WO1994021501A1

WO1994021501A1 - Sealed bearing for fluid-operated brake actuator

Info

Publication number: WO1994021501A1
Application number: PCT/US1994/002092
Authority: WO
Inventors: William J. Hicks; William C. Pierce
Original assignee: Nai Anchorlok, Inc.
Priority date: 1993-03-25
Filing date: 1994-02-28
Publication date: 1994-09-29
Also published as: AU5349194A; WO1994021892A1; AU6516794A

Abstract

An air operated combination diaphragm spring brake (10) has a brake actuator (14) in tandem with a spring brake actuator (16), wherein the brake actuator (14) and spring brake actuator (16) are separated by a dividing wall (35) in which is formed an annular opening (37, 37', 37') having a sealed bearing (38) through which a spring brake actuator rod (56, 56', 56') reciprocates. The sealed bearing (38, 38', 38') further comprises a polymer ring (84, 86, 108, 110, 118) and an elastomeric sealing member (96, 114, 124). The ring (84, 86, 108, 110, 118) has an annular groove (92, 112, 120) which receives the elastomeric sealing member (96, 114, 124). The relationship between the diameter of the aperture (39, 39', 39') in the sealed bearing (38, 38', 38') and the diameter of the annular opening (37, 37', 37') in the dividing wall (35) is such that it prohibits the spring brake actuator rod (56, 56', 56') from contacting any portion of the dividing wall (35) during reciprocation.

Description

SEALED BEARING FOR FLUID-OPERATED ,RAKE ACTUATOR

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to fluid-operated brake actuators for vehicles and more particularly to service and spring brake actuators combined in tandem and having a spring brake actuator rod. State of the Prior Art

An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of a fluid such as compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency or spring brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake. Typically, the spring brake actuator is disposed in tandem with the service brake actuator.

When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm and a pressure plate compresses the spring. In many applications, a spring brake actuator rod is held in a retracted position by a relatively small return spring. In newer applications, the spring brake actuator rod is integral with the pressure plate and held in a retracted position by the air pressure. In both designs, the spring brake actuator rod thus does not affect the normal operation of the brake. Depressing the brake pedal during normal driving operation introduces compressed air into the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and the brakes to be applied with an application force proportional to the air pressure in the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the strong compression spring acting on the spring brake actuator rod which, in turn, acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake. In tandem actuator assemblies, the spring brake push rod typically extends from a chamber in the spring brake portion, through an aperture in a wall separating the spring brake actuator from the service brake actuator, and into a chamber in the service brake portion. Because at least one of the adjoining chambers is usually pressurized, a seal is provided at the aperture around the push rod comprising one or more O-rings positioned in annular channels in the wall around the aperture.

The wall is typically a cast aluminum part in which an aperture is machined. On the other hand, the spring brake actuator rod is typically formed of chromed steel. It sometimes occurs that, as the spring brake actuator rod reciprocates through the aperture, the dissimilar metals of the wall and the push rod will abrade, thereby creating discontinuities in the actuator rod surface as well as loose metal chips. These chips and discontinuities cut the O-rings and otherwise accelerate wear of the O-rings which significantly diminishes their durability. It sometimes occurs that the spring brake actuator rod becomes cocked or skewed as it reciprocates so that the longitudinal axis of the spring brake actuator rod is not coincident with the axis of the wall aperture. This skewing is thought to be caused by lateral forces induced by non-linear expansion of the large force spring when the emergency brake is applied. The abrading and chipping of the O-rings is exacerbated by the skewing of the actuator rod. Also, the machinery of the aperture is a very labor intensive and costly step in the manufacture of spring brake actuators.

