FR2808054A1 - NON-RETURN VALVE FOR A PISTON PUMP - Google Patents

Abstract

The invention proposes to manufacture in one plastic part by injection molding a non-return valve (34) for a piston pump (10) of a vehicle braking system provided with a valve shutter (36 ), a valve spring (38) and a fixing member (46) with which the non-return valve (34) is fixed to a piston (20) of the piston pump (10). This has the advantage of being able to manufacture the non-return valve (34) in one working step by injection molding and assembly of the non-return valve of loose parts is not required.

Description

The present invention is a non-return valve for a piston pump.

  relates to a non-return valve for a piston pump comprising a valve plug. The non-return valve is intended mainly for use in a piston pump of a hydraulic braking system of

  vehicle with a wheel slip control device.

  Such a non-return valve is for example known from the document DE 41079799 A1. The known non-return valve is arranged on the facial end of a piston of a piston pump, the piston pump being intended to transport the brake fluid in a hydraulic vehicle braking installation comprising a sliding control of the wheels. The piston has an axial hole, the opening of which forms a valve seat for the non-return valve. A ball arranged on the valve seat forms a valve plug of the known check valve. When the non-return valve is closed, the ball forming the valve plug is applied in a known manner to the valve seat in a sealed manner and is

  valve seat lifted when the check valve is open.

  It is not advantageous for the valve plug of the known non-return valve to be a separate element and for the non-return valve to be assembled from several elements comprising at least the valve plug, the piston comprising the valve seat, possibly a valve closing spring, a valve body and / or a guide for the valve plug. Another disadvantage of the valve

  known non-return is that the guide for the valve plug must be provided.

  The main object of the present invention is a non-return valve provided with a valve valve, comprising a fixing element for the valve valve with which the valve valve element is movably connected and with

  which the valve plug forms a part.

  The valve plug is then linked with the non-return valve by the fixing element. The non-return valve according to the invention advantageously allows simplified manufacture and in particular assembly. One-piece manufacturing

  the entire check valve is possible for embodiments of the invention.

  The fixing element of the non-return valve according to the invention can for example form a valve body and / or serve to fix the valve plug on

the piston of a piston pump.

  The present invention also relates to a non-return valve, characterized in that the non-return valve comprises a valve spring with which the shutter

of valve forms a part.

  The present invention also relates to a non-return valve characterized in

  that the valve spring forms a part with the fixing element.

  The valve spring links the valve plug with the one-piece fastener. These arrangements of the invention have the advantage that the number of parts of the non-return valve is reduced; thus reducing the number of production stages

  for parts and assembly steps for assembling the non-return valve.

  The present invention also relates to a non-return valve characterized in that the fixing element comprises a valve seat with which the valve shutter cooperates. This arrangement of the invention makes possible the manufacture in

  a part of the non-return valve according to the invention.

  The present invention also relates to a non-return valve characterized in

  that the fastener includes a valve lift limiter.

  The valve lift limiter limits the path by which the valve plug can be lifted from a valve seat. The valve lift limiter

  has the advantage of being able to limit the closing time of the anti-valve

  return. In addition, the valve lift limiter forms a kind of guide which

  holding the valve shutter in a position provided when the anti-valve

  return is closed, which ensures that the non-return valve closes as expected. The present invention also relates to a non-return valve characterized in that the valve plug is linked with the fixing element so as to

be able to rotate.

  For example, the valve plug is connected so that it can be pivoted, by means of a flexible plastic strip or a tongue, to the fixing element. This arrangement of the invention has the advantage, on the one hand, of guiding the valve shutter on the support element and, on the other hand, of releasing a large section of

  passage by pivoting the valve plug.

  The present invention also relates to a non-return valve comprising a valve shutter formed in one piece with the fixing element as well as with the valve spring, if such a spring exists, and is made in one piece. plastic. The manufacturing is carried out quickly and economically, in particular in one step of

  working and by injection molding.

  The present invention also relates to a non-return valve characterized in that the valve plug forms a part by injection molding with

the fixing element.

  The present invention also relates to a non-return valve characterized in that the valve plug is connected at least by a flexible tab forming a

  piece with the fixing element.

  The present invention also relates to a non-return valve characterized in

  that the fixing element is linked by a notch connection to the piston.

