Patent Publication Number: US-2023147861-A1

Title: Crushable body for position adjustment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to, and the benefit of, EP Patent Application No. 21461618.7, filed Nov. 11, 2022 and titled “CRUSHABLE BODY FOR POSITION ADJUSTMENT,” which is incorporated by reference herein in its entirety for all purposes 
     FIELD 
     The following description relates to a crushable body used in a position adjustment assembly for example in a piston assembly with an adjuster assembly. The following description also relates to a piston assembly, an adjuster assembly, a cartridge for an adjuster assembly, a brake system, and a method of assembling a piston assembly. 
     BACKGROUND 
     Assemblies are known in various fields where a position of a moveable member needs to be adjusted due to changes in operation conditions or requirements and where the position adjustment is provided using a crushable body that, in certain operating conditions, is crushed to a shorter axial length which determines the position of the moveable member. One example of the use of such adjustment assemblies is in a brake system. Brake systems typically include friction elements that are brought into engagement with each other when a braking action is required to cause braking. The friction elements, which are typically friction discs, are selectively engaged and disengaged by use of one or more pistons. Such braking systems are known in many applications e.g. in machinery and also in vehicles and aircraft. Such multi-disc braking systems include a plurality of friction discs, also known as a heat sink, which are collectively operated to brake a rotating body such as a wheel e.g. a wheel on the landing gear of an aircraft. 
     During a braking action, wear of the surfaces of friction elements may occur over time. Such wear of the friction discs or other elements causes an increased clearance between friction surfaces of the friction elements when the brake system is disengaged. This increased clearance results in a longer stroke length of a piston or pistons required to engage the brake system, and therefore a longer engagement time. 
     To compensate for the increased clearance periodic manual adjustment of the brake system may be undertaken. Automatic adjusters are also known. One such mechanical automatic adjuster utilizes a tube on the piston and a tube expander element on a rod extending from the housing and through a head of the piston to be received in the tube. As the stroke length of the piston increases, the tube expander element attached to the housing is drawn axially along the bore of the tube and radially outwardly expands the tube to establish a new retracted position for the piston. Such adjuster assemblies may also be found in other applications. Another solution is to use a crushable tube to adjust the stroke length as the brake components wear. The tube is made of a material that crushes under force, but does not return to its original length when the force is removed. Such tubes, however, have been found to have an irregular radial inner and outer diameter when crushed. This irregular shape can result in the crushed tube interfering with other parts of the overall system where the assembly is used e.g., in a brake system, the spring guide and the piston. The forces generated by the crushed tube when in such an irregular crushed shape may also be irregular. 
     SUMMARY 
     According to an aspect of the disclosure, there is provided a crushable body for use in a position adjustment assembly, the crushable body comprising a thin-walled tube comprising a corrugated crushable body portion between two flat end portions, the corrugated crushable body portion configured to form a crushed body of reduced axial length compared to the axial length of the tube when a predetermined force is applied to the tube. In a particular aspect, there is provided a piston assembly comprising a housing defining a piston cylinder; a piston axially moveable within the cylinder responsive to a force applied to an end of the piston; and an adjuster assembly including a piston stroke limiter configured to limit the length of a return stroke of the piston from an extended stroke position, with respect to the cylinder, and a retracted stroke position, with respect to the cylinder; and a crushable body acting between the piston and the piston stroke limiter, the crushable body comprising a corrugated thin walled sleeve extending coaxially with the piston and being arranged to reduce in axial length when a length of an extension stroke exceeds a length of a retraction stroke to set the axial position of the piston stroke limiter to maintain the length of the return stroke. 
     The crushable body may be between the piston and the piston stroke limiter. 
     The crushable body may be a sleeve surrounding at least part of the piston. 
     The crushable body may be in the piston cylinder. 
     Expansion of the crushable body in a radial direction may be constrained by the piston. Expansion of the crushable body in a radial direction may be constrained by the housing. 
     The crushable body may be arranged to progressively deform over a plurality of stroke cycles of the piston. 
     The piston assembly may comprise an extension stroke stop and a retraction stroke stop, wherein the piston stroke limiter comprises a limiting member movable in an axial direction between the extension stroke stop and the retraction stroke stop. 
     The extension stroke stop and the retraction stroke stop may be formed by the housing. 
     The piston may extend through the crushable body 
     The piston assembly may comprise a retraction biasing member arranged to bias the piston stroke limiter from an extended position to a retracted position. 