Other brake designs provide a bearing in the wall aperture through which the spring brake actuator rod reciprocates. For example, the

Black Max^β spring brake actuator manufactured by Overland Brakes, Inc. has a bronze oilite bearing that is press-fit into a machined aperture in the wall. The actuator rod reciprocates through an aperture in the bearing. The seal is provided by an O-ring disposed between one end of the bronze bearing and an annular shoulder formed in the wall. Similarly, a fluid-operated spring brake actuator manufactured by Knorr-Dahl has a plastic bearing mounted within an aperture formed in the wall and has an O-ring disposed at its upper end. The O-ring is trapped between the edge of the bearing and an annular shoulder on the wall extending inwardly of the wall aperture. The other end of the bearing is retained by a deformable flange. Neither of these designs satisfactorily resolve the problem introduced by the skewing of the spring brake actuator rod. The annular shoulder or annular flange typically extends inwardly enough that when the actuator rod becomes skewed, even slightly, it still may abrade against the flange or shoulder and ultimately lead to premature wear on the O-rings.

U.S. Patent No. 3,926,094 discloses a structure which addresses the skewing problems. The bearing and seal assembly comprises a ring with a spherical outer surface received in an opening in the wall having an annular trough complementary in shape to the outer surface of the ring. The spring brake actuator rod reciprocates through the ring with an O-ring received in an annular groove on the ring. However, the design necessarily requires expensive machinery and rather complicated assembly, all of which renders the design very costly to make.

A need exists for a spring brake actuator which can be manufactured relatively inexpensively, for example by casting the wall and aperture without machinery, yet provide for extended durability of the O-ring seals between the pressure chambers.

SUMMARY OF THE INVENTION The invention is a fluid operated brake actuator comprising two internal tandem chambers that are separated by a wall. The wall has a central opemng connecting the two chambers. The opening has an imaginary axis. A first and second flange are mounted to the wall and extend radially inwardly around the opening. The first and second flanges are axially spaced with respect to each other. The first flange at least partially defines a first diameter and the second flange at least partially defines a second diameter.

An actuating rod having an outer surface and a longitudinal axis extends through the opening and is adapted to reciprocate through the opening. The reciprocation of the actuating rod alternately actuates and releases a brake. Ideally, during reciprocation of the actuating rod, the opening axis and the longitudinal axis are coincidental. Sealing means are disposed in the brake to seal the chambers from each other.

The sealing means comprises a body securely fixed within the opening and having a self-lubricating inner surface. The body at least partially defines an annular channel within the opening. The channel defines a third diameter that is less than the first and second diameters. The first, second and third diameters are selectively sized so that when the longitudinal axis is coincident with the opening axis during the reciprocation the actuating rod. The actuating rod will avoid contact with the first and second flanges so that the outer surface of the actuating rod is maintained free of defects otherwise introduced by contact with one of the first and second flanges.

Preferably, the body is a ring. In one aspect of the invention, the body has a first groove in which an elastomeric sealing member is disposed to form a seal between the chambers. Optionally, the ring has an outer surface in which there is an annular groove and a second elastomeric sealing member is disposed within the channel in the outer surface of the ring and bears against the wall to form a second seal between the two chambers.

In another aspect of the invention, the wall comprises one or more tabs to secure the body within the opening. The wall can also have an annular shoulder extending into the opening with the ring securely clamped between the shoulder and the tabs. Preferably, the tabs are deformable.

In another aspect of the invention, the body comprises first and second rings. Preferably, the first and second rings are shaped to form the inner channel when they abut each other axially. The first ring has an elongated depending flange and the second ring has an axial channel into which the depending channel fits. An outer channel can be similarly formed when the first and second rings are coupled. A second elastomeric ring is disposed within the outer channel to form a second fluid seal between the two chambers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an air-operated brake actuator having a sealed bearing according to the invention;

FIG. 2 is an enlarged fragmentary cross sectional view taken from area A in FIG. 1 showing the sealed bearing of FIG. 1;

FIG. 3 is an enlarged fragmentary cross sectional view similar to FIG. 2 showing a second embodiment of a sealed bearing according to the invention; and

FIG. 4 is an enlarged fragmentary cross sectional view similar to FIG. 2 showing a third embodiment of a sealed bearing according to the invention. DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a fluid-operated brake actuator 10 having a general configuration well known in the art. The fluid-operated brake actuator 10 comprises a brake actuator 14 mounted in tandem to a spring chamber or emergency brake actuator 16. A service brake push rod 12 extends from the brake actuator 14 for reciprocating movement between a retracted position and an extended actuating position relative to the brake actuator 14, and is provided with a clevis 17 which is adapted to connect to a conventional brake shoe and drum (not shown in the drawing) in a standard fashion. Reciprocating motion of the service brake push rod 12 will cause the brake to be alternately applied and released.

