GB2113347A

GB2113347A - Piston valve for gas spring

Info

Publication number: GB2113347A
Application number: GB08201974A
Authority: GB
Inventors: L G Nichols
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-01-07
Filing date: 1982-01-25
Publication date: 1983-08-03
Also published as: GB2113347B

Abstract

A piston and piston rod assembly for use in a gas spring comprises a piston rod 15 on which is slidably mounted a piston 13 having a through passageway 19. A restrictor plate 16 is secured to the end of the piston rod and includes an elongate groove 23 which, during outward strokes of the piston, is closed on its normally open side by a land 27 of the piston to define a restrictor channel to restrict flow of fluid through the passageway 19. During inward strokes of the piston, the piston moves into engagement with a shoulder 22 of the piston rod to allow unrestricted flow into the passageway 19 around the edge of the plate 16. <IMAGE>

Description

SPECIFICATION Piston and piston rod assembly This invention relates to a piston and piston rod assembly for use as a flow control device where substantially higher resistance to movement of the piston in one axial direction is required than to movement in the opposite axial direction. The preferred embodiment of the present invention is, for example, suitable for use in a gas spring, for example of the type shown in our British patent specification G.B. 2012913A.

The piston and piston rod assembly shown in British patent specification G.B. 201291 3A has been found satisfactory for many applications, but in some applications difficulty has been experienced in maintaning adequate dimensional control of the orifice provided in the restrictor plate.

According to the present invention a piston and piston rod assembly comprises: a piston rod; a piston mounted on the piston rod; a passageway providing a fluid flow path past the piston; and a restrictor device, the restrictor device and piston being movable relative to each other between a first position in which the restrictor device is spaced from the piston and does not substantially restrict flow through the passageway and a second position in which the restrictor device is in abutment with the piston to restrict flow through the passageway, wherein said restriction to flow is provided by an elongate channel defined by the opposing surfaces of the abutting piston and restrictor device, said channel resticting flow through the passageway.

In the preferred embodiment of the invention the channel is formed by an elongate groove defined by the restrictor device facing the piston. Thus, when the restrictor device and piston are in abutting contact the groove is closed on its normally open side by the surface of the piston to define the elongate channel. The groove may be formed in the surface of a restrictor member, in which case it is preferably shaped such that the channel includes a radially extending exit portion extending from the periphery of the restrictor member, a an arcuate central portion, and a radially extending entry portion extending from the end of the central portion remote from the exit portion. Thus, when the restrictor member is in engagement with the piston fluid flows from the passageway via the entry portion, the central portion, and the exit portion of the elongate channel.

In an alternative embodiment the restrictor device comprises a support plate and a slotted plate which lies against and is supported by the support plate to define the groove.

By suitably dimensioning the groove in the restrictor device, the resistance to flow provided by the channel is substantially determined by the length of the channel. By suitably selecting the length of channel the desired restriction can be provided without the need for highly accurate machining of the groove. Further, because the groove is open along its entire length during unrestricted reverse flow through the passageways, any particles of debris becoming trapped in the groove are flushed clear on each reverse stroke of the piston and piston rod assembly.

The above and further features and advantages of the invention will become clear from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings wherein: Figure 1 shows partially in cross section a piston and piston rod assembly mounted within a cylinder; Figure 2 is a sectional view taken on the line ll-ll of Fig. 1.

Figure 3 is a view corresponding to Fig. 1 showing a second embodiment of the invention; Figure 4 is a plan view of the embodiment of Fig. 3; and Figure 5 shows the slotted disc of the embodiment of Figs. 3 and 4.

Figs. 1 and 2 show a piston and piston rod assembly 10 slidably mounted within a cylinder 11, for example of a gas spring. An 0ring seal 1 2 positioned in a groove in the piston 1 3 ensures sliding sealing contact between the piston 1 3 and the cylinder 11. The piston 1 3 is slidably mounted on a spigot 1 4 formed at the end of piston rod 1 5. A restrictor member 1 6 is secured to the end of the piston rod 1 5 by suitable means, for example screw 1 7. A light spring 1 8 may be provided to bias the piston 1 3 towards the restrictor member 16.

A passageway 1 9 comprising a plurality of axially extending drillings 20 opening into an annular groove 21 is provided to allow fluid to flow past the piston. More particularly, during an upward stroke (as viewed in Fig. 1) of the assembly 10 the piston 1 3 will move away from the restrictor member 1 6 and seat on the shoulder 22 formed at the base of the spigot 1 4. Fluid may then flow freely around the edges of the restrictor member 1 6 and through the passageway 1 9 to the lower side of the piston.The clearance between the restrictor member 1 6 and the cylinder 11 in association with the cross sectional area of the passage 1 9 ensures that during such an upward stroke there is little resistance to movement of the assembly 1 0.

During a downward stroke (as viewed in Fig. 1) of the assembly 10, however, the piston 1 3 moves into abutment with restrictor member 1 6 as shown in Fig. 1 and flow out of the passageway 1 9 is restricted since the only exit passage from the annular groove 21 is via a channel 23 defined between the piston 13 and the restrictor member 16.

