GB2261471A - Spring support in brake actuater - Google Patents

Abstract

In a spring brake actuator includes a cylindrical housing 33 within which a fluid pressure responsive piston 36 moves. The fluid pressure acts in a sense to compress a capture power spring 38 between the piston and one end of the housing. The end of the housing is provided with a spring seat 50 the periphery of which is conical or arcuate and received by complimentary surfaces of the housing so that the force of the spring is converted into pure axial and hoop-stresses of the housing. <IMAGE>

Description

SPRING BRAKE ACTUATORS This invention relates to spring brake actuators.

Spring brake actuators are typically employed as secondary and/or parking brake actuators in compressed air operable brake systems of heavy road vehicles. A typical such actuator comprises a cylindrical body defining an internal bore and a fluid pressure responsive member (typically a piston) sealingly movable in the bore. A heavy spring is retained compressed between one side of the pressure responsive member and a closed end of the cylinder.

Application of sufficient fluid pressure to other side of the pressure responsive member maintains the spring under full compression such as to hold an output rod member fully withdrawn to maintain the brake in a released condition. A brake application is then effected by full or partial release of the fluid pressure.

It has been proposed to construct the cylindrical housing of pressed steel, the spring being contained between the pressure responsive member and an integral closed end of the cylinder. However, the stresses placed by the compressed spring on the curved portion which adjoins the cylindrical or axial part are substantial and failure due to metal fatigue cannot occur in service. One possible solution to this shortcoming is to thicken the metal in relevant locations but this is undesirable from both manufacturing and cost considerations.

It is an object of the present invention to provide an improved construction for a spring brake actuator wherein the above shortcoming is substantially reduced or overcome.

According to the present invention there is provided a spring brake actuator comprising a generally cylindrical housing defining a bore within which a fluid pressure responsive member is axially movable under the influence of fluid pressure acting on one side of the member in opposition to the force of a spring supported in compression between the other side of the member and an integral end of the cylinder to control the output force of the actuator characterised in that the end of the cylinder is formed with a peripheral part the internal surface of which is of frusto conical or arcuate shape the end of the compression spring being supported on a member of complimentary shape abutting said internal surface and via which member the force of the spring is reacted by an axial tension in the cylinder and hoop stress in said peripheral part.

In order that the invention may be more clearly understood and readily carried into effect the same will now be further described by way of an example with reference to the single figure of the accompanying drawing which illustrates a piston and spring brake actuator in combination with a compressed air operable service diaphragm type of brake chamber.

Referring to Fig. 1 of the drawings the combined compressed air operable service brake actuator and spring brake actuator comprises a middle body casting denoted by reference 1. Such a casting is formed by pressure die casting and provides a dividing wall 2 between a pressure chamber 3 of the service brake portion and a pressure chamber 4 of the spring brake portion. These chambers communicate via respective passages (not shown) with twin input ports denoted at 5, 6 formed on the outside of the casting.

On the left hand end of the casting 1 as seen in the drawing there is formed a peripheral lip portion 6 to receive a pressed steel clamp ring 7 to clamp the peripheral bead 9 of a dish-shaped diaphragm 10 between lip 6 and a corresponding lip 8a of the non-pressure portion 8 of the service actuator portion. Diaphragm 10 acts against a push plate 11 carrying the screw-threaded actuator output rod. The push plate is urged towards the normal rest condition as shown by means of a light conical return spring 13 captive between the push plate and the non-pressure portion of the actuator body, the actuator rod extending outwardly through a central aperture 14 thereof for connection to a brake mechanism of a vehicle.

The wider end of the conical spring 14 is located by means of an annular locating plate positioned within the non-pressure portion between the heads of mounting studs 16 which are welded in position to project through respective apertures for mounting the actuator in known manner.

On the right hand end of the middle casting 1 as seen in the drawing there is a further peripheral lip portion 31 to retain a rolled end 32 of a pressed steel cylinder 33 of the spring brake actuator portion. The rolled end 32 and lip 31 trap a retaining ring 31a in known manner and a seal 34 is provided between the casting 1 and cylinder 33.

Axially sealingly slideable in the bore 35 of cylinder 33 there is provided a piston 36 with an annular seal and piston guide assembly 37. A power spring 38 is trapped under compression between the piston 36 and a spring support member the right hand end of the cylinder 33, this spring 38 being shown in its fully compressed condition as it is in use with full pressure applied to the chamber 4.

The piston 36 is formed with a central recess which accommodates a seal 44 and a screw-in spring actuator output member 40 which extends through a axial seal arrangement 41 in the central wall 2 to abut the right hand side of diaphragm 10. The output member 40 is tubular to accommodate and enclose the head 42 of a wind-off bolt 43. The wind-off bolt which is normally in the position shown, is carried in a threaded member 46 captive in the right hand end wall of the cylinder and is provided with a keyed-on nut 47 a snap-on end cover 48, has an aperture which affords access to the nut 47.

In accordance with the invention, the right hand end of the cylinder 33 is formed with a surface portion 51 and the spring support 50 is provided with a complementary external conical surface 52 which is receivable within the surface SI. These conical surfaces extend substantially to the cylindrical surface itself whereas axial clearance is allowed to remain at 'A' between the axially facing surfaces. The result of this is that the stress imposed upon the cylinder 33 by the force of the spring or the spring hold-off pressure in chamber 4 is a hoop-stress.

The force of the spring and for the force exerted on the spring support by the abutting piston 36 in the opposition shown in the drawing is therefore reacted by axial tension in the cylindrical wall and by the hoop-tension in the conical surface of the cylinder end. In order to achieve this most advantageously it has been found that the conical surface makes an angle of approximately 300 to the cylinder axis.

Claims (2)

1. A spring brake actuator comprising a generally cylindrical housing defining a bore within which a fluid pressure responsive member is axially movable under the influence of fluid pressure acting on one side of the member in opposition to the force of a spring supported in compression between the other side of the member and an integral end of the cylinder to control the output force of the actuator characterised in that the end of the cylinder is formed with a peripheral part the internal surface of which is of frusto conical or arcuate shape the end of the compression spring being supported on a member of complimentary shape abutting said internal surface and via which member the force of the spring is reacted by an axial tension in the cylinder and hoop stress in said peripheral part.

2. A spring brake actuator substantially as described with reference to the accompanying drawing.