GB2619301A

GB2619301A - An electric power steering apparatus

Info

Publication number: GB2619301A
Application number: GB2207987.5A
Authority: GB
Inventors: Hinks Derek
Original assignee: ZF Automotive UK Ltd
Current assignee: ZF Automotive UK Ltd
Priority date: 2022-05-30
Filing date: 2022-05-30
Publication date: 2023-12-06
Also published as: DE102023204470A1; GB202207987D0

Abstract

An electric power assisted steering system 10 is provided with a motor having a rotor and a stator, a motor housing, a power take off 410 rotating with the rotor, a gearbox housing with a worm shaft 20 having external helical worm teeth, and a worm wheel. The system also has a main bearing 50 supporting worm shaft 20 at an end closest to the motor, a tail bearing supporting the worm shaft relative to the housing at an end furthest from the motor and a flexible coupler assembly 730 comprising a first coupler 710 and second coupler 720. First coupler 710 is fixed against rotation relative to power take off 410 and second coupler 720 is fixed against rotation to worm shaft 20. and the couplers are interconnected to transfer torque from power take off 410 to worm shaft 20. A biasing assembly 80a acts between power take off 410 and worm shaft 20 to apply a biasing force from the motor through worm shaft 20 to main bearing 50.

Description

AN ELECTRIC POWER STEERING APPARATUS

This invention relates to electric power assisted steering systems of the worm and wheel type.

Electric power steering systems use an electric motor to produce an assistance torque that is applied to a rotating part of the steering system. in a conventional arrangement this torque assists the driver in turning the wheel. Because motors work best at relatively high speeds and because compact motors produce relatively low torques, the connection between the output of the motor and the steering column is usually through a reduction gearbox.

The most widely used type of electric power assisted steering reduction gearboxes arc of a relatively simple worm and gear configuration like that shown in Figure 1 of the IS accompanying drawings. A gearbox assembly I typically comprises a gearbox housing 9 which houses a wormshaft 2 and a worm wheel 3. The wormshaft 2 is connected to the output of an electric motor 4 (shown at the far left). The motor 4 may be secured to an end face of the housing 9 or even located within the housing 9. The wornishaft 2 is supported by two bearing assemblies; one either side of the region where the worm gear of the wormshaft 2 engages the worm wheel 3. The first bearing assembly in this description is called a main bearing assembly 5 and is located at an end closest to the motor 4. The second bearing assembly is referred to as a tail bearing assembly 6 and is located at an end furthest from the motor 4, both bearing assemblies typically comprising bearing elements supported within an inner bearing race that is threaded onto the wormshaft 2 and an outer bearing race that is secured to the housing 9 of the gearbox assembly 1. The function of the bearing assemblies 5, 6 is to allow the wormshaft 2 to rotate whilst to a certain degree limiting axial and radial movement. The worm wheel 3 is connected to an output shaft of the gearbox and located so that teeth of the worm wheel 3 engage teeth of the wormshaft 2.

The connection between the wormshaft 2 and a motor power take off 41 must allow a certain amount of relative movement. This is because the wormshaft 2 is configured to pivot around the main bearing 5 whilst being pressed into engagement with the worm wheel 3 to resist this movement. In many arrangements this is done to ensure the teeth of the wormshaft 2 follow any offset of the worm wheel and remain fully meshed. Without this a rattle can occur as the worm wheel 3 reverses direction.

As shown in Figure 1, and better visible in the exploded partial view of Figure 2, the relative movement can be accommodated by rounding the tip of the motor shaft 42 and locating this within a complimentary socket in a bushing 21 that is connected to the wormshaft 2. in the prior art such as the arrangement disclosed in the applicant's earlier patent application PCT/GB017/052892 from which the image of Figure 1 is taken, the system further comprises two separate couplers; one part fixed to the motor shaft 71 and the other 72 to the wormshaft 2. Each of these two couplers 71, 72 is provided with teeth that interlock the two parts to provide a path for the flow of torque from the motor 4 to the wormshaft 2. Assemblies of this type may also include a flexible coupler 73, disposed between the two couplers 71,72 having a first hub part providing a connection to the wormshaft 2 and a second hub part providing a connection to the power take off 41 from the motor 4. In that case one end of the flexible coupler 73 may be considered to be a first part and the other a second part within the meaning of this application.

An object of the invention is to ameliorate problems associated with prior art steering apparatuses of the kind described above.

