GB2168770A - A system for automatic wear compensation inserted in a hydraulic unit actuating a drum brake - Google Patents

Abstract

A pair of opposing pistons 4, 8, which are connected to the shoes (not shown) to apply braking pressure thereto, are slidingly accommodated in a fixed cylinder 1 connected to a hydraulic braking circuit. A spacer member 17, 18 is connected between the pistons 4, 8, and when the pistons move apart by a distance greater than the spacing between stop rung 21 and spigot 16, due to wear of the linings of the brake shoes, the spacer member will be rotated by the screw threaded engagement between the piston 8 and element 18. If that distance is sufficiently large a spring biassed pawl will move over, and, on brake release, will engage the stop face of, the next one of sawteeth 14, whereas if that distance is not sufficiently large the pawl will ride up the face of the next sawtooth but will return to engage the stop face of the previous tooth on brake release. The compensation mechanism is restored to its original position when the worn linings are replaced by causing one of the two pistons of the hydraulic actuation unit to rotate with respect to the other. <IMAGE>

Description

SPECIFICATIONS A system for automatic wear compensation inserted in a hydraulic unit actuating a drum brake This invention concerns a hydraulic unit for the actuation of vehicle brakes having a drum and shoes, said unit having means for compensating for wear and tear of the linings of the shoes and comprising a fixed cylinder connected to a hydraulic braking circuit, and a pair of opposed pistons which are slidingly accommodated in the cylinder and are connected to the shoes so as to apply braking pressure thereto.

In drum brakes for vehicles, the shoes are generally pivoted by one end and actuated at the other end by a hydraulic cylinder/piston unit which, upon braking, brings the shoes into contact with the drum, cancelling the play normally provided between the said parts, and then applies the braking pressure. As wear and tear of the linings of the shoes progresses, the play between shoes and drum increases, and consequently the idle stroke which precedes the start of the active braking increases, which represents a disadvantage. To obviate this, in the mechanism of the brake there can be inserted compensation devices which bring about shifting of the rest position of the shoes as wear and tear of their linings proceeds.These devices can be installed on the pivot side of the shoes or else on the side of the hydraulic actuation unit, and can be varied types compensating in different ways the play which has become excessive.

In each case, however, the devices are relatively high in cost and introduce considerable complication into the structure of the mechanism of the brake, making assembly of the brake-bearing plate a longer and more costly procedure and hindering replacement of the worn linings of the shoes.

Compensation devices are also known which comprise a member, connected centrally to the cylinder of the hydraulic actuation unit, interposed between the pistons and co-operating with these to modify the stroke allowed so as to provide for compensation of the increase of stroke due to the wear and tear of the linings of the shoes. However, these devices turn out to be cumbersome or bulky and can find application only on hydraulic units of relatively large dimensions.

The object of the present invention is provide drum brakes with means for compensation of play which has become excessive through wear and tear of the linings of the shoes, which brakes nevertheless have a simple construction such that their assembly, as well as replacement of the shoes and their linings is almost as easy as for drum brakes which do not possess such compensating means.

With this object in view, the invention provides a hydraulic unit for the actuation of vehicle brakes having a drum and shoes, said unit having means for compensating for wear and tear of the linings of the shoes and comprising a fixed cylinder connected to a hydraulic braking circuit, and a pair of opposed pistons which are slidingly accommodated in the cylinder and are connected to the shoes so as to apply braking pressure thereto, characterised in that a spacer member, independent of the cylinder, is connected between the said pistons so as to modify its own operative position in relation to the maximum mutual withdrawal reached by the pistons, and to limit the mutual approach stroke of the pistons to a pre-established distance measured from the position of maximum withdrawal reached by the pistons.

In this way, the means for compensating for the increasing play is incorporated inside the hydraulic actuation unit of the brake without substantially modifying the external bulk of the unit itself.

In a preferred practical embodiment of the unit of the invention an escapement wheel having internal sawteeth is fixed to one of the pistons, and the spacer member is mounted rotatably and with limited axial play with respect to the first piston, is coupled with the second piston by way of a rapid threading, and carries at least one radial pawl capable of co-operating with the internal sawteeth of the escapement wheel.

Thanks to these features of construction, during the release stroke the pistons of the unit, thrust by return-springs of the shoes of the brakes, can execute only a limited stroke in a pre-established range, and therefore, at rest, these keep the shoes of the brake at a distance from the drum which lies in a corresponding range. With the progress of the wear and tear of the linings of the shoes, during the braking phase the pistons of the unit shift progressively further during mutual withdrawal, but as their approach stroke is also limited, the minimum distance apart of the pistons increases correspondingly and compensation of the play increased by the wear and tear is achieved.

