The invention relates to a spring brake actuator for a vehicle, in particular parking or emergency brake actuator for use in a commercial vehicle, the actuator comprising: a cylinder housing having a cylinder housing base and a spring seat on the base, a spring brake piston located in said cylinder housing for applying a braking force, a compression spring arranged between the spring seat and the spring brake piston, the compression spring being effective to push the spring brake piston away from the base, wherein the cylinder housing in divided into a spring chamber and a pressure chamber, the compression spring being arranged in the spring chamber and the pressure chamber being supplied with pressurized fluid.

Spring brake actuators of the aforementioned type and generally known in the art. For example, EP 2944529 A1 discloses an exemplary such spring brake actuator. While the spring brake actuators of the aforementioned type generally function in satisfactory manner, it has been observed that particular care needs to be taken when fitting the spring brake piston into the cylinder housing. The sealing in particular on the outer periphery of the spring brake piston in the prior art systems requires attention to the installation step and close tolerances, because the spring brake piston slides with sealing material along the cylinder housing wall. Due to the sliding movement of the sealing member, there was an inherent risk of pressure loss.

In view thereof, it was an object of the invention to improve the spring brake actuator of the initially mentioned type to overcome the deficiencies found in the prior art as much as possible. In particular, it was an object of the invention to provide an improved spring brake actuator which is less of susceptible to pressure leakage, easy to install and maintain.

The invention attains this object by suggesting a spring brake actuator of the initially mentioned type, wherein the cylinder housing comprises an elastically deformable diaphragm that comprises an annular flange which is sealingly mounted to the cylinder housing, and an inner periphery which is sealingly mounted to the spring brake piston such that the pressure chamber is fluid-tightly isolated from the spring chamber. The invention is based upon the realization that optimal sealing can be achieved by preventing any relative movement between the sealing member and the cylinder housing.

Due to the large travel of the spring brake piston, it had previously been found to be unfeasible to install a diaphragm in between the spring chamber and the pressure chamber. Surprisingly, it has however been found out that the diaphragm is unproblematic to use in this function, and it has also been found out that it is surprisingly easy to sealingly mount the diaphragm also to the spring brake piston.

In a preferred embodiment, the cylinder housing comprises a first housing part and a second housing part mounted to the first housing part and the diaphragm is clamped between the first and second housing parts. Clamping in this context is to be understood as any way of mechanically fastening the diaphragm between the first and second housing parts under slight deformation of the diaphragm to achieve a fluid- tight seal between the first housing part and the diaphragm on one side, and the diaphragm and the second housing part on the other side. The clamping can be achieved by flanged connection, by crimping or similar measures.

In a further preferred embodiment, the diaphragm is connected to the spring brake piston by at least one of:

- press fitting,

- threaded fastening,

- material joining, in particular adhesive bonding, thermal bonding, or

vulcanizing,

- shrink fitting,

- wobble riveting, or

a combination of several of the above.

In preferred embodiments, the spring brake piston comprises a piston tube that extends past the diaphragm and is configured to reciprocate between a recessed position and an extended position wherein in the extended position, the piston tube extends out of the first housing and preferably also partially out of the second housing part, to transmit the brake force. Preferably, a release bolt, in particular a mechanical released bolt is mounted inside the cylinder housing, extending into the piston tube, and effective to engage a stop shoulder provided on the spring brake piston upon rotation of the release bolt such as to move the piston tube against the force exerted by the compression spring.

Particularly preferred, the diaphragm is dimensioned such that the piston tube passes past the inner periphery of the diaphragm. The release bolt preferably is threaded engagement with a static nut that is permanently mounted to the cylinder housing base. The presence of the static nut preferably ensures that the cylinder housing itself is always closed, thus no dust or other dirt can enter the cylinder housing. In prior art systems, which were outfitted with a removable stand-alone release bolt that was removed after use, a separate dust plug always needed to be installed in the cylinder housing base to prevent dust from getting into the housing. The dust plug occasionally came loose or was omitted by maintenance personnel, leading to unwanted dirt accumulation inside the housing. By using a permanently installed release bolt and static nut assembly, this problem is solved.

In a further preferred embodiment, the diaphragm comprises an inner annular insert that is sealingly connected to the diaphragm by at least one of:

- material joining, in particular adhesive bonding, thermal bonding or vulcanization, or

- threaded fastening.

The insert intern is preferably sealingly connected to the spring brake piston by at least one of:

- press fitting,

- threaded fastening,

- material joining, in particular adhesive bonding, thermal bonding, or vulcanizing,

- shrink fitting,

- wobble riveting, or

a combination of several of the above. In a further preferred embodiment, the spring brake actuator comprises an internal breather valve fitted into the piston tube.

In further preferred embodiments, the spring brake actuator comprises a third housing part mounted to the second housing part opposite the first housing part, the third housing part containing a second diaphragm which defines a service brake chamber inside the third housing part, and a push rod located in the third housing part, wherein the spring brake piston, preferably through the piston tube, is effective to act upon the second diaphragm such as to transmit the brake force onto the push rod.

Preferably, the second diaphragm defines a working chamber between the diaphragm and the second housing part, wherein the spring brake chamber and the working chamber are in fluid communication through the internal breather valve and the piston tube.

