GB2121896A - Automatic slack adjuster for vehicle drum brake - Google Patents

Abstract

The arrangement has force transmitting devices (7, 8) of adjustable length mounted between a key and respective brake shoes; a wheel (19) is freely journalled on the key shaft (4) and at one side is in engagement with tooth racks of the force transmitting devices (7, 8) and on the other side a spring latch (20) rigidly mounted on the key shaft (4) is arranged to contact ridges and grooves on the whell as in Figure 6. During brake application, teeth (18) of the tooth racks slide in inclined grooves of the wheel (19) to cause rotary movement of the wheel relative to spring latch (20), and at excessive angular movement of key shaft (4) a port (24) of the latch (20) enters into a neighbouring groove of wheel (19) so that, upon brake release, the part (24) causes a tongue on the wheel (19) to increase the lengths of devices (7, 8). Toothed racks may be axially adjustable on the threaded elements (10, 11) of the devices (7, 8) for usage with different keys (6) (see e.g. Figures 9, 10). <IMAGE>

Description

SPECIFICATION Automatic slack adjuster This invention relates to an automatic slack adjuster for a vehicle drum brake of the type actuated by a key rigidly mounted on a key shaft connected to a cylinder-piston activator. The said key is acting upon a pair of brake shoes through force transmitting devices each of which comprises two telescopically displaceable elements, one being an externally threaded rod, while the other is an internally threaded sleeve receiving said rod.Each of the two sleeves of the two elements is provided with a circumferential tooth rack engaging grooves extending radially on an axial shoulder surface of a wheel journalled on the key shaft, the grooves having such shape that any straight radial movements of the tooth rack relative to the wheel causes a torque on both sleeves in the direction causing an increase in the total length of the two telescopically displaceable elements.

U.S. Patent Specifications Nos. 3891068 and 4161999 show slack adjusters of this and similar types.

Slack adjusters of this type should be extremely slow acting i.e. the adjustment performed per braking cycle should be very small. Otherwise, there will be a substantial risk of false adjustments performed partly because of increased slack during short time intervals caused by heating of the brake drum. If the automatic adjustment is sufficiently slow almost no adjustment is carried out during several subsequent braking operations performed with hot braking drums.

However, if the automatic adjustment is set to be extremely slow it will be almost impossible to check that a correct slack adjustment will be obtained because of the very large number of braking cycles required for this purpose. Therefore, it will be necessary to rely on the accuracy of the installation and the elements of the slack adjuster.

As a rule, the slack adjustment may be initiated either because of excessive movement of certain brake force transmitting elements relative to a stationary part of the brake or it may be initiated due to excessive relative movements of two force transmitting elements.

In the first of these two cases it is evident that false mounting of the stationary part or a subsequent displacement of said part will cause errors in the slack adjustment. In the latter of the two cases - the group to which the present invention belongs -- the achievement of the correct slack is dependent on a correct installation as any deviation from movements in the correct planes and angles will cause deviations from the desired slack adjustment. It is almost impossible to check this when mounting the slack adjuster.

The present invention aims to provide a slack adjuster of the type referred to above in which the parts determining the slack adjustment practically cannot be displaced neither during mounting nor afterwards. This will make it possible to rely on obtaining correct slack adjustment without verifying on the spot.

According to the invention, there is provided an automatic slack adjuster for a vehicle drum brake of the type actuated by a key rigidly mounted on a key shaft connected to a cylinder-piston activator, said key acting upon a pair of brake shoes through force transmitting devices each of which comprises two telescopically displaceable elements, one being an externally threaded rod, while the other is an internally threaded sleeve receiving said rod, each of the two sleeves of the two elements being provided with a circumferential tooth rack engaging grooves extending radially on an axial shoulder surface of a wheel journalled on the key shaft, the grooves having such shape that any straight radial movement of the tooth rack relative to the wheel causes a torque on both sleeves in the direction causing an increase in the total length of the two telescopically displaceable elements, wherein said wheel is provided with a second axially extending shoulder surface area having grooves adapted to receive a resilient spring latch rigidly connected to the key shaft.

