GB1586063A - Lining wear adjusters for reversing brakes - Google Patents

Description

(54) LINING WEAR ADJUSTERS FOR REVERSING BRAKES (71) I, LESLIE CYRIL CHOUINGS, a British Subject of Holly Croft, Bourton, Rugby, Warwickshire, do hereby declare the invention for which I pray that a Patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to shoe and drum brakes of the type, hereinafter referred to as being of the type described, in which a given brake actuator input to apply the brake results in a much higher braking torque when the drum is rotating in one direction (hereinafter referred to as the forward direction) than when the drum is rotating in the opposite direction (hereinafter referred to as the reverse direction), the brake mechanism including clearance which is retained during application of the brake during forward rotation of the drum and which is reduced or substantially eliminated upon brake application during reverse rotation of the drum.

The invention finds application in trailer brakes which are applied by the trailer tending to overrun the towing vehicle when the latter slows down. With such trailer brakes it is necessary to make provision for reversing the trailer without applying the brakes or applying them to any substantial extent. The simplest provision is a mechanical spacer which can be moved into position to prevent movement of the two bar of the trailer to apply the brakes. It has also been proposed to include, in the brake, some mechanism which limits the brake torque in reverse. These mechanisms rely, for their operation, on the changes in direction and magnitude of the forces exerted by the shoes when the drum changes from rotating in the forward direction to rotating in the reverse direction. The brakes of the invention include such mechanism and may be of the leading and trailing shoe kind or the duoservo kind having primary and secondary shoes.

The invention is concerned with providing a brake of the type described with an automatic adjuster for compensating for lining wear and which adjusts the brake when the shoe travel on application of the brake exceeds a predetermined distance.

In a brake of the type described the shoe travel is greater when the brake is operated with the drum rotating in reverse than when the brake is operated with the drum rotating.

forwardly. This is because of the circumferential shoe travel introduced into the brake function by the reduction or substantial elimination of the clearance mentioned above.

If, therefore, the wear adjuster were operated when the brake is applied and the drum is rotating in reverse, over-adjustment would take place. Similarly if the brake is operated with the drum stationary by parking brake means this will cause circumferential shoe travel to eliminate the clearance and, if the adjuster were operated, over-adjustment would take place.

It is an object of the invention to overcome this problem.

According to the invention I provide a brake of the type described including an adjuster operable to adjust the brake for lining wear when the shoe travel on brake application with the drum rotating forwardly exceeds a predetermined distance but in which the wear adjuster is not responsive to shoe travel during brake application when the drum is stationary or rotating in reverse.

Also according to the invention I provide a brake of the type described including a single adjuster operable to adjust the brake for lining wear of both shoes when the shoe travel on brake application of the drum rotating forwardly exceeds a predetermined distance, but in which the wear adjuster is not responsive to shoe travel during brake application when the drum is stationary or rotating in reverse, and wherein the degree of adjustment is determined by the extent of movement towards the drum, and with respect to a datum on a fixed part of the brake, of that end of the leading or primary shoe which leads in the forward direction of rotation of the drum.

By arranging the wear adjuster in this manner, the adjuster will be responsive to shoe travel when the drum is rotating forwardly in its normal mode of operation and therefore appropriate adjustment will take place but over-adjustment will be avoided.

In a brake of the leading and trailing shoe kind the wear adjuster may include a pawl pivoted on the shoe which is the leading shoe during forward rotation of the drum and engaging an abutment on a fixed part of the brake, the pawl being casued to oscillate about its pivot to engage and turn an adjustment wheel of a variable length strut on application and release of the brake when the drum is rotating forwardly and where there is sufficient shoe travel to warrant adjustment. The strut may extend between an operating lever, which is pivoted to the other shoe, and said leading shoe.

In such a leading and trailing shoe brake there may be a pivoted lever between the ends of the shoes remote from the actuator arranged so that the shoe which is the trailing shoe when the drum is rotating forwardly engages the lever at a greater distance from the latter's pivot than the shoe which is the leading shoe during forward rotation of the drum; the lever being forced against a stop during brake application during forward rotation of the drum but moving away from the stop and reducing the distance between the ends of the shoes on reverse rotation.

