GB2220452A - Improvements in self-energising disc brakes - Google Patents

Abstract

A self-energising disc brake is disclosed in which axial movement of pressure plates (2, 3) relatively away from each other is achieved in response to relative angular movement between the plates by the cooperation of balls with ramps (45) defined by the bases of pairs of recesses in which the balls are located. The radial cross-section of each recess is of non-circular outline having at least two discrete faces with which the ball co-operates as it moves along its respective ramp. There is therefore a reaction between the ball and each of the two faces, which enables the direction and magnitude of each reaction to be determined. <IMAGE>

Description

IMPROVEMENTS IN SELF-ENERGISING DISC BRAKES This invention relates to self-energising disc brakes of the kind in which rotatable friction discs provided with friction linings are adapted to be brought into engagement with spaced oppcsed braking surfaces in a housing by pressure plates located between the friction discs and centred by stationary pilot lugs, application of the brake being initiated by angular movement of the pressure plates effected by operation of a brake-applying mechanism, and balls are located in complementary pairs of recesses in adjacent faces of plates such that on operatic 0 the brake-applying mechanism to move the pressure plates angular in opposite directions, the balls tend to ride up ramps defined by the bases of the recesses to cause the pressure plates to move apart into engagement with the braking surfaces, the pressure plates then being carried round with the friction discs until one is arrested by the engagement of a lug on the plate with a dra-taking stop abutment in the housing and the continued angular movement of the other pressure plate provides a servo action.

Self-energising brakes of the kind set forth are commonly used in tractors and like vehicles, and may be "dry", liquid cooled, or of the oil immersed type.

In self-energising disc brakes of the kind set forth it is desirable for each ball to move centrally along the ramp defined by the inclined base of the recess or pocket in which it is located. In practice, however, this is very difficult to achieve with recesses of conventional, substantially circular, radial cross-sections since positioned tolerances of the recesses, and tolerance variations due to mis-match between the pressure plates, cause the balls to deviate from a desired direction, namely a centrally located path. This deviation increases the "out of parallelism" of the braking faces defined by the outer surfaces of the two pressure plates. As a result, the actuator mechanism may expand unevenly when the brake is applied which would cause fluctuations in the braking torque resulting in unpredictable performance.

In addition the resultant force due to the reactions of the pilot lugs and due to the clamp forces between the plates and its direction, may vary for each recess.

Since the position and the magnitude of each resultant force cannot be determined, difficulties arise ir.

correcting the tolerances.

According to cur invention, in a sel~-e-.ercXsing disc brake of the kina set forth tes radial cross-section of each recess is of non-circular outline having at least two discrete faces with which the ball co-operates as the ball moves along the respective ramp.

There is therefore a reaction force be.wee.i the ball ad each of the two faces, which enables the direction and the magnitude of each reactIon force to be determined.

One of the reaction forces comprises the reaction of the respective plate at a respective pilot lug on the housing, and the other comprises the reaction due to the clamp loads between the pressure plates.

The faces may be substantially normal to each other, or they may subtend an obtuse angle between them.

The faces may be of straight or of part circular profile.

Normally each recess has a radial cross-section of channel outline having a base face which takes the reaction due to the clamp loads, and a pair of side faces, one of which takes the reaction at the pilot lug depending upon the direction of rotation of the brake.

The three faces are contiguous and preferably the faces are symmetrically arranged to define each recess.

The base face of each recess may be of elliptical form.

Some embodiments of brake are illustrated il the accompanying drawings in which: Figure 1 IS an end elevation ca a self-energising brake of known construction; Figure 2 is a section on the line 2-2 of Figure 1; Ficure 3 is a section on the line 3-2 of Figure 1; Figure 4 is a section similar to Figure 3 but illustrating one embodiment of our invention; Figure 5 shows the brake profile in the region of the drag taking stop abutment; Figure 6 is a section on the line 6-6 of Figure 5; Figure 7 is a section on the line ,-, of Figure 6; Figure 8 is a view of the cutter profile; and Figure 9 is a section on the line 9-9 cf Figure 8.