According to a first aspect the invention provides an electric power assisted steering apparatus comprising: a motor having a rotor and a stator, a motor housing and a power take off that rotates with the motor rotor; a gearbox housing which houses a wormshaft, incorporating one or more external helical worm teeth, and a worm wheel; a main bearing assembly that supports the wormshaft relative to the housing at an end closest to the motor: a tail bearing assembly that supports the wormshaft relative to the housing at an end furthest from the motor; a flexible coupler including a first coupler and a second coupler, wherein the first coupler is fixed against rotation relative to the power take off of the motor and the second coupler is fixed against rotation to the wormshaft, the flexible coupler providing a path for the transfer of torque from the power take off to the wormshaft; and a biasing assembly that acts between the power take off of the motor and the wormshaft to apply a biasing force from the motor through the wormshaft to the main bearing assembly.

The applicant appreciates that provision of such a biasing force allows the inner race of the main bearing to be preloaded in a direction away from the motor into its seat in the housing of the gearbox. The provision of the biasing assembly enables this to be controlled precisely.

The biasing force may be substantially parallel to the axis of the power take off The biasing force may act to increase the distance between the wormshaft and the motor The biasing assembly may include a first annular part that is coaxial with a motor shaft of the motor and engages the power take off or the first coupler fixed relative to the power take off.

The biasing assembly may further include a second annular part that engages the wormshaft or the second coupler fixed relative to the wormshaft.

One or both of the annular parts may include a ring, or a disc or a cylinder. In case there will be a central hole allowing the annular part to slide onto the power take off of the motor. The term ring will be used hereinafter to define any such part annular part that has a hole in it.

The biasing assembly may additionally include a biasing means arranged such that a force is generated the two rings that biases the two rings away from each-other.

The biasing means may be configured such that the force is generated by compression of the biasing means by the two rings of the biasing assembly.

In one advantageous arrangement, the biasing means may comprise a coil spring having a length in which the long axis of the coils of the coil spring is coaxial with the power take off. The power take off in such an arrangement therefore extends through the coil spring which has a larger diameter than the power take off.

Each of the first and second rings may have a radially extending shoulder or flange that faces towards a shoulder or flange of the other ring with the coil spring held in compression between and acts upon the two shoulders.

Either one of the first and second ring of the two rings may define an annular chamber in which an annular piston portion of the other ring is slidably located. The chamber may be at least partially filled with a grease that provides damping to movement of the piston in the chamber.

To define the chamber a first ring may comprise two concentric cylindrical portions that define an annular chamber thercbetwcen having an open end that faces the second ring, the two cylinders concentric with the power take off shaft. The second ring may comprise a cylinder that has is sized to slide telescopically into the annular chamber.

In an alternative to the provision of a coil spring that is compressed to directly generate the required axial biasing force, the biasing means may comprise a resilient member. The biasing means may be configured such that the force is generated by compression of the resilient member. Alternately, the biasing assembly may be configured such that the force is generated through tension of the resilient member.

In this arrangement the biasing means may be compressed by a force parallel to the axis of the power take off, the biasing means may comprise a loop or band or ring that is stretched or compressed by a force that is generally orthogonal to the axis of the power take off.

The resilient member may comprise an elastically resilient 0-ring that is coaxial with 30 the motor power take off. By 0-ring we mean any loop which can be expanded in diameter or reduced in diameter by applying a radial force at locations around the loop and which will resist such a change in size by generating an opposing force.

The 0-ring may comprise a rubber ring or a ring of other elastomeric material. It may have a circular cross section that is constant around the whole ring. It may be toroidal in its free state when not subject to any external forces. it may be concentric with the power take off shaft of the motor.

The 0-ring may engage chamfered surfaces on both the first and second rings whereby movement of the two rings towards or away from each other applies a radial force to the 0-ring that is opposed by the elasticity of the 0-ring.

Each of the first and second ring may comprise a single continuous chamfered surface that extends around the whole ring. The two chamfered surfaces may both contact the 0-ring and form a V-shaped channel into which the 0-ring is located. The width of the V-shaped channel may increase or decrease as the two rings move axially relative to one another. The width of the channel may be smaller than the cross-sectional width of the 0-ring. The width of the channel and the 0-ring being measured parallel to the power take-off axis. I5

Each of the first and second ring may comprise a plurality of fingers that each define a chamfer that engages the 0-ring. The fingers of one ring may be spaced circumferentially in between the fingers of the other ring. In this way, the fingers of one ring may be located at the same distance from the axis of the power take off to the fingers of the other ring. The 0-ring may move along the chamfers as the two rings move axially.

in the arrangements described hereinbefore the two rings move relative to one another against a biasing force but other arrangements are possible within the scope of the present invention.