In another arrangement of a leading and trailing shoe brake, the wear adjuster comprises a lever pivoted on the shoe which is the leading shoe during forward rotation of the drum, limiting means for permitting pivoting of the lever in one sense about its pivot but preventing the lever pivoting in the opposite sense, a strut engaged with clearance at one end with the lever and engaged adjacent the other end with a ratchet device which enables the strut to move towards said leading shoe but not away therefrom, and an actuator arranged to engage the lever to apply said leading shoe to the drum, the limiting means preventing rotation of the lever during said application but permitting pivoting movement on release of the actuator, the strut being arranged to move through the ratchet device on application of the brake with the drum rotating forwardly to vary the rest position of said leading shoe as wear takes place and the lever progressively pivoting relative to the shoe as wear takes place so that the lever is always engaged with the actuator, the reverse movement of the leading shoe during braking when the drum is rotating in reverse being permitted by the clearance between said one end of the strut and the lever.

In this other arrangement, said leading shoe may carry a second lever pivoted thereto and engaged with the other shoe by a second strut, the second lever being mounted to pivot with respect to a fixed part of the brake and engaging said leading shoe at a position nearer the lever pivot than its similar engagement with the other shoe.

The means for limiting braking torque in reverse may be as described in my Patent Application No. 9197/74, (Serial No. 1493831) to which reference should be had.

In a brake of the duo-servo kind, the adjuster may form part of a strut between the shoe ends remote from the actuator and include a pawl pivoted to the shoe which is the primary shoe when the drum is rotating forwardly and which is connected to one end of the cable the other end of which is connected to a fixed part of the brake, the cable passing around a guide on said primary shoe and going slack when said clearance is reduced but moving the pawl during braking when the drum is rotating forwardly, said movmeent being against spring means provided to urge the pawl to an operating position. The spring means may be provided by a coil spring surrounding the cable.

The invention will now be described in detail by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a leading and trailing shoe brake embodying the invention with the brake in the 'off' position; Figure 2 is a diagram of the brake of Figure 1 showing the brake in its applied position with the drum rotating forwardly; Figure 3 is a diagrammatic view of another leading and trailing shoe brake embodying the invention in the 'off' position; Figure 4 is a partial section in the line 44 of Figure 3;and Figure 5 is a diagrammatic view of a duoservo brake embodying the invention.

Referring now to Figures 1 and 2 the brake comprises a back plate or torque reaction member 10 and a drum 11.There are two shoes 12 and 13. A lever 14 is pivoted at 15 to the torque reaction member 10. The lever 14 is engaged by the end 16 of the shoe 12 and the end 17 of the shoe 13. It will be seen that the distance 1 t from the centre of the pivot 15 to the point of engagement of the end 17 with the lever 14 is greater than the distance 12 between the centre of the pivot 15 and the point of engagement of the end 16 with the lever. In the position shown in Figure 1, the lever engages an abutment 19 fixed to the torque reaction member. The shoes are biased to this position by means of a spring 20 acting between the shoe 12 and the torque reaction member 10. A further spring 21 acts between the ends 22 and 23 of the shoes which are remote from the lever 14.

The combined actuator and wear adjustor comprises a lever 24 which is pivotally connected at 25 to the shoe 13 adjacent to the end 23 thereof and is pivoted to one end 27 of a threaded pin 28 received in a threaded part of a bore 29 of a sleeve 30. Received in a plain part of the bore 29 is a spigot 31 which passes freely through an adjusting wheel 32 secured to the right hand end of the sleeve 30. The spigot 31 has a head 33 which carries a pivot pin 34 for a pawl 35. The pin 34 bears on shoe 12 (being located in a notch in the shoe web) and provides the brake-operating force for this shoe. One arm 36 of the pawl is slotted at 37 and a pin 38, secured to the torque reaction member 10, is received in the slot 37.