Referring now to Fig. 2 the restrictor member 16 is formed with a groove 23 comprising an inlet portion 24, a central arcuate portion 25, and an outlet portion 26. When the piston 1 3 is in abutment with the restrictor member 1 6 the land 27 formed radially outwardly of the annular groove 21 closes the groove 23 over the majority of its length to define a restrictor channel. Fluid flowing through the passageway 1 9 enters the channel via the entry portion 24 of the groove, flows around the arcuate portion 25 of the groove to exit via the exit portion 26. By suitably selecting the length of the groove 23 the desired degree of restriction of flow during a downward movement of the piston can be obtained.In the embodiment shown in Fig. 2 a groove of substantially 180= arcuate extent is shown in solid lines, but this length can be extended up to at least 340 (as shown in broken lines) if it is desired to increase the restriction aforded by the groove.

The groove 23 may be formed by any suitable technique.

It will be observed that during upward strokes of the piston during which the piston is spaced from the restrictor member, the groove 23 is open along its entire length, and accordingly any debris whcih may have become lodged within the groove can fall free and be carried away by the fluid sweeping past the open groove.

Referring now to Figs. 3 to 5 a modified embodiment of the invention is shown which is identical to the embodiment of Figs. 1 and 2 except that the restrictor member 1 6 of Figs. 1 and 2 has been replaced by a restrictor device 1 6A comprising a support plate 30 and a slotted plate 31.

The slotted plate 31 is cut from thin strip material and, as can best be seen from Fig. 5, is formed with a slot 32 comprising a radially extending entry portion 33 and an arcuate portion 34. The arcuate portion is illustrated as having an arcuate extent of 180 but may have a greater arcuate extent up to, for example, 340 (as indicated by broken lines) or a lesser arcuate extent as desired. The support plate 30 is in the form of a relatively thick disk and is formed with an axially extending hole 35 which registers with the arcuate portion 34 of the groove 32.

The support plate 30 and slotted plate 31 are secured to the piston rod 1 5 by the screw 1 7 such that the slotted plate 31 lies against and is supported by the support plate 30. The surface of the support plate closes one side of the slot 32 whereby the plates 30, 31 together define a groove 36 which extends from the entry portion 33 to the hole 35. By manufacturing the slotted disc 31 from material having a known and substantially constant thickness and by forming the slot 32 to close tolernces the dimensions of the groove 36 formed can be accurately controlled more easily than if the groove is formed in a solid restrictor member as shown in Figs. 1 and 2.

Further, by relatively rotating the plates 30, 31 the effective length of the groove 36 i.e.

the arcuate extent of the groove 36 from the entry portion 33 to the hole 35, can be adjusted to a desired value. After the desired relative position has been achieved, the screw 1 7 is fully tightened to lock th plates relative to each other.

It should be noted that the hole 35 is of sufficiently large diameter for the contribution it makes to flow resistance to be very small compared with that of the groove 36 and thus close control of the tolerences of the hole 35 is not necessary, and the resistance to flow provided by the restrictor device is substantially proportional to the effective length of the groove 36.

Claims

1. A piston and piston rod assembly comprising: a piston rod; a piston mounted on the piston rod; a passageway providing a fluid flow path past the piston; and a restrictor device, the restrictor device and piston being movable relative to each other between a first position in which the restrictor device is spaced from the piston and does not substantially restrict flow through the passageway and a second position in which the restrictor device is in abutment with the piston to restrict flow through the passageway, wherein said restriction to flow is provided by an elongate channel defined by the opposing surfaces of the abutting piston and restrictor device, said channel restricting flow through the passageway.

2. A piston and piston rod assembly according to claim 1 wherein the restrictor device is fixed relative to the piston rod and the piston is slidable on the piston rod to provide the relative movement between the restrictor device and the piston.

3. A piston and piston rod assembly according to claim 2 wherein the restrictor device is secured to the end of the piston rod.

4. A piston and piston rod assembly according to any preceding claim wherein the channel is formed by an elongate groove defined in the surface of the restrictor device.

5. A piston and piston rod assembly according to claim 4 wherein the elongate groove includes a radially extending entry portion which, when the piston and restrictor device are in abutment registers with the passageway, and an arcuate portion which is closed on its normally open side by a land on the piston when the piston and restrictor device are in abutment.

6. A piston and piston rod assembly according to claim 5 wherein the restrictor device is a unitary restrictor member and the groove includes a radially extending exit por tion which extends from the end of the arcuate portion remote from the entry portion of the periphery of the restrictor member.

7. A piston and piston rod assembly according to claim 5 wherein the restrictor device comprises a support plate and a slotted plate abutting the support plate, the slotted plate including a slot which is closed on one side by the support plate to define the entry portion and the arcuate portion of the groove.

8. A piston and piston rod assembly according to claim 7 wherein the support plate includes an axially extending through hole which registers with the arcuate portion of the groove to form an exit from the channel.

9. A piston and piston rod assembly according to claim 8 wherein the support plate and the slotted plate are rotatable relative to each other to vary the position of the hole along the arcuate portion of the groove and thus vary the effective length of the channel.

1 0. A piston and piston rod assembly, substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 to 5 of the accompanying drawings.