EP0442908B1

EP0442908B1 - Fluid-power cylinder provided with a cushioning device

Info

Publication number: EP0442908B1
Application number: EP89912017A
Authority: EP
Inventors: Sven-Erik Overstrom
Original assignee: OVERSTROM SVEN ERIK
Current assignee: OVERSTROM SVEN ERIK
Priority date: 1988-11-08
Filing date: 1989-11-02
Publication date: 1994-04-20
Anticipated expiration: 2009-11-02
Also published as: DE68914848D1; SE8804035L; CA2002278A1; SE8804035D0; SE465529B; WO1990005245A1; DE68914848T2; EP0442908A1

Abstract

A fluid-power cylinder comprising a power cylinder member (11) and a power piston member (12) reciprocable therein between end positions includes a device for cushioning the relative movements of the power piston member and the power cylinder member adjacent the end positions. The cushioning device comprises a cushioning cylinder (13) movable together with the power piston member (12) and a cushioning piston (14) which is stationary with respect to the power cylinder member (11) and which is receivable and axially movable in the cushioning cylinder to define a cushioning cylinder compartment (31A, 31B) having a restricted discharge passage (37/38, 40/41). A seal (36A, 36B) on the cushioning piston (14) acts as a one-way valve to prevent flow of cushioning fluid between the cushioning piston (14) and the cushioning cylinder (13) when the cushioning piston moves inwardly in the cushioning cylinder compartment (31A, 31B) and thereby reduces the volume of the compartment, but allows substantially unrestricted fluid flow when the cushioning piston moves outwardly.

Description

This invention relates to a fluid-power cylinder provided with a cushioning device for cushioning the movements of the piston in the cylinder.
Fluid-power cylinders often require a cushioning device which retards the piston at one or both ends of its stroke within the cylinder. In most cushioning devices, a residual volume of fluid trapped in a cushioning compartment is allowed to escape from the cushioning compartment only through a restricted discharge passage in order to retard the movement of the piston during the final portion of its stroke.
In a prior art cushioning device of this kind (SE-B-347046), a single cushioning piston is provided on one side of a power piston member which is reciprocable between opposite end positions in a power cylinder member. As the power piston member approaches one end position, the cushioning piston is received in a cushioning cylinder provided at one end of the power cylinder member so that fluid is trapped in a cushioning compartment defined by the cushioning cylinder. A restricted discharge passage for the trapped fluid is formed by grooves or bores which communicate with the narrow annular gap between the cushioning piston and the wall of the cushioning cylinder.
A problem which is present in this and many other prior art cushioning devices is that the cushioning effect is dependent on the dimensional accuracy of components which move relative to one another and define the restricted discharge passage between them. Wear of these components also affects the cushioning effect which may therefore be gradually reduced in the course of time.
Another problem which is present in many prior art cushioning devices is that it is difficult to adjust the cushioning effect such that the desired cushioning is achieved for both end positions or stroke ends.
An object of this invention is to provide a fluid-power cylinder provided with a cushioning device which is improved in respect of the above-mentioned problems.
According to the invention, there is provided a fluid-power cylinder comprising a power cylinder member, a power piston member, which includes a power piston body reciprocable in the power cylinder member between opposed end positions and a piston rod secured to the power piston body and projecting from one end of the power cylinder member, and a device for cushioning the relative movements of the power cylinder member and the power piston member at the end positions, characterised by the following features:

