GB2131119A

GB2131119A - A fluid-damped piston and cylinder assembly

Info

Publication number: GB2131119A
Application number: GB08234069A
Authority: GB
Inventors: Anthony Thomas Mallett
Original assignee: Wipac Group Sales Ltd
Current assignee: Wipac Group Sales Ltd
Priority date: 1982-11-30
Filing date: 1982-11-30
Publication date: 1984-06-13
Also published as: CA1207612A; GB2131119B; DE3343050A1; FR2536810B1; FR2536810A1

Abstract

A fluid-damped piston and cylinder assembly comprises a cylinder within which a piston mounted on a piston rod 12 is movable to displace grease or other damping fluid through or past the piston between a first chamber in front of the piston 21 and a second chamber 22 behind the piston and surrounding the piston rod. The increase in overall length of the first and second chambers as the length of the first chamber is reduced is taken up by a free piston 117 whose displacement compresses a spring 20. The free piston may surround the piston rod and define one end of the chamber 22. The assembly can be used for generally supporting or for restricting or assisting the opening and closing of various lids and doors such as the boot lid or bonnet or tail-gate of a vehicle. <IMAGE>

Description

SPECIFICATION A fluid-damped piston and cylinder as sembly This invention relates to a fluid-damped cyiin der and piston assembly, for generally sup porting or restricting or assisting the opening or closing of various lids or doors, for example a vehicle tail-gate, boot lid or bonnet.

Broadly, the present invention provides a fluid-damped cylinder and piston assembly comprising a cylinder having opposite end walls, a control piston slidable along the axis of the cylinder, a piston rod extending within the cylinder and supporting said control piston at the inner end of the rod, the rod extending slidably through an opening in a first one of said end walls, a charge of grease or other damping fluid filling a first chamber formed on the forward side of the control piston and filling a second chamber formed on the rear ward side of the control piston around said piston rod, said chambers being intercon nected by a restricted passage through which said fluid can flow in response of movement of the piston and piston rod relative to the cylinder to vary the relative axial lengths of the chambers, and resilient means arranged to oppose the displacement. of said damping fluid in one direction between said chambers.

More particularly the present invention pro vides a fluid-damped cylinder and piston as sembly comprising a cylinder having opposite end walls, a control piston slidable along the axis of the cylinder, a piston rod extending slidably within the cylinder through an aper ture in one of said end walls and supporting the control piston at its inner end, the space within the cylinder comprising a first chamber on the forward side of the control piston and an annular chamnber on the rearward side of the control piston surrounding the piston rod, a restricted passageway interconnecting the two chambers through which said fluid can flow in either direction between said chambers in response to movement of the control piston relative to the cylinder and a free piston forming a movable wall of one of said cham bers, whereby the difference in the axial lengths of said chambers resulting from the movement of the control piston and piston rod relative to the cylinder results in a movement of the free piston proportional to the volume of damping fluid displaced by the piston rod, and a spring biassing said free piston in one direction along the axis of said cylinder.

Certain embodiments of the present inven tion are illustrated in the accompanying draw ings in which: Figure 1 is a section along the axis of a piston and cylinder assembly showing the assembly in its fully extended position for supporting a lid or other part in an open position; Figure 2 is an axial section through the assembly of Fig. 1 showing the assembly in its fully contracted position; Figure 3 is a fragmentary section through a second embodiment of the piston and cylinder assembly; Figure 4 is an axial section along a third embodiment of the piston and cylinder assembly; Figure 5 is an axial section along a fourth embodiment of the piston and cylinder assembly; and Figures 5a and 5b are enlarged sections through the control piston of Fig. 5, whereas Figure Sc is an end view of the control piston.

As shown in Fig. 2, the piston and cylinder assembly comprises a cylinder 10 containing a control piston 11, the piston being mounted on the inner end of a piston rod 1 2 which extends through a central aperture 1 3a of an end wall 1 3 of the cylinder. The cylinder is closed at its opposite end by an end wall 14 which is permanently secured in the end of the cylinder and is integral with a trunion 1 5 for the pivotal support of the cylinder at that end. A seal 1 6 is provided between the end wall 14 and the adjacent part of the cylinder.