For example, in a still further alternative arrangement, the biasing assembly may comprise a first ring that is coaxial with a motor shaft of the motor and engages the power take off or the first coupler fixed relative to the power take off, a second ring that engages the wormshaft or the second coupler fixed relative to the wormshaft; wherein the first ring and/or the second ring comprises one or more guide parts which limit the relative axial motion of the first ring with respect to the second ring, and a resilient biasing means located between one of the rings and the wormshaft, or between the power take off and one of the rings, that is held in compression so that the two rings and biasing means together provide the biasing force to the main bearing.

The two rings may be arranged such that during installation they may be pushed together into any one of a plurality of pre-set configurations. Each configuration may set a fixed distance between the outward facing ends of the two rings. The outwardly facing end of the first ring may be defined as the surface that engages the power take off or the first coupler. The outwardly facing end of the second ring may be defined as the surface that engages the wormshaft or the second coupler. The biasing force is this arrangement is generated directly by compression of the biasing means.

One of the first and second rings may be provided with a plurality of circumferential ribs and the other of the first and second rings may be provided with resilient tang that are disposed at spaced locations around the circumference of the ring. The resilient IS tangs may be snapped into a position between any adjacent pair of ribs to set the configuration of the two rings.

Each rib may extend continuously around a circumference of the ring. Alternatively, each rib may extend around only a part of the circumference of the ring, each rib being offset axially by a small distance from a circumferentially adjacent rib.

The biasing means may comprise a resilient spacer that acts between an outwardly facing end of the ring that faces the wormshaft and is held in compression between the wormshaft and that end face of the ring.

The biasing assembly may be located in an annular void between the first and second coupler parts and the power take off of the motor shaft.

In at least one arrangement the flexible coupler may include a third coupler part that is resilient, and which connects the first coupler to the second coupler. For example, the first and second couplers may define interlocking dog teeth and the flexible coupler may include elements that fit fill spaces between the dog teeth. Torque is therefore transferred between the first and second couplers through these elements. The dog teeth and elements may be spaced circumferentially around the axis of the motor power take off The biasing assembly may be fixed to the third coupler part. In some arrangements one of both of the first and second rings of the coupler part may form the third coupler part.

The power take off may comprise an extension of the motor rotor or s a separate component that is fixed to an end of the motor rotor.

The free end of the power take off may include a plurality of axially extending splines that engage splines on the first coupler. The free end of the power take off may be defined as the end not directly connected to the motor shaft or motor rotor.

There will now be described, by way of example only, four embodiments of a biasing assembly for a gearbox that falls within the scope of the present invention with reference to and as illustrated in the accompanying drawings of which: Figure 1 is cross section view of a prior art electric power assisted steering gearbox; Figure 2 is an enlarged cross-sectional view of portion of an electric power assisted steering apparatus according to a first embodiment of the invention; Figures 3a and 3b are views of a biasing assembly of the first embodiment of an electric power assisted steering apparatus according to an aspect of the invention; Figure 4 is a view of a biasing assembly of a second embodiment of an electric power assisted steering apparatus, according to an aspect of the invention, in an extended configuration; Figure 5 is a view of a biasing assembly of a second embodiment of an electric power assisted steering apparatus, according to an aspect of the invention, in a compressed configuration; Figure 6 is a view of a biasing assembly of a third embodiment of an electric power assisted steering apparatus according to an aspect of the invention; and Figure 7, is a view of a biasing assembly of a fourth embodiment of an electric power assisted stccring apparatus according to an aspect of the invention.

A first embodiment of the invention is shown in Figures 2 to 3b.

Figure 2 shows part of an electric power assisted steering apparatus 10 according to a first embodiment of the invention. The steering apparatus 10 provides a geared reduction in the output of an electric motor of the steering apparatus, allowing torque generated by the motor to be transferred to the steering column or rack (or other part of the steering system), the torque assisting the driver to turn the wheel or providing the principal source of steering torque.

The steering apparatus 10 comprises a gearbox housing (not shown) which houses a wormshaft 20 incorporating one or more external helical worm teeth, and a worm wheel which is carried by an output shaft. The gear ratio of the steering assembly is C\J set by the relative shape and number of teeth of the wormshaft 20 and the wheel, each C\I rotation of the wormshaft 20 causing the wheel to advance by a set number of teeth M) corresponding to a fraction of a whole revolution of the wheel.