With such a unit incorporating the compensation mechanism, the remainder of the brake structure can be entirely the same as that of a conventional brake (without a compensation mechanism) with all the advantages which derive therefrom. One particular advantage which follows from this is that a hydraulic actuation unit with the compensation mechanism can be substituted for a normal actuation unit in a brake which lacks a compensating mechanism, thereby introducing therein a new function.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-section, on a scale larger than the natural one, of a preferred embodiment of the hydraulic unit of the invention for actuating a drum brake; Figure 2 is a side view on an even larger scale of an isolated spacer member of such a unit, seen in the direction of the arrow II of Figure 1; Figure 3 is a cross-section of an isolated escapement wheel having an internal toothing which is part of the unit of Figure 1; and Figure 4 is a front view, in the direction of the arrow IV of Figure 3, of the escapement wheel and the pawl which co-operates therewith.

With reference first of all to Figure 1, a preferred embodiment of the hydraulic unit of the invention comprises a cylinder 1 which is intended to be fixed in a brake mechanism having a drum and shoes and is provided with means 2 for connection to a hydraulic braking circuit which will supply the internal chamber of the cylinder 1 by way of an aperture 3. A first piston 4 is slidingly mounted in the cylinder 1. This piston 4 is provided with a sealing ring 5 and with an external protective packing 6, as well as with a tappet 7 suitable for applying pressure to one end of a brake shoe (not shown). At the opposite end of the cylinder 1 to the piston 4 there is a second piston 8 provided in similar manner with a sealing ring 9, protective packing 10 and a tappet 11 to act on another shoe of the brake.In known manner, the two pistons 4 and 8 mutually withdraw into the cylinder 1 in the braking phase under the action of a pressurised liquid introduced into the cylinder 1, and, in the release phase, approach their respective brake shoes under the action of return-springs connected to the shoes.

The first piston 4 has an internally threaded cavity 12 into which a threaded tang 16 of an escapement wheel 13 having internal sawteeth 14 is screwed so as to fix the wheel 13 to the piston 4.

The escapement wheel 13 has opposed flattened surfaces 15 (see Figure 4) to facilitate the screwing and locking of the tang 16 with respect to the first piston 4. A spacer member 17 extends through the escapement wheel 13 and is freely rotatable relative thereto. One end of this spacer member 17 extends into the threaded cavity 12 and is provided with a stop ring 21 whilst the opposite end 18 of this spacer member has a rapid threading having several threads (for example five threads) and is screwed in a corresponding internal rapid threading having several threads of the second piston 8.

The stop ring 21 is disposed so as to allow the spacer member 17-18 a pre-established play with respect to the escapement wheel 13 and therefore to the first piston 4. This play is selected to correspond to the minimum stroke desired in the operation of the unit, namely to the sum of the stroke part destined to bring the shoes nearer to the drum and of the stroke part required to apply the braking pressure.

The central portion 19 of the spacer member, situated between the end portions 17 and 18 thereof, has a radial seat 20, in which is installed a pawl 22.

This pawl 22 is shaped so as to be able to co-operate with the internal sawteeth 14 of the escapement wheel 13, and is urged outwardly of the spacer member by a spring 23, which thus maintains the pawl 22 elastically in engagement with the toothing 14.

The operation of the device is as follows. As long as wear and tear does not occur, the withdrawal stroke and the approach stroke of the pistons 4 and 8 are allowed by the axial play of the spacer member 17, 18 with respect to the first piston 4, as defined by the position of the stop ring 21. Progressively, as the linings of the shoes wear down, the distance apart of the pistons 4, 8 increases, and exceeds this pre-established play. At each mutual withdrawal of the pistons 4 and 8 the rapid threading 18 (which constitutes a reversible coupling) causes the spacer member 17, 18 to rotate slightly in a first direction so that the pawl 22 carried by the spacer member rotates with respect to the escapement wheel 13, whilst a corresponding reverse rotation takes place during the approach of the pistons 4, 8.The inclination of the sawteeth 14 is chosen so that during rotation of the spacer member in the said first direction, the pawl 22 tends to surmount a tooth. This does not occur so long as the rotation of the spacer member is less than the angular distance between successive teeth 14, but when the rotation impressed on the spacer member becomes sufficient, the pawl 22 goes beyond a tooth of the escapement wheel 13, and this tooth then prevents the corresponding reverse rotation during the subsequent approach of the pistons 4, 8. The spacer member 17, 18 then holds the pistons 4, 8 at a greater distance apart than previously thus compensating for the wear and tear which has occurred in the linings of the brake shoes.After this has happened, the play (at rest) of the shoes with respect to the drum, which had been increasing with the progress of the wear and tear, returns to its initial value. This occurrence is repeated each time that the wear and tear of the linings of the shoes reaches a predetermined value, which depends on the inclination of the threading 18 and on theangular distance between the successive teeth 14 of the escapement wheel 13. When, finally, the maximum wear and tear allowed having been reached, the shoes or their linings have to be replaced, all that needs to be done to restore the compensation mechanism to its starting condition is to cause one of the pistons 4 and 8 of the unit to rotate with respect to the other so that the threading 18 of the spacer member is again screwed into the second piston 8.Of course, such a relative rotation between the pistons, 4, 8 cannot happen spontaneously in use, because of the engagement of the tappets 7 and 11 with the shoes of the brakes.

