GB2276685A - Master cylinder - Google Patents

Abstract

A master cylinder has a body 11 with a bore 12 with a mouth 34 at one end through which a portion of a piston 15 extends to receive a load input rod. The piston portion passes through an annular seal 32 mounted at the mouth of the master cylinder bore 12, the seal having an inner portion 53 that seals against the piston, and is mounted on an end cap 33 located within the mouth of the bore. The seal also has at least one locating portion 55 which is accommodated by a like cavity 66 in the end cap 33 so that the end cap can only be located within the mouth of the bore when the seal is in the correct orientation with respect to the end cap and cannot be assembled back to front in the end cap. <IMAGE>

Description

A MASTER CYLINDER The present invention relates the master cylinders for hydraulic control system in particular as used on motor vehicles.

Master cylinders are used in both brake and clutch systems in vehicles in which hydraulic fluid is used for operating the brakes or clutch of the vehicle. Known master cylinders usually comprise a housing having a bore therein which extends longitudinally and which is closed at one end. One or more pistons are slidably positioned in the bore, and an actuating rod engages the piston closest to the open end for actuation of the brakes or clutch. A spring associated with each piston biases the piston towards the open end of the bore.

The actuating rod is connected to a foot pedal or other means operable by the vehicle operator. The rear end of the piston closest to the open end may extend longitudinally through the open end of the bore. A rigid end cap closes the open end of the master cylinder and acts as a form of bearing for the rear end of the piston where it extends through the open end. An elastomeric seal between the end cap and the rear end of the piston, and between the end cap and a surface of the bore form a substantially fluid tight seal at the open end of the bore to substantially prevent egress of hydraulic fluid and ingress of dirt and moisture.

When assembling the end cap and seal into the open end or mouth of the bore it is possible to place seal so that it faces in the wrong direction.

The present invention seeks to overcome the above problem.

Accordingly there is provided a master cylinder having a body with a bore with a mouth, at one end through which a portion of a piston extends to receive a load input rod, the piston portion passes through an annular seal mounted at the mouth of the master cylinder bore, the seal has an inner portion that seals against the piston, and is mounted on an end cap located within the mouth of the bore, characterised in that the seal has at least one locating portion extending axially outwardly and which is accommodated by a like cavity in the end cap such that the end cap can only be located within the mouth of the bore when the seal is in the correct orientation with respect to the end cap.

Preferably the axially inward sealing surface is formed on an outer annular seal lip which extends axially inwardly, and preferably abuts against an internal shoulder formed on the master cylinder bore adjacent the mouth thereof. Conveniently the locating portion is a raised annular rib.

Preferably the master cylinder further includes a second piston both pistons being reciprocal within the bore, and being biased apart by a spring having one end abutting one of said pistons and the other end abutting a spring end cap connected to said one piston by a coaxial tie bar, wherein the end cap is limited in its movement away from said one piston by a resilient clip which engages the tie bar.

The invention will be described by way of example and with reference to the accompanying drawings in which: Fig 1 is a longitudinal section through a master cylinder according to the invention.

Fig 2 is a detailed section showing an alternative form of end cap for clipping externally of the master cylinder body.

Fig 3 is a detailed section of the first piston assembly as shown in Fig 1.

With reference to Fig 1, there is illustrated a tandem master cylinder 10 for a vehicle hydraulic brake system. The master cylinder 10 comprises a housing 11 having a master cylinder bore 12 with two pistons 14 and 15 sealingly slidable therein.

The master cylinder bore 12 is supplied with hydraulic fluid from a reservoir 16 via passageways 17 and 18 which feed into the pressure chambers 19 and 21 respectively disposed in front of the primary seals 22,23 of the pistons 14 and 15 respectively.

The piston 14 is in the form of a spool and is located in the closed end of the master cylinder bore 12. (The piston 14 will hereinafter be called the second piston) and is biased into an inoperative position by a spring 24 housed in the closed end of the bore 12 which constitutes the pressure chamber 19. The inlet passageway 17 opens into the chamber 19 immediately in front of the primary piston seal 22 when the piston 14 is in the inoperative position. A supply chamber 25 is formed around the mid section 26 of the piston and is connected to the reservoir 16 by a passageway 27 which is substantially (but not necessarily parallel) to the passageway 17.

The supply chamber 25 is sealed from the pressure chamber 21 of the other piston 15 (hereinafter called the first piston) by a reverse seal 28.

The piston 15 is again in the form of a spool with the primary seal 23 located on the front of the piston and a supply chamber 31 located around the mid section of the piston.

The supply chamber 31 of the first piston 15 is fed by a passageway 30 substantially parallel (but not necessarily parallel) to the passageway 18 and is sealed from the atmosphere by a gland seal 32 held in an end cap 33 secured in position in the mouth 34 of the bore 12.

When the first piston 15 is in its inoperative position its primary seal 23 is located immediately behind the fluid inlet 18.

The first piston 15 is biased towards the end cap 33 by return spring 35 which at one end abuts a spring seat 36 located on an axially inner surface of the piston 15 and at its other end abuts a spring end cap 37. As is best seen in Fig 3 the spring end cap 37 is connected to the first piston 15 by a coaxial tie bar 38.

