GB2080894A - Vehicle braking actuator - Google Patents

Abstract

An actuator incorporates an automatic adjuster to compensate for wear of the brake linings. Instead of operating the adjuster in response to movement of one tappet assembly relative to the housing, which with known constructions can cause over-adjustment in some circumstances, it is arranged that the adjuster is responsive only to relative movement of the two tappet assemblies (12, 13). In order to facilitate sensing of the relative movements of the tappet assemblies, one tappet assembly (12) is provided with a depending sleeve (25) located on the opposite side of the wedge member (22) from the body (23) of that tappet, and within which a sleeve (26) of the second tappet assembly (13) is housed. The adjuster incorporates a drive ring (41) which is responsive to excessive relative axial movement of the sleeves 25, 26 to rotate sleeve 26 for increasing the effective length of the second tappet assembly (13). <IMAGE>

Description

SPECIFICATION Vehicle brake actuator This invention relates to a vehicle brake actua tor of the kind comprising a housing, first and second tappet assemblies working in bores in the housing and adapted to engage at their outer ends with respective brake friction mem I bers, and actuating means located in the housing between substantially opposed oper ating faces of the respective tappet assemblies for urging the tappet assemblies in opposite directions to apply the brake friction members against a brake rotor. Such a brake will herei nafter be referred to as a 'brake actuator of the kind set forth'.

The invention is particularly concerned with automatic adjuster mechanism in such an ac tuator for adjusting the retracted positions of the tappets to compensate for wear of the brake friction members.

Many adjuster mechanisms have been pro posed for use in conjunction with actuators of the kind set forth, particularly for use in internal shoe drum brakes.

Often a strut of adjustable length is posi tioned between abutments on the brake shoes of an internal shoe drum brake, and the length of the strut is arranged to be adjusted in response to excessive relative movement apart of the abutments. Such an adjuster has the advantage that it is responsive to relative movement of the brake shoes, and it is there fore substantially unaffected by a reversal of the torque on the brake shoes that may occur during vehicle rollback when stopping on a gradient. However, the strut is exposed to brake lining dust and may seize in use.

It is therefore common practice to incorpo rate the adjuster mechanism in the actuator itself. The mechanism is then usually arranged to be operated in response to excessive move ment of a part of one tappet assembly relative to the housing, and the mechanism is ar ranged to alter the length of the tappet assem bly to compensate for brake lining wear.

However, again torque reversal on the brake shoes can cause overload of the ad I juster mechanism with such arrangements, and it is often necessary to provide such an adjuster mechanism with an overload clutch which is arranged to be released during exces sive movement in the same direction of the tappet assemblies.

The invention aims to provide an adjuster mechanism for an actuator of the kind set forth which is located substantially within the actuator housing and which is operated in response response to relative movement of the tappet assemblies, rather than to movement of a tappet assembly relative to the housing.

According to the invention in a vehicle brake actuator of the kind set forth the first tappet assembly is provided with a projection which is located within the housing and which overlaps with a part of the second tappet assembly located within the housing, and the adjuster mechanism is responsive to relative movement of the projection and said part.

Preferably the projection and said part are disposed towards the outer end of the second tappet assembly from the operating face of the second tappet assembly.

The actuating means of the actuator is preferably a wedge member, but it may be a cam or an hydraulic piston and cylinder assembly.

When the actuating means is a wedge member, then said opposed operating faces are faces which are substantially parallel to the respective wedge member surfaces, and bearing elements are arranged between the wedge member surfaces and the corresponding operating faces.

The projection and said part preferably comprise co-axiai first and second sleeves, and the first sleeve associated with the first tappet assembly is preferably the radially outer of the two sleeves.

The first sleeve is preferably arranged to be non-rotatable in use, and the second sleeve is arranged to be rotatable and to have a threaded engagement with another part of the second tappet assembly to adjust the length of the second tappet assembly.

The rotation of the second sleeve relative to the first sleeve is preferably effected by a drive ring which has a threaded engagement with the second sleeve and a clutch engagement with the first sleeve.

The first tappet preferably comprises a body provided at its inner end with the respective operating face and with a pair of diametrically opposed, axially extending arms that connect the body with the first sleeve, and between which the wedge member is disposed.