  The present invention also relates to a non-return valve characterized in that there is a clamping between the non-return valve and a piston which maintains the

piston valve.

  The non-return valve according to the invention is provided as an intake or exhaust valve of a piston pump, in particular to be fixed on a front side of a piston of a piston pump. According to the invention, the piston pump is in particular provided as a pump in a braking installation and is used to control the pressure in the wheel brake cylinders. Depending on the type of brake system, the designations ABS, ASR, FDR or EHB are used. In the braking system, the pump is used, for example, to bring brake fluid from a wheel brake cylinder or from several wheel brake cylinders into a master cylinder (ABS) and / or to supply brake fluid. brake of an accumulator tank in a cylinder or several wheel brake cylinders (ASR or FDR or EHB). The pump is necessary, for example, in a braking installation with a wheel slip control (ABS or ASR) and / or in a braking installation with pilot assistance function (FDR) and / or in a electro-hydraulic braking (EHB). With the wheel slip control (ABS or ASR), one can for example avoid a locking of the vehicle wheels during a braking action accompanied by a strong pressure on the brake pedal (ABS) and / or a rotation the vehicle's drive wheels when the accelerator pedal (ASR) is pressed hard. In a braking system with pilot assistance function (FDR), a braking pressure is established in one or more wheel brake cylinders, independently of the actuation of the brake pedal or the accelerator pedal, in order to avoid for example a stall of the vehicle with respect to the path desired by the driver. The pump can also be used in an electro-hydraulic braking system (EHB), in which the pump brings the brake fluid into the wheel brake cylinder (s), each time an electric sensor associated with the brake pedal detects an actuation of said brake pedal or in which the pump ensures the

  filling a brake system accumulator.

  In the following, the invention is explained by means of embodiments chosen in a preferred manner and represented in the drawings and in which FIG. 1 is an axial section of an exemplary embodiment of a piston pump

  with a non-return valve according to the invention.

  Figure 2 the check valve of Figure 1 is shown in perspective view Figure 3 a detail view along arrow II in Figure 1 of a modified embodiment Figure 4 an axial sectional view of a second example of realization of the invention. Figure 5 an axial sectional view of a third embodiment of the invention. Figure 6 a sectional view of a non-return valve of the piston pump according to the invention of Figure 5 in axial section as a spare part Figure 7 a sectional view of a modification of the embodiment of the invention shown in Figure 5 Figure 8 a sectional view of a check valve according to the invention

  of the piston pump of figure 7 as a spare part in side view.

  In Figure 1, we can see a first example of embodiment according to the present

invention.

  The piston pump 10 shown in Figure 1 is housed in a hydraulic block 12 forming the pump body and so named in the following. The hydraulic block 12, only part of which is shown in the drawing and which surrounds the piston pump 10, comprises a hydraulic connection for controlling a vehicle braking installation by slip control and which is also not shown. In the hydraulic block 12, other hydraulic construction elements are not shown, such as, for example, valves

  magnetic; these elements being connected together hydraulically.

  The hydraulic block 12 forming the pump body 12 is provided with a pump bore into which a jacket 16 in the form of a hollow cylinder is introduced.

  having a bottom of shirt 18 and forming a part with said shirt.

  The piston pump 10 comprises a piston 20 which is guided over part of its length in the jacket 16, which piston moves axially in the pump bore 14 over another part of its length, which part projects the jacket 16. The pump bore 14 and the jacket 16 form a piston guide (14, 16). The piston 20 is essentially composed of two parts; it comprises a deformed part 22 in the form of a socket and a valve seat element 24 housed in the deformed part 22. The deformed part is produced in a sheet of metal by stamping; it has the shape of a cylindrical and hollow socket which is closed

  on one side by a front wall 26 and forms a part with the deformed part 22.

  To increase the resistance to wear, the deformed part 22 is hardened and it is not necessary to carry out another treatment because the quality of the surface of the part

distorted is sufficient.

  The valve seat element injected into the deformed element 22 is made of plastic, of roughly cylindrical shape and protrudes from the deformed element 22 with part of its length. The valve seat member 24 is traversed by a continuous axial hole 28 opening into a conical valve seat 30 on a side projecting from the

deformed part 22.