The pawl has a further arm 39 which is arranged to engage the wheel 32 as will be described.

Clearance 40a and 40b are shown between the shoes 12 and 13 and the drum respectively.

With the brake in the off position the clearance 40b between the shoe 13 and the drum will be small as described below. There is a further clearance 41 between a face 1 3a of the shoe 13 and the abutment 19. The forward rotation of the drum is indicated by the arrow 43 so that during forward rotation the shoe 12 is the leading shoe and the shoe 13 is the trailing shoe.

The brake is shown in its off position in Figure 1. The shoes are applied to the drum by applying a force to the lever 24 as indicated by the arrow 24a. This applies a force to the shoe 13 at 25 and to the shoe 12 via the strut constituted by the pin 28, the sleeve 30, the spigot 31 and the pin 34. When the shoes are applied with the drum rotating forwardly, the moment about the pivot 15 due to the force applied to the lever 14 by the shoe end 16 is greater than the equivalent moment applied by the shoe end 18 so that the lever is held against the abutment 19. As the shoe ends 22 and 23 move apart, the centre of the pin 34 moves to the right of a datum line A as shown in Figure 2 and as a result the pawl is moved to the position shown in that figure, in which the arm 39 of the pawl has moved part the point of maximum diameter of the wheel 32. The teeth of the wheel are shaped so that the pawl may move clockwise during brake application past the teeth but, as the pawl moves anti-clockwise during its return, the end of the lever 39 will engage the wheel and will rotate it thus rotating the sleeve 30 relative to the pin 28 and thus moving the ends 22 and 23 of the shoes apart.

Whether adjustment takes place or not depends on the clearance 40 between the shoes and the drum and its relation to the pitch of the ratchet wheel teeth.

During operation of the brake with the drum rotating in reverse, the shoe 13 becomes the leading shoe and the shoe 12 the trailing shoe. Due to the known reversal of the magnitude and directions of the forces, the moment applied by the shoe end 17 to the lever 14 about the pivot 15 is greater than the moment applied by the shoe end 16 about the pivot 15 and the lever 14 pivots in an anti-clockwise direction. The ends 16 and 17 are moved closer together because the ends 17 moves in an anti-clockwise direction through a greater distance than does the end 16 due to the difference between the lever arms 11 and 12 with the result that the face 1 3a of the shoe 13 approaches the abutment 19 reducing the clearance 41. As described in detail in said earlier application the maximum travel which can be imparted to the lever 24 by the service operating means will be insufficient to enable the clearance 41 to be eliminated and therefore this prevents the attainment of any substantial brake torque during brake application with the drum rotating in reverse. The shoe 12 will move slightly in an anti-clockwise sense so that the pin 34 will move to the left of the datum line A. The pawl may have a tendency to turn slightly in an anti-clockwise direction during this show movement and if this anti-clockwise movement should tend to turn the wheel 32 spring means (not shown) may be provided between the arm 39 and the rest of the pawl which will yield to prevent overload of the mechanism. The strut 28, 29 etc will be under load during reverse braking and a considerable force would be required to turn the wheel 32.

Alternatively a clearance may be provided between the pin 38 and the slot 37 to prevent anti-clockwise movement of the pawl during reverse braking.

The trailing shoe 13 (in forward rotation of the drum) will normally so adjust itself that a close clearance 40b is maintained in the off position between it and the drum as shown in Figure 1. Thus the clearance 40a will represent the total wear of both shoes and the point from which the strut constituted by the sleeve 30, pin 28 and spigot 31 will start its movement automatically compensates for wear of both shoe linings. Adjustment between the shoes takes place by lengthening of the strut and this occurs to the extent that the pin 34 moves during operation outwardly of the datum line A which is at right angles to the longitudinal axis of the strut. It will be noted that the pin 34 is at the leading end, considered in the direction of forward rotation 43, of the shoe 12 and the degree of adjustment by the movement of this leading end towards the drum during braking with the drum rotating forwardly. Movement of pin 25 to take up the movement of the shoe 13 towards abutment 19 will not cause any noticeable movement of the pin 34 so no adjustment occurs on this account.

