Spring brake actuator with guide and bearing for an actuator rod

A bearing (104) is mounted to the caging bolt head (86) within a hollow actuator rod (60) of a spring brake actuator (14). The bearing (104) is self-lubricated to guide movement of the actuator rod during application and release of the spring brake and to minimize friction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a spring brake actuator. More specifically, the 
invention relates to a spring brake actuator with a guide and bearing for 
an actuator rod. 
2. State of the Prior Art 
A brake system for a vehicle such as a bus, truck or the like typically 
includes a brake shoe and drum assembly which is controlled by a brake 
actuator. Conventional air brake actuators have both a service brake 
actuator and an emergency brake actuator. The service brake actuator 
applies the brakes under normal driving conditions by the application of 
compressed air. The emergency brake actuator includes a strong compression 
spring which applies the brake when air is released. This actuator is 
often referred to as the spring brake actuator. Typically, the spring 
brake actuator is disposed in tandem with a service brake actuator. 
A chamber in the spring brake is normally pressurized with air. One wall of 
the chamber is moveable and typically comprises either a diaphragm or a 
piston. It, in turn, acts against the compression spring so that pressure 
in the chamber compresses the spring. 
In the event of a loss of air pressure or an intentional exhaustion of air 
from the spring brake actuator chamber, the brake will be mechanically 
activated by the force of the strong compression spring acting on a spring 
brake actuator rod which, in turn, acts upon the service brake actuator or 
directly to apply the brakes. Thus, the spring brake actuator serves both 
as a parking brake and an emergency brake. In some applications the spring 
brake actuator is designed to operate solely as a parking brake, as for 
instance, when the service brake actuator is hydraulically operated. 
U.S. Pat. No. 5,105,727 to Bowyer, issued Apr. 21, 1992, discloses a known 
spring brake assembly which includes both a spring brake actuator and a 
service brake actuator. The service brake actuator includes an air chamber 
partially defined by a flexible diaphragm acting against a service brake 
push rod and a return spring to assure proper release of the brake when 
air is exhausted from the air chamber. The spring brake actuator also 
includes an air chamber comprising a spring brake housing and a spring 
brake actuator rod disposed integrally with a spring brake pressure plate 
and secured to the spring brake diaphragm. The spring brake diaphragm is 
provided with a centrally disposed aperture and engages the actuator rod 
and pressure plate to form an airtight seal. A strong power spring acts 
between an end of the spring brake housing and the pressure plate, and 
tends to urge the actuator rod out of the air chamber to engage the 
diaphragm and push rod of the service brake and to cause the brake to be 
applied. During normal operation, the spring force is counteracted by air 
pressure in the spring brake air chamber which acts against the diaphragm 
to compress the power spring, retracting the actuator rod. When system air 
pressure is lost, or manually released, the power spring extends to apply 
the brakes. 
The actuator rod is hollow and provided with a central bore. The proximal 
end (nearest the spring brake housing) of the actuator rod is partially 
closed by an inwardly directed annular flange on a pressure plate. A brake 
releasing caging bolt extends into the central bore from outside of the 
spring brake housing and has a head for engaging the annular flange. The 
bolt is threaded and extends out of the actuator rod, through an aperture 
in the spring brake housing, through a threaded nut, and terminates in a 
wrench nut. 
The compression spring does not always extend evenly in an axial direction 
so that the pressure plate and the actuator rod attached to it can wobble 
or become cocked. Consequently, the caging bolt head may abrade the 
interior wall of the actuator rod as it moves when the emergency brake is 
applied or released. 
SUMMARY OF THE INVENTION 
In accordance with the invention, the foregoing problem is overcome by 
providing a bearing on the caging bolt head, between the head and the 
interior wall of the actuator rod. The bearing serves to guide reciprocal 
movement of the actuator rod over the head and also minimizes friction 
between the two surfaces. 
Preferably, the bearing is formed of nylon and attaches to the head by 
snap-fit engagement. Also, the bearing surface has longitudinal grooves to 
permit the flow of air from one side of the head to the other within the 
actuator rod. 
