Patent Publication Number: US-2022212650-A1

Title: Piston tube assembly for a spring brake actuator, and spring brake actuator

Description:
FIELD 
     The present invention relates to a piston tube assembly for a spring brake actuator. More particularly, the present invention relates to piston tube assembly including 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. 
     BACKGROUND 
     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 need to be conducted. 
     Also, known piston tubes that 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. 
     SUMMARY 
     With that in mind, it was an object of the invention to provide a piston tube assembly of the initially mentioned type that is more cost efficient to manufacture without sacrificing functionality. 
     In one aspect, the object is achieved by providing a piston tube assembly having a tubular body that is a first tubular body, and the piston tube assembly further includes a second tubular body that is of a different material than the first tubular body, encloses the first tubular body, and mechanically supports the first tubular body, wherein an 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. According to the approach of the present disclosure, the functionality of the original monolithic tubular body of the prior art may be preserved or even enhanced. In a preferred embodiment, according to an aspect, the second tubular body consists partially or completely of steel, and the first tubular body consists partially or 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 handled 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, according to an aspect, the first tubular body includes a head section, and the head section includes 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, according to an aspect, 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, according to an aspect, at least one of the tubular bodies has a circular outside profile. 
     The invention has been described above in a first aspect with respect to a piston tube assembly. According to a second aspect, the invention also achieves the object mentioned above by providing a spring brake actuator for a commercial vehicle, including: 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, the compression spring 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 includes a piston tube assembly according to any one of the preferred embodiments described above. 
     The spring brake actuator shares the same benefits and preferred embodiments of the piston tube assembly described above. Preferred embodiments of the piston tube assembly are therefore also preferred embodiments of the spring brake actuator and vice versa, such that reference is made to the description of the preferred embodiments of the piston tube described above in order to prevent unnecessary repetitions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the invention, the invention will now be described in more detail with reference to the accompanying drawings. 
       In brief, the figures to which reference will be made are as follows: 
         FIG. 1  illustrates a schematic cross-sectional view of a spring brake actuator according to an aspect of the invention, 
         FIG. 2 a    is a schematic cross-sectional view of a piston tube assembly for the spring brake actuator of  FIG. 1 , 
         FIG. 2 b    is a perspective view of the piston tube assembly; 
         FIG. 3 a    is a cross-sectional view of a first tubular body of the piston tube assembly, and 
         FIG. 3 b    is an end view of the first tubular body. 
     
    
    
     DETAILED DESCRIPTION 
     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. Further, the features described in the description, the drawings, and the claims disclosing the invention may be a part of the invention when 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  FIG. 1 , a spring brake actuator  1  for a commercial vehicle is depicted. The spring brake actuator  1  includes 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 is 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  includes a piston tube assembly  17 . Inside the piston tube assembly  17 , a mechanical release bolt  19  is rotatably housed and is 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 is effective to allow for fluid transport through the piston tube assembly  17 . 
     The spring brake actuator  1  further includes a third housing part  27 , which is connected to the second housing part  13 . The spring brake actuator  1  includes a flexible diaphragm  29  which is sealingly located between the second housing part  13  and the third housing part  27  and is 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  includes 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 , thereby 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. 2   a, b.    
     In  FIGS. 2 a  and 2 b   , it is shown that the piston tube assembly  17  includes 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. 2 a   , an internal breather valve  41  is mounted inside the internal breather valve assembly  25 . The internal breather valve assembly  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  0 -rings  45 , are positioned in spaced-apart relationship next to the lateral opening  43 . 
     As can be seen from  FIG. 2 b   , the second tubular body  37  preferably has a circular or cylindrical outside profile  49 . 
     As can further be seen from  FIG. 3 a   , b, the first tubular body  35  comprises a recess  51  sized to accommodate the internal breather valve  41  therein. Preferably, the internal breather valve  41  is built in accordance with U.S. Pat. No. 8,011,483 B2. Adjacent to the lateral ventilation holes  47 , the first tubular body  35  preferably includes a number of annular recesses  57  for accommodating correspondingly shaped sealing elements such as the  0 -rings  45  shown in  FIG. 2   a.    
     Adjacent to the head section  39 , the first tubular body  35  preferably comprises a pressing section  57  which is sized 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. 3 b   , 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 .