  The valve seat member 24 has on a protruding area of the deformed part 22 a circular sealing lip which forms a part with said valve seat element. The sealing lip 32 which forms a piston lining has the shape of a hollow truncated cone and is directed with a free edge towards the bottom of the jacket 18. The sealing lip 32 is applied with pretension to the inside the jacket 16 and seals the piston 20 in the jacket 16. The sealing lip 32 forms a piston lining on the side of the high pressure, it is stressed on an inside side by a fluid (brake fluid) which must be transported by the piston pump 10. If the brake fluid pressure increases during transport, the sealing lip 32 urged by the brake fluid is in addition to its preload pressed harder against the jacket 16 so that the sealing lip 32 provides a good seal even under high pressure. Thanks to its preload, the sealing lip 32 seals at low pressure or at equal pressure. A good seal is thus obtained for the different pressures of brake fluid present during operation with a

  low level of friction and wear for the sealing lip.

  A non-return valve according to the invention, shown in enlarged form in FIG. 2, is fixed on the facial end situated in the jacket 16 of the piston 20 or of the valve seat element 24. The anti-valve return 34 has a hemispherical valve plug 36 which cooperates with the valve seat 30 of the valve seat member 24. The valve plug 36 has a valve spring 38 on a flat circular surface away from the valve seat 30 , which valve spring forms a part with the valve shutter 36. The valve spring 38 has a zig-zag shape and presses the valve shutter 36 with a low force of

  stress in axial direction against valve seat 30.

  On one end remote from the valve shutter 36, the valve spring is connected in one piece to a spring support 40 which has the shape of an angle in the embodiment. A branch 42 of the spring support 40 is arranged in a radial plane of the piston pump 10 and another branch 44 of the spring support is approximately parallel to the axis in the region of a circumference of the element of valve seat 24. The spring support 40 is integrally formed with a tubular support element 46 which is placed on the end of the valve seat element 24 located in the jacket 16. The valve shutter 36 , the valve spring 38, the valve holder 40 and the support member 46 are made together in one

  piece of plastic material injected into a melting or injection mold.

  The tubular support element 46 of the non-return valve 34 has on its circumference an annular step 48 which forms a bearing surface 48 for a piston return spring 50 of the piston pump 10. The return spring of piston is formed as a helical pressure spring which is introduced into the jacket 16. The piston return spring 50 rests on the bottom of the jacket 18 and

  press against the annular step 48 of the support element 46 of the check valve

  return 34; said step forms the bearing surface 48. The piston return spring 50 presses by means of the fixing element 46 of the piston 20 against the circumference of an eccentric 52 rotated by an electric motor; said eccentric being disposed on a front side of the piston 20 projecting from the jacket 16. By a rotary drive of the eccentric 52, the piston 20 is driven in a known manner in an alternating reciprocating movement axially in the pump bore 14 and jacket 16 which in a known manner causes the transport of brake fluid. The piston return spring 50 simultaneously holds the fixing element 46 of the non-return valve 34 on the face end of the valve seat element 24

  of the piston 20 located in the jacket 16.

  To bring in the brake fluid, the valve seat element 24 is provided with axial grooves 54 on the circumference and radial grooves 56 on the front side of the part injected into the deformed part 22; said grooves leading to the stamped through hole 58 in the circumferential wall of the deformed part 22 towards the axial hole 28 of the valve seat element 24. The inlet of the fluid takes place through an inlet bore 60 arranged in the pump body 12, which bore opens radially into the pump bore 14 and then through the through holes 58 and the axial and radial grooves 54, 56 in the axial hole 28 towards the non-return valve 34

  according to the invention which forms an inlet valve 34 of the piston pump 10.

  The fluid to be transported arrives from the axial hole 28 through the non-return valve 34 in a

pump chamber 61.

  The outlet of the pump chamber 61 from the piston pump 10 is effected by a central hole 62 in the bottom of the jacket 18, which hole opens with a valve seat 64 conical to an outlet valve 66 on an outside side of the bottom of the liner 18. The outlet valve 66 is formed as a spring-loaded non-return valve. A compression coil spring as a valve spring 68 presses a valve ball 70 forming a valve plug against the valve seat 64. The valve ball 70 and the valve spring 68 are introduced into an axial blind hole 72 in a closure plug 74 which is placed on the bottom of the jacket 18. The closure plug is fixed with a circular portion 76 stamped on the pump body 12 in the pump bore 14 and made watertight. The brake fluid outlet from the outlet valve 66 is effected by radial channels 78 arranged in the shape of a star between the closure plug 74 and the jacket bottom in the pump bore 14 through a outlet bore 80 arranged in

  the pump body radially with respect to the pump bore 14.