The use of an escapement wheel with internal toothing, co-operating with a pawl which is disposed in its interior, makes it possible (the overall size of the unit being constant) to maximise the radius of the toothing 14, and therefore (the dimensions of the teeth being constant) to reduce the angular distance between successive teeth. It therefore becomes possible, the other conditions being equal, to reduce the degree of wear and tear of the linings of the shoes, by progressively compensating for the increased play and thus keeping to the minimum the variation in the idle stroke of the brake pedal before the start of effective braking. A further reduction of this degree of wear and tear can be obtained, if it is considered useful, by providing two mutually opposed pawls 22 co-operating with two diametrically opposite points of the toothing 14, and assigning to this toothing an odd number of teeth.

The use of a rapid threading 18 having several threads is advantageous with respect to the alternative possibility of a coupling having a helicoidal groove and a pin engaging in this, because with the former the engagement pressure in the coupling is reduced thereby enhancing its sliding ability. For the same reason the use of a stop spring 21 is preferred as a means of limiting axial mobility compared to a pin engaging in an annular groove.

The described device makes it possible to achieve the desired result in the most satisfactory manner, while still not involving excessive manufacturing costs.

It will be appreciated that the above-described compensation mechanism is entirely contained in the hydraulic actuation unit, which does not require an increase in dimensions nor any unusual conformation on the outside. Thus it can be used in any drum brake device currently lacking means for compensating for wear and tear to introduce therein this function. As a result, both the mounting of the brake-bearing plate and any operation which requires intervention on the related mechanisms are considerably simplified, whilst the production cost of the compensation mechanism incorporated in the hydraulic unit is considerably less than hitherto known compensation mechanisms of equal performance.

Of course, the form shown constitutes only an example, and other embodiments are possible within the scope of the invention.

Claims (6)

1. A hydraulic unit for the actuation of vehicle brakes having a drum and shoes, said unit having means for compensating for wear and tear of the linings of the shoes and comprising a fixed cylinder connected to a hydraulic braking circuit, and a pair of opposed pistons which are slidingly accommodated in the cylinder and are connected to the shoes so as to apply braking pressure thereto, characterised in that a spacer member, independent of the cylinder, is connected between the said pistons so as to modify its own operative position in relation to the maximum mutual withdrawal reached by the pistons, and to limit the mutual approach stroke of the pistons to a pre-established distance measured from the position of maximum withdrawal reached by the pistons.

2. A hydraulic unit for brakes as claimed in claim 1, characterised in that an escapement wheel having internal sawteeth is fixed to one of the pistons, and the spacer member is mounted rotatably and with limited axial play with respect to the first piston, is coupled with the second piston by way of a rapid threading, and carries at least one radial pawl capable of co-operating with the internal sawteeth of the escapement wheel.

3. A hydraulic unit for brakes as claimed in claim 2, characterised in that the escapement wheel is fixed to the first piston by means of a threaded tang screwed into a threaded cavity of the piston, in that the spacer member extends axially through the threaded tang, and in that the spacer member has near its end a stop ring which defines its axial play with respect to the escapement wheel and the first piston.

4. A hydraulic unit for brakes as claimed in claim 2 or 3, characterised in that the spacer member has a central portion having at least one radial seat in which is installed the said pawl together with a spring which urges it outwards.

5. A hydraulic unit for brakes as claimed in claim 2, 3 or 4, characterised in that the spacer member bears two diametrically opposed pawls, and in that the escapement wheel has an odd number of teeth.

6. A hydraulic unit for the actuation of drum brakes substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.