GB2113798A

GB2113798A - An hydraulic damper

Info

Publication number: GB2113798A
Application number: GB08201814A
Authority: GB
Inventors: Lawrence George Nicholls
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-01-22
Filing date: 1982-01-22
Publication date: 1983-08-10
Also published as: FR2520464A1

Abstract

An hydraulic damper for controlling movement between relatively movable parts of a hospital bed or the like comprises a piston 4, carried by a piston-rod 6, working in a sealed, liquid-filled chamber 13 within a cylinder 1. The piston 4 includes at least one axially extending passage 16 permitting an unrestricted flow of liquid through the piston 4. During movement of the piston 4 in the cylinder 1 in one direction a relatively movable valve plate 17 engages with the piston 4, and a restricted orifice in the plate 17 permits a restricted flow through the passage 16. During movement in the opposite direction the plate 17 is spaced from the piston 4, permitting a free flow. The chamber 13 is expansible to compensate for expansion and/or displacement of the liquid, one end of the chamber 13 comprising an axially movable annular seal 10 through which the rod 6 passes. A spring 14 urges the seal 10 towards the other end of the chamber 13 to pressurise the liquid. <IMAGE>

Description

SPECIFICATION An hydraulic damper This invention relates to an improved hydraulic damper of the kind in which damping is produced by the restricted flow of liquid between opposite ends of the cylinder in which works a piston carried by a piston-rod which, in turn, extends through one end of the cylinder.

Dampers of the kind set forth are used to control movement between relatively movable parts of hospital beds, accident trollies or the like, for example to control movement of the patient supporting platform into the Trendelenburg tilt positon.

The said movement may be controlled by dampers of the kind set forth in which the liquid is pressurised by gas or air under pressure. Such dampers are unsatisfactory in maintaining the desired damping characteristics in the event of the excape of the gas or air to atmosphere, such as upon failure of a seal.

According to our invention in a damper of the kind set forth the piston is provided with a valve which permits a restricted flow of liquid between opposite ends of the cylinder upon movement of the piston in the cylinder in one direction and a free flow of liquid between the ends upon movement of the piston in the opposite direction, and the liquid is contained in the cylinder between an annular seal through the piston-rod works and a closure for the end of the cylinder on the opposite side of the piston, the seal having an outer peripheral edge in slidable sealing engagement with the cylinder and an inner peripheral edge in slidable sealing engagement with the piston-rod, and a spring acts to urge the seal inwardly towards the closure in order to pressurise the liquid in the cylinder, the seal being movable with respect to the cylinder to compensate for expansion of the fluid and/or displacement of the same due to the volume of the piston-rod.

Since the pressurisation of the liquid depends entirely upon the load in the spring pressurisation of the liquid will be maintained even in the event of leakage of liquid past the seal which will result in external leakage, visible to the user.

The spring may act between the moveable seal and a closure for the adjacent end of the cylinder with the closure acting as a guide for the pistonrod but permitting the free passage of air into a chamber within the cylinder in which the spring is housed.

Preferably the valve is of known type comprising a plate which is movable relative to the piston, and the plate is provided with a restricted orifice to permit a restricted flow through passages in the piston when the plate is in engagement with the piston which occurs during movement of the piston in the said one direction, the plate being spaced from the piston to permit free flow the passages during movement of the piston in the said opposite direction.

When the damper is used to control movement between relatively movable parts of a hospital bed, we arrange for the restricted flow to occur during contraction of the damper which may occur as a patient supporting platform is returned to a normal horizontal position.

One embodiment of our invention is illustrated in the single Figure of the drawing which is a longitudinal section through an hydraulic damper.

The hydraulic damper illustrated in the drawing comprises a cylinder 1 which is closed at one end by a closure 2 provided with a connecting eye 3, and a piston 4 adapted to work in the bore 5 of the cylinder 1 is carried by a piston-rod 6 which is guided to slide through a closure bush 7 for the opposite end of the cylinder 1. A connecting eye 8 is carried by the free, outer, end of the piston-rod 6 and a tubular dirt shield 9 for enclosing the portion of the piston-rod 6 which projects from the bush 7 projects inwardly from the eye 8 and overlaps the adjacent end of the cylinder 1.

An annular seal 10 of elastomeric material is arranged within the cylinder 1, between the bush 7 and the piston 4. The seal 10 is provided with radially spaced double dynamic sealing lips 11 and 1 2 which are directed towards the piston 4 with the outer lip 11 slidably engaging sealingly with the bore 5, and the inner lip 1 2 slidably engaging sealingly with the piston-rod 6.

A damper chamber 1 3 is defined within the cylinder 1, between the seal 10 and the closure 2, and the chamber 1 3 is filled with a liquid, suitably hydraulic fluid.

A compression spring 14 of tapered outline acts at its inner end of smaller diameter on abutment plate 1 5 carried by the seal 1 O at that end, and at its outer end of greater diameter abuts against the bush 7. The load in the spring 14 acts through the seal 10 to pressurise the liquid in the chamber 1 3.

The portion of the cylinder 1 in which the spring 1 4 is housed is in free communication with atmosphere, for example through passages in the bush 7.