The known self-energising brake shown in Figures 1 and 2 of the drawings comprises a housing 1 through which a rotatable shaft 16 extends axially. A pair of axially spaced friction discs 17,18 provided on opposite sides with linings of friction material are slidably splined on the shaft. The discs are adapted to be brought into engagement with axially spaced radial braking surfaces i9 at opposite ends of the housing 1 by an actuator. The actuator comprises a pair of annular pressure plates 2 and 3 which arc located between the discs 17,18 and are centred by three stationary pilot lugs 4, 5, 6 on the housing 1.

Balls 7, 8, c, 10 are housed in complementary pairs nf recesses 11, 12, 13, 14 in adjacent faces c tile pressure plates 2, 3. The recesses 11, 12, 13, 14 are each of generally circular radial cross-section, and are relatively inclined in a circumferential direction to define ramp surfaces with which the ball co-operate.

The pressure plates 2 and 3 are ncrmally urged towards each other by means of annularly spaced tension return springs 15 and, in the "off" positicn of brake as shown in Figure 2, with the friction linincs substantially unworn the plates 2 and 3 are in close proximity.

The application of the brake is initiated by moving the pressure plates 2 and 3 angularly in opposite directions. This causes the balls 7, 8, 9, 10 to ride up the ramps of the respective recesses in which they are located to urge the plates 2 ad 3 relatively al.Gy from each other and e > :pand the actuator. The plates 2 and 3, in turn, move apart into engagement with the friction discs to urge them into engagement with the radial braking surfaces 19 in the housing.

When the discs 17,18 are rotating in one direction, the plates 2 and 3 are carried round with them until one plate is arrested by the engagement of a radial lug 20, 21 on that plate with an anchor pin, defining the lug 5. Continued angular movement of the other plate provides a servo-action to increase the relative axial separation between the plates 2 and 3 and increase the braking torque.

When the shaft 16 is rotating in the opposite direction ano the brake is applied, the sequence described above is repeated except that the er.gular movement of the other plate is arrested by the anchcr pin 5.

The angular movement of the plates 2 and 2 to apply the brake is initiated by a brake-applying mechanism 30. The type of mechanism illustrated is one of many and comprises a pair of toggle links 31 and 32 which are symmetrically arranged and are of equal lengths. The link 31 is pivotally connected at its innermost end to a radial lug 35 on the pressure plate 2 by a pivotal connection 33, and the link 32 is pivotally connected at its innermost end to a radial lug 36 on the pressure plate 3 by a pivotal connection 34. The free ends of both links 30 and 31 are pivotally connected between the limbs of a bifurcated fitting by means of a pivot pin 37 and the fitting is coupled to the inner end of a radially moveable pu rod (not shown) , which may be operated mechanically or hydraulically.

In the application of the brake, a pull applied to the pull-rod in a radial direction is transmitted to both toggle links 31 and 32. This urges the lugs 35 and 36 towards each other in a circumferential direction to effect application of the brake with a self-energising action as described above.

The brake described above may be of the "dry", liquid cooled, or oil-immersed type.

As will be seen from Figure 3, when the brake is applied, there s a tendency, due to pcsitional tolerances cf the circular recesses 13 and mis-match cf the two pressure plates 2, 3 for the ball 9 to deviate from the ideal direction and increase any cut c parallelIsm of the braking faces 40, 41 defined b7 the outer faces of the pressure plates 2, 3 by an amount 2S, namely the sum of the lift S for each recess 13 of that pair.The pressure plates 2, 3 are expanded unevenly causing fluctuations in the braking torque, which result in unpredictable performances. Ir effect a second reaction R2 exists and the total reaction is a resultant, R, which comprises a combinatIon of R2 with the normal reaction R1, due to the clamp load between the two pressure plates 2 and 3.