the cushioning device comprises (a) for each end position a cushioning piston on the power cylinder member and a cushioning cylinder on the power piston member, the cushioning cylinder defining a cushioning compartment having a closed inner end and an open outer end in which the cushioning piston is receivable when it approaches the end position, and (b) a cushioning fluid reservoir in constant fluid flow communication with both cushioning compartments, a restricted discharge passage providing the sole path for escape of cushioning fluid from the associated cushioning compartment to the cushioning fluid reservoir when the cushioning piston is received in the cushioning cylinder,
the piston rod of the power piston member is hollow, its interior being in constant unrestricted communication with the cushioning fluid reservoir,
the cushioning cylinders are located within the piston rod of the power piston member and axially spaced apart, adjacent cylinder ends being open to define the open outer ends of the cushioning compartments,
each cushioning piston is provided exteriorly with a fluid seal adapted to sealingly engage the interior wall of the associated cushioning cylinder when the cushioning piston is moving in the cushioning cylinder towards the inner end of the cushioning compartment and to permit substantially unrestricted fluid flow between the cushioning piston and the interior wall of the cushioning cylinder when the cushioning piston is moving in the cushioning cylinder towards the outer end of the cushioning compartment, and
the discharge passage is a passage which is provided in the cushioning piston and bypasses the fluid seal.

Other objects, features and advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the drawing.

Fig. 1 is a longitudinal sectional view of a fluid-power cylinder embodying the invention;
Fig. 2 is an enlarged longitudinal sectional view of a portion of the fluid-power cylinder of Fig. 1.

