The present invention relates to a piston tube assembly for a spring brake actuator, comprising a tubular body having a contoured inside profile for non- rotationally guiding a running nut of a mechanical release mechanism, and an internal breather valve mounted to the piston tube assembly for allowing fluid transport into and out of the piston tube and any volume in fluid communication therewith.

Piston tubes are known in the industry as an essential part of a spring brake actuator. The piston tube is often integrally formed or alternatively permanently connected to a spring guiding portion of a spring brake piston which is actuated by either a compression spring in the spring brake actuator or positive pressure in an adjacent pressure chamber. The piston tube itself is responsible for transmitting the brake force applied by the compression spring to either an external brake mechanism or a further functional module of a spring brake actuator, such as a service brake portion.

Known piston tube assemblies, while functioning generally satisfactorily, face the challenge of having to undergo a comparatively complicated manufacturing procedure. If the piston tube is integrally formed with the remaining structure of the spring brake actuator, complicated molding forms need to be provided, or complicated machining procedures conducted.

Also, those piston tubes in the art which are provided in the form of piston tube assemblies - which are separate parts that have to be permanently fastened to the spring brake piston structure - have a comparatively complicated layout. With that in mind, it was an object of the invention to provide a piston tube assembly of the initially mentioned type which is more cost efficient to manufacture without sacrificing functionality. The invention attains the object mentioned before by suggesting that the piston tube assembly as initially mentioned has a tubular body which is a first tubular body, and the piston tube assembly further comprises a second tubular body that is of a different material than the first tubular body, encloses the first tubular body and is effective to mechanically support the first tubular body, wherein the internal breather valve is mounted to the first tubular body. The invention is based upon the realization that while counterintuitive, it is surprisingly more cost efficient to replace a monolithic tubular body with two distinct tubular bodies engaging one another. Firstly, both separate tubular bodies can be

manufactured at lower cost respectively as compared to a more complicated monolithic tubular body. Secondly, the second, outer tubular body may be selected to be of a stronger material than the first tubular body such that the different functionalities required in the piston tube can be distributed among the two tubular bodies. By this, the functionality of the original monolithic tubular body of the prior art may be preserved or even enhanced. In a preferred embodiment, the second tubular body consists partially or completely of steel, and the first tubular body consists partially of completely of a nonmetallic component, preferably a polymer. The polymeric tubular body has the advantage of having a low friction coefficient which leads to an improved slidable guiding of the running nut of the mechanical release mechanism. The torque transfer asserted by the running nut is assumed by the first tubular body and transferred to the second tubular body. In order to achieve this, the second tubular body preferably engages the first tubular body in a torque-transmitting connection, in particular in a positive connection such as by mutually engaging mating geometries, such as groove and tongue, or snap-fit connectors.

Alternatively or additionally the connection may also be a non-positive connection such as one by frictional engagement, particularly preferred by thread engagement or press fitting.

In a further preferred embodiment, the first tubular body comprises a head section and in the head section comprises a recess for accommodating the internal breather valve, and preferably at least one lateral ventilation hole opening into the recess for the internal breather valve. In a further preferred embodiment, the contour of the inside profile of the first tubular body is polygonal, preferably rectangular, pentagonal, hexagonal, heptagonal or octagonal.

In a further preferred embodiment, at least one of the tubular bodies has a circular outside profile.

The invention has herein above been described in a first aspect with respect to a piston tube assembly. According to a second aspect, the invention also achieves the object mentioned further above by suggesting a spring brake actuator for a commercial vehicle, comprising: a cylinder housing having a housing base, a spring brake piston located inside the cylinder housing, a compression spring located inside the cylinder housing, between the housing base and the spring brake piston, and effective to push the spring brake piston away from the housing base, the spring brake piston being adapted to transmit a brake force exerted by the compression spring, and wherein the spring brake piston comprises a piston tube assembly according to any one of the preferred embodiments described herein above.

The spring brake actuator shares the same benefits and preferred

embodiments of the piston tube assembly. Preferred embodiments of the piston tube assembly are also preferred embodiments of the spring brake actuator and vice versa, which is why reference is made to the description of the preferred embodiments of the first aspect herein above in order to prevent unnecessary repetitions.

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 prefered embodiment,

Fig. 2a, b show schematic representations of a piston tube assembly for the spring brake actuator of Fig. 1 , and

Fig. 3a, b show schematic representations of a first tubular body of the piston tube assembly of Fig. 2a, b.

