GB2206166A - A hydraulic unit for the actuation of a vehicle's brakes including a device for taking up excess play caused by wearing of the brake shoes - Google Patents

Abstract

A unit for drum brakes comprises a fixed cylinder 1 connected to the hydraulic braking circuit, a pair of opposed movable pistons 5, 8 sealed in the cylinder 1 and a device inserted between the pistons, 5, 8 for taking-up of play due to wearing of the brake shoe linings. The device includes an expansible bush 12 inserted in a seat of the first piston 5 so as to be longitudinally slidable therein and connected to the second piston 8 by connection means, such as a pin 14 having a longitudinal play corresponding to the normal working stroke of the unit. The bush 12 is provided with expansion means, preferably a sphere 17 and a spring 18, inserted into the bush 12 and arranged to urge it radially outwards with a pre-established pressure. This produces a frictional force between the bush 12 and the first piston 5 greater than the force of the return springs. <IMAGE>

Description

A HYDRAULIC UNIT FOR THE ACTUATION OF A VEHICLE'S BRAKES INCLUDING A DEVICE FOR TAKING UP EXCESS PLAY CAUSED BY WEARING OF THE BRAKE SHOES The present invention relates to a hydraulic unit for the actuation of a vehicle's brakes, of the type having a drum and shoes, comprising a fixed cylinder connected to the hydraulic braking circuit, a pair of opposed movable pistons sealed in the said cylinder and connected to the said shoes in order to apply the braking pressure to them in opposition to return springs, and a device inserted between the said pistons, for taking-up of excess play caused by wearing of the brake shoe linings.

In drum brakes for vehicles, the shoes are generally pivoted by one end and are actuated at their other end by a hydraulic cylinder/piston unit. Upon commencement of the braking action, the hydraulic unit brings the shoes into contact with the drum, cancelling the play which is normally maintained between the said parts by return springs connected to the shoes, and then causes the application of the braking pressure to the drum. With the progressive wearing of the linings of the shoes, the play between shoes and drum increases, and also the idle stroke which precedes the commencement of the active braking. To obviate this disadvantage there can be inserted in the brake mechanism recovery devices which provide for shifting the rest position of the shoes as the wear of the linings progresses.In a first type of commonly known form, these devices, of various types, are installed on the pivot side of the shoes or else between the hydraulic actuating unit and the shoes. Italian Patent No. 1,158,804 discloses a second type of recovery device, which is inserted in the hydraulic actuating unit itself, interposed between the pistons and co-operating with these in order to modify the stroke allowed, thus compensating for the increase in stroke due to the wear of the linings of the shoes.

The first type of device introduces a considerable complication into the structure of the mechanism of the brake, making the assembly of the brake-bearing plate both lengthy and costly and hindering the replacement of the worn-out linings of the shoes; moreover, as they are not installed in a protected position, these devices can be easily damaged. The devices of the second type overcome these disadvantages but, like the first ones, are characLerlsad by a relatively high number of components and are somewhat expensive. Moreover, all the devices hitherto known act in an intermittent and irreversible manner.

The aim of the present invention is that of permitting the construction of drum brakes which, while still being endowed with the ability to take up play which has become excessive through wear of the linings of the shoes, retain the simplicity of construction, assembly and ease of replacement of the shoes and of their linings, characteristic of drum brakes which do not include recovery devices or have recovery devices of the second type described. A further object is to permit the use of a substantially simple and economical control unit, constituted by few components and which acts in a continuous manner and is also able to take up the play recovered in the cases where a change in the amount of play has been caused by an accidental disturbance rather than by the wear of the linings of the shoes.

With these objects in view the present invention provides an hydraulic unit for the actuation of a vehicle's brakes of the type having a drum and shoes, comprising a fixed cylinder connected to the hydraulic braking circuit, a pair of opposed movable pistons sealed in said cylinder and connected to the said shoes in order to apply braking pressure to these in opposition to return springs, further including a device for the taking up of play due to wear of the brake shoe linings, said device being disposed between said pistons and characterised in that it includes an expansible bush inserted in a corresponding seat of the first piston so as to be longitudinally slidable therein and connected to the second piston by connection means having a longitudinal play corresponding to the normal working stroke envisaged for the hydraulic unit and also including expansion means inserted in the said bush and arranged to urge it radially outwards with a preestablished pressure so as to generate a frictional force between the bush and the first piston which is greater than the force applied to the pistons by the return springs of the shoes.