Application of a parking brake causes substantially equal forces to be applied at the shoe ends 16 and 17 and because of the greater moment arm of the end 17 about the pivot 15, the lever 14 pivots in an anti-clockwise direction and the shoes are moved bodily in an anti-clockwise direction until the face 1 3a engages the abutment 19. Braking torque can then be obtained. It follows that the range of movement of the parking brake means must be greater than that of the service operating means to apply the brake in reverse, i.e. the parking brake means must be able to move the lever 24 through a greater distance than can the service operating means whose range of movement is limited as described above.

Referring now to Figures 3 and 4, these show a further brake of the leading and trailing shoe kind. The brake comprises a torque reaction member or back plate 45 on which are mounted two shoes 46 and 47. The direction of forward rotation of the drum, not shown, is indicated by the arrow 48 so that during forward rotation the shoe 46 is the leading shoe and the shoe 47 is the trailing shoe.

Between the lower ends of the shoes 46 and 47 is a torque limitation means indicated generally at 49 and these are substantially of the form shown and described in my patent application No. 9197/74 (Serial No 1493831) to which reference should be made. Briefly, a lever 50 is pivoted to the shoe 46 at 51 and is fulcrummed at 52 on an abutment 53 secured to the back plate 45. A spring 54 engages at one end with a pin 55 on the back plate and at the other end in an aperture 56 in the shoe 46 thus forcing the lower end 57 thereof into engagement with an abutment surface 58 on the abutment 53. A strut 59 extends between the lever 50 and the shoe 47 and as shown there is a clearance 60 between the end 61 of the shoe 47 and an abutment surface 62 in the abutment 53. A hairpin spring 63 on the pin 55 holds the end 61 of the shoe 47 away from the abutment 62 by a distance determined by a stop 63a on the abutment 53 and which is engaged by the right hand limb of the spring.

A floating actuator 64 is located between the upper ends of the shoes 46 and 47 and engages at 65 the upper end of the shoe 47.

The actuator does not engage the shoe 46 directly but engages the end 66 of a lever 67 which is pivoted at 68 to the shoe 46. The other end 69 of the lever is formed with ratchet teeth and these engage with complementary teeth on a pawl 70 pivoted at 71 on the shoe 46 and urged to rotate in a clockwise sense by a spring 72. It will be seen from Figure 4 that the lever 67 comprises spaced plates 74 arranged on either side of the web 75 of the shoe 46. Similarly, the pawl 70 comprises plates 76 spaced on either side of the web 75.

A pin 77 passes through the plates 74 and through a distance piece 78. The pin has a head 79 at one end and a cylindrical abutment 80 at its other end, the abutment having a flange 81. The abutment is received in an eye 82 at one end of a strut 83. As will be clear from the drawings there is considerable clearance around the abutment 82 within the eye.

The other end of the strut is received within a U-shaped member 84 pivoted at 85 to the back plate, the strut passing between the limbs of the U. Mounted between the limbs of the U on a pivot 86 is a pawl 87 having a serrated surface 88. The pawl is urged in a clockwise direction by a spring 89 so that the serrated surface 88 engages the strut 83. The serrated surface is such that the strut can move to the left in Figure 3 but not to the right. A return spring 90 is engaged at its end with the shoes 46 and 47 and serves to force the upper end of the shoe 47 and the end 66 of the lever into engagement with the actuator 64.

During application of the brake when the drum is rotating forwardly in the direction of the arrow 48, the actuator will force the shoe 46 outwardly via the lever 66. The arrangement of the ratchet 70 and the lever 67 is to prevent rotation of the lever in an anti-clockwise direction during this brake application. The strut 83 will be moved to the left as the shoe moves after the clearance between the head 80 and the eye 82 has been taken up.

