GB2282648A

GB2282648A - Knock-back/retraction of a piston in a disc brake

Info

Publication number: GB2282648A
Application number: GB9320852A
Authority: GB
Inventors: Ashley Way; Stephen Bryan
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1993-10-09
Filing date: 1993-10-09
Publication date: 1995-04-12
Anticipated expiration: 2013-10-09
Also published as: GB9320852D0; GB2282648B

Abstract

A disc brake caliper which includes a pad-operating piston (3) slideable in a cylinder bore (2), the piston carrying a seal (6) to seal the bore. A friction ring (10) acts between the piston and the cylinder to provide, in combination with the seal (6), the required knock-back/retraction characteristics for the control of piston movement within the bore. The friction ring (10) may be carried on the piston or may be mounted in the wail of the cylinder bore. <IMAGE>

Description

DISC BRAKE CALIPER This invention relates to a disc brake caliper including a pad-operating piston which is axially slidable in a cylinder bore under the action of pressurised fluid introduced into the bore from an associated brake pressure source such as a master cylinder. The piston when displaced by the pressurized fluid forces an associated friction pad into contact with a brake disc to generate the braking effect.

Such calipers tend to rely on the so-called "retraction" effect when elastic deflection of the piston/bore seal which occurs under the action of brake applying pressure is used to move the piston back from the friction pad when the brake applying pressure is released.

The so-called "knock-back'1 effect when vibration in the brake or contact between the disc and pad (due for example to deflection of the caliper mount) causes the piston to be knocked-back or to shuffle back from the pad is also used to move the piston back from the friction pad when the brake applying pressure is released.

Problems arise due to excessive "knock-back" which increases the stroke of the piston (and thus momentarily delays the operation of the brakes) on the next brake application and also due to inadequate "retraction" which results in the friction pad exerting a drag effect on the disc after release of the brake applying pressure.

It is an object of the present invention to provide an improved form of disc brake caliper which mitigates the above excessive "knock-back" and inadequate "retraction" problems.

Thus in accordance with the present invention there is provided a disc brake caliper including a padoperating piston slidable in a cylinder bore, said piston carrying sealing means to seal said bore, the caliper being characterised in that friction means act between the piston and the cylinder to provide, in combination with the sealing means, the required knock-back/retraction characteristics for the control of piston movement within the bore.

By careful tuning of the total frictional resistance and deflection characteristics of the sealing means and the friction means the required overall knock-back/retraction characteristics of the piston can be obtained so that the above discussed knock-back/retraction problems are mitigated.

In a preferred arrangement the friction means is also carried on the piston. Alternatively the friction means may be mounted in the wall of the cylinder bore.

The friction means may comprise an annulus of polymeric material (e.g. high temperature rubber) set in an annular groove in the piston or cylinder bore.

In an alternative arrangement the friction means may comprise an annulus of resilient metallic material (eg. spring steel) set in an annular groove in the piston or cylinder bore.

The present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a section through part of a disc brake caliper embodying the present invention; Figure 2 shows a section through an alternative caliper construction embodying the present invention, and Figures 3 and 4 diagrammatically illustrate the forces exerted on the seal and friction means of the caliper during and after a brake application.

Figure 1 shows a brake caliper body 1 provided with a cylinder bore 2 in which a piston 3 is slidable.

Bore 2 has an inlet (not shown) through which pressurised hydraulic fluid is introduced from an associated brake pressure source such as a master cylinder (not shown) to move piston 3 into contact with an associated friction pad 4 which in turn engages an associated brake disc in the known manner.

The piston 3 has an outer surface 5 which is a close sliding fit in the bore 2. Hydraulic fluid is prevented from passing between the bore 2 and the piston 3 by an annular pressure seal 6. The pressure seal 6 is located in a first annular groove 7 in the piston 3 and maintains a seal with the bore 2 as the piston 3 slides within the bore 2. Since the seal is carried by the piston it is possible to site the seal closely adjacent the inner end of piston 2 so that the hydraulic fluid is kept well away from the hotter regions of the brake adjacent friction pad 4.