As shown in the drawing, the invention is embodied in a double-acting pneumatic cylinder provided with a hydraulic device for cushioning the stroke of the power piston member of the cylinder at both end positions or stroke ends. Two separate cushioning cylinders are provided within a tubular piston rod of the power piston member and reciprocate therewith, and a dual cushioning piston is stationary with respect to the power cylinder member in which the power piston member is reciprocable. It is to be understood, however, that the invention may be embodied in other types of fluid-power cylinders, e.g. cylinders in which the same kind of fluid is used both as working fluid and cushioning fluid.
The illustrated fluid-power cylinder comprises a power cylinder member generally designated by 11 and a power piston member 12 which is reciprocable in the power cylinder member. It also comprises a hydraulic cushioning device including a pair of cushioning cylinders 13, a dual cushioning piston 14 with an associated support tube 15 and a cushioning liquid reservoir 16.
Main components of the power cylinder member 11 are a cylindrical tube 20 of circular cross-section and front and rear end walls 21 and 22, respectively, which sealingly close the ends of the tube and are provided with connectors 23 and 24, respectively, through which the working fluid, namely compressed air, is supplied and discharged. The rear end wall 22 is made integral with the cushioning liquid reservoir 16. Mounted on the reservoir 16 are a pair of aligned pivot pins 25 by which the power cylinder member 11 may be pivotally mounted on a bracket (not shown).
The power piston member 12 comprises a power piston body 26 which sealingly engages the inner wall of the cylinder tube 20 and divides the cylinder space between the end walls into front and rear cylinder compartments 27 and 28, respectively, which communicate with respectively the front air connector 23 and the rear air connector 24, and a tubular piston rod 29, one end of which, the rear end, is secured to the power piston body and projects forwardly through a sliding seal in the front end wall 21 of the power cylinder member 11. Secured to the front end of the piston rod 29 is an eye bolt 30, through which the power piston member 12 may be connected to a part to be actuated, such as a door or the like.
The end positions of the reciprocating movements of the power piston member 12 in the power cylinder member 11 are defined by a pair of abutments which are provided on the cylinder end walls 23 and 24 and are adapted to be engaged by the power piston body 26.
The cushioning cylinders 13 of the cushioning device are formed by front and rear cylindrical sleeves which are secured in the hollow piston rod 29 adjacent respectively the front and the rear end thereof. The adjacent ends of the two sleeves are internally bevelled or widened. Together with the front part of the piston rod 29 and the power piston body 26 the sleeves 13 define a pair of cushioning compartments 31A, 31B, which are open towards the interior 32 of the piston rod at their outer, adjacent ends but are otherwise closed.
The dual cushioning piston 14 is secured to the front end of the support tube 15 which in turn has its rear end secured to the cylinder end wall 22 and is in constant open communication with a cushioning liquid compartment 33 in the cushioning liquid reservoir 16. The support tube 15 extends through a sliding seal 34 in the power piston body 26 and through the rear cushioning cylinder sleeve 13.
The cushioning piston 14 comprises a front piston section 14A and a rear piston section 14B. When the power piston member 12 is moved inwardly to its inner end position in the power cylinder member 11 as shown in the drawing, the front cushioning piston section 14A is moved inwardly in the front cushioning cylinder compartment 31A to the vicinity of the inner end or bottom of the compartment. Similarly, in the outer end position of the power piston member 12, the rear cushioning piston section 14B is moved inwardly in the rear cushioning cylinder compartment 31B to the vicinity of the inner end or bottom thereof.
The dual cushioning piston 14 comprises a dumbbell-shaped metal body, the enlarged end portions of which form the front and rear cushioning piston sections 14A, 14B and have a diameter slightly smaller than the inner diameter of the cushioning cylinder sleeves 13. The difference in diameters is sufficient to permit a substantially unrestricted liquid flow through the gap between each cushioning cylinder sleeve 13 and the associated cushioning piston section 14A, 14B received therein.
Each cushioning piston section 14A, 14B is provided with an external circumferential groove 35A, 35B, which accommodates a frusto- conical sealing ring 36A, 36B of relatively soft material. The sealing rings are opposed such that their larger ends, which are adapted to engage sealingly the interior wall of the cushioning cylinder sleeves 13, are directed away from one another, i.e. towards the inner ends of the cushioning compartments.
Extending through entire cushioning piston 14 is a central axial passage 37 which is in constant open communication with the interior of the support tube 15 and thus with the cushioning liquid compartment 33. Inserted in the front end of this passage is a calibrated restrictor 38. By way of a transverse passage 39 the central portion of the passage 37 is in constant unrestricted communication with the space around the cushioning piston.
From the rear end face of the rear piston section 14B extends an axial passage 40, which is in constant unrestricted communication with the piston rod space 32 and is in constant but restricted communication with the space around the cushioning piston, the last-mentioned communication being by way of a calibrated restriction 41 located between the two sealing rings a small distance from the sealing ring 36B.
The piston rod space 32 and the spaces communicating with it are always filled with the cushioning liquid. When the power piston 12 moves outwardly relative to the power cylinder member 11 from the illustrated inner end position under action of compressed air supplied to the rear power cylinder compartment 28, the front cushioning cylinder compartment 31A defined in front of the front cushioning piston section 14A expands. During this movement, the cushioning compartment 31A is successively filled with cushioning liquid which flows against very little resistance into the compartment past the sealing ring 36A because this sealing ring is pressed into its groove 35A. Thus, the sealing ring 36A acts as a valve (one-way valve) which opens automatically to permit liquid flow into the front cushioning cylinder compartment 31A.
When the power piston member 12 approaches its outer end position, the rear cushioning piston section 14B is received in the rear cushioning cylinder sleeve 13. The rear sealing ring 36B, because of its frusto-conical shape, is caused to engage sealingly the inner wall of the sleeve (one-way valve action). The only path through which the cushioning liquid trapped in the rear cushioning cylinder compartment 31B can escape is therefore formed by the passage 40 containing the calibrated restriction 41. By virtue of its cross-sectional area, this restriction determines the rate at which the power piston member 12 continues its movement to the outer end position.
Similarly, during the retraction of the power piston member 12 to its inner end position, the calibrated restrictor 38 in the front cushioning piston section 14A determines the rate at which the final portion of this retraction takes place.
In use, there is no wear of those elements, namely the restrictors 38 and 41, of the cushioning device which determine the speed at which the power piston member 12 moves near the end positions, assuming that the forces acting on the power piston member 12 are given. These elements therefore do not undergo any change in respect of their predetermined cross-sectional flow area during the useful life of the cushioning device. Moreover, because it is also easy to design the restrictors with the desired cross-sectional flow area and to ensure that the cushioning liquid trapped in the cushioning cylinder compartments cannot escape except through the restrictors, the cushioning device can be designed to provide the desired cushioning effect. Moreover, it is easy to ensure that any wear that takes place does not cause the cushioning effect to change over the time in a way that is difficult to compensate. If the cushioning effect of the device should be reduced because of wear of the sealing rings of the cushioning pistons, the desired cushioning effect can readily be restored by replacement of the sealing rings.
In the illustrated embodiment the calibrated restrictors are fixed, but it is of course within the scope of the invention to make them adjustable.
The cushioning liquid reservoir 16, the main body of which is made integral with the rear cylinder end wall 22, has a substantially circular cylindrical exterior shape and is sealingly closed at its rear end by means of a screw-threaded cover 42.
The two aligned pivot pins 25 are secured to a circular clamp 43 which is held in position about the cushioning liquid reservoir 16 by means of a bolt 44 which is passed through a pair of outwardly bent flanges 45 of the clamp. The clamp 43 is secured axially in its position, and possibly also against rotation, on the cushioning liquid reservoir by a ridge 46 on the inner side of the clamp or by any other suitable projection which engages a corresponding groove or recess 47 on the outer side of the cushioning liquid reservoir 16.

