. Thus, the rolling elements of Figure 8 will always coincide with the line XX of intersecting valleys. This permits the actuator to operate properly.

Figures 9 and 9A illustrate the arrangement of Figure 8 but with each of the first and second members rotated relatively through an angle θ. Figure 9A is an enlarged view of the section of Figure 9 showing the angular change. For clarity, the chain-line spiral is kept stationary. From these figures, it can be

Q seen that the line XX of intersecting valleys has shifted through an angle of I2 and the rolling elements have moved radially the amount R indicated. The radial shift R is quite small, even in the enlarged view of Figure 9A, for the amount of rotation shown. Each of the rolling elements 120 and 130 has moved the distance R radially inwardly. This confirms the above discussion that the rolling elements and line XX rotate at half the speed and therefore through half the angle that the first member rotates, when the second member remains stationary.

Figure 10 illustrates an alternative embodiment of an actuator according to the invention, in which four rolling elements are employed. In this embodiment, the first and second members are formed in an identical manner to the members of Figure 8 and therefore the members each include a single- start spiral groove formed on their respective tapered thrust bearing surfaces.

As shown in Figure 10, two identical pairs of rolling elements are provided. The rolling elements 220 are disposed on an axis YY, while the rolling elements 230 are disposed on the axis ZZ.

The axis YY, is disposed perpendicularly to the axis ZZ and the arrangement is such that the axis ZZ represents the overlying intersection of groove valleys between the first and second members, while the axis YY represents a 90° out of phase relationship, where a valley of one of the first and second members overlies a peak of the other of the first and second members. Figures 10a and 10b respectively illustrate cross-sections taken through the rolling elements 230 and 220, to show the above described relationships. Figures 10a and 10b also illustrate the bearing engagement between the tapered rolling surfaces and the rolling elements. In each of these figures, there is effectively 50% engagement. This differs from the arrangement shown in Figure 8a, which shows 100% engagement. The Figure 10 arrangement includes one clear advantage over the Figure 8 arrangement, in that the load of the actuator is distributed over four rolling elements. However, that load is distributed across only 50% of the rolling element bearing surface. The Figure 8 and 10 arrangements therefore have similar overall load bearing surface area. In the event that the load bearing capability of the actuators illustrated in Figures 8 and 10 is not sufficient, Figure 11 illustrates an alternative embodiment in which four rolling elements are employed, each with full bearing surface engagement with the respective first and second members. This arrangement employs a two-start spiral on each of the tapered rolling surfaces of the first and second members. The respective starts are disposed 180° apart and by this arrangement, each of the rolling elements can be disposed along a rolling axis in which the valleys of the first member overly the valleys of the second member.

The arrangements of the figures illustrate that the number of rolling elements can be increased from a pair of elements, to any number of elements including odd numbered elements as required to bear the actuator load. The main limiting factor is the size of the actuator and in brake applications, the diameter of the first and second members will need to be in the region of 40mm. Additionally, the numbers of rolling elements will affect the cost of the actuator, although not significantly. In the spiral groove arrangements, Figures 8, 10 and 11 all illustrate that the rolling elements cannot all be identical. In the Figure 8 arrangement, two differently phased rolling elements are provided. In the Figures 10 and 11 arrangements, one pair each of differently phased rolling elements are provided. It is envisaged that a six rolling element actuator with a three start spiral groove will require three pairs of differently phased rolling elements, and so on for an eight, ten, etc. The consistent aspect of these arrangements is that an even number of rolling elements is provided and that the number of spiral starts required for full bearing surface engagement is equal to half the total number of rolling elements. The invention however, is not limited to the Figures 8, 10 and 11 arrangements and it is permissible that an actuator according to the invention can have an odd number of rolling elements. The Figure 7 arrangement could be employed with an uneven number of rolling elements, preferably with equal angular spacing. Even the arrangements of Figures 8, 10 and 11 could have an uneven number of rolling elements, subject to appropriate groove and phase construction.

Likewise, the invention is not limited to a grooved profile on each or any of the first and second members and the or each rolling element. Figure 12 represents such an arrangement in which the respective bearing surfaces of the first and second members and the rolling element include no grooved profile.