     The retraction biasing member may be a helical compression spring. 
     According to an aspect of the disclosure, there is provided an adjuster assembly for maintaining a release clearance between selectively engageable friction parts to compensate for wear in the frictional parts, the assembly comprising a stroke limiter configured to limit the length of a stroke of an extendable member between an extended stroke position and a retracted stroke position; and a crushable body comprising a corrugated thin walled tube arranged to act between the piston and the piston stroke limiter and configured to be at least partially crushed to reduce the axial length of the crushable body in response to an extension stroke movement due to wear of the frictional parts exceeding a retraction stroke movement. 
     According to an aspect of the disclosure, there is provided a cartridge for an adjuster assembly of a piston assembly comprising a crushable body in the form of a corrugated thin walled tube arranged to reduce in axial length when compressed in an axial direction. 
     According to an aspect of the disclosure, there is provided a piston assembly comprising a cartridge for an adjuster assembly with a crushable body in the form of a corrugated thin walled tube arranged to reduce in axial length when compressed in an axial direction 
     According to an aspect of the disclosure, there is provided a brake system for an aircraft comprising at least one of a piston assembly as recited above, an adjuster assembly as recited above, and a cartridge as recited above. 
     According to an aspect of the disclosure, there is provided a method of assembling a piston assembly comprising: receiving a piston in a piston cylinder of a housing through a first side of the housing in a first direction; receiving an adjuster assembly comprising a crushable body in the form of a corrugated thin walled tube on the piston in the first direction; and enclosing the piston cylinder at the first side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the disclosure, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings. The description relates to the use of the crushable body of the disclosure in an adjuster of a brake system. The crushable body may, however, have many other applications. 
         FIG.  1    is a schematic side view of a multi disc brake system; 
         FIG.  2    is a cross-sectional schematic side view of a piston assembly of the multi disc brake system incorporating an adjustor assembly having a crushable body according to the disclosure, in the neutral position; 
         FIG.  2 A  shows the forces acting when the assembly is in the state shown in  FIG.  2   ; 
         FIG.  3    is a cross-sectional schematic side view of a piston assembly of the multi disc brake system incorporating an adjustor assembly having a crushable body according to the disclosure, in the braking position; 
         FIG.  4    show the forces acting to retain the piston when it is full extended; 
         FIG.  5 A  is a perspective schematic view of a crushable tube of an adjuster assembly according to the disclosure; 
         FIG.  5 B  shows the shape of a non-corrugated tube, when crushed, for comparison; 
         FIG.  6 A  shows a plot of load vs. crushed length for an example non-corrugated tube; and 
         FIG.  6 B  shows a plot of load vs. crushed length for an example corrugated tube according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG.  1   , a brake system  10  is described below. The brake system  10  is a multi-disc brake system. The brake system  10  comprises a friction brake assembly  20  and piston assemblies  30 . One of a plurality of piston assemblies  30  is shown, and the number of piston assemblies  30  may differ and may be a single piston assembly  30 . The piston assembly  30  is arranged to act on the brake assembly  20  to actuate a braking action. 
     The brake system  10  is operable to restrict rotation of a rotating body such as a wheel. In the example shown, the rotating body is an aircraft wheel  11 , but the brake system of this disclosure may have other applications. The wheel  11  includes a series of axially extending rotor splines  13  (one shown). Wheel  11  is supported for rotation about an axle  12  by bearings  14 . Axle  12  defines an axis A of the brake system  10  and of various components thereof. Reference to the terms axis and axial may include an axis of rotation defined by axle  12  or a dimension parallel to such axis. 
     The brake assembly  20  includes pressure plate  21 , flange  22 , torque tube  23 , and end plate  24 . Torque tube  23  may be an elongated annular structure that includes a reaction plate  25  and a series of axially extending stator splines  26  (one shown). The brake assembly  20  also includes a plurality of friction disks  27 . These comprise at least one non-rotatable friction disk (stator)  28 , and at least one rotatable friction disk (rotor)  29 . The friction disks  27  each include an attachment structure  27   a.    