The invention will now be further described, by way of example, with reference to the drawings, in which: Fig. 1 schematically shows the brake force transmitting part of a brake provided with one embodiment of a slack adjuster according to the invention; Fig. 2 shows the brake of Fig. 1 with the brake force transmitting parts in a different position; Fig. 3 is a section taken along the line Ill-Ill of Fig. 1 showing additional parts of the automatic slack adjuster according to the invention: Fig. 4 shows to an enlarged scale one side of a wheel forming an element of the adjuster; Flg. 5 shows the other side of the wheel shown in Fig. 4; Fig. 6 shows the side of the wheel shown in Fig.

5 with a co-operating latch; Fig. 7 is a section taken along the line VIl-VIl in Fig. 6 to an enlarged scale; Fig. 8 shows a force transmitting device of the type shown in Figs. 1, 2 and 3 but to an enlarged scale and partly in section; and Figs. 9 and 10 are diagrams illustrating how modifications to the element shown in Fig. 8 can make it suitable for different types of brakes.

The brake shown in Fig. 1 comprises a cylinderpiston activator 1 having a piston rod 2 which at its outer end is pivotally connected to a lever 3 rigidly connected to a key shaft 4 protruding into a brake drum 5 rigidly connected to a wheel (not shown). The key shaft 4 carries a key 6 having concave depressions adapted to receive convex end surfaces of force transmitting devices 7, 8 each of which comprises two telescopically displaceable elements 9, 10 and 11, 1 2 respectively. Said devices 7, 8 act upon brake shoes 13, 14 provided with linings 1 5 and 1 6 respectively. Fig. 1 shows the parts of the brake when in their released brake positions leaving gaps between the linings 15, 16 and the inner surface of the drum 5.

A braking cycle is initiated by supplying compressed air to the activator 1. This will cause the rod 2 to be expelled and turn the key shaft 4 and the key 6 in such a direction as to cause movement of the devices 7, 8 so that the brake shoes 13, 14 will force their linings 1 5, 1 6 against the drum 5. This position is shown in Fig 2.

It will be understood that wear on the linings 1 5, 16 and the drum 5 inherently may have a detrimental influence on the brake as it may cause a substantial change in the force transmitted and in the time necessary for obtaining application of the brake.

However, if it is possible to increase the length of the devices 7, 8 corresponding to the wear of the brake surfaces, the brake will maintain its braking properties. This possibility is realised by an automatic angular displacement of the elements 9, 10 and 11, 12 respectively. The elements 10 and 11 are interiorly threaded sleeves adapted to receive the correspondingly threaded elements 9 and 12 which are prevented from rotation by means not shown. Each of the two sleeves 10, 11 is provided with a circumferential tooth rack 1 7 carrying a number of teeth 1 8.

Fig. 3 shows a section along the line Ill-Ill of Fig. 1, but in Fig. 3 the key 6 has been omitted.

However Fig. 3 shows two further items not shown in Fig. 1 and 2. One of said items is a wheel 19 freely journalled on an extension of the shaft 4.

The other item is a resilient spring latch 20 rigidly connected to the outer end surface of the shaft 4.

The spring latch 20 urges the wheel 19 in the direction away from the end of the shaft 4. The wheel 1 9 is on its lower side (which is shown in Fig. 4) provided with inclined grooves 21 extending between radii A and B and adapted to receive at least one tooth 1 8. On its opposite surface -- shown in Fig. 5 - the wheel 1 9 is provided with radially directed grooves 22 and ridges 23. The grooves 22 are adapted to receive a part 24 of the spring latch 20. Fig. 6 shows a view along the line VI--VI of Fig. 3 but to the same scale as the one used in Figs. 4 and 5.

An end 25 of the spring opposite the part 24 slides against the tops of the ridges 23 and thus balances the spring load on the wheel 1 9 caused by the part 24.

The relative influence of the spring latch 20 and the wheel 19 is illustrated in Fig. 7. It will be understood that relative angular movements greater than the angular distance between two ridges 23 will cause the part 24 of the spring latch 20 to enter into a neighbouring groove 22. Only relative movements in one direction are possible.

Forces of the spring latch part 24 in the direction towards the left in Fig. 7 will cause contact between the surface of a ridge side and a surface 26 of the part 24. This surface 26 will not be substantially worn, because of its rather great area. An edge 27 of the part 24 determines when the part 34 should enter into a neighbouring groove 22. This edge 27 is protected against wear during the movement of the wheel 19 caused by the latch 20.

The function of the device described is as follows: During application of the brake, the force transmitting devices 7, 8 are moved outwardly relative to the axis of the key shaft 4 thus causing the said braking surfaces to engage each other.