  The valve plug of the non-return valve 34 according to the invention does not

  does not necessarily have a ball shape, other shapes are possible.

  The valve shutter 82 may, for example, as shown in the individual representation of Figure 3, be formed as a circular disc. For this variant embodiment, the valve seat 30 is formed by a circular edge provided on the valve seat element 24. The edge forming the valve seat 30 is on a front side of the turned seat element 24 towards the pump chamber 61. The non-return valve 34 and the piston pump are also formed identically in FIGS. 1 and 3 and operate in the same way; to avoid repetition, we

  can refer to the description given above for figure 1. The same references

  are used for the same items.

  Between the piston 20 and the fixing element 46 of the non-return valve 34 there exists a notch connection 81. The notch connection is formed by a circular bead 85 on an inner circumferential face of the fixing element 46 which meshes in a circular groove on the outer circumference of the valve seat member 24. Due to the notched connection 81, the piston 20 and the check valve 34 can be firmly inserted before the piston 20 is mounted in the pump body 12. This

  will essentially facilitate the assembly of the piston pump 10.

  In Figure 2, we can see a second embodiment according to the present invention.

  For the description of the piston pump 10 represented in FIG. 4, we will use

  the same references as in FIG. 1 to designate the components which are identical. The piston pump 10 of Figure 4 is formed, unlike the piston pump of Figure 1 made as a single piston pump, as a step piston pump. This means that the piston 20 of the piston pump 10 of FIG. 4 is guided in the jacket 16 over a larger diameter and sealed outside the jacket 16 in the pump bore 14 in the body of pump 12. In forming the stepped piston pump, the piston pump 10 of FIG. 4 comprises in the pump bore 14 or in the jacket 16 an annular chamber 83 surrounding the deformed part 22 of the piston 20. A volume of this annular chamber 83 changes during the alternating reciprocating stroke of the piston 20 and causes suction

  brake fluid during a discharge stroke of the piston pump 10.

  The non-return valve 34 according to the invention of the piston pump 10 of FIG. 4 forming the inlet valve comprises a valve shutter 84 which has the shape of a circular disc. The valve shutter 84 is linked by a plastic tab 86 forming a part with the valve seat element 24; which element

  is part of the piston 20 and is injected into the deformed element 22 of the piston 20.

  The valve seat element 24 of the piston 20 simultaneously forms the support element 46 of the non-return valve 34 according to the invention. The valve plug 84 is fabricated in one piece in an injection molding manufacturing step with the valve seat member forming the support member 46, which

  valve shutter 84 is linked in a pivoting movement to tongue 86.

  Seen in the radial direction, the tongue 86 is quite thin. It is thus possible to obtain low bending stresses during a pivoting movement of the valve shutter 84 inside the tongue 86 which clearly prolongs the fatigue strength of the tongue 86. thickness of the tab 86 is comprised of

  preferably only between 0.5 and lmm.

  When the non-return valve 34 is closed, the shutter 84 is applied in leaktight manner to the opening of the axial hole 28 in the valve seat element 24. A circular front edge of the opening forms the seat of valve 30 of the non-return valve 34. When the non-return valve 34 is opened the shutter 84 rises from the valve seat 30 in a pivoting movement; the shutter 84 is pivotally guided on the valve seat element by the tab 86 which connects said valve

valve seat plug 24.

  The pivot angle with which the valve 84 can be lifted from the valve seat is limited by a hook 88 which also forms a part with the seat element 24 of the piston 20 constituting the fixing element of the anti-valve return 34; said hook being parallel to the axis of the valve seat element 24 on the circumference of the shutter 84. The hook 88 forms a valve lift limiter 84. It limits a valve lift, that is to say -to say the pivoting angle or the path by which the valve 84 can be lifted from the valve seat. For this arrangement of the invention, all the parts of the non-return valve, that is to say its support element 46, the shutter 84 which is pivotally connected by the tongue 86 with the element of fixing 46 and the valve lift limiter are manufactured in a single manufacturing step in a plastic material by injection molding. The non-return valve 34 thus does not require steps for assembling these parts; it is immediately ready for use after being molded. Because the shutter 84 rises little from its valve seat 30 due to the valve lift limiter 88, it is possible to very quickly close the non-return valve 34 at the start of a discharge lift. For this reason, it is possible to dispense with a special valve closing spring and the tongue 86 is flexible during bending. The notched connection 81 provided between the support ring and the valve seat element 24 of the piston 20 guarantees an essential reduction in the efforts for the assembly of

the piston pump 10.

