GB2268230A

GB2268230A - Spring brake actuator

Info

Publication number: GB2268230A
Application number: GB9317206A
Authority: GB
Inventors: Stephen Walter Fogg
Original assignee: Knorr Bremse Systems for Commercial Vehicles Ltd
Current assignee: Knorr Bremse Systems for Commercial Vehicles Ltd
Priority date: 1993-08-18
Filing date: 1993-08-18
Publication date: 1994-01-05
Also published as: GB9317206D0

Abstract

A compressed air operable spring brake actuator has a piston (3) sealingly slideable in a cylinder bore (2) under the influence of compressed air acting in a chamber (4) against the force of helical spring means (11, 12) and the spring means comprises two helical spring portions with an interconnecting member (5, 14) permitting one spring portion (12) to be located under compression at least partially within the other portion (11) to provide a shorter assembly than may otherwise be required for a given actuator stroke. Thus the overall length of the actuator can be reduced. <IMAGE>

Description

FLUID PRESSURE OPERABLE ACTUATORS.

This invention relates to fluid pressure operable spring force actuators and especially but not exclusively to spring brake force actuators.

Fluid pressure operable spring brake force 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 under compression between one side of the pressure responsive member and one end of the cylinder. Application of fluid pressure to the other side of the pressure responsive member maintains the spring under full compression and holds an output rod member in a fully withdrawn position.

In such position no force is applied to the brake by the spring. A brake application is then effected by partial or full venting of the fluid pressure. In order to effect release of the brake without applying fluid pressure it is usual to provide a screw-threaded member which is accessible from the outside operable to draw the pressure responsive member towards its full released position in the cylinder.

It is readily appreciated that any known force actuator of the above type has an overall length which is determined by three main design aspects, the fully withdrawn length of output rod, the fully compressed length of the heavy spring and the necessary lengths of the ends of the cylindrical body together with any ancillary projections such as the mentioned screw threaded member. Optimum design has hitherto been centred upon achieving optimum power stroke with minimal compressed length of the power spring. To this end the wire used for the spring has been designed to be highly stressed and the spring has been designed to be partially barrel shaped to minimise fully compressed length.Control of spring material and treatment is therefore critical any appreciable departure from a given specification resulting either in premature failure of the spring or non-attainment of specified force output characteristics.

The object of the present invention is to provide an improved spring force actuator wherein the effective stroke for a given fully compressed spring length may be improved.

According to the present invention there is provided a spring force 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 spring means supported in compression between the other side of the member and an end wall of the cylinder to control the output force of the actuator characterised in that the spring means comprises a first helical portion acting in series with a second helical portion via interconnecting means enabling one said helical portion to be located at least partially within the other said helical portion.

Preferably, said interconnecting means comprises a body having axially spaced radially inwardly and outwardly extending flanges which receive said other ends.

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 diaphragm operable and spring force brake actuator in accordance with the invention.

Referring to the drawing, the spring force actuator comprises a generally cylindrical body 1 provided by a middle casting having a bore 2 within which a piston 3 is sealingly axially slideable under the effect of compressed air applied to å volume 4. The piston 3 has a tubular output member sealingly axially slideable in the end wall 6 and is provided with an end cap 7 which bears against the pressure side of a diaphragm 8 of an integral but independently operable service brake chamber with a common casting.

The cylindrical body 1 has an open end which receives a closure member 9 retained by an internal circlip 10 to trap a compressed spring means between 9 and the piston 3.

The spring means comprises a first helical spring portion 11 and a second helical spring portion 12. Both the springs 11 and 12 are parallel helical with respective ends ground flat normal to the common axis.

Interconnecting means comprising a cylindrical member 13 with axially spaced radially outward and inward flanges 14 and 15 is arranged between the springs so that they act in series, as shown. In the shown full retracted position of the piston 3 some predetermined clearance exists between the coils of the springs.

A spring-brake wind-off bolt of known form is indicated by reference 16. This bolt has a head 17 relative to which the tube 5 is normally movable. The bolt shown passes freely through the piston 3, threadedly engages a fixed bush 18 in the closure member 9 and terminates with a keyed-on nut, 19. The latter is readily accessible to mechanically retract the piston 3 towards the position shown in the event of no compressed air supply being available.

The mentioned service portion of the combined actuator includes the diaphragm 8 clamped at its periphery 21 between the middle casting and a non-pressure portion 22 of the housing. The non-pressure portion 22 is provided with mounting studs 23 for mounting the combined actuator to a vehicle and the output is transmitted via a final push rod assembly 24, either by compressed air acting on the right of the diaphragm 8 of the service portion or by the end member 7 acting thereon due to the force of springs 11 and 12 in series acting via the piston 3 and output tube 50 of the spring brake portion.

In operation of the spring brake portion, in the absence of compressed air pressure in the chamber 4 the full force of the spring 11 and 12 in series is applied to the output tube 5. This corresponds to a typical vehicle parking condition with the brakes applied. In order to release the brakes, compressed air is applied to chamber 4 via an input port (not shown) and at a predetermined pressure value the output force is completely over-ridden. A further increase of input pressure results in full hold-off with the piston 3 in the position shown against end stops.

The diameter and treatment of the helical coils of the springs 11 and 12 are chosen to provide approximately equivalent rates and it is found that with an arrangement of spring means as described above the length occupied by the spring means can be appreciably less than a single spring affording a similar actuator output characteristic.

Accordingly, the available stroke of a spring force actuator constructed in accordance with the invention can be appreciably greater than the stroke of a prior art actuator of similar length.

Whilst in the foregoing the invention is described with spring means comprising two helical springs arranged in series, for certain applications even greater advantage may be gained from similarly arranging three or more springs in series. In order to do this an additional interconnecting means is required for each extra spring used.

Whilst in the foregoing, the interconnecting means between one spring and the next in series is provided by a cylindrical member with respective axially spaced inward and outward projecting flanges alternative forms of interconnecting means may be used. Indeed in the event of sufficient radial space existing between one spring and the next, the springs may be constructed on continuous wire a joining section extending axially from one end of one spring to the opposite end of the next spring.

Again, although not described in the foregoing, the helical spring means may be arranged to be of semi-barrel or conical shape as distinct from the cylindrical shape described by way of particular example. In that case the interconnecting means required to be shaped to accommodate the fully compressed condition of the spring means without mutual fouling between one spring and the next.

Claims

1. A spring force 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 spring means supported in compression between the other side of the member and an end wall of the cylinder to control the output force of the actuator wherein the spring means comprises a first helical portion acting in series with a second helical portion via interconnecting means enabling one side helical portion to be located at least partially within the other said helical portion.

2. A spring force actuator as claimed in claim 1, said interconnecting means comprising a generally cylindrical member having a radially outward extending spring supporting flange at one end and a radially inward extending spring supporting flange at the other end thereof.

3. A spring force aetustor substantially as described herein with reference to the accompanying drawing.