C\I 20 A main bearing assembly 50 supports the wormshaft 20 at an end closest to the motor C\J and a tail bearing assembly supports the wormshaft at an end furthest from the motor, the bearings enabling the shaft to rotate about its long axis. The main bearing assembly 50 may comprise a first holding part 51, or outer race, fixed relative to the housing, a second holding part 52, or inner race, fixed relative to the wormshaft, and one or more bearings 53 accommodated by recesses in the first and second holding part 51, 52. The tail bearing assembly is free to move relative to the housing through a limited range of motion that enables the wormshaft 20 to move radially away from the axis of the wheel gear.

The wormshaft 20 is connected to a power take off from an electric motor such that torque is transferred from the power take off 410 to the wormshaft by a flexible coupler assembly. This comprises a first coupler 710 directly connected to the power take off 410, a second coupler 720 directly connected to the wormshaft 20; and a third (flexible) coupler part 730 disposed therebetween. The first coupler 710 is fixed against rotation relative to the power take off 410 and the second coupler 720 is fixed against rotation relative to the wormshaft 20. The first 710, second 720 and third 730 couplers are arranged so at to transfer torque from the power take off 410 to the wormshaft 20. The flexible coupler assembly may be of the kind described in PCT/GB017/052892. The third coupler part 730 may be separate from or integrally formed with one or both of the first 710 and second 720 couplers.

The steering apparatus 10 further comprises a biasing assembly 80a that acts between the power take off 410 of the motor and the wormshaft 20 to apply a biasing force from the motor through the wormshaft 20 to the main bearing assembly 50. This biasing force may reduce or eliminate rattle at the main bearing assembly through relative shear motion of the first holding part 51 relative to the second holding part 52 of the main bearing assembly 50. This shear may increase the area through which the one or more bearings 53 contacts the recesses of the first and second holding parts 51, 52 of the main bearing assembly 50, thereby ameliorating rattle.

This has the advantage of preloading the main bearing assembly 50 independently of the flexible coupler 730 and may eliminate the need for spacer shims previously utilised for this role.

The biasing assembly 80a is located in an annular void between an inner diameter of the first 710 and second 720 coupler parts and the power take off 410 of the motor shaft 420. In this embodiment, the biassing assembly 80a resides completely within the inner diameter of the first 710, second 720 and third 730 couplers.

The biasing assembly 80a comprises a first ring 81a that is coaxial with a motor shaft of the motor and engages the power take off 410 or the first coupler 710. The biasing assembly 80a further comprises a second ring 82a that may be coaxial with the wormshaft 20 and engage the wormshaft 20 or the second coupler 82a fixed relative to the wormshaft 20.

By ring we mean a component that has a central hole allowing it to be fitted onto the motor power take off 410. it may be an annular ring as shown in the embodiments but could have other shapes as long as it can slide over the power take off 410 unimpeded. To locate the ring in position the shape of this central hole should be complimentary to, and slightly larger than, the shape of the power take off 410 or the end of the motor shaft 420 onto which it is fitted.

Figure 2 shows the first ring 81a directly contacting the power take off 410 and the second ring 82a directly contacting the wormshaft 20. The motor shaft 420 extends through the centre of the first 81a and second ring 82a. The biasing assembly 80a is located in an annular void between the first 710 and second 720 coupler parts and the power take off 410 of the motor shaft. in this embodiment, the biassing assembly 80a resides substantially completely within that annular void.

A biasing means 83a is included in the biasing assembly 80a, arranged to generate a force 84a between power take off 410 and the wormshaft 20 that biases the wormshaft 20 away from the power take off 410. As shown by the arrows in Figure 2, the force 84a may be generated between the two rings 81a, 82a and bias the two rings 81a, 82a away from each-other. in the first embodiment, the biasing member 83a is a resilient member, more specifically an elastic 0-ring. This may comprise a rubber or other material. In an alternative it may comprise a coil spring that is wrapped around to form a continuous loop, allowing the loop to be expanded or compressed to vary its inner and outer circumference. Such a device is considered to fall withing the meaning of the term 0-ring as used herein, along with any other device that has an internal size that can be enlarged and which will provide a resisting force.