Slidably mounted on the piston rod is a sleeve-shaped free piston 1 7 which engages the wall of the cylinder through ring seals 1 8.

A coil spring 20 acting between the free piston 1 7 and the end wall 1 3 of the cylinder biasses the free piston 1 7 resiliently toward the control piston.

The control piston thus divides the interior of the cylinder into a first chamber 21 on the forward side of the control piston, and a second chamber 22 on the rearward side of the control piston and surrounding the piston rod. The first chamber is closed at its opposite end by end wall 1 4 and the second chamber is closed at its opposite end by the free piston 17.

A passageway for the flow of grease from one side of the control piston to the other is formed by one or more apertures 11 a in the control piston or by one or more slots 11 b formed in the periphery of the control piston.

The piston rod, at its outer end, is formed with a second trunion 23 for pivotally supporting the opposite end of the piston and cylinder assembly.

In operation, if the trunnions 15, 23 are caused to move towards each other, for example by closing the boot lid of a vehicle, the control piston is caused to move in a sense to shorten the length of the first chamber 21 compressing the grease therein and causing it to be expelled from the first chamber 21 through or around the drive piston into the second chamber 22. Since the volume per unit length of the first chamber is greater than the volume per unit length of the second chamber (by virtue of the space occupied by the piston rod), movement of the control piston 11 causes the free piston 1 7 to be pushed away fromn the control piston 11 against the return force of the spring 20.

The force applied between the ends of the assembly to move them towards each other must be sufficient to overcome the force of the spring and the viscosity of the grease. The fully contracted position of the assembly is illustrated in Fig. 2 wherein the control piston is seen to be located close to the end wall 14 of the cylinder, and the spring 20 is fully compressed.

If then the force used to move the trunions towards each other is removed, and possibly with the application of a smaller force in the opposite direction, the potential energy of the spring will cause the spring to expand urging the free piston 1 7 toward the control piston 11 and causing the grease within the second chamber 22 to be expelled through the restricted passageway of the control piston 11 back into the first chamber 21.

Let us assume that the control piston of diameter D, mounted on a piston rod of diameter d, moves a distance L forwardly causing the free piston to move rearwardly through a distance 1, then

bearing in mind that the displacement of the free piston results from the rearward displacement of a volume of grease equal to the volume swept by the forward movement of the piston rod. The remainder of the grease in distance L is merely transferred from one side of the control piston to the other without affecting the displacement of the free piston.

Therefore the displacement ratio

Furthermore if the load in the compressed spring is P, the force acting on the piston rod is Pl L In the illustrated embodiment, as an example, for a cylinder of 16.0 units internal diameter and a piston rod diameter of 8.0 units, the displacement ratio is 3:1, or in other words the amount of grease displaced by a contraction of the length of the first chamber 21 by 3 units will enter the second chamber 22 and compress the spring through a distance of one unit. This displacement ratio can, however, be varied according to individual requirements by varying the diameter of the pisotn rod and the diameter of the cylinder.

In order to provide an easier flow of grease through the control piston in one direction than the other, a floating valve can be provided on one side of the control piston to further restrict the passageway through the control piston when pressure is applied to the valve on that side of the control piston.

The control piston 11 illustrated in Fig. 1 has such a floating valve 25 located on the side of control piston away from the free piston 1 7 so as to create a greater resistance to flow of grease when the assembly is being contracted than when it is being expanded.

In the embodiment of Fig. 3, the control piston 11 is seen to be spaced from the surrounding wall of the cylinder at intervals to leave peripheral slots 1 9 for the flow of grease.

In the embodiment of Fig. 4, the control piston is provided with a resilient cup shaped valve 26 extending on the side away from the piston rod to create a greater resistance to flow of grease when the assembly is being contracted than when it is being expanded.

During contraction, grease flows through holes in the base of the control piston, where during expansion, grease flows through the same holes and, by deflecting the resilient wall of the piston, also around its periphery.