The piston 4 is provided with radially spaced axially extending ports 1 6 which permit an unrestricted flow of liquid through the piston 4, and with a valve plate 1 7 which is movable through a limited distance with respect to the inner end of the piston 4, which is adjacent to the closure 2, in order to control flow through the ports 1 6. The plate 1 7 is provided with a restricted orifice (not shown) which permits a restricted flow of liquid through the piston 4 as the damper is contracted with the piston 4 moving inwardly towards the closure 2 and the valve plate 1 7 in engagement with the adjacent face of the piston 4 otherwise to prevent flow through the ports 1 6.

When the piston 4 is moved in the opposite direction on extension of the damper the plate 17 moves relatively away from the piston 4 so that an unrestricted flow can take place through the ports 16.

The seal 1 Moves in the bore 5 to compensate for expansion and contraction of the liquid and to accommodate changes in the effective volume of the liquid, due to the piston-rod 6. For example, upon contraction of the damper, the seal 10 moves towards the bush 7 against the ioad in the spring 14 to accommodate liquid displaced by the volume of the piston-rod 6 as it enters the chamber 1 3.

Our damper is suitable for controlling movement of the patient supporting platform of a hospital bed, accident troiley and the like with respect to a wheel mounted chassis. The eyes 3 and 8 are pivotally connected between the two parts in a position such that the chamber is extended freeiy as the platform is moved into the Trendelenburg tilt position, but a damping effect is achieved by restricted flow through the orifice in the plate 1 7 as the platform is returned to a horizontal position.

Claims

CLAIMS 1. A damper of the kind set forth in which the piston is provided with a valve which permits a restricted flow of liquid between opposite ends of the cylinder upon movement of the piston in the cylinder in one direction and a free flow of liquid between the ends upon movement of the piston in the opposite direction, and the liquid is contained in the cylinder between an annular seal through the piston-rod works and a closure for the end of the cylinder on the opposite side of the piston, the seal having an outer peripheral edge in slidable sealing engagement with the cylinder and an inner peripheral edge in slidabie sealing engagement with the piston-rod, and a spring acts to urge the seal inwardly towards the closure in order to pressurise the liquid in the cylinder, the seal being movable with respect to the cylinder to compensate for expansion of the fluid and/or displacement of the same due to the volume of the piston-rod. 2. A damper as claimed in claim 1, in which the spring acts between the movable seal and a closure for the adjacent end of the cylinder with the closure acting as a guide for the piston-rod but permitting the free passage of air into a chamber within the cylinder in which the spring is housed. 3. A damper as claimed in claim 1 or claim 2, in which the valve comprises a plate which is movable relative to the piston, and the plate is provided with a restricted orifice to permit a restricted flow through passaages in the piston when the plate is in engagement with the piston which occurs during movement of the piston in the said one direction, the plate being spaced from the piston to permit free flow through the passages during movement of the piston in the said opposite direction. New claims filed on 16/12/82 Superseded claims 1-3 New or amended claims:

1. An hydraulic damper of the kind set forth in which the piston is provided with at least one axially extending passage permitting an unrestricted flow of liquid through the piston, and a valve plate which is movable relative to the piston, the valve plate being provided with a restricted orifice to permit a restricted flow of liquid through the passage when the valve plate is in engagement with the piston which occurs during movement of the piston in the cylinder in one direction, and the valve plate being spaced from the piston to permit a free flow of liquid through the passage during movement of the piston in the opposite direction, the liquid being contained in the cylinder between an annular seal through which the piston-rod works and a closure for an end of the cylinder on the opposite side of the piston, the seal having an outer peripheral edge in slidable sealing engagement with the cylinder and an inner peripheral edge in slidable sealing engagement with the piston-rod, and a spring acts to urge the seal inwardly towards the closure in order to pressurise the liquid in the cylinder, the seal being movable with respect to the cylinder to compensate for expansion of the liquid and/or displacement of the same by the volume of the piston-rod.

2. An hydraulic damper as claimed in Claim 1, in which the spring acts between the movable seal and a closure for the said one end of the cylinder with the closure acting as a guide for the pistonrod but permitting the free passage of air into a chamber within the cylinder in which the spring is Housed

3. An hydraulic damper as claimed in Claim 2, in which the spring comprises a compression spring of tapered outline acting at its inner end of smaller diameter on an abutment plate carried by the seal, and abutting at its outer end of greater diameter against the closure for the said one end of the cylinder.

4. An hydraulic damper as claimed in any preceding claim, in which the seal is provided with outer and inner radially spaced double dynamic sealing lips which are directed towards the piston, with the outer lip slidably engaging sealingly with the cylinder and the inner lip slidably engaging sealingly with the piston-rod.

5. An hydraulic damper as claimed in any preceding claim, in which a tubular dirt shield projects inwardly from the free, outer end of the piston-rod and overlaps the said one end of the cylinder to enclose the portion of the piston-rod which projects from the cylinder.

6. An hydraulic damper as claimed in any preceding claim, in which the restricted flow of liquid takes place during contraction of the damper.

7. An hydraulic damper substantially as herein described with reference to and as illustrated in the accompanying drawing.