     The flange  22  is fixedly mounted to the axle  12 . The pressure plate  21  is also non-rotatable. The piston assemblies  30  are connected to the flange  22  at circumferentially spaced positions around the flange  22 . The piston assemblies  30  face axially toward the wheel  10  and contact a side of pressure plate  21 . The piston assemblies  30  are hydraulically powered, however they may be powered mechanically, electrically, or pneumatically. The piston assemblies  30  are configured to exert a force on the friction disks  27  towards reaction plate  25 . Actuation of the piston assemblies  30  causes the rotors  29  and stators  28  to be pressed together between the pressure plate  21  and the end plate  24 . Although the piston assemblies  20  are shown as part of brake system  10 , it is contemplated that the piston assemblies, as disclosed herein, may be employed in a variety of other systems. For example, piston assemblies  30  may be employed in a multi-disk clutch system. 
     One of the piston assemblies  30  is described in further detail below with reference to  FIG.  2   . The piston assembly  30  includes a piston  31 . The piston  31  is configured to translate axially within a piston housing  32 . The piston assembly comprises an adjuster assembly  50  located axially around the piston between the piston and the housing. The piston  31  moves axially within and relative to the adjustor assembly  50 . 
     The piston housing  32  includes a main housing body  33  and a locker  34 . The locker  34  is secured to an open end of the housing  32  to close the end. 
     The housing  32  defines a piston cylinder and defines an axis B of the piston assembly  30 . The piston  31  is translatable in a longitudinal direction along axis B in the piston cylinder. 
     The piston  31  has a piston head  36  and a piston rod  37 . The piston rod  37  extends from the piston head  36 . The piston rod  37  is elongate and extends from the housing  32 . A distal end  38  of the piston rod  37  is attached to a disc  39 , also known as an insulator, by means of an insulator support  40  on the distal end  38  of the piston rod  37 . The disc  39  is secured to the end of the distal end  38  by the insulator support  40  and moves together with the piston rod  37 . The insulator is connected to the insulator support by a fastener such as a clip. The disc  39  comprises an engaging surface  39   a . The engaging surface  39   a  is arranged to move into and out of engagement with the friction brake assembly  20 . The engaging surface  39   a  of the disc  39  is arranged to contact and bias pressure plate  21  when moved into engagement in dependence on operation of piston assembly  30 . 
     As is known in the art, movement of the piston relative to the housing is caused by application of a brake or working fluid, for example hydraulic fluid, to the piston head  36 . A fluid supply (not shown) is arranged to supply working fluid to the volume. 
     A seal  41  extends around the piston head  36 . The seal  41  is a slidable seal which is configured to slide along and seal with the interior wall of the housing  32 . The seal  41  is shown in the example as comprising an o-ring, however alternative sealing arrangements may be provided. The arrangement of the piston  31  and the adjuster assembly  50  with the housing  32  mean that a single sealing arrangement provided by seal  41  is required to fluidly seal between the adjuster assembly  50  and the cavity (not shown) where the pressurized fluid is applied to the piston head. As such, a simple sealing configuration of the piston assembly  30  is provided. Working fluid in the cavity is prevented from flowing to other parts of the assembly by the seal  41 . 
     The piston assembly  30  includes the adjustor assembly  50 . The adjustor assembly  50  is received in the housing  32 . The adjuster assembly  50  extends in an axial direction. The adjuster assembly  50  includes a spring  52 , a spring guide  53 , a crushable body  60  and a piston stroke limiter  70  defined at the end of the spring guide. The spring  52  is a biasing member which is arranged to bias the piston  31  into a retracted stroke position, for example as shown in  FIG.  2   . The retracted stroke position is a position in which the piston  31  has moved in a retracted position into the piston chamber. 
     The spring  52  is a compressive helical spring. The housing  32  defines an annular chamber  54  around the piston that extends in a longitudinal direction about axis B. The annular chamber  54  is accessible at the open end of the housing when the locker  34  is removed. The spring  52  is received in the annular chamber  53  and is retain by the spring guide  53 . 
     The crushable body  60  will now be described in detail with reference to  FIGS.  2  to  6   . The crushable body  60  comprises a corrugated thin walled sleeve structure  61  and is received in the annular chamber around the spring  52  and the spring guide  53 , between the spring guide and the housing. Crushable body  60  forms a sleeve through which the piston  31  extends. Crushable body  60  is an elongate corrugated tube  61  in which corrugations  64  are formed around the tube along the length of the tube. In an uncrushed condition the crushable body  60  defines a gap between the tube and the housing. This provides for radial expansion of the crushable body  60  when the crushable body is at least partially in a crushed condition. 