In Fig. 1 , a spring brake actuator 1 for a commercial vehicle is depicted. The spring brake actuator 1 comprises a cylinder housing 2 having a first housing part 3 and a cylinder housing base 5. On the cylinder housing base 5, a spring seat 7 is located. Inside the cylinder housing 3, a spring brake piston 9 is slidably arranged. A compressions spring 11 is interposed between the spring seat 7 and the spring brake piston 9 and effective to push the spring brake piston 9 away from the housing base 5.

The spring brake piston 9 separates the interior of the cylinder housing 2 into a spring chamber 12 and a pressure chamber 14. A sealing element 10 slidably rests against the inside wall of the first housing part 3.

The first housing part 3 is connected to a second housing part 13 which in the present embodiment is a flange connector. The first housing part 3 and the second housing part 13 are sealingly connected through a sealing element 15, for example an O-ring. The spring brake piston 9 comprises a piston tube assembly 17. Inside the piston tube assembly 17, a mechanical release bolt 19 is rotatably housed and effective to drive a running nut 21 for mechanically releasing and tensioning the compression spring 11. The running nut 21 is configured to engage a

corresponding stop shoulder 23 of the spring brake piston 9.

Inside the piston tube assembly 17, an internal breather valve assembly 25 is located and effective to allow for fluid transport through the piston tube assembly 17.

The spring brake actuator 1 further comprises a third housing part 27 which is connected to the second housing part 13. The spring brake actuator 1 comprises a flexible diaphragm 29 which is sealingly located between the second housing part 13 and the third housing part 27 and preferably mounted thereto by way of clamping, e.g. crimping.

The flexible diaphragm 29 separates the interior volume defined by the second housing part 13 and the third housing part 27 into a service brake chamber 32 and a working chamber 33. The second housing part 13 comprises a recess configured to allow a reciprocating movement of the piston tube assembly 17 into the interior defined between the second housing part 13 and the third housing part 27, manipulating the volume of the working chamber 33 and service brake chamber 32. The piston tube assembly 17 comprises a front face 59 that pushes against the flexible diaphragm 29, which in turn transmits the brake force applied to it to a push rod 31 which can be connected to an external brake mechanism.

Details of the piston tube assembly 17 are shown in Fig. 2a, b.

In Fig. 2a and 2b, it is shown that the piston tube assembly 17 comprises a first tubular body 35 mounted inside a second tubular body 37. Preferably, the first tubular body 35 consists of a polymer, wherein the second tubular body 37 consists of steel. The first tubular body 35 comprises a head section 39 in which the internal breather valve assembly 25 is located. In the state shown in Fig. 2a, and internal breather valve 41 is mounted inside the internal breather valve assembly 25. The internal breather valve 25 communicates with the

environment through a number of lateral ventilation holes 47, which open into an annular space that is in fluid communication with a further lateral opening 43 provided in the second tubular body 37. Preferably, a number of sealing elements, such as O-rings 45, are positioned in spaced-apart relationship next to the lateral opening 43.

As can be seen from Fig. 2b, the second tubular body 37 preferably comprises a circular outside profile 49.

As can further be seen from Fig. 3a, b, the first tubular body 35 comprises a recess 51 dimensioned to accommodate the internal breather valve 41.

Preferably, the internal breather valve is built in accordance with US 8011483 B2. Adjacent to the lateral ventilation holes 47, the first tubular body 35 preferably comprises a number of annular recesses 57 for accommodating correspondingly shaped sealing elements such as the O-rings 45 shown in Fig. 2a.

Adjacent to the head section 39, the first tubular body 35 preferably comprises a pressing section 57 which is dimensioned for being press-fitted into the second tubular body 37 and has a correspondingly selected outside diameter and surface roughness, depending on the corresponding inside surface of the second tubular body 37 and the desired tightness of the press fit.

As can further be seen from Fig. 3b, the inside surface profile 55 of the first tubular body 35 has a polygonal shape, in this example a hexagonal shape, for non-rotationally guiding the running nut 21 along piston tube assembly 17.

List of Reference Signs

1 Spring brake actuator

2 Cylinder housing

3 First housing part

5 Housing base

7 Spring seat

9 Spring brake piston

10 Sealing element

1 1 Compression spring

12 Spring chamber

13 Second housing part

14 Pressure chamber

15 Sealing element

17 Piston tube assembly

19 Mechanical release bolt

21 Running nut

23 Stop shoulder

25 Internal breather valve assembly 27 Third housing part

29 Flexible diaphragm

31 Push rod

32 Service brake chamber

33 Working chamber

35 First tubular body

37 Second tubular body Head section

Internal breather valve

Lateral ventilation opening

Sealing element

Lateral ventilation hole

Outside surface profile

Recess for internal breather valve Annular recess for sealing element Internal surface profile

Press-fit section

front face, piston tube assembly