When the brake is released, the spring 90 will tend to draw the shoe 46 to the right and the lever 67 will pivot in a clockwise direction and the clearance between the abutment 80 and the eye 82 will prevent the lever tending to move the strut 83 to the right and the strut will determine the rest position of the shoe.

As wear takes place and the rest position of the shoe becomes further and further to the left in Figure 3 the lever 67 will pivot clockwise so as to remain in engagement with the actuator 64 but the end 91 of the shoe 46 will become further spaced from the actuator 64.

During braking in reverse, as described in said earlier application, the shoe 47 becomes the leading shoe and the clearance 60 is substantially taken up so that there will be a slight clockwise movement of the shoe 46 and this will not move the strut 83 so that there will be no adjustment during braking in reverse, this clockwise movement is permitted by the clearance between the eye 82 and the abutment 80. Similarly, during application of the brake by a parking brake means the clearance 60 will be taken up as described and the shoe 46 will move clockwise so again there will be no adjustment for wear.

Referring now to Figure 5, this shows a duo-servo brake of the type described in detail in my published specification (Serial No. 1,540, 595).

Briefly, the brake comprises a back plate 92 on which are mounted shoes 93 and 94.

A strut 95 extends between the one ends of the shoes and is of the construction described in general in Figures 1 and 2 and had an adjustment wheel 96 which, when turned, adjusts the length of the stmt. The direction of forward rotation of the drum (not shown) is indicated by the arrow 97 and under these circumstances the shoe 93 is the primary shoe and the shoe 94 is the secondary shoe. The brake is operated by an actuator comprising a rotatable head 98 which engages via a strut 99 a lever 100 pivoted at 101 to the shoe 94.

The actuator also engages via a strut 102 with the end of the shoe 93. During braking with the drum rotating forwardly, the shoe 94 engages an abutment 103 on the back plate and there is a clearance 104 between the shoe 93 and an abutment 105. During braking when the drum is rotating in reverse, as described in said application, the shoes 93 and 94 and the strut 95 move circumferentially of the drum as a unit in an anti-clockwise direction thus substantially eliminating the clearance 104. The range of movement of the actuator is limited to prevent the total elimination of the clearance. Similarly, if the brake is applied via the parking brake means the shoes and strut rotate anti-clockwise to take up the clearance 104 and for this purpose the parking braking means must have a greater range of movement than the service braking means.

All this is described in detail in said specification No. 1 540 595.

The automatic wear adjuster comprises a pawl 106 pivoted at 107 to the shoe 93. One end 108 of the pawl is arranged to co-operate with the adjustment wheel 96 and the other end 109 of the pawl is connected to one end of a cable 110 the other end 111 of which is anchored to the back plate 92. The anchorage 112 for the cable enables the length of the cable to be adjusted by slackening a clamping screw 113. The cable passes over a guide 114 on the shoe 93 and a coil spring 115 surrounds part of the cable 110 and operates between a clevis 116 which is interposed between the cable and the end 109 and an abutment 117 on the guide 114.

During braking with the drum rotating forwardly in the direction of the arrow 97, the shoe 93 is moved outwardly away from the anchorage 112 so that the cable is tightened and this causes the pawl to rotate in an anticlockwise direction. Upon release of the brake the pawl will pivot in a clockwise direction under the influence of the spring 115. If there has been sufficient wear, the pawl end 108 will have passed a tooth on the adjustment wheel 96 during operation of the brake and upon release the wheel 96 will be turned as the pawl is returned by the spring 115 to the position shown.

During braking with the drum rotating in reverse, however, the shoes and strut 95 move in a counter clockwise direction as described to reduce the clearance 104 and as a result, the cable 110 goes slack and there is no operation of the pawl. A similar movement of the shoes and the strut takes place when the brake is applied by parking brake means, so that again no pivoting of the pawl takes place and no adjustment for wear.

It will be seen that the invention provides a wear adjuster for a reversing brake of either the leading and trailing shoe kind or the duoservo kind.