Preferably the body, arms and sleeve are formed as an integral member.

The integral member is preferably provided with an abutment which is engaged by the second tappet assembly to determine the retracted positions of the tappets relative to each other.

An internal shoe drum brake incorporating an actuator in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a side elevation of the brake, with the drum removed; Figure 2 is asection of the line 2-2 of Fig.

1; Figure 3 is a longitudinal cross-section of the actuator of the brake of Fig. 1; Figure 4 is a perspective view, with a 90t sector cut away, of the first tappet of the actuator of Fig. 3; and Figure 5 is a part section on the line 5-5 of Fig. 3.

With reference to Figs. 1 and 2 the brake shown is of the kind in which a leading shoe 1 and a trailing shoe 2 have adjacent ends 3 and 4 respectively in engagement with a fixed abutment block 5 carried by a back plate 6.

The opposite ends 7 and 8 of the shoes are operated by an actuator assembly 9 comprising a housing 10 rigidly mounted on the back plate 6. The usual tension springs 11 maintain the ends of the shoes in engagement with the abutment 5 and actuator 9.

With reference to Fig. 3, the housing 10 is provided with a longitudinal through bore 11 in which work first and second tappet assemblies 1 2 and 1 3 respectively, and with an intersecting transverse bore 14 through which extends a conventional wedge actuating means 1 5. The first and second tappet assemblies 1 2 and 1 3 are provided in the usual manner with projecting outer end portions 1 6 and 1 7 respectively that are slotted to have a keyed engagement with the respective ends 8 and 7 respectively of the shoes 2 and 1, and with opposed inclined operating surfaces 18 and 1 9 which are arranged to be urged apart in the usual manner by rollers, carried in a cage 21, in response to inward movement of the wedge member 22 of the wedge actuating means 15.

However, as distinct from conventionai arrangements, the first tappet, as shown in Fig.

4 comprises a cylindrical body 23 connected by a pair of diametrically opposed, axially extending arms 24 to a first sleeve 25, and as shown in Fig. 3 the first sleeve 25 overlaps with, and is coaxial with a second sleeve 26 of the second tappet assembly 1 3. It will be seen that the first tappet 1 2 has been formed as an integral member by turning, and that the arms 24 have been produced by broaching a hole 27 of rectangular transverse crosssection through the centre of the tappet.

As shown in Fig. 3, the cage is guided for movement by the opposed planar faces 28 of the arms 24.

The inclined operating faces 18 and 1 9 are provided respectively on independent ramp members 29, 30 respectively which are each provided with a short locating spigot 31. The ramp members 29, 30, as shown in Fig. 5 are of complementary shape in transverse cross-section to that of the aperture 27. The spigot 31 of the ramp member 29 is received within a blind bore 32 provided in the body 23 of first tappet 12, whereas the spigot 31 of the ramp member 30 is received within the bore 33 at the inner end of second sleeve 26, with the rear face 34 of the ramp member 30 in engagement with the inner end face of sleeve 26.

In addition to the sleeve 26, the second tappet assembly 1 3 comprises an externally threaded output member 35 which extendsoutwardly of a flexible sealing boot 36 engaged over an annular cap 37 secured to the housing 10, and rigidly carries a starwheel 38. The outer end of the output member 38 is provided with a blind bore 39 which in which a spigot 40 on the tappet end 1 7 is fixediy secured.

A drive ring 41 is internally threaded and is engaged with threads 42 formed on the radially outer end of the axially outer end portion of second sleeve 25. The threads 43 of the engagement between member 35 and second sleeve 26 are of a tighter helix than those of the threads 42. Thus, the threads 42 may be said to be 'fast' and threads 43 may be said to be 'slow'.

The drive ring 41 is biassed axially inwardly by a light compression spring 42 abutting cup 37 at its outer end and ring 41 is provided with an inclined radially outer surface 43 which engages with a complementarily inclined clutch face 44, shown best in Fig. 4, on the free end of first sleeve 25.

