GB2030667A

GB2030667A - Wheel brake cylinder

Info

Publication number: GB2030667A
Application number: GB7932680A
Authority: GB
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co OHG
Priority date: 1978-09-29
Filing date: 1979-09-20
Publication date: 1980-04-10
Also published as: GB2030667B; DE2842433C2; FR2437529B1; IT1123748B; ES245833Y; IT7926036A0; JPS5547026A; DE2842433A1; SE437863B; SE7907785L; FR2437529A1; ES245833U; JPS6227290B2

Abstract

A wheel brake cylinder (9) includes a piston (4) guided on a pin (3). To compensate for friction lining wear a friction ring (2) is arranged on the pin (3) and held captive in a recess (1) of the piston (4). The piston (4) is able to move relative to the ring (3) by an amount (L) equal to the desired brake play. When the brake play exceeds the distance L, actuation of the piston (4) causes the friction ring (2) to be moved along the pin (3) in order to reduce the brake play to distance L. The friction ring (2) is a slotted plastically-deformable ring which initially has an external diameter sufficiently small to allow it to be arranged in the recess (1). The ring is expanded by temporary insertion of a fitting pin (not shown) into it so that the ring is held captive in the recess (1). Finally, the piston, with ring inserted, is pushed on to pin 3, so that the ring is held frictionally thereon. <IMAGE>

Description

SPECIFICATION Wheel brake cylinder The invention relates to a wheel brake cylinder including a device for automatically adjusting the brake unactuated position of a piston in the cylinder whereby to compensate for friction lining wear, wherever the piston has an axial bore with an internal annular recess in which a slotted friction ring having axial play and embracing a guide pin fixed at the cylinder base is fitted.

Arrangements of this nature are desirable because they guarantee a uniform working stroke by the piston and thus maintain the distance which the pedal must be depressed in a hydraulic brake system constant, despite wearing of the friction linings.

In one such known wheel brake cylinder (German printed and examined patent application 1 260 240) a helical spring is used as the friction ring, said helical spring consisting of at least one turn of spring having a smaller diameter than the outer adjacent turns of spring, said outer turns of spring having equal diameters. With the larger turns the helical spring fits into the annular recess of the bore of the piston, whilst with its inner turn it makes frictional contact with a cylindrical projection at the base of the cylinder. When a pressure medium is introduced, the piston overcomes the free play between inner turn and outer turn, so putting the helical spring under tension.

If the distance to be covered by the piston is greater than the avaiiable play, the inner turn is pushed along the cylindrical projection. When the pressure medium is relieved the helical spring returns the piston only that distance corresponding to the free play, since the inner turn of the helical spring is again fixed in position due to frictional contact with the cylindrical projection and further sliding back of the piston is prevented.

It is expensive to make the helical spring for this arrangement and besides this, tolerances can occur with respect to the diameter of the inner turn which severely impair the functional safety. In addition, it is difficult to mount the helical spring in the piston.

In another known arrangement (our publication ATE-Kundendienst-Mitteilung, pages 1 0.6-01 and 02 from 25.1 1.76) the internal annular recess ends on the side facing the outside edge of the bore in an annular recess which is open to the outside. The guide pin fixed at the base of the cylinder is accommodated in the piston bore and holds the friction ring, positioned with axial play, in the internal annular recess. The latter is closed off by a detent plate, through which the guide pin projects freely, said plate being held firmly in the outer annular recess by means of caulking in the piston. This sort of assembly necessitates unfavourable lengthening of the piston, since in order to caulk the detent plate in the outer recess, the piston must be longer.

According to the present invention there is provided a wheel brake cylinder including a device for automatically adjusting the brake unactuated position of a piston in the cylinder whereby to compensate for friction lining wear, wherein the piston is axially guided on a pin fixed to the cylinder and extending into a bore in the piston, the bore including a first portion, adjacent to a piston end wall, of a first diameter and a second portion adjacent to the first portion of a larger second diameter, and wherein a slotted plastically-deformed friction ring is arranged on the pin and held captive in the second bore portion whereby to determine the position of the piston, the piston being axially movable with respect to the ring by a predetermined amount.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows a section through a wheel brake cylinder for a drum brake with an adjusting device for friction linings, and Fig. 2 shows an alternative form of friction ring before fitting.