The brake actuator 14 comprises a cup-shaped service housing section 18 and a double cup-shaped adapter housing 20 joined together by a clamp 22 to form a service brake chamber 23. The adapter housing 20 is also sometimes known as a flange case.

A first elastomeric diaphragm 24 (also known as the service brake diaphragm) is suspended within the service brake chamber 23, the peripheral edge thereof secured in fluid tight engagement between the cup- shaped service housing section 18 and the service side of the adapter housing 20 by the clamp 22. The first elastomeric diaphragm 24 thus separates the service brake chamber 23 into two portions: a first service chamber portion 26 and a second service chamber portion 28. The first service chamber portion 26 communicates with a source of pressurized air (not shown) through an air service port 42 in the adapter housing 20. The second service chamber portion 28 is vented to the atmosphere through at least one opening 32 in the cup-shaped service housing section 18. In FIG. 1, the first service chamber portion 26 is shown evacuated so that the first elastomeric diaphragm 24 is forced against the adapter housing 20 because of the force from compression spring 46. The service brake push rod 12 extends through a central opening

30 in the cup-shaped service housing section 18 and has a pressure plate 44 at the end thereof within the second service chamber portion 28. The pressure plate 44 bears against the first elastomeric diaphragm 24. A compression spring 46 extends between the pressure plate 44 and the interior surface of the cup-shaped service housing section 18. A push rod guide 34 having an annular seat 40 is disposed within the central opening 30 to guide reciprocal movement of the service brake push rod 12 within the central opening 30 and also to receive the end of the compression spring 46 and retain it in position around the central opening 30. The compression spring 46 thus urges the pressure plate 44 and the service brake push rod 12 to a fully retracted position as depicted in FIG. 1.

To operate the service brake, compressed air is introduced through the air service port 42 into the first service chamber portion 26 to force the first elastomeric diaphragm 24 and pressure plate 44 against the force of the compression spring 46, thereby extending the service brake push rod 12 toward the actuating position. The openings 32 permit rapid evacuation of air from the second service chamber portion 28 as the service brake is actuated. Mounting studs 47 are provided to mount the fluid- operated brake actuator 10 onto a vehicle. The spring chamber or emergency brake actuator 16 is defined by the spring side of the adapter housing 20 and a generally cylindrical head 48 or spring housing, which is clamped to the spring side of the adapter housing 20 by a clamp 50 to form the spring brake chamber 51. A second elastomeric diaphragm 52, known as the spring diaphragm, is suspended within the spring brake chamber 51, the peripheral edge thereof secured in fluid tight engagement between the cylindrical head 48 and the spring side of the adapter housing 20 by the clamp 50. The second elastomeric diaphragm 52 thus separates the spring brake chamber 51 into two portions: a first spring chamber portion 62 and a second spring chamber portion 63. The second spring chamber portion 63 is filled with pressurized air supplied through an air service port 54 in the adapter housing 20 when the emergency brake is in its normal released position as depicted in FIG. 1.

The adapter housing 20 includes a divider wall 35 which separates the adjoining service brake chamber 23 and spring brake chamber 51. A spring brake actuator rod 56, aligned with the service brake push rod

12, has one end extending from the spring brake chamber 51 through a central opening 37 in divider wall 35 for reciprocating motion through the central opening 37 between a retracted position and an actuating position. A sealed bearing 38 is provided in the central opening 37 through which the spring brake actuator rod 56 reciprocates. The sealed bearing 38 will be described in greater detail below.

The spring brake actuator rod 56 is cylindrical and has a longitudinal axis. The central opening 37 has an axis defined by its center line. The sealed bearing 38 also defines an aperture 39 that is concentric with the central aperture 37. Thus, the sealed bearing 38 and the central aperture share the same axis. Ideally, the longitudinal axis of the actuator rod coincides with the central opening axis during reciprocation. However, the tolerances between the central opening 37, sealed bearing 38 and spring brake actuator rod are such that the axes can become skewed with respect to each other during reciprocation of the spring brake actuator rod 56.