As best shown by Figures 3a and 3b, each of the first and second rings 81a, 82a comprise a plurality of interdigitating fingers 811a, 821a projecting generally towards the other ring. The fingers 811a, 821a of one of the first and second ring being spaced circumferentially in between the fingers of the other of the first and second ring such that the fingers of one ring are located at the same distance from the axis of the power take off to the fingers of the other ring These fingers 811a, 821a each comprise a chamfer that engages the 0-ring 83a. The slope of the chamfers is such that the outer diameter of each of the first and second rings 81a, 82a decreases toward the other of the first and second ring 81a, 82a. The effect of the chamfers of the first and second rings 81a, 82a in conjunction with the resilient biasing member 83a is that, on compression of the biasing assembly 80a along the axis of either or both the first and second rings 81a, 82a, the biasing means 83a in placed in tension (i.e., the elastic ring is made to expand), the 0-ring 83a moving along the chamfers as the two rings 81a, 82a move axially. This in turn generates a force substantially parallel to the motor shaft 420 and/or wormshaft 20 that acts to push the wormshaft 20 away from the motor. In this way, a biasing force is transferred from the motor through the wormshaft 20 to the main bearing assembly 50.

This process is best viewed through comparison of Figure 3a and 3b which show the biasing assembly 80a according to the first embodiment in an uncompressed and compressed configuration respectively. Compression of the biasing assembly 80a during installation or use reduces the axial distance between outward facing ends of the two rings 81a, 82a. The outwardly facing end of the first ring 81a being defined as the surface that engages the power take off 410 or the first coupler 710 and the outwardly facing end of the second ring 82a being defined as the surface that engages the wormshaft 20 or the second coupler 720. The resultant biasing force 84a produced by the elastic 0-ring 83a shown by the block arrows in Figure 3b.

The chamfers may be substantially planar, or may be arcuate, as in Figures 3a and 3b. A convex arcuate shape may better engage the 0-ring 83a and ameliorate problems associated with high stress sites between the 0-ring 83a and the two rings 81a, 82a, at the edges of the chamfers. Such high stress sites could otherwise reduce the lifetime of the elastic 0-ring 83a.

The fingers 81 I a, 821a of the first and second ring 8 I a, 82a act in conjunction with raised portions 85a disposed on the first and second rings 81a, 82a to limit the relative axial motion of the first ring 81a with respect to the second ring 82a. The interaction of the plurality of interdigitating fingers 85a disposed on each of the first and second rings 81a, 82a will act to limit the relative coaxial rotation of the first ring 8 la to the second ring 82a. Similarly, the raised portions 85a on one of the first and second rings 81a, 82a may interact with the plurality of fingers 85a on the other of the first and second ring 81a, 82a to limit coaxial rotation of the first ring 81a with respect to the second ring 82a.

Figures 4 and 5 show a biasing assembly 80b of an electric power assisted steering apparatus according to a second embodiment of the claimed invention. This assembly comprises a first ring 81b that is coaxial with the motor shaft 420 of the motor and engages the power take off 410 or the first coupler 710 fixed relative to the power take off 410 and a second ring 82b that engages the wormshaft 20 or the second coupler 720 fixed relative to the wormshaft 20. As with the first embodiment the two rings 81b, 82b can slide along the motor power take off 410. Unlike the previous embodiment the two rings 81b, 82b are fixed axially in position relative to one another when in use. A biasing means 83b is provided in the form of a resilient spacer that fits between the second ring 82b and an end face of the wormshaft 20. As shown, this comprises an O-ring bonded to a flange 822b on the end of the second ring 82b that faces the wormshaft 20. The overall length of the biasing assembly 80b is therefore the combined length of the two rings 81b, 82b in their fixed relationship plus the thickness of the 0-ring 53b.

The first ring 81b comprises a cylindrical body that extends away from a flange 812b that engages the first coupler part 710. This carries a plurality of ribs 87b on an outer circumferential surface. As shown, there are multiple sets of ribs 87b, each set comprising three axially spaced ribs, and each set extending around a part of the circumference of the cylindrical body of approximately 20 degrees. The ribs in each set are offset axially from the ribs of the immediately adjacent set by a distance less than the spacing between each rib in a set.

The second ring 82b comprises a set of fingers 82l b that project from the flange 822b that engages with the end of the wormshaft 20. These fingers 821b are spaced around the circumference of the flange 822b and extend in parallel with the axis of the power take off 410 over the cylindrical body of the first ring 81b. Each finger 821b carries a tang 88b at a free end that projects radially inwards. The fingers 821b are resilient and this allows the two rings 81b, 82b to be pushed together with each tang SRI) snapping in between two ribs 87b of a set on the cylindrical body. The overall length of the two rings 81b. 82b, defined as the spacing between the outer faces of the two flanges 812b, 822b, can be adjusted according to which position the tangs 88b are located.