     The crushable body  60  extends in an axial direction in the housing  32 . The crushable body  60  has a first end  62  and a second end  63 . The thin walled tube  61  is collapsible in an axial direction when the crushable body  60  is compressed in the axial direction between the first and second ends  62 ,  63 . The crushable body  60  defines a tubular arrangement through which the piston rod  37  is extendible. 
     The term thin walled is used herein to describe elements that have a small thickness in comparison to other dimensions such as length and width. Such elements are deformable in a controlled manner when a compressive force is applied to opposing, distal ends of the element. It will be understood that the thickness of the wall will be dependent on the compressive force required to crush the body  60  as well as the dimensions of the corrugations. In the present arrangement, it is anticipated that the thickness of the walls of the crushable member is in the range of approximately 0.1 mm-0.5 mm (0.004-0.02 inch). The walls may be a sheet material, with a thickness of less than approximately 6 mm (0.24 inch). Other dimensions are also possible provided the crushing performance is such as to maintain a substantially regular shape. It will also be understood that the material of the crushable body  60  may differ, in the present arrangement the crushable body  60  is formed from a ductile metal sheet arrangement, such as stainless steel, for example. Other materials include plastics, composites and foam. 
     A crushable body is intended to mean a body which is able to reduce in axial length when a compressive force is applied to the crushable body in the axial direction. Upon application of a compressive force in the axial direction above a predetermined value, the body reduces in axial length in a controlled manner, but does not collapse. Such an arrangement is non-resilient, that is, upon release of or reduction of the compressive force on the crushable body, the body  60  does not substantially expand in axial length. The predetermined force that causes crushing is largely independent of the deformed shape of the crushable tube—i.e. its shape at any stage of crushing, and is, therefore as independent of the number of times the axial length has previously been adjusted. It is also desirable that this force value is independent of temperature to avoid loss of adjustment force at high temperatures or to avoid variations in adjustment force due to different temperatures. 
     The crushable body  60  and the spring guide  53  defining the piston stroke limiter  70  are housed in the housing  32 . 
     The piston stroke limiter  70  transfers the force applied by the piston and the spring  52  between the components. 
     The limiting member  72  is received in the housing between a retraction stroke stop  76  and an extension stroke stop  77 . The retraction and extension stroke stops  76 ,  77  limit the axial movement of the piston stroke limiter  70 . The retraction stroke stop  76  and the extension stroke stop  77  are spaced apart from each other in the axial direction. The spacing, together with the thickness of the spring guide  53  defines the stroke length of the piston stroke limiter  70 . It will be understood that the configuration of the stoke stops  76 ,  77  may differ. 
     In a neutral condition, as shown in  FIG.  2   , in which the hydraulic pressure applied to the piston assembly  30  is less than the force exerted by the spring  52 , the piston  31  is biased into the retraction position. The retraction position is limited by retraction stroke stop  76 . 
     In use, the piston assembly  30  is assembled with the friction brake assembly  20 . The disc  39  is disposed adjacent to but spaced from the pressure plate  21  as shown in  FIG.  2   . A gap defining a release clearance is defined between the engaging surface  39   a  and the pressure plate  21 . Such a release clearance is predetermined. 
     Operation of the piston assembly  30  will now be described briefly. The piston  31  is initially in its neutral position, as shown in  FIGS.  1  and  2   , with the piston  31  in the retracted stroke position and the predetermined release clearance being formed between the disc  39  and the pressure plate  21 . The disc  39  and pressure plate  21  act as selectively engageable parts. Components of the brake assembly  20  act as selectively engageable parts in dependence of the action of the piston assembly  30 . The spring  52  biases the piston  21  into the retracted stroke position by means of the piston stroke limiter  70  and crushable body  60  through which the biasing force acts. The acting forces are indicated by arrows. Here, the load from the pre-loaded spring  52  acts on the piston  31  on one side and on the spring guide  53  on the other side ( 1 ,  FIG.  2 A ). The load of the spring acting on the spring guide loads ( 2   a ,  FIG.  2 A ) the insulator support  40  which is threaded to the piston  31  and so the load is transferred ( 3   a ,  FIG.  2 A ) through the thread to the piston. In this state, pressure applied to the end of the piston is not enough to compress the spring  52 . Until the spring force is exceeded, pressure applied to the end of the piston by the compressed fluid is transferred through the spring  52  to the spring guide  53  ( 1 ,  FIG.  2 A ) and from the spring guide to the corrugated crushable tube  60  ( 2   b ,  FIG.  2 A ). The force acting on the crushable tube is passed to the locker  34  ( 3   b ,  FIG.  2 A ) and then to the housing through the threaded connection ( 4   b ,  FIG.  2 A ). The insulator is not loaded in this state. 