The one end of the spring brake actuator rod 56 terminates in a reaction plate 66 in the first service chamber portion 26. The reaction plate_# 66 is received in an annular seat 41 when the spring brake actuator rod 56 is in the retracted position as depicted in FIG. 1. The other end of the spring brake actuator rod 56 terminates in a push plate 58. The push plate 58 bears against the second elastomeric diaphragm 52 and a return spring 61 is disposed in the second spring chamber portion 63 between the push plate 58 and the divider wall 35 to bias the push plate 58 against the second elastomeric diaphragm 52. A pressure plate 59 bears against the other side of the second elastomeric diaphragm 52 and a large force compression spring or power spring 60 is disposed in the first spring chamber portion 62 between the pressure plate 59 and the cylindrical head 48.

During normal operation of the fluid-operated brake actuator 10, the spring brake actuator rod 56 will be in the fully retracted position, as depicted in FIG. 1, by means of compressed air which is maintained in the second spring chamber portion 63. When the compressed air is exhausted, the compression spring 60, one end of which engages the outer end wall of the cylindrical head 48, forces the push plate 58 and spring brake actuator rod 56 in the direction of the service brake push rod 12. The force of the compression spring 60 causes the spring brake actuator rod 56 to be extended through the central opening 37, thereby causing the reaction plate 66 to apply a force to the first elastomeric diaphragm 24 and pressure plate 44 of the brake actuator 14. This action causes the service brake push rod 12 to be extended toward the actuating position, thereby applying the brake. When the brake is to be released, compressed air is once again introduced into the second spring chamber portion 63 to a pressure that exerts a force greater than the force of the compression spring 60. The force of the compressed air against the second elastomeric diaphragm 52 causes the pressure plate 59, to be returned to the position depicted in FIG. 1. The force of the return spring

61 causes the spring brake actuator rod 56 to also be retracted.

The pressure plate 59 is adapted to receive a threaded caging bolt 70, which passes through an opening 72 in the cylindrical head 48. A hex head nut 78 is threadably received on the threaded caging bolt 70. To manually release the spring brake or otherwise cage the compression spring 60 in the compressed position shown in FIG. 1, the hex head nut 78 is rotated to bear against the cylindrical head 48, thereby drawing the threaded caging bolt 70 outwardly and the pressure plate 59 toward the cylindrical head 48. When the threaded caging bolt 70 is not installed, a plug 74 is provided to plug the opening 72.

Looking now more closely at FIG. 2, the sealed bearing 38 is mounted within the central opening 37 of the adapter housing 20 and is in sealing engagement between the adapter housing 20 and the spring brake actuator rod 56. If the sealed bearing 38 does not seal the second spring chamber portion 63 from the first service brake chamber 26, the pressurized fluid in the second spring chamber portion 63 will leak into the first service brake chamber 26, resulting in the accidental exhausting of the second spring chamber portion 63 and the application of the spring brake.

The sealed bearing 38 comprises a non-metallic body 80 made from a synthetic polymer such as nylon. The non-metallic body 80 thus has a softer, self-lubricating inner surface 82 against which the spring brake actuator rod 56 bears as it reciprocates. This structure significantly reduces the potential for abrasions characteristic of the prior art metal-to-metal contact between the actuator rod and the adapter housing. Also, the relative size of the body 80 extends a sufficient distance beyond the smallest diameter portion of the central opening 37 to ensure the actuator rod does not contact the wall of the central opening 37 even after the body 80 is worn or when the spring brake actuator rod skews during reciprocation. Further, the bearing is sized to prohibit the skewing of the spring brake actuator rod by lateral forces during reciprocation. In the embodiment of FIG. 2, non-metallic body 80 comprises a first ring 84, T-shaped in cross-section, and a second ring 86, U- shaped in cross-section. An elongated flange 88 forming the cross-sectional T" of the first ring 84 is received in a shallower axial channel 90 of the 5 second ring 86, thereby forming an inner annular channel 92 and outer annular channel 94. An inner O-ring 96 is disposed within the inner annular channel 92 and an outer O-ring 98 is disposed within the outer annular channel 94. It will be understood that ring seals having different cross-sections are functional equivalents of conventional O-rings, such as ring seals having

10 elliptical or star shaped cross-sections.