Claims

A fluid-power cylinder comprising
a power cylinder member (11),
a power piston member (12), which includes a power piston body (25) reciprocable in the power cylinder member between opposed end positions and a piston rod (29) secured to the power piston body and projecting from one end of the power cylinder member, and
a device for cushioning the relative movements of the power cylinder member (11) and the power piston member (12) at the end positions,
characterised in that
the cushioning device comprises
(a) for each end position a cushioning piston (14A,14B) on the power cylinder member (11) and a cushioning cylinder (13) on the power piston member (12), the cushioning cylinder (13) defining a cushioning compartment (31A,31B) having a closed inner end and an open outer end in which the cushioning piston is receivable when it approaches the end position, and

(b) a cushioning fluid reservoir (16) in constant fluid flow communication with both cushioning compartments (31A,31B), a restricted discharge passage (37/38,40/41) providing the sole path for escape of cushioning fluid from the associated cushioning compartment to the cushioning fluid reservoir when the cushioning piston is received in the cushioning cylinder,
the piston rod (29) of the power piston member (12) is hollow, its interior being in constant unrestricted communication with the cushioning fluid reservoir (16),
the cushioning cylinders (13) are located within the piston rod of the power piston member (12) and axially spaced apart, adjacent cylinder ends being open to define the open outer ends of the cushioning compartments (31A,31B),
each cushioning piston (14A,14B) is provided exteriorly with a fluid seal (36A,36B) adapted to sealingly engage the interior wall of the associated cushioning cylinder (13) when the cushioning piston is moving in the cushioning cylinder towards the inner end of the cushioning compartment (31A,31B) and to permit substantially unrestricted fluid flow between the cushioning piston and the interior wall of the cushioning cylinder when the cushioning piston is moving in the cushioning cylinder towards the outer end of the cushioning compartment, and
the discharge passage (37/38,40/41) is a passage which is provided in the cushioning piston and bypasses the fluid seal (36A,36B).
A fluid-power cylinder according to claim 1, characterised in that the discharge passage (37/38,40/41) includes a calibrated restrictor (38,41).
A fluid-power cylinder according to claim 2, characterised in that the restrictor (38) is provided on a restrictor member replaceably mounted on the cushioning piston (14A).
A fluid-power cylinder according to any one of claims 1-3, characterised in that the fluid seal (36A,36B) is engageable with the interior wall of the cushioning cylinder (13) under action of fluid pressure acting axially on the fluid seal and directed towards the inner end of the cushioning compartment (31A,31B), and displaceable away from that wall under action of fluid pressure acting axially on the fluid seal towards the outer end of the cushioning compartment.
A fluid-power cylinder according to claim 4, characterised in that the fluid seal (36A,36B) is a frusto-conical ring of resilient material, the larger end of the ring facing the inner end of the cushioning compartment (31A,31B).
A fluid-power cylinder according to any one of claims 1 to 5, characterised in that the cushioning fluid reservoir (16) forms an essentially cylindrical axial extension of the other end of the power cylinder member (11).
A fluid-power cylinder according to claim 6, characterised in that the major portion of the cushioning fluid reservoir (16) is made integrally with an end wall (22) of the power cylinder member (11).