During application of the brakes by inward movement of input rod 45 the ramp members 29 and 30 are forced apart to urge the tappet assemblies 1 2 and 1 3 in opposite directions to urge the actuated ends 8 and 7 of the brake shoes 2 and 1 respectively apart. Ramp member 30 acts on second sleeve 26 through face 34, and the output member 35 is carried outwards by the slow threads 43. Frictional force in the slow threads prevent any rotation of the second sleeve 26 by the slow threads, output member 35 being held against rotation by shoe end 7.Leftward movement of second sleeve 26 relative to first sleeve 25 will initially take drive ring 41 clear of clutch face 44 to disengage the drive ring and enable it subsequently to rotate relative to the second sleeve 26, if the relative axial movement of the first and second sleeves 25 and 26 is greater than the preset backlash in the fast threads 42. If the brake shoes are already correctly adjusted then no rotation of the drive ring 41 will take place. Thus the preset backlash determines the normal brake lining clearances.

On retraction of the actuator rod 45, the first and second sleeves 25 and 26 will move axially relative to each other in the opposite direction, and drive ring 41 will quickly lock against clutch face 44 so that additional retractive movement of second sleeve 26 relative to first sleeve 25 will result in rotation of the second sleeve 26 by the fast threads of the fixed drive ring 41 to cause tuming of the slow threads 43, which are now under reduced friction forces. Turning of second sleeve 26 relative to output member 35 produces an effective lengthening of'the second tappet assembly 13.

As shown in Fig. 4, first tappet 12 is provided on arms 24 with diametrically op posed left-wardly facing abutment faces 47 which act as stops engaged by the inner end 34 of second sleeve 26 to determine the retracted positions of the first and second tappet assemblies.

Since the adjustment of the length of the second tappet assembly 1 3 is reponsive entirely to relative movement between the first and second sleeves 25 and 26, and not to movement of either tappet assembly relative to housing 1, the adjustment is unaffected by bodily movement of both tappet assemblies in the same direction. Thus it will be appreciated that the actuator assembly may, with suitable monor modification, be employed in a twoleading shoe brake or in other configurations of brake where a servo action is transferred from one brake shoe to the other shoe through the tappet assemblies.

As shown in the drawings the adjustable tappet assembly 1 3 is preferably arranged to engage with the trailing shoe, in a leading trailing arrangement, because the wear rate of the trailing shoe lining is less than that of the leading shoe.

In a leading/trailing shoe brake the arrangement is preferably such that when the shoe linings are new the actuating rod 45 adopts, as shown in Fig. 3, a small angle with respect to the axis of bore 14, and in the direction towards the second tappet assembly 1 3. Then, as the linings wear, the leading shoe lining will wear faster than the trailing shoe lining, and the rod 45 will progressively swing, in its unactuated condition, through the axis of bore 1 4 towards the first tappet body 23. Thus the angle of inclination of the unactuated rod 45 to the axis of bore 14 will be kept to a minimum. The angular change during lining wear is further minimised by using a long rod 45 so that the pivot end 46 is at a maximum distance from wedge member 22.

On replacement of the brake linings, the starwheel 38 is rotated to screw output member 35 fully in to second sleeve 26. The starwheel 38 may be engaged by a clickspring of well-known arrangement.

In a modification, not shown, the surface 34 of the ramp member 30 is replaced by a clutch face in order to increase the friction opposing rotation of second sleeve 26 relative to ramp member 30 during re-setting of the adjuster on replacement of the brake shoes.

Claims (3)

1. A vehicle brake actuator of the kind set forth in which the first tappet assembly is provided with a projection located within the housing and which overlaps with a part of the second tappet assembly located within the housing, and the adjuster mechanism is responsive to relative movement of the projection and said part.

2. A vehicle brake actuator as claimed in Claim 1 in which the projection and said part are in the form of co-axial first and second sleeves respectively.

3. A vehicle brake actuator as claimed in Claim 2 in which the first sleeve has a clutch face engaged by a drive ring which has a threaded engagement with the second sleeve, and the second sleeve has a further threaded engagement with an output member of the second tappet assembly, the arrangement be ing such that during excessive relative move ment of the first and second sleeves during application of the brake the drive ring is turned to a new position, and during release of the brakes the drive ring is held and rotates the second sleeve relative to the output member to alter the length of the second tappet assembly.