The sealed bearing 38 is securely fixed within the central opemng 37 by any of several different means. For example, it can be press fit, snap fit, adhesively bonded, or staked. In FIG. 2, the adapter housing is formed with an annular shoulder 100 on the service side to define a first

15 diameter of the central opening 37, and a plurality of annular tabs 102 on the spring side of the opening to define a second diameter of the central opening. Preferably, the first and second diameters are less than the diameter of the central opening located between the shoulder 100 and the tabs 102 and greater than the diameter of the spring break actuator rod. Alternatively, the

20 tabs 102 can be replaced with a continuous ring or flange that can be rolled inwardly. Preferably, the annular shoulder 100 and the annular tabs 102 can be cast with the adapter housing, thereby avoiding unnecessary and costly machining operations. The sealed bearing 38 is received in the central opemng 37, one end thereof abutting the annular shoulder 100. The annular

25 tabs 102 are cast so that they extend axially and are then bent inwardly toward the central opening 37 and against the other end of the sealed bearing 38 to securely fix the sealed bearing 38 within the central opening 37. It will be apparent that the inner O-ring 96 establishes a fluid tight seal between the t spring brake actuator rod 56 and the non-metallic body 80, even as the spring

30 brake actuator rod 56 reciprocates. Similarly, the outer O-ring 98 establishes a fluid tight seal between the non-metallic body 80 and the adapter housing 20.

The difference between the diameter of the bearing aperture 39 and the first and second diameters is sufficient to prevent the spring brake

35 actuator rod 56 from contacting any portion of the dividing wall 35 during reciprocation, even if the spring brake actuator rod skews during reciprocation. Preferably, the difference between the diameters is approximately 0.120 inches. Prior art devices only disclosed a difference of approximately 0.020, which when combined with the tolerances of the prior art actuator rods and their dividing wall construction did not prohibit the actuator rod from contacting a portion of the dividing wall. An additional benefit of the large difference in the diameters is that it tends to increase the cross sectional width of the sealed bearing 38, which prolongs the life of the sealed bearing and the brake. FIG. 3 illustrates a second embodiment of a sealed bearing 38' according to the invention. In this and subsequent embodiments, like numerals are used to identify components similar in structure and function to those of the first embodiment. Here, the adapter housing 20' is formed with an annular rib 104 protruding radially into the central opening 37'. Annular tabs 102' are formed on both the service and spring sides of the central opening 37' to define a first diameter on the service side of the central opening 37' and a second diameter on the spring side of the central opening 37'.

The sealed bearing 38' comprises first and second polymer rings 108, 110 that define a bearing aperture 39'. The first ring 108 abuts the spring brake side of the annular rib 104 and can be securely fixed in the central opening 37' by the annular tabs 102' in the manner described above. Similarly, the second ring 110 abuts the service brake side of the annular rib 104 and is secured by the annular tabs 102'. The first and second rings 108, 110 are dimensioned radially to be greater than the depth of annular rib 104. Thus, the first and second rings 108, 110 and the annular rib 104 define a channel 112 which receives an O-ring 114. The O-ring 114 simultaneously establishes a fluid tight seal between the spring brake actuator rod 56' and the adapter housing 20', while the rings 108, 110 provide a self-lubricated bearing surface for the spring brake actuator rod 56'. Like the first embodiment, the rings 108 and 110 of the second embodiment define a diameter of the aperture 39' whose difference between the first and second diameters is sufficient to prevent the spring brake actuator rod from contacting any portion of the inner surface of the central opening 37' or the divider wall 35' during the ideal or skewed reciprocation of the spring brake actuator rod 56'. Furthermore, the positioning of the O-ring 114 between the polymer rings 108 and 110 results in the axial separation of the polymer rings to be greater than in a construction where the O-ring is not disposed between the polymers 108 and 110, thereby, effectively increasing the axial length of the polymer rings 108 and 110. The increased axial length further reduces the skewing of the spring brake actuator rod by supporting the spring brake actuator rod along a greater portion of the length of the spring brake actuator rod without increasing size of the bearing, the O-rings or the thickness of the central opening.