In the construction shown in Figure 4 of the accompanying drawings, which illustrates one embodiment of the invention, each recess 13 is of channel section comprising a base face 45, which defines the ramp surface, and spaced end faces 46, 47 which are contiguous with, and are normal to, the base face 45.

As illustrated the ball 9 co-operates with the base face 45, and a corresponding one of the two end faces, suitably the face 46, when the brake is applies in a normal forward direction of shaft rotation. This separates the two reactions R1 and R2, namely the reaction R1 due to the clamp load between the two pressure plates 2, 3 and the reaction R2 at a pilot lug 4, 5 or 6, respectively, making them normal to each other. The straight line or linear base face 45 permits the ball 9 to move radially to produce R2 with no separation S = 0 resulting.

In the construction illustrated in Figures 4 to 9 of the accompanying drawings, the base face 45 is elliptical in a radial direction, and both end faces 46 and 47 are straight, terminating in an outer portion 48, 49 of circular profile, with the straight portions inclined or angled away from the conical base face 45.

It follows, therefore, that the reaction 2 3s, correspondingly, angled. This means that the value of lift, S, will be greater than zero, but, nevertheless, will be negligible.

As shown in Figures 8 and 9 the recesses 11, 12, 13, 14 can be produced by the use of a normal machine process, namely a cutter 50.

In a non-expanded actuator condition, each ball 7,8,9 and 10 may rest on the conical portion 45 of each respective recess 11,12,13 and 14 (Figure 9).

However, since the point angle of the cutter 50 and the generated ramp angle may not coincide exactly, there may be a slight variation in the effective ramp angle as the actuator is expanded. This can be overcome by increasing the depths of the recesses, and partially expanding the actuator to fit over the anchor pin 5 (see Figure 5). This ensures that each ball 7, 8, 9, 10 is positioned on the generated base face 45 of the respective recess which defines the ramps, for example as shown in Figure 5.

In the brakes illustrated in Figures 4-9 above, the pressure plate which forms the servo N self-energising plate when the brake is applied can be located solely by the balls and the pairs of recesses.

This improves the relative performance and efficiency of the brake.

Claims (14)

1. A self-energising disc brake of the kind set forth, in which the radial cross-section of each recess is of non-circular outline having at least two discrete faces with which the ball co-operates as the ball moves along the respective ramp.

2. A brake as claimed in claim 1, in which there is a reaction force between the ball and each of the two faces.

3. A brake as claimed in claim 2, in which one of the reaction forces comprises the reaction of the respective plate at a respective pilot lug on the housing, and the other comprises the reaction due to the clamp loads between the pressure plates.

4. A brake as claimed in any preceding claim, in which the faces are substantially normal to each other.

5. A brake as claimed in any of claims 1-3 r in which the face subtend an obtuse angle between there.

6. A brake as claimed in any preceding claim, in which the faces are of straight profiles.

7. A brake as claimed in any of claims 1-5, in which the face are of part circular profiles.

8. A brake as claimed in any preceding claim, in which each recess has a radial cross-section of channel outline having a base face which takes the reaction due to the clamp loads, and a pair of side faces, one of which takes the reaction at the pilot lug depending upon the direction of rotation of the brake.

9. A brake as claimed in claim 8, in which the three faces are contiguous.

10. A brake as claimed in claim 8 or claim 9, in which the faces are symmetrically arranged to define each recess.

11. A brake as claimed in any of claims 8-10, in which the base face of each recess is of elliptical form.

12. A self-energising disc brake substantially as described herein with reference to and as illustrated in Figure 4 of the accompanying drawings.

13. A self-energising dise brake substantially as described herein with reference to and as illustrated in Figures 5-7 of the accompanying drawings.

14. A seif-energising disc brake as claimed in claim 13, in which recesses are machined by a cutter substantially as described herein with reference to and as illustrated in Figures 8 and 9 of the accompanying drawings.