     Upon actuation of the piston assembly  30  ( FIG.  3   ) a hydraulic force is applied to the piston head  36  through supply of pressurized fluid to the piston head ( 1   a ,  FIG.  3   ). The piston  31  is urged to move and so translate along in an axial direction from the retracted stroke position to an extended stroke position. The piston  31  translates in the axial direction until the engaging surface  39   a  of disc  39  engages with the pressure plate  21  and causes actuation of the friction brake assembly  20 . The force required to crush the crushable structure is greater than the force generated by the spring  52  and less than a maximum force which can be generated by pressurized fluid. The force required to actuate the friction brake assembly  20  to cause the predetermined braking effect is less than the compressive force required to cause crushing action of the crushable body  60 . The compressive force required to cause a crushing of crushable body  60  is less than the maximum force that may be applied to the piston  31  and the maximum actuating force that may be applied by the piston to the friction brake assembly  20 . The force generated by the applied pressure to actuate the brake is passed to the spring and the spring is compressed until the insulator comes into contact with the pressure plate  21  or until the piston contacts the spring guide. When the piston is sufficiently extended to cause the insulator to engage the pressure plate the braking force is applied through the insulator to the pressure plate ( 9   a ,  FIG.  3   ). Load  3   b  from the compressed spring acts on the spring guide ( 3   b ,  FIG.  3   ). This load is limited and depends on the length of the compressed spring and spring stiffness. The spring guide transfers this load to the crushable tube (load  4   b ). If there is a gap between the insulator and the pressure plate (i.e. if, due to wear, the piston stroke is insufficient to bring the insulator into contact with the pressure plate) additional load is applied from the piston. The maximum force  4   b  is the force required to crush the tube. The force is passed from the tube to the locker  34  ( 5   b ,  FIG.  3   ). 
     When the actuation of the friction brake assembly  20  is no longer required, the actuating force applied by the working fluid supply on the piston  31  is removed. As such, the biasing force of the spring  52  exceeds the actuating force applied on the piston  31  and the piston  31  is urged to move from its extended stroke position to the retracted stroke position. Such stroke actions may be performed in a plurality of cycles. 
     As the components of the friction brake assembly wear, such wear is required to be compensated in order to maintain the length of the piston stroke over repeated cycles, and in particular to maintain the release clearance irrespective of the extent of the wear. When such wear occurs, initially the piston  31  will move from its retracted stroke position when pressure is applied and overcomes the biasing force of the spring  52  ( 2   b ,  FIG.  3   ) to reduce the clearance between the disc  39  and the pressure plate  21 . This movement is limited by the action of the piston stroke limiter  70 , and abutment against the extension stroke stop  77 . 
     If wear has occurred then the actuating stroke length is greater than the stroke length of the piston stroke limiter  70 . When this occurs, the actuating force applied to the piston increases and applies a compressive force on the crushable body  60  above a predetermined crush limit between the piston head and the piston abutting the extension stroke stop  77 . When this force exceeds the resistance to crushing of the crushable body  60  ( FIG.  4   ) then the crushable body  60  is caused to crush and so reduce in axial length as a result of the partial deformation of the thin walled structure  61  until the required engagement provided by disc  39  with the friction brake assembly  20  to cause the braking action. At this point, axial movement of the piston  31  is prevented due to the reaction force of the friction brake assembly  20 . In this state, the spring is compressed and acts on the piston (force  2   a ,  FIG.  4   )) to retain the piston in the housing and the force is also acting on the spring guide on the other side (force  2   b ) and is passed from the spring guide to crush the crushable tube  60  (force  3 ). The load  3  to crush the tube  60  is the sum of the load  1  from the pressure applied to the piston force  1 ), the load  2   b  applied through the spring guide, minus the load  2   a  acting to retain the piston. 
     In the crushed state, the crushable tube  60  transfers the whole load  3  to the locker (load  4 ). 