To assemble the gearbox assembly, the two rings 81b, 82b are threaded onto the power take off 410 with a small space between them. The gearbox and motor are pushed together and the rings 81b, 82b will move into engagement. Once fully tightened the tangs 88b will snap into one of the grooves formed between two axially spaced ribs 876. The 0-ring 83b can then compress as required to take up any pivoting movement of the wormshaft 20 about the main bearing assembly 50. Of course, the two rings 81b, 82b can be pre-assembled after taking a measurement of the required overall length during assembly of the gearbox to the motor.

The skilled person will understand that the two rings 81b, 82b may be arranged such that during installation they may be pushed together into any one of a plurality of preset configurations. There is no requirement for selection of a specific width of spacer shim during assembly as required in the prior art.

Figure 6 shows a biasing assembly 80c of an electric power assisted steering apparatus according to a third embodiment of the claimed invention. This biasing assembly 80c is substantially similar to the biasing assembly 80a according to the first embodiment of the invention (as shown in Figures 2 to 3b) and achieves a substantially similar result. The biasing assembly 80c of the third embodiment differs from the biasing assembly 80a of the first embodiment in the configuration of the first and second rings 81c, 82c. Notably these rings 81c, 82c serve double duty by also acting as part of the flexible coupler 730 through which the torque is transferred from the motor to the wormshaft 20.

Similarly to the biasing assembly 80a of the first embodiment, the biasing assembly 80c of the third embodiment comprises a first ring 81c that has the form of a disc being relatively thin compared with its external diameter-that is coaxial with a motor shaft 420 of the motor and engages the power take off 410 or the first coupler 710.

The biasing assembly 80c further comprises a second ring 82c that may be coaxial with the wormshaft 20 and engage the wormshaft 20 or the second coupler 720 fixed relative to the wormshaft 20. This also has a disc shape and the same outer diameter as the first ring 81c. The two are sandwiched together face to face. The first ring 81c directly contacts a radially extending shoulder of the first coupler 710 which limits movement of the first ring 81c away from the wormshaft 20. The second ring 82c directly contacts the wormshaft 20. The motor shaft 420 extends through the centre of the first and second ring 81c, 82c.

A biasing means 83c is included in the biasing assembly 80c, arranged to generate a force between the power take off 410 and the wormshaft 20 that biases the wormshaft away from the power take off 410. The force being generated between the two rings 81c, 82c and biasing the two rings 81c, 82c away from each-other. The biasing member 83c is a resilient member, more specifically an elastic 0-ring.

Unlike the biassing assembly 80a of the first embodiment, the ring 81c, 82c of the biasing assembly 80c of the third embodiment does not comprise a plurality of interdigitating fingers projecting generally towards the other ring 81c, 82c. Instead, each of the first and second rings 81c, 82c comprise a single continuous chamfered surface 86c that extends around the whole ring 81c, 82c. The two chamfered surfaces 86c both contact the 0-ring 83c and form a V-shaped channel into which the 0-ring 83c is located. The slopes of the chamfered surfaces 86c are such that the inner diameter of each of the first and second rings 81c, 82c increases toward the other of the first and second ring 81c, 82c. The effect of the chamfered surfaces 86c of the first and second rings 81c, 82c in conjunction with the resilient biasing member 83c is that, on compression of the biasing assembly 80c along the axis of either or both the first and second rings 81c, 82c, the biasing member 83c in placed in compression. The compression of the 0-ring 83c is axial and circumferential. The width of the V-shaped channel decreases as the two rings 81c, 82c move axially toward one another. The width of the channel is smaller than the cross-sectional width of the 0-ring 83c. The width of the channel being measured parallel to the power take-off axis.

This compression in turn generates a force substantially parallel to the motor shaft 420 and/or wormshaft 20 that acts to push the wormshaft 20 away from the motor. in this way, a biasing force is transferred from the motor through the wormshaft 20 to the main bearing assembly 50.

in effect, the two discs 81c, 82c are pushed away from each other constantly due to compression of the 0-ring 83c, which generates the required biasing force on the main bearing assembly 50.