Claims (12)

WHAT I CLAIM IS:

1. A brake of the type described including an adjuster operable to adjust the brake for lining wear when the shoe travel on brake application of the drum rotating forwardly exceeds a predetermined distance, but in which the wear adjuster is not responsive to shoe travel during brake application when the drum is stationary or rotating in reverse.

2. A brake of the type described including a single adjuster operable to adjust the brake for lining wear of both shoes when the shoe travel on brake application of the drum rotating forwardly exceeds a predetermined distance, but in which the wear adjuster is not responsive to shoe travel during brake application when the drum is stationary or rotating in reverse, and wherein the degree of adjustment is determined by the extent of movement towards the drum, and with respect to a datum on a fixed part of the brake, of that end of the leading or primary shoe which leads in the forward direction of rotation of the drum.

3. A brake according to Claim 1 or Claim 2 of the leading and trailing shoe kind in which the wear adjuster includes a pawl pivoted on the shoe which is the leading shoe during forward rotation of the drum and engaging an abutment on a fixed part of the brake, the pawl being caused to oscillate about its pivot to engage and turn an adjustment wheel of a variable length strut on application and release of the brake when the drum is rotating forwardly and when there is sufficient shoe travel to warrant adjustment.

4. A brake according to Claim 3 wherein the strut extends between an operating lever, which is pivoted to the other shoe, and said leading shoe.

5. A brake according to either of Claims 3 or 4 including a pivoted lever between the ends of the shoes remote from the actuator and arranged so that the shoe which is the trailing shoe when the drum is rotating forwardly engages the lever at a greater distance from the latter's pivot than the shoe which is the leading shoe during forward rotation of the drum; the lever being forced against a stop during brake application during forward rotation of the drum but moving away from the stop and reducing the distance between the ends of the shoes during brake application on reverse rotation of the drum.

6. A brake according to Claim 1 or Claim 2 of the leading and trailing shoe kind wherein the wear adjuster comprises a lever pivoted on the shoe which is the leading shoe during forward rotation of the drum, limiting means for permitting pivoting of the lever in one sense about its pivot but preventing pivoting of the lever in the opposite sense, a strut engaged with clearance at one end with the lever and engaged adjacent the other end with a ratchet device which enables the strut to move towards said leading shoe but not away therefrom, and an. actuator arranged to engage the lever to apply said leading shoe to the drum the limiting means preventing rotation of the lever during said application but permitting pivoting movement on release of the actuator, the strut being arranged to move through the ratchet device on application of the brake with the drum rotating forwardly to vary the rest position of said leading shoe as wear takes place and the lever progressively pivoting relative to the shoe as wear takes place so that the lever is always engaged with the actuator, the reverse movement of the leading shoe during braking when the drum is rotating in reverse being permitted by the clearance between said one end of the strut and the lever.

7. A brake according to Claim 6 wherein said leading shoe carries a second lever pivoted thereto and engaged with the other shoe by a second strut, the second lever being mounted to pivot with respect to a fixed part of the brake and engaging said leading shoe at a position nearer the lever pivot than its similar engagement with the other shoe.

8. A brake according to Claim 1 or Claim 2 of the duo-servo kind wherein the adjuster forms part of a strut between the shoe ends remote from the actuator, the adjuster changing the effective length of the strut and being operated by a pawl pivoted to the shoe which is the primary shoe during forward rotation of the drum and connected to one end of the cable the other end of which is connected to a fixed part of the brake, the cable passing around a guide on said primary shoe and going slack when said clearance is reduced but moving the pawl during braking when the drum is rotating forwardly, said movement being against spring means provided to urge the pawl to an operating position.

9. A brake according to Claim 8 wherein the spring means comprises a coil spring surrounding the cable.

10. A brake of the kind described substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

11. A brake of the kind described substantially as hereinbefore described with reference to and as shown in Figures 3 and 4.

12. A brake of the kind described substantially as hereinbefore described with reference to and as shown in Figure 5.