Fig. 1 shows a cylinder 9 in which a piston 4 sealed with a rubber ring 6 slides, the cylindrical surface of said piston being sealed off from dirt by a protective sleeve 5. In a bore in the base of cylinder 9 a guide pin 3 is rigidly fixed. The pin is axial to the piston and extends into a stepped bore 8 in the piston. A first section, as seen from the cylinder base, of stepped bore 8 has a diameter dK greater than the diameter dB of the pin, a second section has a diameter greater than dK and a third section, with a diameter a little larger than the diameter de of guide pin 3 which serves to accommodate the pin 3. A step 11 is between the second and third bore portions. The second portion of the stepped bore defines an annular slot 1.Fitted over the guide pin so as to make frictional contact therewith is a friction ring 2 which, in annular slot 1 of piston 4, exhibits as well as an amount of radial play also a certain amount L of axial play, which corresponds to the brake play. If pressure medium is now introduced through inlet 7 into pressure space 10, piston 4 slides out of cylinder 9 and overcomes the axial play L. If a friction lining (not shown) rendered active by the piston has not yet made contact at the friction surface provided (not shown), the frictional force between the guide pin and the friction ring is overcome by further increasing the pressure and the friction ring is pulled by the piston over the guide pin until the friction linings make contact.

On relieving the pressure, the piston is returned, by a restoring force provided from the exterior, by that distance corresponding to the axial play L, and at step 11 comes up againstfriction ring 2.

Since the frictional force is greater than the restoring force, the piston remains in the adjusted position.

Before fitting, the friction ring has an external diameter dA a little smaller than the diameter of the bore comprising the first section of stepped bore 8. The internal diameter, on the other hand, is considerably smaller than the diameter of guide pin 3. After friction ring 2 has been inserted into stepped bore 8, its internal diameter is expanded by means of a fitting pin (not shown) to about 97% of the diameter dB of guide pin 3. Since the friction ring is slotted and consists of plastically deformable material, its external diameter increases too so that the friction ring lies captive in slot 1. A chamfer at the end of guide pin 3 facilitates insertion of the piston into the cylinder.

Only at this stage of assembly is friction ring 2 expanded to its final external diameter d'A, then, as a result of its elastic properties, it is held frictionally in position on guide pin 3.

By choosing a different friction ring in the shape of a sleeve (Fig. 2) with a radial, encircling collar 20, the frictional force can be increased on account of the greater over-all length. This makes considerably greater frictional forces available, just as are required in the field of drum brakes where very strong restoring springs are used for adjusting the amount of play. As a result of the simple manufacture and fitting, the production costs are considerably less than for conventional adjusting devices for friction linings. Due to the simple construction a small over-all length as is required for drum brakes is possible.

This arrangement renders further special fixing means unnecessary. Very high frictional forces can be supported and the total length of the piston is reduced to a minimum, this being of particular advantage in drum brake systems.

By constructing the ring of elastic metal, in particular of soft-annealed steel C 35, extremely high frictional forces are attained.

As a result of the internal diameter of the friction ring's expanding to approximately 97% of the diameter of the guide pin, fitting of the piston is considerably facilitated.

Since expansion of the friction ring to its final external diameter only occurs during fitting of the piston, by means of the guide pin fixed at the cylinder base, possible tolerances in the diameter of the guide pin are compensated for. The frictional force is now only dependent on the elastic properties of the material used for the friction ring, and for the manufactured adjusting devices for friction linings can thus be considered almost constant.

Claims

1. A wheel brake cylinder including a device for automatically adjusting the brake unactuated position of a piston in the cylinder whereby to compensate for friction lining wear, wherein the piston is axially guided on a pin fixed to the cylinder and extending into a bore in the piston, the bore including a first portion, adjacent to a piston end wall, of a first diameter and t second portion adjacent to the first portion of a larger second diameter, and wherein a slotted plastically-deformed friction ring is arranged on the pin and held captive in the second bore portion whereby to determine the position of the piston, the piston being axially movable with respect to the ring by a predetermined amount.

2. A wheel brake cylinder as claimed in claim 1, wherein the ring is of an initial diameter less than the first diameter but is expanded, after insertion into the second bore portion, to a final external diameter less than the second diameter but greater than the first diameter.

3. A wheel brake cylinder as claimed in claim 1 or 2, wherein the friction ring consists of an elastic metal.

4. A wheel brake cylinder as claimed in any one of the preceding claims, wherein the friction ring consists of soft-annealed steel C 35.

5. A wheel brake cylinder as claimed in claim 2, or claims 3 to 4 as appendant to claim 2, wherein the internal diameter of the friction ring is still smaller after expanding then the diameter of the guide pin.

6. A wheel brake cylinder as claimed in claim 2, or any one of claims 3 to 5 as appendant to claim 2, wherein expansion of the friction ring to its final external diameter occurs during fitting of the piston onto the guide pin.

7. A wheel brake cylinder as claimed in any one of the preceding claims wherein the friction ring is a friction sleeve which has a radial encircling collar that is held captive in the second bore portion.

8. A wheel brake cylinder including an automatic adjusting device and substantially as herein described with reference to and as illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.

9. A slotted plastically-deformable friction ring for use in a wheel brake cylinder as claimed in any one of the preceding claims.