  The piston return spring 50 is supported for the piston pump 10 shown in FIG. 4 by a support ring 90 on the piston 20; the support ring 90 being placed in the inner zone of the sealing lip 32 on the seat element of

valve 24.

  The piston pump 10 shown in Figure 4 also corresponds to the piston pump 10 shown in Figure 1 and operates in the same manner. To avoid

  repetitions we therefore refer to the explanations given in Figure 1.

  In Figure 2, we can see a second embodiment according to the present

invention.

  The piston pump 10 shown in Figure 5 is formed like the piston pump in Figure 4 as a step piston pump. Its piston 20 comprises deformed parts 24, 92 in the form of a socket, which are advantageously produced by stamping a metal sheet. The two deformed pieces 24, 92 have a hollow cylindrical step shape and form a piece on a facial side with a facial wall 26, 94. The two deformed pieces 24, 92 are pressed one against the other or both deformed parts 24, 92 are open in the same direction, that is to say in the direction of the jacket 16. The face wall 94 of the internal deformed part 92 is provided with a central hole for the passage of brake fluid. The non-return valve 34 according to the invention is pressed in the internal deformed part 92 of the piston 20. The non-return valve 34 of the piston pump 10 shown in FIG. 4 is shown individually in axial section in FIG. 6 The non-return valve 34 comprises a fixing element 98 in the form of a hollow cylinder which is applied against the face wall 94 of the deformed part 92 inside the piston 20; the fixing element being provided with a central hole 96 for the passage of the brake fluid. A facial edge of the fixing element 98 diverted from the facial wall 94 forms a valve seat 99 of the non-return valve 34; the fastener 98 is thus also the valve seat member. On the side of the fixing element 98 diverted from the face wall 94 is disposed a cylindrical shutter the valve 100 of the non-return valve 34. The valve shutter 100 is linked by a tab 102 disposed at a point of its circumference so that it can be lifted and pivoted with the fixing element. The tongue 102 is arranged parallel to the axis on a circumference of the valve plug and the tongue 102 is formed in a wavy fashion. The valve shutter 100 can be lifted from the fixing element 98 by expanding the tongue 102 in an axial direction and the shutter 100 can be pivoted relative to the fixing element 98 of the

non-return valve.

  In this arrangement of the non-return valve 34 according to the invention, all the parts of the non-return valve 34, that is to say the fixing element 98 and the valve shutter 100 linked by the tab to fastening element 98 are produced together in one piece from plastic and in one working step

  by injection molding; it is therefore no longer necessary to assemble the anti-valve

  return 34 from spare parts.

  When the check valve 34 is closed, its valve shutter 100

  applies tightly to the valve seat 99 of the fixing element 98.

  When opened, the valve shutter 100 is raised relative to the fixing element 98 by expansion of the tongue 102 and pivots so as to release a

large passage section.

  The jacket 16 of the piston pump 10 shown in Figure 5 advantageously manufactured in a metal sheet by stamping. The jacket 16 has a cylindrical shape with a bottom 104 on a front side. A free edge 106 of the jacket 16 is clamped towards the inside and maintains the piston 20 on an annular step in the jacket 16. A piston return spring 50 formed as a helical pressure spring is introduced into the jacket 16. The valve return spring 50 rests on the bottom 104 of the jacket 16 and presses against an annular step 110 inside the piston 100; said step being formed on the piece

  internal deformation 92 of piston 20.

  A bottom hole 112 has been arranged in the bottom 104 of the jacket 16, the opening of which forms a valve seat of a non-return valve 66. The non-return valve 68 forms an outlet valve of the piston pump 10 ; said valve is formed like the outlet valve 66 of the piston pump 10 shown in FIG. 1 as a non-return valve, biased by a spring with a spring ball 70

as a valve plug.

  The piston pump 10 shown in FIG. 5 is also formed identically and functions in the same way as the piston pump 10 shown in

  Figure 1 and we therefore refer to the explanations given in Figure 1.