In Figure 12, first and second members 325 and 331 include tapered rolling surfaces 332 and 333 respectively. Each of these surfaces in cross- section is flat, or ungrooved. Disposed between and in bearing arrangement with the surfaces 332 and 333 is a rolling element 337. The first member 325 is in threaded connection with a shaft 317, while a ring 338 is disposed between the first and second members and includes a threaded bore for receipt and threaded connection with one end of a threaded shaft 301. The threaded shaft 301 is in threaded connection with the rolling element 337, which can move radially toward or away from the centreline £ by rotation along the shaft 301.

In this arrangement, the rolling element 337 rotates through frictional engagement between its tapered rolling surface and the tapered rolling surfaces of the first and second members 325, 331 upon relative rotation between the first and second members. In one direction of relative rotation, the rolling element will move along the threaded shaft 301 toward the centreline £ , causing the first and second members 325, 331 to separate and by that separation, the actuator can apply an actuating load as discussed previously. Relative rotation in the reverse direction will cause the rolling element 337 to shift away from the centreline <L and so permit the first and second members to move closer together, generally under the influence of biasing means. The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.

Claims

Claims:

1. An actuator including first and second members, each of which has a rolling surface, said first and second members being arranged so that the respective rolling surfaces are disposed in substantially opposed relationship and for relative rotation, said actuator further including rolling elements disposed between said first and second members and in engagement with the respective rolling surfaces thereof, that engagement causing said rolling elements to roll relative to said rolling surfaces upon relative rotation between said first and second members, at least one of said rolling surfaces being at least part conical so as to define a generally curved and tapered surface, each said rolling element being constrained to roll in a spiral path upon relative rotation of said first and second members so as to traverse the gradient of said tapered rolling surface, and the other of said rolling surfaces having a profile so that traversal of said tapered rolling surface by said rolling elements causes a shift in the spacing between said first and second members.

2. An actuator according to claim 1 , each of said rolling surfaces defining a generally curved and tapered conical rolling surface.

3. An actuator according to claim 2, wherein each of said tapered rolling surfaces is formed circular in plan view and is aligned coaxially.

4. An actuator according to any one of claims 1 to 3 wherein the or one of said tapered rolling surfaces defines a plurality of concentric grooves of diminishing diameter and each said rolling element is of generally frustoconical form and defines a spiral groove on its outer surface and the respective grooved surfaces of said tapered rolling surface and said rolling elements being arranged for meshing engagement, the engagement being such as to cause each said rolling element to traverse said tapered rolling surface upon relative rotation between said first and second members.

5. An actuator according to any one of claims 1 to 3, wherein the or one of said tapered rolling surfaces defines a spiral groove and each said rolling element is of generally frustoconical form and defines a plurality of grooves coaxially spaced on its outer surface and the respective grooved surfaces of said tapered rolling surface and said rolling elements being arranged for meshing engagement, the engagement being such as to cause each said rolling element to traverse said tapered rolling surface upon relative rotation between said first and second members.

6. An actuator according to claim 5 wherein said tapered rolling surface includes a single-start spiral groove.

7. An actuator according to claim 5 wherein said tapered rolling surface includes multiple spiral grooves.

8. An actuator according to any one of claims 4 to 7, wherein each of said rolling surfaces defines grooves for meshing engagement with said rolling elements.

9. An actuator according to any one of claims 4 to 8, each of said rolling elements being respectively slidably mounted on a radially extending shaft which is rotatable in an orbit in the direction of relative rotation between said first and second members and which permits rolling and radial movement of said rolling element along said shaft, each said shaft being fixed relative to each other so that said rolling elements roll, upon relative rotation between said first and second members, in a fixed angular relationship relative to each other.

10. An actuator according to claim 9, a plurality of rolling elements being mounted on one or more of said shafts.

11. An actuator according to claim 9 or 10, each said shaft extending radially from a ring disposed about the axis of rotation of said rolling elements.