The pressure seal 6 is preferably of polymeric material (eg. high temperature rubber). The seal may include an anti-extrusion ring 8 of metal or relatively rigid plastics material the purpose of which is to prevent the tendency for the seal 6 deform under the action of the pressure within the bore 2 and to be extruded between the piston 3 and the bore 2.

In accordance with the present invention the piston 3 has a second annular groove 9 which is concentric to the first annular groove 7 and in which is located a friction means in the form of an annular ring 10. Ring 10 is also preferably formed from polymeric (e.g. high temperature rubber) or resilient metallic material (e.g. spring steel) to create friction within the bore 2.

Referring to Figure 3, introduction of hydraulic fluid at pressure P into bore 2 from the associated master cylinder (not shown) forces piston 3 to the right as indicated by arrow X to in turn force friction pad 4 into contact with the associated brake disc (not shown) to generate the braking effect. Fluid pressure P causes that portion of seal 6 outside groove 7 to slip on bore 2 and thus be deflected towards brake pad 4. During movement of piston 3 to the right the friction ring 10 grips the bore 2 so that the portion of the friction means outside groove is deflected in effect away from brake pad 4.

Since both the deflection of seal 6 and friction ring 10 is elastic deflection, when the brake is released (see Figure 4), the elastic energy stored in seal 6 and friction ring 10 produces two restoring forces. FS and FR respectively which act on piston 2 in opposition te each other. Force FS produced by the seal 6 cts toward pad 4 and force FR produced by ring 10 acts away from pad 4.

As will be appreciated, the above seal and friction ring arrangement enables the problem of inadequate "retraction" to be overcome since the force FR can be tuned to be greater than the force FS thus giving a net "retraction" force of the required level.

Also, the addition of the friction ring 10, with is increased frictional grip on bore 2, provides control over excessive knock-back.

Thus by tuning the relative values of forces FS and FR and the frictional characteristics of seal 6 and ring 10 for each particular caliper design it is possible to ensure the required overall knock-back/retraction characteristics of the piston 3 are achieved so that pad drag is minimised and caliper performance is significantly improved over a conventional caliper. Careful tuning allows the ideal condition in which the pad just contacts the disc with virtually zero pad drag to be obtained.

Also, by using seals and friction rings manufactured from silicone rubber, and locating the piston seal adjacent the inner end of the piston, the caliper can continue to operate acceptably at the high temperatures developed in high performance/racing vehicles.

Figure 2 shows an alternative arrangement in which the friction ring 10, instead of being mounted on piston 2, is mounted in a second annular groove 9 formed in the wall of cylinder bore 2. As will be evident, brake pressure P will cause seal 6 to deflect towards pad 4 and ring 10 to deflect away from pad 4 which will again produce opposing restoring forces FS and FR respectively As will be appreciated, the invention is applicable to single or multiple piston calipers of the single sided or double sided type.

Claims

1. A disc brake caliper including a pad-operating piston slidable in a cylinder bore, said piston carrying sealing means to seal said bore, the caliper being characterised in that friction means act between the piston and the cylinder to provide, in combination with the sealing means, the required knock-back /retraction characteristics for the control of piston movement within the bore.

2. A caliper as claimed in claim 1 wherein the friction means is also carried on the piston.

3. A caliper as claimed in claim 1 wherein the friction means is mounted in the wall of the cylinder bore.

4. A caliper as claimed in any one of claims 1 to 3 wherein the friction means is an annulus of polymeric material set in an annular groove in the piston or cylinder bore wall.

5. A caliper as claimed in any one of claims 1 to 3 wherein the friction means is an annulus of resilient metallic material set in an annular groove in the piston or cylinder bore wall.

6. A disc brake caliper constructed and arranged substantially as herein before described with reference to and as shown in the accompanying drawings.