  The non-return valve 34 is pressed with its cylindrical fixing element 98 into the deformed part 92 in the form of a cup. This has the effect of creating a clamp 103 between the non-return valve 34 and the piston 20. The clamp 103 has the effect of preventing the fluid (bypassing the non-return valve 34) from returning from the pump chamber 61 in the inlet bore 60. The tightening 103 additionally maintains the non-return valve 34 not only during the operation of the piston pump 10 but before and during the assembly of the piston pump firmly on the piston 20. This essentially reduces the effort required for manufacturing

piston pump 10.

  In the piston pump shown in FIG. 7, the non-return valve 34 which is shown in detail in FIG. 8 in side view, is formed differently from the non-return valve 34 of the piston pump 10 represented in FIG. 5. Unlike the embodiment of FIG. 5, the non-return valve 34 of the piston pump of FIG. 7 has several tabs, for example three, tabs 102 which link the valve shutter 100 to the fixing element 98. The tongues 102 are distributed around the circumference and are formed in a wavy fashion. In order to be able to open the non-return valve 34, the tongues 102 may, due to their undulation, be expanded so as to lift the valve shutter axially from the fixing element 98. In this case, it is not possible to rotate the valve shutter 100. It is advantageous to provide several tabs 102 because it can better guide the valve shutter 100 on the fixing element and it is also advantageous to be able to better exert the spring force of the tabs 102 in the closing direction of the valve shutter 100. The tabs 102 simultaneously form a valve limiter 88 which limits the lifting of the valve and therefore the path by which the valve shutter 100 can be lifted from the element

fixing 98.

  In the embodiment shown in FIG. 8, the valve lift limiter also has the effect of lifting the valve shutter 100 only from the valve seat in a limited manner so that the non-return valve 34 can be close very quickly at the start of a delivery lift; like this

  is explained by means of the embodiment shown in FIG. 4.

  With the exception of the difference indicated that the non-return valve 34 of the piston pump 10 shown in Figure 7 has several (three) tabs 102 which connect the valve shutter 100 in one piece with the fixing element 98 of the non-return valve 34, this non-return valve is constructed and operates in the same manner as that in FIGS. 5 and 6 and reference is made in this direction to the explanations of FIGS. 5 and 6. The piston pump 10 of figure 7 is also constructed and works in the same way as that of figure 5 and one is returned

  therefore to the explanations of figure 5.

Claims (11)

Claims   1. Non-return valve for a piston pump comprising a valve valve characterized in that the non-return valve (34) comprises a fixing element (46, 98) for the valve valve (36, 82, 84 ,) with which fixing element the valve shutter (36, 82, 84,) is movably connected and with which element the valve shutter   valve (36, 82, 84, 100) forms a part.   2. Non-return valve according to claim 1, characterized in that the non-return valve (34) comprises a valve spring (34, 102) with   which the valve plug (36, 82, 100) forms a part.   3. Non-return valve according to claim 2, characterized in that the valve spring (38, 102) forms a part with the fixing element (46, 98).   4. Check valve according to claim 1, characterized in that the fixing element (46, 98) comprises a valve seat with which   the valve shutter (36, 82, 84, 100) cooperates.   5. Check valve according to claim 1, characterized in that the fixing element (46, 98) comprises a valve lift limiter (88, 102).   6. Check valve according to claim 1, characterized in that the valve shutter (84, 100) is linked with the fixing element (46, 98) so that it can pivot.   7. Check valve according to claim 1, characterized in that the valve plug (36, 82, 84, 100) is made in one piece in   plastic with the fixing element (46, 98).   8. Non-return valve according to claim 6 or 7, characterized in that the valve shutter (36, 82, 84, 100) forms a part produced by   injection molding with the fixing element (46, 98).   9. Check valve according to any one of the preceding claims,   characterized in that the valve shutter (36, 82, 84, 100) is connected at least by a flexible tongue (86, 102) forming a part with the fixing element (46, 98).   10. Check valve according to any one of claims, characterized   in that the fastening element (46, 98) is linked by a notched connection (81) to the piston (20, 22, 24, 92).   11. Non-return valve according to claim 1, characterized in that there is a clamping (103) between the non-return valve (34) and a piston (20)   which holds the valve (34) to the piston (20).