In the embodiment of Fig. 5, the free piston takes the form of a disc-shaped free piston 11 7 which is located between the control piston 11 and the end wall 14 of the cylinder so that the free piston 117 forms an end wall of the chamber 21 opposite the control piston 11. The spring 20 in this embodiment acts between the disc-shaped free piston 11 7 and the end wall 14.

In the details shown in Figs. 5a, 5b and 5c, it will be seen that the control piston 11 is provided with peripheral slots 11 b and inwardly spaced apertures 11 a, as well as with an annular resilient flap valve 27. The valve 27 is arranged to be displaced by the flow of grease through apertures 11 a from the rearward side of piston 11 to the forward side thereof, but closes the apertures 11 a in response to a flow of grease in the opposite direction. This therefore means that there is substantially greater resistance to flow in the latter direction. Peripheral slots 11 b, however, are open at all times.

In operation of the embodiment of Fig. 5, movement of the control piston to shorten the length of first chamber 21 displaces grease through the piston to the second chamber 22 on its rearward side thereby increasing the combined lengths of the first and second chambers. This overall increase in length is absorbed by movement of the free piston 11 7 against the biassing force of spring 20. After the assembly has been fully contracted, the potential force of spring 20 will urge free piston 11 7 towards the control piston 11 forcing grease to flow from the second chamber into the first chamber thereby expanding the assembly, as soon as it is free to expand.

Using the same terminology as was used in the case of Fig. 1, if the control piston moves forward by a distance L, the free piston is moved by a distance 1, where Lid2 = 1 D2 and the displacement ratio is

If, as before, D is 16 units and d is 8 units, the displacement ratio for the assembly is 4:1.

Although the piston and cylinder assembly has been shown and-described as containing a spring arranged to bias the assembly towards an expanded state, the spring could be arranged to bias the assembly towards a contracted state.

Claims

1. A cylinder and piston assembly capable of storing energy and releasing the energy to lift one external member relative to another said assembly comprising means defining a hollow cylinder including a first end closure at one end thereof, first connection means external to the cylinder and rigid with said first end closure and serving to connect the assembly to an external member, a second end closure at the other end of the hollow cylinder having an aperture therein co-axial with the cylinder longitudinal axis, a control piston slidable within the cylinder having at least one flowlimiting aperture therein, a piston rod connected to the piston, the rod passing slidably through the aperture-in the second closure, said piston rod carrying at an end thereof external to the cylinder, second connection means serving to connect the assembly to another external member, a free piston slidable within the cylinder independently of the control piston and of the piston rod, spring means resiliently loading the free piston, and a charge of non-gaseous flowable material within the cylinder, said charge being contained within a first chamber on one side of the control piston and within a second chamber on the other side of the control piston, said free piston additionally serving to limit one of the chambers, said flow-limiting aperture serving to interconnect the said chambers for flow in either direction in dependence upon external loading on the said first and second connection members, the relative axial lengths of the chambers being thereby varied, the spring means acting on the free piston serving to oppose flow of the flowable material in one direction between the chambers, the difference in the axial lengths of said chambers resulting from the movement of the control piston and piston rod relative to the cylinder resulting in a corresponding movement of the free piston proportional to the volume of damping fluid displaced by the piston rod.

2. An assembly according to claim 1, wherein the free piston has a central aperture and is slidable on the piston rod, said free piston having first sealing means cooperating with the internal wall of the cylinder, second sealing means cooperating with the external periphery of the piston rod, and means for receiving one end of the spring means.

3. An assembly according to claim 1 or claim 2, wherein the piston rod has a reduced diameter end portion and a terminal stop, the control piston being slidable on said reduced diameter portion and, having peripheral, restricted slots which are permanently open to flow of the non-gaseous flowable material and having sector-shaped openings surrounding said reduced diameter portion and tapering so that when the control piston abuts the terminal stop flow can take place through the sector-shaped openings and when the control piston abuts the shoulder formed between the main portion of the piston rod and the end reduced diameter portion no flow can occur except through said slots in the periphery of the piston.