In this embodiment, the biassing assembly 80c extends beyond the inner diameter of the first and second couplers 710, 720 so that it can function as the flexible coupler part of the flexible coupler assembly. The first and second disc shaped annular rings 81c, 82c comprise apertures 89c that extend through the complete thickness of the biasing assembly 80c, with thickness measured parallel to the first and second rings' 81c. 82c axis. The apertures 89c of the first ring 8 lc are aligned with the apertures 89c of the second ring 82c and configured to accommodate protruding dog teeth of either or both the first and second couplers 710, 720.

The apertures 89c may be substantially wedge shaped and disposed circumferentially at regular intervals around the first and second rings for optimised transferal of torque from the motor to the wormshaft. The size and configuration of the apertures 89c may be adapted to optimise mechanical properties of the biasing assembly for a certain motor and gear ratio.

Figure 7 shows a biasing assembly 80d of an electric power assisted steering apparatus according to a fourth embodiment of the claimed invention. This biasing assembly 80d is substantially similar to the biasing assembly 80a according to the first embodiment of the invention (as shown in Figures 2 to 3b) and achieves a substantially similar result. The biasing assembly 80d of the fourth embodiment differs from the biasing assembly 80a of the first embodiment in the configuration of the first and second rings 81d, 82d Similarly to the biasing assembly 80a of the first embodiment, the biasing assembly 80d of the fourth embodiment comprises a first ring 8Id that is coaxial with a motor shaft 420 of the motor and engages the power take off 410 or the first coupler 710. The biasing assembly 80d further comprises a second ring 82d that may be coaxial with the wormshaft 20 and engage the wormshaft 20 or the second coupler 720 fixed relative to the wormshaft 20. The first ring 81d directly contacting the power take off 410 and the second ring 82d directly contacting the wormshaft 20. The motor shaft 420 extends through the centre of the first and second ring 81d, 82d.

A biasing means 83d is included in the biasing assembly, arranged to generate a force between power take off 410 and the wormshaft 20 that biases the wormshaft 20 away from the power take off 410. The force being generated between the two rings 81d, 82d and biasing the two rings 81d, 82d away from each-other. The biasing member 83d is a resilient member, more specifically a coil spring that is coaxial with the power take off 410.

Unlike the biassing assembly 80a of the first embodiment, the rings 81d, 82d of biasing assembly 80d of the fourth embodiment does not comprise a plurality of interdigitating fingers projecting generally towards the other ring 81d, 82d. Instead, each of the first and second rings 81d, 82d comprise a radially extending shoulder or flange 812d, 822d that faces towards the shoulder of the other ring 81d, 82d. The spring is located between and acting upon the two shoulders 812d. 822d.

The effect of the shoulders or flanges 8I2d, 822d of the first and second rings 81d, 82d in conjunction with the spring 83e is that, on compression of the biasing assembly 80d along the axis of either or both the first and second rings 81d, 82d, the spring 83d is placed in compression. The compression of the spring 83d being parallel to the axis of the spring 83d. The width of spring 83d decreases as the two rings 8 Id, 82d move axially toward one another. The width of the spring 83d being measured parallel to spring axis. I5

On compression of the spring 83d, one of the rings 81d, 82d moves telescopically over the other. As shown the second ring 82d telescopes over the first ring 81d but this can be reversed.

To control this telescopic movement, the first ring 81d has a twin wall construction defining an annular chamber 813d into which an annular piston portion 823d of the second ring is slidably located. The chamber-piston arrangement /1 I 3d, 823d ensures the axis of the first ring 8Id remains substantially; coplanar with the axis of the second ring 82d. This arrangement will also increase the force required for the first ring 81d to rotate relative to the second ring 82d along the axis of both rings 81d, 82d.

The chamber 813d is at least partially filled with a grease or other suitable material that provides damping to movement of the piston 823d in the chamber.

Any 0-rings 83a, 83b, 83c, 83d discussed herein may be made from rubber or any other resilient material.