In another aspect of the invention, the head is elongated, and the bearing 
comprises two discrete rings, one near each end of the head.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to the drawings and to FIG. 1 in particular, a cross-sectional 
view of an air-operated brake actuating unit 10 having a general 
configuration well known in the art is illustrated. The actuating unit 10 
comprises a service brake portion 12 mounted in tandem to a spring brake 
or emergency brake portion 14. A service brake push rod 18 extends from 
the service brake 12 for reciprocating motion and is provided with a 
clevis 20 which is adapted to connect to a conventional brake system (not 
shown) in a standard fashion. Reciprocating motion of the push rod 18 will 
cause the brake to be alternately applied and released. 
The service brake 12 comprises a pair of facing cup-shaped housing sections 
21 and 22, each having an outwardly directed ranged edge 25. The housing 
sections 21 and 22 are clamped together at their ranged edges by means of 
a clamp 27 to form a service brake inner chamber 30. Flanged edges 25 of 
the housing sections 21 and 22 compress and clamp a peripheral edge of an 
elastomeric diaphragm 32 suspended within the inner chamber 30. 
The service brake push rod 18 extends through a central opening 34 in 
housing section 21 and into the service brake chamber 30 where it 
terminates in a pressure plate 36. A compression spring 40 extends between 
pressure plate 36 and the interior surface of the housing section 21. A 
spring seat 48 disposed around the central opening 34 receives the end of 
the compression spring 40 and retains it in position around the opening 
34. The spring 40 thus urges the pressure plate 36 and the service brake 
push rod 18 to a fully retracted position. 
To operate the service brake, compressed air is introduced through an air 
service port 42 in housing section 22 to force the diaphragm 32 and 
pressure plate 36 against the force of spring 40 to actuate the push rod 
18. Openings 44 are provided in the housing section 21 to allow for the 
rapid evacuation of air from the inner chamber 30. Mounting studs 46 are 
provided to mount the brake actuating unit 10 onto a brake bracket. 
The spring brake 14 comprises a pair of facing cup-shaped housing sections 
51 and 52 joined at their edges to form an inner chamber 53. Housing 
section 51 is provided with an outwardly directed circumferential flanged 
edge 54. The housing sections 51 and 52 are clamped together by means of a 
peripheral curved edge 56 on housing section 52 engaging the edge 54 on 
housing section 51. An elastomeric diaphragm 58 is suspended within the 
spring brake chamber 53 and is compressed at its peripheral edge between 
the edges 54, 56 of housing sections 51 and 52. The portion of the chamber 
53 between the diaphragm 58 and housing section 51 forms a pressure 
chamber 55 which is filled with compressed air supplied through an air 
service port 57 in housing section 51 when the emergency brake is in its 
normal released position. 
An actuator rod 60, aligned with push rod 18 extends through a central 
opening 64 in an end wall of the housing section 51 and has a first end 90 
adjacent the housing section 52. A second end 92 of the actuator rod 60 
terminates in a reaction plate 62 disposed in an aligned central opening 
63 in an end wall of housing section 22. The opening 64 is provided with a 
bearing 66 having seals 68 to form an airtight seal for the actuator rod 
60. The actuator rod 60 extends through a centrally disposed aperture 81 
in the diaphragm 58 and carries a pressure plate 70. A powerful 
compression spring 72 extends between the housing section 52 and the 
pressure plate 70. A substantially flat portion 73 of the pressure plate 
70 engages one end of the spring 72 and a tubular portion 74 extends 
generally axially along the axis of spring 72. The tubular portion 74 is 
press-fit onto an end portion of actuator rod 60 such that the pressure 
plate 70 and the rod 60 form an integral unit. The pressure plate 70 may 
be formed of cast aluminum. 
During normal operation of the brake 10, compressed air in the pressure 
chamber 55 holds the actuator rod 60 in the fully withdrawn position as 
shown in FIG. 1. However, in the event that system pressure is lost, 
either accidentally or due to manual release, the compressed air will 
escape from the pressure chamber 55 allowing the spring 72 to extend the 
actuator rod 60. When the compressed air is exhausted, the compression 
spring 72 forces the pressure plate 70 and rod 60, integrally attached to 
the pressure plate, in the direction of the brake push rod 18 of the 
service brake 12. When the brake is to be released, compressed air is once 
again introduced in the space between housing section 51 and diaphragm 58. 