4. An assembly according to claim 1 or claim 2, wherein the piston rod has a reduced diameter end portion and a terminal stop at the free end of said reduced diameter end portion, the control piston including a rigid disk with peripheral permanently open slots and apertures arrayed around the said end portion of the piston rod, and a flexible, deformable, disk mounted on the said end portion which co-operates with the apertures in the rigid disk to prevent flow of the nongaseous flowable material in one direction of motion of the control piston and permit flow in the opposite direction of motion of the control piston.

5. An assembly according to claim 1 wherein the free piston has a central aperture and is slidable on the piston rod, said free piston having, first sealing means cooperating with the internal wall of the cylinder, second sealing means cooperating with the external periphery of the piston rod, and means for receiving one end of the spring means, said control piston being in the form of a cup with a flexible side wall and the base having permanently open apertures, the flexible side wall permitting passage of the non-gaseous flowable material in one direction of motion and preventing such flow in the opposite direction, flow and hence motion being limited to passage through the permanently open apertures.

6. An assembly according to claim 4 wherein the free piston is slidable within the cylinder within the first chamber.

7. An assembly according to any one of the preceding claims, wherein, in operation, the spring means is compressed through a distance of one unit in response to displacement of the control piston and piston rod through a distance in the range three to four units.

8. A cylinder and piston assembly capable of storing energy and releasing the energy to lift one external member relative to another said assembly comprising a hollow cylinder including a first end closure at one end thereof first connection means external to the cylinder and rigid with said first end closure and serving to connect the assembly to an external member, second end closure at the other end of the hollow cylinder having an aperture therein co-axial with the cylinder longitudinal axis, a control piston slidable within the cylinder having at least one flowlimiting aperture therein, a piston rod connected to the piston, the rod passing slidably through the aperture in the second closure, said piston rod carrying at an end thereof external to the cylinder, second connnection means serving to connect the assembly to another external member, a free piston with a central aperture slidable on the piston rod and within the cylinder independently of the control piston, spring means resiliently loading the free piston towards the control piston, and a charge of non-gaseous flowable material within the cylinder, said charge being contained within a first chamber on one side of the control piston and within a second chamber on the other side of the control piston, said free piston additionally serving to define the second chamber, said flow-limiting aperture serving to interconnect the said chambers for flow in either direction in dependence upon external loading on the said first and second connection members, the relative axial lengths of the chambers being thereby varied, the spring means acting on the free piston serving to oppose flow of the flowable material in one direction between the chambers, the difference in the axial lengths of said chambers resulting from the movement of the control piston and piston rod relative to the cylinder resulting in a corresponding movement of the free piston proportional to the volume of damping fluid displaced by the piston rod.

9. A cylinder and piston assembly capable of storing energy and releasing the energy to lift one external member relative to another said assembly comprising a hollow cylinder including a first end closure at one end thereof, first connection means external to the cylinder and rigid with said first end closure and serving to connect the assembly to an external member, a second end closure at the other end of the hollow cylinder having an aperture therein co-axial with the cylinder longitudinal axis, a control piston slidable within the cylinder having at least one flowlimiting aperture therein, a piston rod connected to the piston, the rod passing slidably through the aperture in the second closure, said piston rod carrying at an end thereof external to the cylinder, second connection means serving to connect the assembly to another external member, a free piston slidable within the cylinder independently of the control piston and spaced from the control piston between the piston and the first end closure, spring means resiliently loading the free piston, and a charge of non-gaseous flowable material within the cylinder, said charge being contained within a first chamber on one side of the control piston and within a second chamber on the other side of the control piston, said free piston additionally serving to limit the first chamber, said flowlimiting aperture serving to interconnect the said chambers for flow in either direction in dependence upon external loading on the said first and second connection members, the relative axial lengths of the chambers being thereby varied, the spring means acting on the free piston serving to oppose flow of the flowable material in one direction between the chambers, the difference in the axial lengths of said chambers resulting from the movement of the control piston and piston rod relative to the cylinder resulting in a corresponding movement of the free piston proportional to the volume of damping fluid displaced by the piston rod.

10. A cylinder and piston assembly substantially as hereinbefore described with reference to Figs. 1 and 2; 3; 4; or 5, 5a and 5b of the accompanying drawings.