FIG. 4 illustrates a third embodiment of a sealed bearing according to the invention. The adapter housing 20" has annular tabs 102" formed on both sides of the central opening 37" as described above. The sealed bearing 38" comprises a single, molded, polymer annular ring 118 having a radial inner channel 120 and radial outer channel 122. The sealed bearing 38" defines a sealed bearing aperture 39". An inner O-ring 124 is received within the radial inner channel 120 and an outer O-ring 126 is received within the radial outer channel 122. The sealed bearing is secured within the central opening 37" by the annular tabs 102" on the service side and spring side of the dividing wall 35". The tab 102" on the service side defines a first diameter and the tab 102" on the spring side defines a second diameter. Thus, the inner O-ring 124 will be in sealing engagement with the spring brake actuator rod 56 and the outer O-ring 126 will be in sealing engagement with the adapter housing 20". The annular ring 118 can also be used without the radial outer channel 122 and outer O-ring 126.

Like the first and second embodiments, the difference in the diameter of the sealed bearing aperture 39" and the first and second diameters is sufficient to prohibit the spring brake actuator rod 56" from contacting any portion of the dividing wall 35" during ideal or skewed reciprocation.

The sealed bearing according to the invention is structurally much less complex than the prior art sealed bearings. Furthermore, the manufacturing of the sealed bearing and its retaining structure is greatly simplified over previous sealed bearings. Specifically, the central opening and the retaining tabs or flanges can be cast as a portion of the dividing wall, eliminating the need for expensive machining or milling techniques. Also, the dividing wall can be cast without a central opening, which can be subsequently drilled.

The assembly of the sealed bearing is also simplified in that the sealed bearing need only be placed within the central opening and one or more annular ribs or tabs be bent over to secure the sealed bearing with respect to the central wall. The assembly is accomplished without any special support or mounting pieces for the bearing.

In addition to the simplified construction and assembly of the sealed bearing according to the invention, the sealed bearing further prohibits the spring brake actuator rod from contacting the central wall during the ideal or skewed reciprocation of the spring brake actuator rod. The particular shape of the sealed bearing also reduces the skewing of the spring brake actuator rod induced by lateral forces. The extra cross sectional width of the sealed bearing advantageously provides a greater amount of material that the actuator rod can wear away to further prolong the life of the O-rings and the sealed bearing.

Thus, the sealed bearing according to the invention greatly increases the life of the sealed bearings, O-rings and spring brake actuator rod by reducing the abrading and chipping of the spring brake actuator rod to prolong the life of the seal, thereby increasing the life of the spring brake.

The increased life and reliability of the spring brake is accomplished in a manner that reduces the complexity and expense of manufacturing and assembling the spring brake.

Reasonable variation and modification are possible within the scope of the foregoing disclosure without departing from the spirit of the invention which is defined in the accompanying claims.

Claims

CLAIMS The embodiments for which an exclusive property or privilege is claimed are defined as follows:

1. In a fluid operated brake actuator comprising two interna tandem chambers separated by a wall, the wall having an opening therein between the two chambers along an imaginary opening axis, said wall having a first and second flange extending radially inwardly around the opening and spaced axially from each other, said first flange at least partially defining a first diameter and said second flange at least partially defining a second diameter, an actuating rod having an outer surface and a longitudinal axis extending through the opening and adapted to reciprocate through the openin for alternately actuating and releasing a brake, ideally with the opening axis and the longitudinal axis coincident, and sealing means to seal the chambers from each other, the improvement wherein: the sealing means comprises a body securely fixed within the opening and having a self-lubricating inner surface, said body at least partially defining an annular channel within the opening, said channel defining a third diameter less than the first and second diameters, the first, second, and third diameters being selectively sized so that when the longitudinal axis is not coincident with the opening axis as the actuating rod reciprocates, the actuating rod will avoid contact with the first and second flanges, whereby to maintain the outer surface of the actuating rod free of defects otherwise introduced by contact with one of the first and second flanges.