For a more complete understanding of the invention, the invention will now be described in more detail with reference to the accompanying drawings. The detailed description will illustrate and describe or is considered as a preferred embodiment of the invention. It should of course be understood that various modifications and changes in form or detail could readily be made without departing from the scope of the invention. It is therefore intended that the invention may not be limited to the exact form and detail shown and described herein, nor to anything less than the whole of the invention disclosed herein and disclaimed hereinafter. Further, the features described in the description, the drawings and the claims disclosing the invention may be essential for the invention considered alone or in combination. In particular, any reference signs in the claims shall not be construed as limiting the scope of the invention. The word “comprising” does not exclude other elements or steps. The wording“a” or“an” does not exclude a plurality.

In brief, the figures to which reference will be made show: Fig. 1 shows a schematic cross sectional view of a spring brake actuator according to a preferred embodiment, and

Fig. 2 shows a schematic three dimensional view of a diaphragm for use in the

spring brake actuator of Fig.1 .

Fig. 1 shows a spring brake actuator 1 for use in a commercial vehicle. The spring brake actuator 1 comprises a cylinder housing 2 having a first housing part 3. The first housing part 3 comprises a cylinder housing base 5. On the cylinder housing base 5, a spring seat 7 is provided. Inside the cylinder housing 2, the spring brake actuator 1 comprises a spring brake piston 9 which is configured to reciprocate between a retention position (shown in Fig. 1 ) and a release position (not shown) which would be more to the right as compared to the state shown in Fig. 1 .

In between the spring brake piston 9 and the cylinder housing base 5, a compression spring 1 1 is arranged engaging the spring seat 7. The compression spring 1 1 is biased against the spring brake piston 9 and effective to push the spring brake piston 9 away from the cylinder housing base 5.

The cylinder housing 2 comprises a second housing part 13 which preferably is a flange portion. The first housing part 3 is sealingly connected to the second housing part 13. The sealing function is provided by a diaphragm 15 which is clamped in between the first housing part 3 and the second housing part 13 with an annular flange 17 provide on its outer periphery.

The diaphragm 15 is further sealingly connected to the spring brake piston 9 by in insert 19 positioned on an inner periphery 18 (Fig. 2) of the diaphragm 15. Accordingly, the diaphragm 15 compartmentalizes the volume defined by the first and second housing parts 3,13 into a spring chamber 12 and a pressure chamber 14.

The spring brake piston 9 comprises a piston tube 20. An internal breather valve assembly 21 is fitted into the piston tube 20. The internal breather valve assembly 21 comprises an internal breather valve 22 which is configured to allow fluid transport into and out of the piston tube 20. Since the piston tube 20 is in fluid communication with the spring chamber 12, fluid transport is allowed to bypass the pressure chamber 14 through the piston tube 20 and the internal breather valve 22.

The spring brake actuator 1 comprises a release bolt 23 permanently installed on the cylinder housing 2 and in threaded engagement with a static nut 25 fastened to the cylinder housing base 5. The release bolt 23 comprises one or more engagement members 27 inside the piston tube 20 which are configured to engage a correspondingly positioned stop shoulder 29 on the spring brake piston 9. By rotating the release bolt, it is possible to manually move the spring brake piston 9 into the release position or back into the retention position, depending on the sense of rotation of the release bolt 23.

The spring brake actuator 1 further comprises a third housing part 31 which is sealingly connected to the second housing part 13. The spring brake actuator 1 further comprises a second diaphragm 33 which is sealingly connected, namely interposed, between the second housing part 13 and the third housing part 31 with an annular flange 34 provided on its outer periphery. The diaphragm 33 is in contact with a push rod 35. The push rod 35 is configured to reciprocate between a retained position (shown in Fig. 1 ) and an extended position (not shown in Fig. 1 ) in which the push rod 35 transmits the brake force coming from the spring brake piston 9 onto an external brake mechanism (not shown).

A working chamber 37 and a service brake chamber 39 are defined between the diaphragm 33 and the second housing part 13. The working chamber 37 is in fluid communication with the piston tube 20 through the internal breather valve assembly 21 .

Fig. 2 provides a more detailed view of the diaphragm 15. The diaphragm 15 and the insert 19 found on the inner periphery 18 of the diaphragm 15 are preferably bonded by material joining. The diaphragm 15 preferably consists of the insert 19 and an elastically deformable base body 16 which is substantially cup-shaped and comprises the outer annular flange 17 for fastening the diaphragm sealingly to the cylinder housing 2. As can be seen from the figures above, the invention suggests a simple yet very reliable manner of sealingly separating the pressure chamber 14 from the spring chamber 12 while at the same time maintaining ease of installation and keeping the number of parts involved under control. Risks of leakage are significantly reduced due to the prevention of relative movement between sealing parts and sealed surfaces.

List of reference signs

1 spring brake actuator

2 cylinder housing

3 first housing part

5 cylinder housing base

7 spring seat

9 spring brake piston

1 1 compression spring

12 spring chamber

13 second housing part

14 pressure chamber

15 diaphragm

16 base body

17 annular flange, diaphragm

18 inner periphery, diaphragm

19 insert, diaphragm

20 piston tube

21 internal breather valve assembly

22 internal breather valve

23 mechanical release bolt

25 static nut

27 engagement member

29 stop shoulder

31 third housing part

33 second diaphragm

34 annular flange, second diaphragm

35 push rod

37 working chamber

39 service brake chamber