Claims

CLAIMS1. An electric power assisted steering apparatus comprising: a motor having a rotor and a stator, a motor housing and a power take off that rotates with the motor rotor; a gearbox housing which houses a worm shaft, incorporating one or more external helical worm teeth, and a worm wheel; a main bearing assembly that supports the worm shaft relative to the housing at an end closest to the motor; a tail bearing assembly that supports the worm shaft relative to the housing at an end furthest from the motor; a flexible coupler assembly comprising a first coupler and second coupler, whereby the first coupler is fixed against rotation relative to the power take off of the motor and the second coupler is fixed against rotation to the worm shaft and the two are interconnected so at to transfer torque from the power take off to the worm shaft; and a biasing assembly that acts between the power take off of the motor and the worm shaft to apply a biasing force from the motor through the worm shaft to the main bearing assembly.
2. An apparatus according to claim I wherein the biasing assembly comprises: a first ring that is coaxial with a motor shaft of the motor and engages the power take off or the first coupler fixed relative to the power take off; a second ring that engages the worm shaft or the second coupler fixed relative to the a biasing means arranged such that a force is generated the two rings that biases the two rings away from each-other.
3. An apparatus according to claim 2 wherein the biasing means is configured such that the force is generated by compression of the biasing means by the two rings of the biasing assembly.
4. An apparatus according to claim 3 in which the biasing means comprises a coil spring that is coaxial with the power take off
5. An apparatus according to claim 4 in which each ring has a radially extending shoulder that faces towards the shoulder of the other ring and in which the spring is located between and acts upon the two shoulders.
6. An apparatus according to claim 5 in which one of the two rings defines an annular chamber in which an annular piston portion of the other ring is slidably located.
7. An apparatus is which the chamber is at least partially filled with a grease that provides damping to movement of the piston in the chamber.
8. An apparatus according to claim 3 in which the biasing means comprises a resilient member. I59.
An apparatus according to claim 8 wherein the biasing assembly is configured such that the force is generated by compression of the resilient member
10. An apparatus according to claim 2 wherein the biasing means comprises a resilient member and the biasing assembly is configured such that the force is generated through tension of the resilient member.
11. An apparatus according to claim 9 in which the resilient member comprises an elastically resilient 0-ring that is coaxial with the motor power take off.
12. An apparatus according to claim 11 in which the 0-ring engages chamfered surfaces on both the rings whereby movement of the two rings towards or away from each other applies a radial force to the 0-ring that is opposed by the elasticity of the
13. An apparatus according to claim 12 in which each ring comprises a single continuous chamfered surface that extends around the whole ring, the two chamfered surfaces both contacting the 0-ring and forming a V-shaped channel into which the O-ring is located, the width of the V-shaped channel increasing or decreasing as the two rings move axially relative to one another and in which the width of the channel is smaller than the cross sectional width of the 0-ring.
14. An apparatus according to claim 12 in which each ring comprises a plurality of fingers that each define a chamfer that engages the 0-ring, the fingers of one ring being spaced circumferentially in between the fingers of the other ring, the 0-ring moving along the chamfers as the two rings move axially.
15. An apparatus according to claim 1 in which the biasing assembly comprises: a first ring that is coaxial with a motor shaft of the motor and engages the power take off or the first coupler fixed relative to the power take off; a second ring that engages the worm shaft or the second coupler fixed relative to the worm shaft; wherein the first ring and/or the second ring comprises one or more guide parts which limit the relative axial motion of the first ring with respect to the second ring; and a resilient biasing means located between one of the rings and the worm shaft or between one of the rings and the first coupler that is held in compression so that the two rings and biasing means together provide the biasing force to the main bearing.
16. An apparatus according to claim 15 in which the two rings are arranged such that during installation they may be pushed together into any one of a plurality of preset configurations, each configuration setting a range of axial distances between the outward facing ends of the two rings or setting a fixed distance between the outward facing ends.
17. An apparatus according to claim 16 in which one of the first and second rings is provided with a plurality of circumferential ribs and the other of the first and second rings is provided with resilient tangs that can be snapped into a position between any adjacent pair of ribs to set the configuration of the two rings.
18. An apparatus according to any one of claims 15 to 17 in which the biasing means comprises a resilient spacer that acts between an outwardly facing end of the ring that faces the wormshaft and is held in compression between the wormshaft and that end face of the ring.
19. An apparatus according to any preceding claim in which the biasing assembly is located in an annular void between the first and second coupler parts and the power take off of the motor shaft.
20. An apparatus according to any preceding claim in which the power take off comprises an extension of the motor rotor or is a separate component that is fixed to an end of the motor rotor.
21. An apparatus according to claim 13 in which the free end of the power take off includes a plurality of axially extending splines that engage splines on the first coupler.
22. An apparatus according to any preceding claim in which the flexible coupler assembly include a third flexible coupler part through which torque is transferred between the first and second couplers.
23. An apparatus according to claim 22 in which one or both of the rings of the biasing assembly define the third flexible coupler.