DISC BRAKE RETRACTION DEVICE

Abstract: A disc brake calliper 10 includes a reaction member 34 slidably contacting the inner wall of the brake cylinder and a retraction spring 38 acting between the reaction member and piston 16 to retract the piston a predetermined distance following a brake application. The device reduces brake drag and improves vehicle fuel economy.

Description: DISC BRAKE RETRACTION DEVICE This invention relates to disc brakes and is particularly suitable for application to motor vehicle disc brakes. Disc braking systems currently in use in motor vehicles include a calliper with a piston working in a cylinder and arranged to urge a brake pad into pressure contact with a brake disc when hydraulic fluid is supplied to the cylinder. There is usually no return device for displacing the brake pad from the brake disc after a brake application and the brake pad remains held in contact with the disc by friction in the hydraulic actuating system unless urged back by run out of the disc or by vibration of the calliper relative to the disc. It is found that the frictional drag between brake pad and disc when the brakes are not actuated gives rise to an increase of average fuel consumption of the order of one per cent. In city driving on smooth roads and at relatively low speeds the percentage increase is higher. There have earlier been proposed means for positive retraction of the brake pad, examples of these being described in GB-A-2,097,876 and GB-A-2,036,898. Such prior solutions have resulted in an imprecise amount of retraction and this in turn caused variation in the amount of pedal movement required from one actuation to the next and different amounts of play in the pads of the different wheels. The present invention seeks to mitigate the above disadvantages and to permit a precise retraction of the brake pad following each actuation. In accordance with the present invention, there is provided a disc brake calliper including a housing formed with a cylinder, a piston for moving a brake pad into pressure contact with a brake disc and a device for positively retracting the brake pad from the disc to reduce brake drag when the brake is released, characterised in that the device for retracting the brake pad comprises a reaction member, means on the reaction member for frictionally engaging the cylinder, is movable with the piston, and is connected to the piston by a lost motion connection, and a retraction spring acting between the reaction member and the piston to retract the piston upon release of the brake, the retraction distance being precisely predetermined by the amount of lost motion in the connection between the reaction member and the piston. The invention will now be described further, by way of example, with reference to the accompanying drawing which is a fragmentary cross-section along the axis of the cylinder in a brake calliper embodying the invention. A disc brake calliper 10 comprises a housing 12 formed with a cylinder 14. A piston 16 is arranged to urge brake pads 18 and 20 into pressure contact with opposite faces of a brake disc 22 when hydraulic fluid 24 is forced through inlet 26 into the cylinder 14 by a conventional brake pedal and master cylinder arrangement (not shown). The piston 16 comprises and inner element 28 and an outer element 30 joined by a spigot element 32. An annular reaction member 34 is positioned in the annular space between the inner and outer piston elements 28 and 30 and has an axial length which is less than the spacing between the inner and outer elements by a distance X equal to the required predetermined retraction distance of the piston. The connection between the piston 16 and the reaction member 34 is therefore a lost motion connection allowing a predetermined distance X of movement of the piston relative to the reaction member 34. An "0" ring friction element 36 is received in a groove in the outer surface of the reaction member and frictionally engages the inner surface of the cylinder 14. A coil shaped retraction spring 38 is located in a recess 40 in the reaction member 34 open towards the inner piston element 28 so that the retraction spring urges the reaction member 34 and the inner piston element 28 apart. The force exerted by the spring 38 on the reaction member 34 is insufficient to overcome the friction between the "0" ring 36 and the cylinder 14 but is sufficient to overcome the frictional forces affecting the piston 16 and the hydraulic actuating system. In operation, after the brakes are released the retraction spring 38 moves the piston 16 back from the disc 22 until the outer piston element 30 contacts the reaction member 34. The distance by which the piston 16 is moved is equal to X, the free movement of the reaction member relative to the piston 16. As the brake pads wear the reaction member 34 is moved along the cylinder 14 by contact with the inner piston element 28 upon successive applications of the brakes. Alternatively, the reaction member and retraction spring may be formed by a plate spring device in friction contact with the cylinder 14.

Claims: CLAIMS 1. A disc brake calliper including a housing (12) formed with a cylinder (14), a piston (16) for moving a brake pad (18) into pressure contact with a brake disc (22) and a device for positively retracting the brake pad (18) from the disc (22) to reduce brake drag when the brake is released, characterised in that the device for retracting the brake pad comprises a reaction member (34), means on the reaction member (34) for frictionally engaging the cylinder (12), is movable with the piston (16), and is connected to the piston by a lost motion connection, and a retraction spring (38) acting between the reaction member (34) and the piston (16) to retract the piston (16) upon release of the brake, the retraction distance being precisely predetermined by the amount of lost motion in the connection between the reaction member (34) and the piston (16). 2. A disc brake calliper as claimed in Claim 1, in which the reaction member (34) comprises an annular element (34) retained within a recess in the piston (16) and having limited axial movement relative to the piston (16), the limited axial movement being equal to the required predetermined retraction distance of the piston (16). 3. A disc brake calliper as claimed in Claim 2, in which the annular element (34) is formed with a recess for receiving the retraction spring. 4. A disc brake calliper as claimed in any preceding claim, wherein the means for frictionally engaging the cylinder (12) is an "0" ring (36) mounted in the outer surface of the reaction member (34).

Inventors: STACE, RICHARD JOHN LESLIE
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1984
Main classification: F16D65
IPC codes: F16D65