     Upon release of the actuating force on the piston  31 , then the biasing force of the spring  52  exceeds the actuating force and urges the piston stroke limiter  70 , the crushable body  60  and piston  31  away from the extended stroke position, into the retracted stroke position. As the crushable body  60  is non-resilient, then the reduced axial length of the crushable body  60  is maintained. As such, the release clearance is defined by the movement of the piston from contact with the extension stroke stop  77  to contact with the retraction stroke stop  76 . As such, the release clearance in the retracted stroke positon is maintained. The adjustment gap corresponds to the release clearance. 
     Upon continued operating cycles of the piston assembly  30 , and therefore the brake system  10 , wear of the discs of the friction brake assembly  20  continues. Upon each cycle, the compression of the crushable body  60  in the axial direction, and therefore the reduction in the axial length of the crushable body  60  corresponds to the wear of the friction brake assembly  20 . The prescribed clearance is reestablished upon return of the piston assembly  30  to the retracted stroke position. Such continued wear causes the piston  31  to telescope from the housing  32  relative to adjuster assembly  50  and the piston stroke limiter  70  in which the release clearance is maintained in the retracted stroke position, and the disc  39  engages with the friction brake assembly  20  in the extended stroke position. 
     The crushable body  60 , best seen in different degrees of being crushed in  FIG.  5 A , comprises a corrugated crushable body portion  600  defined between two end portions  62 ,  63 . The corrugated crushable body portion  600  has a regular corrugated form along its axial length. The end portions  62 ,  63  are formed with flat (non-corrugated) sections. These flat ends allow for correct tube positioning to prevent lateral movement and to avoid force decrease at the start of crushing. The geometry of the end portions  62 ,  63  is different from that of the corrugated crushable body portion  600 , and this flat geometry controls the initial load performance at the start of crushing, to avoid load spikes. As the crushable body  60  is crushed and therefore reduces in its axial length the force required to crush the crushable body  60  is stable over the crushable length and therefore the force required to maintain the release clearance is stable. The corrugated structure has been found to result into a crushed form which has a substantially regular inner and outer diameter as the peaks and troughs of the corrugations essentially cancel each other out when crushed. With the arrangement of the crushable body  60 , the stroke length possible is maximized within the same envelope of the housing. The ratio of maximum adjustment length to physical assembly length is maximised. The tube diameter, depths of corrugations, lengths of flat ends, wall thickness, corrugation length and tube length can all be varied as required and will affect the force paths described above. In a preferred example, that has been found to provide particularly good crush characteristics, the ratio of the corrugation depth to external tube diameter is between 0.01 and 0.3, the ratio of corrugation depth to corrugation length (in the axial direction) is between 0.1 and 1.85; the ratio of wall thickness to external tube diameter is between 0.001 and 0.2 and the ratio of the sum of the length of the flat end portions to the overall tube length is between 0.001 and 1. The resulting, regular shape of the crushed tube can be compared with the irregular shape of a non-corrugated tube when crushed as seen in  FIG.  5 B . 
     The behavior of the corrugated crushable tube of this disclosure can be compared to that of a non-corrugated tube by viewing the load vs. axial length plots of  FIGS.  6 A and  6 B . With a smooth (non-corrugated) tube, as seen in  FIG.  6 A , there is a high peak force at the time the tube starts to be crushed and the force oscillates during crushing. In contrast, with a corrugated tube according to the disclosure, the flat end portions  62 ,  63  ensure that there is a gradual increase in force at the start of crushing and then the force remains very stable during the crushing of the corrugated crushable body portion  600 . Without the flat end portions  62 ,  63 , there may be uneven drops in force at the initial phase of crushing, but after this initial phase, the force will also remain very stable during the crushing of the corrugated crushable body portion  600 . 
     The fully crushed crushable body  60  prevents under adjustment of fully worn brakes. The adjustment force increases in the event of over-adjustment of the system. To replace the fully crushed crushable body  60 , the disc  39 , locker  34 , spring guide  53  and used crushable body  60  are all removed from the same end of the piston assembly  30  without the need to remove the piston  31  from the housing  32 . As such, ease of maintenance of the piston assembly  30  is maximized. The number of parts of the system is minimized. 
     While the disclosure is provided in detail in connection with only a limited number of embodiments, it should be readily understood that the disclosure is not limited to such disclosed embodiments. Rather, the disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the claims. Additionally, while various embodiments of the disclosure have been described, it is to be understood that the exemplary embodiment(s) may include only some of the described exemplary aspects. Accordingly, the disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.