During the release stroke which follows a braking action, the pistons of the unit, impelled by the return springs of the shoes of the brake, carry out a stroke corresponding to the longitudinal play with which the expansible bush is connected to the second piston. This play corresponds to the normal working stroke of the unit. If, due to wearing of the linings of the shoes the release stroke tends to assume an amplitude greater than that thus established, the pistons are stopped by the action of the expansible bush, because the frictional force present between it and the first piston is greater than the force due to the action of the return springs. The idle stroke of the brake pedal is thus prevented from increasing.In the course of a subsequent braking action, the hydraulic force exerted on the pistons, which is greater than the force of the return springs, then prevails over the frictional force present between the expansible bush and the first piston. The bush is thus slightly displaced in the piston and assumes a new postion which corresponds to the new condition of the linings of the shoes and compensates for the wear which has occurred. This taking up of increasing play is continuous and takes place every time only to the minimum extent required, contrary to the known devices in which the recovery action is discontinuous or intermittent and necessarily has to have a relatively large displacement. A more perfect operation of the brake is therefore obtained.

Furthermore, if there has occurred an excessive play because of unique or accidental conditions occuring, the non-irreversible character of the frictional connection between the expansible bush and the first piston ensures that the excessive play will be taken up and corrected during the release phase.

The invention will be described further by way of example, with reference to the accompanying drawings in which:- Fig. 1 shows, in a longitudinal section taken along the. line I-I of Fig. 2, a hydraulic unit in accordance with the invention for the actuation of a drum brake; Fig. 2 shows a cross-section thereof taken along the line II-II of Fig. 1; and Fig. 3 shows a cross-section thereof taken along the line III-III of Fig. 1.

With reference to the drawing, the number 1 indicates a cylinder, destined to be securely mounted in the mechanism of a drum and shoes type brake. The cylinder is provided with a connector 2 for connection to a hydraulic braking circuit (not shown) in order to supply the cylinder's internal chamber 3 which is also provided with a discharge connector 4. Slidably disposed in the cylinder 1 is a first piston 5, provided with a seal, an outer protective cowling and a tappet 7 suitable for applying a pressure to one end of a brake shoe (not shown). Also located in the cylinder 1 opposite the piston 5, is a second piston 8, this too provided with a seal, a protective cowling 9 and tappet 10, by means of which this acts on the second brake shoe (not shown). During the braking phase, the two pistons 5, 8 draw away mutually under the action of a pressurised liquid introduced into the chamber 3 of the cylinder 1 in the known manner. In the release phase subsequent to the braking action, the two pistons 5, 8 mutually draw nearer under the action of return springs (not shown) connected, as usual, to the brake shoes.

The first piston 5 has a blind cavity 11, into which there is inserted a bush 12 so as to be longitudinally slidable therein. The inner end of the bush 12 is. slightly tapered and is made expansible by the provision of slots 13. The opposite end of the bush 12 projects from the cavity 11 of the first piston 5, passes through the chamber 3 and slidably penetrates into a cavity 15 provided in the second piston 8. A pin 14 traverses the bush 12 passing into a hole 16 of the second piston 8. The hole 16 provides a degree of play for the pin 14 accommodated therein, the amplitude of which is selected equal to the normal working stroke of the hydraulic unit. In the inner end of the bush 12 there is arranged a sphere 17, held in place by a spring 18 which is supported on the pin 14. The pressure of the spring 18 urges the sphere 17 to wedge itself in the tapered end of the bush 12.Said bush 12, being expansible thanks to the slots 13, is expanded and is thus kept pressed against the wall of the cavity 11 of the first piston 5 with a force dependent upon the force exerted by the spring 18. Therefore, between the bush 12 and the first piston 5 a frictional force is produced which can be chosen during design of the device by suitable dimensioning of the interengaging parts and especially the spring 5 < 8. This selection of the parts dimensions is to be such that the frictional force generated is somewhat greater than the force transmitted to the pistons 5, 8 by the return springs of the brake shoes.