The force of the compressed air against the diaphragm 58 causes the 
pressure plate 70, the rod 60 and the spring 72 to be returned to the 
position depicted in FIG. 1. 
The actuator rod 60 is a hollow tube or rod provided with a central bore 61 
to accommodate a brake release or caging bolt 78. The bolt 78 is adapted 
to engage an end edge 80 of the tubular portion 74 of the pressure plate 
70 to maintain the spring 72 in a compressed position whenever such is 
desired. Thus, the bolt 78 may be used to manually release the spring 
brake or to ensure that the compression spring 72 will remain compressed 
when maintenance functions are performed on the brake assembly. The bolt 
78 threads into a fixed threaded opening or nut such as a nut 77 affixed 
by welding or staking to the end wall of housing section 52. A second nut 
or wrench head 79 is fixedly attached to a proximal end of the threaded 
bolt such that the bolt may be rotated in nut 77 by a common wrench or the 
like. 
The end edge 80 provides positive engagement with the walls of actuator rod 
60 when the rod is actuated and further serves as an engagement surface 
for a caging bolt head 86 at the distal end of the caging bolt 78. The 
caging bolt head 86 engages the edge 80 when the bolt 78 is withdrawn to 
retain the compression spring 72 in its compressed state. Otherwise, the 
bolt 78 extends into the central bore 61 of actuator rod 60 by a 
sufficient distance to allow the actuator rod 60 to extend to its full 
length of travel without engagement between the edge 80 and caging bolt 
head 86. 
Turning now to FIGS. 2 and 3, the caging bolt head 86 comprises a radially 
outwardly extending flange 100 at an inner end 102 of the caging bolt 78. 
A guide 104 encircles the flange 100 and has an outer bearing surface 114 
formed of a self-lubricating material. Preferably, the guide 104 is molded 
from a polymer such as nylon, and when disposed between the caging bolt 
head 86 and an interior wall 106 of the actuator rod 60, it serves to 
guide reciprocal movement of the rod over the head and also minimizes 
friction between the two elements. The guide 104 is generally cylindrical 
and has an inner surface 108 with a shallow annular groove 110 between 
opposed annular flanges 112, 113. The flange 100 of the head 86 is nested 
within the annular groove 110 with the guide 104 being secured onto the 
flange 100 by a snap-fit engagement. 
An outer surface 114 of the guide 104 is provided with a plurality of 
axially extending grooves 116. The axial grooves 116 allow any air which 
may be trapped within the actuator rod 60 to move freely past the caging 
bolt head 86 as the actuator rod 60 reciprocates. 
FIG. 4 illustrates a second embodiment of the invention. As in the earlier 
embodiment, like numerals will be used to illustrate like elements of the 
second embodiment. 
The caging bolt 78 extends coaxially into the actuator rod 60 to terminate 
in an elongated caging bolt head 270. The caging bolt head 270 is 
generally cylindrical having a proximal end 280 adjacent the caging bolt 
threads and an opposite distal end 282. A first Teflon.TM. guide 284 
encircles the proximal end 280 and a second Teflon.TM. guide 286 encircles 
the distal end 282. The Teflon.TM. guides 284 and 286 bear against the 
inner wall 106 of the actuator rod 60. The first and second Teflon.TM. 
guides 284 and 286 are spaced apart a distance greater than the diameter 
of the caging bolt head 270 to stabilize any off-centered movement of the 
actuator rod 60. Preferably, the guides 284, 286 are fixed to the head 270 
by an adhesive, or by staking or by being recessed in corresponding 
annular grooves (not shown) 
While particular embodiments of the invention have been shown, it will be 
understood that the invention is not limited thereto since modification 
can be made by those skilled in the art, particularly in light of the 
foregoing teachings. For example, the spring brake actuator can be any one 
of a variety of configurations, such as a stand alone actuator (not in 
tandem with a service brake), piston-operated rather than diaphragm 
operated, hydraulic rather than pneumatic, or in an application known as 
air-over-hydraulic where a pneumatic spring brake actuator is adapted to 
operate in conjunction with a hydraulic service brake actuator. Reasonable 
variation and modification are possible within the foregoing disclosure of 
the invention without departing from its true spirit and scope.