12. An actuator according to any one of claims 1 to 3 wherein said rolling elements are spherical ball bearings and said tapered rolling surface includes spiral grooves substantially complementary to the shape of said bearings for receipt thereof so that said rolling elements roll in said spiral grooves and traverse said tapered rolling surface upon relative rotation between said first and second members.

13. An actuator according to claim 12, wherein a separate spiral groove is provided to receive each separate rolling element.

14. An actuator according to claim 12 or 13, said ball bearings being confined within a bearing cage so that they roll, upon relative rotation between said first and second members, in a fixed angular relationship relative to each other.

15. An actuator according to any one of claims 12 to 14, wherein each of said rolling surfaces includes a spiral groove or grooves for receipt of said rolling elements.

16. An actuator according to any one of claims 1 to 3, said rolling elements being of generally frustoconical form and each being mounted on a radially extending shaft which is rotatable in the direction of relative rotation between said first and second members, each said rolling element cooperating with said shaft on which it is mounted so that rolling rotation of said rolling element relative to said shaft results in movement of said rolling element along said shaft for traversal of said tapered rolling surface by said rolling element.

17. An actuator according to claim 16, said rolling element cooperating with said shaft through threaded engagement.

18. A disc brake assembly including a pair of brake shoes disposed for engagement on either side of a rotor, an actuator for actuating said brake shoes into engagement with the braking surface of said rotor, said actuator including first and second members, each of which has a rolling surface, said first and second members being arranged so that the respective rolling surfaces are disposed in substantially opposed relationship and for relative rotation, said actuator further including rolling elements disposed between said first and second members and in engagement with the respective rolling surfaces thereof, that engagement causing said rolling elements to roll relative to said rolling surfaces upon relative rotation between said first and second members, at least one of said rolling surfaces being at least part conical so as to define a generally curved and tapered surface, each said rolling element being constrained to roll in a spiral path upon relative rotation of said first and second members so as to traverse the gradient of said tapered rolling surface, and the other of said rolling surfaces having a profile so that traversal of said tapered rolling surface by said rolling elements causes a shift in the spacing between said first and second members, said first member being drivable to rotate by electric driving means and said second member being coupled to one of said brake shoes and being fixed against rotation, whereby rotation of said first member by said electric driving means in a first direction increases the spacing between said first and second members so that said second member is caused to shift and thereby to actuate said brake shoes into engagement with said rotor, and whereby rotation of said first member in a second direction, reverse to said first direction, decreases the spacing between said first and second members for withdrawal of said brake shoes from engagement with said rotor.

19. A disc brake assembly according to claim 18, said first member being mounted on a shaft, and said second member being aligned coaxially with said first member.

20. A disc brake assembly according to claim 18, said first member being mounted in threaded engagement on a threaded section of a shaft which is driven by said electric drive means, said threaded engagement being such that rotation of said shaft in a first direction is operable to take up lost travel as necessary by rotating relative to said first member and shifting said actuator axially with respect to said shaft to advance said brake shoe to engage said rotor whereafter said threaded engagement locks to prevent further relative rotation between said shaft and said first member, further rotation of said shaft in said first direction being operable to cause rotation of said first member relative to said second member.

21. A disc brake assembly according to claim 20, said rolling elements having a home position at a point of least spacing between said first and second members, and wherein rotation of said first member in said first direction causes said rolling elements to roll away from said home position toward an end position at a point of maximum spacing between said first and second members for brake actuation, and whereby rotation of said first member in said second direction causes said rolling elements to roll toward said home position, said assembly including wear compensation means which is operable to ensure return of said rolling elements to said home position despite friction lining wear that occurs during a brake application, said wear compensation means being operable to maintain rotation of said first member with said shaft until said rolling elements return to said home position after said brake shoes have disengaged from said rotor in said second direction of rotation.

22. A disc brake assembly according to claim 21 , said wear compensation means including a drag device which is fixed to one of said first member or said shaft and which can slip relative to the other, the device imparting a drag load to said first member in said second direction of rotation to maintain rotation of said first member with said shaft until said rolling elements return to said home position, whereafter said drag device is operable to slip relative to said other of said first member or said shaft.