During said outward movement at least one tooth 18 of each device 7, 8 is sliding radially in a groove 21 of the wheel 19.

It will be understood that, owing to the inclined shape of the grooves 21, radial opposite movements of the two teeth 1 8 of the devices 7, 8 will cause a rotary movement of the wheel 19.

However, the turning of the key 6 will also cause a rotary movement of the devices 7, 8 in the same plane and in the same direction. Therefore, the actual rotation of the wheel 1 9 relative to a stationary part - e.g. a not-shown backing plate in which the shaft 4 is journalied -- will be smaller. Also the rotation of the key shaft 4 and thus the rotation of the spring latch 20 rigidly secured thereto will be performed in the same direction. However, owing to the shape of the grooves 21, a slight relative rotation will be performed between the wheel 1 9 and the spring latch 20 and said relative rotation is a function of the angular travel of the key shaft 4.

At excessive angular movements of the key shaft 4, the said relative movement is sufficient to cause the spring latch part 24 to enter into a neighbouring groove 22 of the wheel 19. Upon release of the brake the key shaft 4 is moved in the opposite direction and the latch part 24 will cause a torque on the wheel 1 9 and thus on the tooth carrying elements 10, 11 relative to the elements 9, 1 2 so as to increase the total lengths of the devices 7, 8.

Owing to the symmetrical design of the elements conveying slack adjustments and owing to the lack of stationary reference elements deciding the adjustment, the descirbed device may be made extremely accurate and the proper function will be ensured even in the case of greater variations of the geometry of the brake causing leverage variations due to tolerances in the manufacture of the force transmitting parts.

The same parts such as the devices 7, 8, the wheel 1 9 and the spring latch 20 may even be used for brakes of varying types having keys 6 of different size and shape. This is possible by making the tooth rack 1 7 axially adjustable relative to the elements 10 and 11. As shown in Fig. 8 this is done by loosening a set screw 28.

axially displacing the rack 1 7 to its desired position and mounting the set screw 28 in the appropriate one of a number of holes 29.

Figs. 9 and 10 show two different keys 6. The key 6 of Fig. 9 having a rather great active length and a larger c' angle between the active radius R and an imaginary line 30 connecting the axis 31 of the key shaft 4 with the axis 32 of rotation of the device 7 relative to the brake shoe 1 3. The key 6 of Fig. 10 is shorter and has a smaller angle 5 between the active radius rand the imaginary line 30.

In Fig. 9 the displaceable rack 1 7 is in its extreme end position and during a braking operation a tooth 1 8 will move to a final position 1 8a while the axis 32 will move to the position 32a. The radial distances of the tooth positions 1 8 and 1 8a from the axis 31 determine the radial path of the teeth in the grooves 21 of the wheel 19 (not shown in Fig. 9).

Fig.10 shows how the displaceable rack 1 7 makes it possible to use the same wheel 19 having the same grooves 21 also for a brake having the smaller angle 83 and the shorter active radius r. It will be understood that the dispiaceable rack 1 7 in Fig.10 has been displaced so that a tooth end position 1 8a is located near the outer end of the grooves 21 , whereas the start position 18 is between the ends of the grooves 21. It will also be understood that the position of the rack 1 7 shown in Fig. 9 would not be suitable for a brake having the key of Fig. 10. In such case the wheel 1 9 would have insufficient diameter.

Claims (3)

1. An automatic slack adjuster for a vehicle drum brake of the type actuated by a key rigidly mounted on a key shaft connected to a cylinderpiston activator, said key acting upon a pair of brake shoes through force transmitting devices each of which comprises two telescopically displaceable elements, one being an externally threaded rod, which the other is an internally threaded sleeve receiving said rod, each of the two sleeves of the two elements being provided with a circumferential tooth rack engaging grooves extending radially on an axial shoulder surface of a wheel journalled on the key shaft, the grooves having such shape that any straight radial movement of the tooth rack relative to the wheel causes a torque on both sleeves in the direction causing an increase in the total length of the two telescopically displaceable elements, wherein said wheel is provided with a second axially extending shoulder surface area having grooves adapted to receive a resilient spring latch rigidly connected to the key shaft.

2. A slack adjuster according to claim 1, wherein said circumferential tooth racks are axially adjustably connected to the sleeves.

3. An automatic slack adjuster for a vehicle drum brake substantially as described herein with reference to the drawings.