2. In a fluid operated brake actuator according to claim 1 wherein the body comprises a ring.

3. In a fluid operated brake actuator according to claim 1 wherein the body has an annular groove and an elastomeric sealing member is disposed in the annular groove to form a seal between the two chambers.

4. In a fluid operated brake actuator according to claim 3 wherein the ring has an outer surface with a second annular groove in which is disposed a second elastomeric sealing member that bears against the wall to form a second seal between the two chambers.

5. In a fluid operated brake actuator according to claim 4 wherein the wall has an annular shoulder extending into the opening and at least one tab extending into the opening spaced axially from the shoulder, with the ring securely clamped within the opening between the shoulder and the at least one tab.

6. In a fluid operated brake actuator according to claim 5 wherein the at least one tab is deformable metal and clamps the ring within the opening by being bent beyond its elastic limit into the opening and against the ring.

7. In a fluid operated brake actuator according to claim 1 wherein the body is formed of a low friction polymer and the elastomeric sealing member comprises an O-ring.

8. In a fluid operated brake actuator according to claim 7 wherein the body is a polymer ring with an inner annular groove and an outer annular groove, said inner annular groove and outer annular groove each having an O-ring.

9. In a fluid operated brake actuator according to claim 1 wherein the body comprises a first ring and a second ring.

10. In a fluid operated brake actuator according to claim 9 wherein the first ring and second ring are shaped to form the annular groove when they abut each other axially.

11. In a fluid operated brake actuator according to claim 10 wherein the first ring has an elongated depending axial flange and the second ring has an axial channel into which the elongated depending flange fits.

12. In a fluid operated brake actuator according to claim 11 wherein the body has an outer groove formed between the first ring and the second ring and a second elastomeric ring is disposed within the outer groove to form a second seal between the two chambers.

13. In a fluid operated brake actuator according to claim 12 wherein the wall has an annular shoulder extending inwardly at one portion of the opemng and at least one tab extending into the opening and spaced axially from the shoulder, with the body securely clamped within the opening between the shoulder and the at least one tab.

14. In a fluid operated brake actuator according to claim 9 wherein the first and second rings are a low friction polymer.

15. In a fluid operated brake actuator according to claim 14 wherein the wall has an annular shoulder extending inwardly at one portion of the opening and at least one tab extending into the opening and spaced axially from the shoulder, with the body securely clamped within the opening between the shoulder and the at least one tab.

16. In a fluid operated brake actuator according to claim 15 wherein the at least one tab is deformable metal and clamps the body within the opening by being bent beyond its elastic limit into the opening and against the body.

17. In a fluid operated brake actuator according to claim 9 wherein the wall has an annular rib extending inwardly at one portion of the opemng and the first ring and second ring abut the rib on opposite sides, each ring having an annular width greater than the annular width of the rib, wherein the annular channel is defined by the first and second rings and the

18. In a fluid operated brake actuator according to claim 17 wherein the wall further comprises at least one tab spaced axially from one side of the rib, and the first ring is clamped within the opening and between the at least one tab and the rib, and at least one other tab spaced axially from the other side of the rib, the second ring being clamped within the opening and between the at least one other tab and the rib to secure the second ring within the opening.

19. In a fluid operated brake actuator according to claim 1 wherein the rod has a diameter and the rod diameter is less than the third diameter.

20. In a fluid operated brake actuator according to claim 1 wherein the rod has a diameter and the rod diameter is less than the first and second diameters.

21. A method of sealing adjoining spring and service chambers in a fluid operated brake actuator, wherein the chambers are separated by a wall having an opening therein between the two chambers and wherein an actuating rod is adapted to extend through the opening and to reciprocate therethrough for alternately actuating and releasing a brake, the method comprising the steps of: securing a body having a central aperture defined by a self-lubricating inner surface within the opening, said body having an annular channel in the inner surface, placing an elastomeric sealing member within the annular channel, and extending the actuating rod through the aperture wherein the actuating rod will bear against the elastomeric sealing member and the inner surface as it reciprocates through the opening.

22. The method of claim 21 wherein the body is secured within the central opening by deforming a portion of the wall over the body.