23. A disc brake assembly according to claim 22, said drag device being in frictional engagement with said other of said first member or said shaft, which engagement is of greater frictional resistance in said second direction of shaft rotation than in said first direction so that said drag device can slip more readily in said first direction than in said second direction.

24. A disc brake assembly according to claim 23, said drag device including an elongate member which extends at least partly about said first member or said shaft in said frictional engagement therewith and which extends into substantially fixed engagement with said other of said first member or said shaft.

25. A disc brake assembly according to claim 23, said drag device including an annular member which is fixed to said first member or said shaft and which has at least one finger extending into said frictional engagement with said other of said first member or said shaft.

26. A disc brake assembly according to claim 18, said first member being mounted in fixed relationship on a shaft which is driven by said electric drive means, said shaft being in threaded engagement with a drive member of said drive means and said threaded engagement being such that rotation of said drive member in a first direction is operable to take up lost travel as necessary by rotating relative to said shaft and shifting said shaft and said actuator axially with respect to said shaft to advance said brake shoe to engage said rotor, whereafter said threaded engagement locks to prevent further relative rotation between said drive member and said shaft, further rotation of said shaft in said first direction being operable to cause rotation of said first member relative to said second member.

27. A disc brake assembly according to claim 26, wherein said drive member is the rotor of said electric driving means.

28. A disc brake assembly according to claim 26, wherein said drive member is the output member of a gear assembly.

29. A disc brake assembly according to any one of claims 26 to 28, said rolling elements having a home position at a point of least spacing between said first and second members, and wherein rotation of said first member in said first direction causes said rolling elements to roll away from said home position toward an end position at a point of maximum spacing between said first and second members for brake actuation, and whereby rotation of said first member in said second direction causes said rolling elements to roll toward said home position, said assembly including wear compensation means which is operable to ensure return of said rolling elements to said home position despite friction lining wear that occurs during a brake application, said wear compensation means being operable to maintain rotation of said shaft with said drive member until said rolling elements return to said home position after said brake shoe has disengaged from said disc brake rotor in said second direction of rotation. .

30. A disc brake assembly according to any one of claims 18 to 29, said brake assembly including a hollow piston defining an end face and a skirt depending from said end face, said brake shoe being mounted against the outside of said piston end face and said actuator being mounted within said piston, said second member being in engagement with said end face on the opposite side to said brake shoe and said first member being mounted and biased toward said second member for limited movement relative to said skirt in a direction toward or away from said end face, rotation of said first member by said electric drive means to increase the spacing between said first and second members causing said second member to shift said piston toward said rotor and said first member to shift relative to said skirt.

31. A disc brake assembly according to claim 30, said first member being mounted against said skirt with biasing means that biases said first member toward said end face and said second member towards said first member so that when said drive means rotates said shaft and said first member to reduce the spacing between said first and second members said piston shifts away from said rotor.

32 A drum brake assembly including a brake shoe arrangement disposed within a brake drum, which arrangement is expandable into braking engagement with a braking surface of the drum, and an actuator for actuating said brake shoe arrangement between expanded and contracted conditions, said actuator including first and second members, each of which has a rolling surface, said first and second members being arranged so that the respective rolling surfaces are disposed in substantially opposed relationship and for relative rotation, said actuator further including rolling elements disposed between said first and second members and in engagement with the respective rolling surfaces thereof, that engagement causing said rolling elements to roll relative to said rolling surfaces upon relative rotation between said first and second members, at least one of said rolling surfaces being at least part conical so as to define a generally curved and tapered surface, each said rolling element being constrained to roll in a spiral path upon relative rotation of said first and second members so as to traverse the gradient of said tapered rolling surface, and the other of said rolling surfaces having a profile so that traversal of said tapered rolling surface by said rolling elements causes a shift in the spacing between said first and second members, said actuator being disposed substantially between opposed brake shoe ends and being coupled thereto by coupling means for increasing and decreasing the separation between those ends, said first member being drivable to rotate by electric driving means and said second member being coupled to a first of said brake shoe ends against rotation, whereby rotation of said first member by said electric driving means in a first direction increases the spacing between said first and second members to thereby increase the spacing between said shoe ends to expand said brake shoe arrangement, and whereby rotation of said first member in a second direction, reverse to said first direction, decreases the spacing between said first and second members to thereby reduce the spacing between said shoe ends to contract said brake shoe arrangement.