Following the actuation of the pedal of the brake, a hydraulic pressure is sent to the chamber 3 and the two pistons 5, 8 are forced apart. After having taken up the play existing between the shoes and the drum, the pistons 5, 8 act on the shoes to apply the braking pressure. During this mutual drawing apart of the pistons 5, 8, the bush 12 entrained through friction with the piston 5 thus bringing the pin 14 into contact with a wall (the left-hand wall as shown in Fig. 1) of the hole 16. If the travel during this mutual drawing apart of the pistons 5, 8 continues, the bush 12 is constrained to slide in the cavity 11 of the piston 5 against the frictional force which retains it.In fact, the said frictional force is somwhat greater than the force exerted on the pistons 5, 8 by the return springs of the shoes, but the hydraulic actuation force of the brakes is by far greater and overcomes without any difficulty said frictional force. When the braking action has been concluded the hydraulic pressure is released and the return springs of the shoes repel the pistons 5, 8 towards each other. This can happen freely because the pin 14 is displaced in the hole 16 for the entire amplitude of the play existing there. Since this play corresponds to the normal working stroke of the unit, as long as there is no wearing of the linings of the shoes or another disturbing element, the entire release stroke is allowed.However, when the release stroke, for whatsoever reason, tends to become greater than the play of the pin 14 in the hole 16, the pistons 5, 8 are stopped as soon as such play has been taken up, because the frictional force which opposes the sliding of the bush 12 in the piston 5 is greater than the return force of the springs. The release stroke is therefore, limited to its normal amount. In the course of a subsequent further braking action, if this turns out to be necessary, the bush 12 slides in the piston 5, as before, by the amount necessary to re-establish a correct position of the ends of the working stroke allowed by the play between the parts 14 and 16.

Normally the sliding described takes place in the sense of a progressive emergence of the bush 12 from the cavity 11 of the piston 5, to compensate for the progressive wear of the linings of the shoes. It should be noted however that, if the preceding recovery has been excessive because of disturbing actions, such as a thermal and/or mechanical stressing of the parts during the braking action, the excess recovery. can be taken up due to the fact that the frictional connection between the bush 12 and the piston 5 is not irreversible. For this reason the sliding of the bush 12 can take place in both directions as soon as there is applied to it an urging greater than the frictional force present between said bush 12 and the piston 5.This special feature of the device in accordance with the invention ensures that, even if a disturbing action accidentally occurs there is no danger that the brake could be locked by the action of the recovery device.

It can be seen from the foregoing that the taking up of the excess play caused by wearing of the brake shoes does not take place in an intermittent manner, i.e. when the play has exceeded a pre-determined value, as in the known recovery devices, but in a completely continuous and gradual manner. The recovery device is additionally entirely contained in the hydraulic actuating unit and is therefore protected from damage by an external action. Furthermore the recovery device is constituted by few components, the industrial realisation of which and the assembly thereof are simple, and therefore relatively economical. The device is also not subjected in any part to concentrated stresses such that it had a long useful life.

Naturally various modifications can be made to the exemplified embodiment. For example, a sphere and spring mechanism represents an advantageous expansion means for the expansible bush, but other expansion means could also be used. The pin 14 could be made integral with the piston 8 and inserted in a hole, having an amount of play, in the bush 12, provided that there is provided alternative means for the support of the spring 18. Furthermore, a pin inserted with play in a hole represents a simple and sure means for providing for the connection of the expansible bush with a longitudinal play corresponding to the normal working stroke of the unit. However, other types of connection with play could be applied.

Claims (7)

1. A hydraulic unit for the actuation of a vehicle's brakes of the type having a drum and shoes, comprising a fixed cylinder connected to the hydraulic braking circuit, a pair of opposed movable pistons sealed in the said cylinder and connected to the said shoes in order to apply braking pressure to these in opposition to return springs, further including a device for taking up of play due to wearing of the brake shoe linings, said device being disposed between the said pistons and characterised in that it includes an expansible bush inserted in a corresponding seat of the first piston so as to be longitudinally slidable thereon and connected to the second piston by connection means having a longitudinal play corresponding to the normal working stroke envisaged for the hydraulic unit and also including expansion means inserted into the said bush and arranged to urge it radially outwards with a preestablished pressure, so as to generate a frictional force between the bush and the first piston which is greater than the force applied to the pistons by the return springs of the shoes.

2. A hydraulic unit according to claim 1, characterised in that the said connection means between the expansible bush and the second piston are constituted by a pin fixed to the expansible bush and located in a hole provided in the second piston, the hole having the desired longitudinal play with respect to the pin.

3. A hydraulic unit according to claim 1, characterised in that the said expansible bush is a tube provided with slots, of predetermined length, open at the end of the tube intended to expand.

4. A hydraulic unit according to claim 1, characterised in that the said expansion means is a body urged by a spring to wedge itself into the end, suitable tapered, of the expansible bush.

5. An hydraulic unit according to claim 4, characterised in that the said body urged by the spring is a sphere.

6. A hydraulic unit according to claims 2 and 4, characterised in that the said pin provides a support for the spring which forms part of the expansion means.

7. A hydraulic unit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.