The. tie bar 38 is formed integrally with the piston 15 and extends towards the closed end of the bore.

The spring end cap 37 has a inverted top-hat section with a radially outwardly projecting flange 39 at one end forming the spring seat, and end wall 41 at the other end having a coaxial hole 42 therein to receive the end portion of the tie bar.

The axially inner end portion of the tie bar is formed with a raised annular shoulder 43, which can pass through said hole 42 in the spring cap, with an adjacent groove 44. The function of the groove 44 is to remove any residual radius between the shoulder 43 and the outer surface of the tie bar, so that a resilient retaining clip 45 can abut the shoulder 43 for an accurate determination of the length of the piston assembly comprising cap 37, spring 35 and piston 15. The resilient clip 45, preferably a circlip, retains the spring end cap 37 on the tie bar against the bias of the compression spring 35.

The primary piston assembly of known desired length is set so that the two seals 22,23 are located immediately in front of the fluid inlets 17 and 18.

The return spring 35 typically has a higher fitted spring load than the return spring 24 for the second piston 14 so that the second piston return spring effectively returns both pistons to an inoperative position as a single assembly.

The portion of the first piston 15 extending through the sealed end cap 33 has recess 71 therein to receive a push rod from a brake booster, or brake pedal to operate the master cylinder.

In the invention position as shown as Fig 1, both pistons 14 and 15 are biased against the push rod (not shown) to the respective chambers 19 and 21.

When the operating load is applied to the first piston 15 both pistons move into the bore 12, closing the inlets 18 and 19, allowing a pressure build up in the pressure chambers for operation of the brakes.

The gland seal 32 is in the form of an annular seal of 'U' shaped cross section having inner and outer annular lips 51 and 53 respectively that project axially inwardly. The outer lip 51 in use abuts against an internal shoulder 52 in the bore 12 adjacent the mouth thereof 34. The outer seal lip 51 has a raised lip 56 on its outer periphery that seals against the bore (best seen in Fig 2) The inner lip 53 has a radially inwardly annular rib 54 thereon. (Best seen in Fig 2) which in use wipes against the outer surface of the first piston 15. The seal 32 has an annular rib 55 projecting axially outwardly from the base 50 thereof and is offset relative to the outer annular lip 51. The rib 55 serves as a location portion which locates in a like annular cavity 66 formed in the end cap 33.

The end cap 33 is an annular disc having an axially inner surface in which the cavity 66 is formed, and an axially extending radially inner portion 57 which guides the piston 15. The radially outer portion 58 of the end cap 33 has axially outwardly extending resilient fingers 59 each with a detent thereon. In use the detents engage in a groove 61 adjacent the mouth of the bore to hold the end cap 33 in position. The axially outer end of the inner portion 57 has a wiper seal 67 thereon for cleaning the outer surface of the piston 15.

If the seal 32 is inadvertently put into the master cylinder bore 12 the wrong way round, since the groove 66 in the end cap cannot accommodate the seal lip 52, then it will not be possible to easily snap the end cap 33 in position. Hence the assembly operator will correct the mistake.

An alternative form of end cap 133 is shown in Fig 2. The end cap 133, is substantially similar to that shown in Fig 1 except that the outer periphery of the end cap is formed with a wall portion 158 that extends over the outer surface 112 of the master cylinder 10 adjacent the mouth 34 and is resilient so that it can snap fit into a groove 161 in the outer surface.

Claims (8)

1. A master cylinder having a body with a bore with a mouth at one end through which a portion of a piston extends to receive a load input rod, the piston portion passes through an annular seal mounted at the mouth of the master cylinder bore, the seal has an inner portion that seals against the rod, and is mounted on an end cap located within the mouth of the bore, characterised in that the seal has at least one locating portion extending axially outwardly and which is accommodated by a like cavity in the end cap such that the end cap can only be located within the mouth of the bore when the seal is in the correct orientation with respect to the end cap.

2. A master cylinder as claimed in Claim 1 wherein the seal has an axially inner sealing surface formed on an outer annular seal lip which extends axially inwardly.

3. A master cylinder as claimed in Claim 1 or Claim 2 wherein the master cylinder bore has an internal shoulder adjacent the mouth thereof, and said axially inward sealing surface abuts against said shoulder.

4. A master cylinder as claimed in any one of claims 1 to 3, wherein the locating portion contains a raised annular rib.

5. A master cylinder as claimed in any one of claims 1 to 4 wherein the end cap has a clip means which is engagable in a groove in the master cylinder body.

6. A master cylinder as claimed in Claim 5 wherein the end cap has flexible resilient fingers which extend axially outwardly and have a detent surface thereon which engages in an internal groove in the bore adjacent the mouth thereof.

7. A master cylinder as claimed in any one of claims 1 to 6 and further includes a second piston, both pistons being reciprocal within the bore, and being biased apart by a spring having one end abutting one of said pistons and the other end abutting a spring end cap connected to said one piston by a coaxial tie bar, wherein the end cap is limited in its movement away from said one piston by a resilient clip which engages the tie bar.

8. A master cylinder as claimed in Claim 8 wherein the clip is a circlip which engages the outer surface of an end portion of the tie bar, and can abut a raised shoulder adjacent the end of the tie bar.