33. A drum brake assembly according to claim 32, said first member being mounted in threaded engagement on a shaft which is driven by said electric drive means, said threaded engagement being such that rotation of said shaft in a first direction is operable to take up lost travel as necessary by rotating relative to said first member and extending said coupling means in engagement with said shoe ends to expand said brake shoe arrangement to engage said braking surface whereafter said threaded engagement locks to prevent further relative rotation between said shaft and said first member, further rotation of said shaft in said first direction being operable to cause rotation of said first member relative to said second member.

34. A drum brake assembly according to claim 33, said rolling elements having a home position at a point of least spacing between said first and second members, and wherein rotation of said first member in said first direction causes said rolling elements to roll away from said home position toward an end position at a point of maximum spacing between said first and second members for brake actuation, and whereby rotation of said first member in said second direction causes said rolling elements to roll toward said home position, said assembly including wear compensation means which is operable to ensure return of said rolling elements to said home position despite friction lining wear that occurs during a brake application, said wear compensation means being operable to maintain rotation of said first member with said shaft until said rolling elements return to said home position after said brake shoe arrangement has disengaged from said braking surface in said second direction of rotation. .

35. A drum brake assembly according to claim 34, said wear compensation means including a drag device which is fixed to one of said first member or said shaft and which can slip relative to the other, the device imparting a drag load to said first member in said second direction of rotation to maintain rotation of said first member with said shaft until said rolling elements return to said home position, whereafter said drag device is operable to slip relative to said other of said first member or said shaft.

36. A drum brake assembly according to claim 35, said drag device being in frictional engagement with said other of said first member or said shaft, which engagement is of greater frictional resistance in said second direction of shaft rotation than in said first direction so that said drag device can slip more readily in said first direction than in said second direction.

37. A drum brake assembly according to claim 36, said drag device including an elongate member which extends at least partly about said first member or said shaft in said frictional engagement therewith and which extends into substantially fixed engagement with said other of said first member or said shaft.

38. A drum brake assembly according to claim 36, said drag device including an annular member which is fixed to said first member or said shaft and which has at least one finger extending into said frictional engagement with said other of said first member or said shaft.

39. A drum brake assembly according to any one of claims 33 to 38, said shaft including a gear wheel mounted coaxially thereon and in meshing engagement with a drive gear driven by said electric drive means, said gear wheel shifting axially with said shaft upon relative movement between said first and second members and for friction lining wear compensation and said drive gear having an axial toothed width sufficient to maintain engagement with said gear wheel over said axial shift.

40. A drum brake assembly according to claim 32 , said first member being fixed relative to a shaft which is driven by said electric drive means and a gear wheel mounted on said shaft in threaded engagement therewith being driven by said electric drive means through a geared arrangement, said threaded engagement being such that rotation of said gear wheel in a first direction is operable to take up lost travel as necessary by rotating relative to said shaft and extending said coupling means in engagement with said shoe ends to expand said brake shoe arrangement to engage said braking surface whereafter said threaded engagement locks to prevent further relative rotation between said gear wheel and said shaft and whereby further rotation of said gear wheel in said first direction causes said shaft to rotate therewith which is operable to cause rotation of said first member relative to said second member.

41. A drum brake assembly according to claim 40, said gear wheel being rotatably coupled to a first of said coupling means associated with one of said shoe ends and being operable to shift said first coupling means axially and away from a second of said coupling means associated with the other of said shoe ends to extend said coupling means for lost travel take up and brake application.

42. A drum brake assembly according to claim 41 said gear wheel being rotatably coupled to said first coupling means by an axial bearing.

43. A brake assembly of the disc or drum kind including an actuator according to any one of claims 1 to 17.