IE20190074A1 - A cushioning device - Google Patents

Abstract

A cushioning device for a hydraulic cylinder comprises an outer bushing 7, an inner bushing 5, a cushioning chamber 19 between the bushings, and a compression spring 6 located in the cushioning chamber 19. The bushings have a flow path 25, 26 for pressure medium to and from he cushioning chamber. The bushings 5, 7 move between an extended configuration in which the bushings are axially spaced-apart a first distance under the bias of the compression spring 6 to define an enlarged cushioning chamber 19, and a retracted configuration in which the bushings 5, 7 are spaced-apart a second distance which is shorter than the first distance and the volume of the cushioning chamber is thereby reduced. The bushings 5,7 are mounted to the internal end 27 of a piston rod 11 of a hydraulic cylinder and the bushings 5, 7 are movable relative to one another as the piston rod 11 approaches the end of the stroke to bring the piston rod 11 to a gradual stop.

Description

“A Cushioning Device” Introduction The invention relates to a cushioning device for use in hydraulic cylinders. The cushioning 5 device is particularly suitable for use in displacement or plunger cylinders.

Cushioning systems for double acting cylinders are well known. Double acting cylinders have two ports and two separate pressure chambers; one at either side of a piston head which hydraulically separate each chamber from the other. Typically, in a double acting cylinder one 10 of these ports will be in the exhaust position when the other port is in the pressure position. This common cylinder design allows a cushioning effect to be created by selectively restricting the pressure medium flow at exhaust port. One undesirable side feature of some of the known systems is that an elevated pressure “spike” is created in the pressure medium when the cushioning is engaged. This pressure spike is exerted on the piston seal and reduces its life 15 expectancy.

In the case of displacement or plunger cylinders, while they may have a ring on the piston rod, which acts as a guide and also acts as a stroke limit, they do not have any piston head in the hydraulic sense. In a displacement cylinder, the complete cylinder space at both sides of the 20 guide ring is a common pressure vessel and therefore there is only one single pressure chamber into which pressure medium is pumped through one or more ports. The displacement cylinder has no hydraulic power in the opposite direction but relies on an external force or weight to retract it to its original closed length. The oil, or other pressure medium, is returned from the single chamber into which it was injected. Therefore the common method used to restrict the 25 exhaust flow from the second port as in double acting cylinders is not possible for a displacement or plunger cylinder.

Furthermore, it will be apparent that since a displacement cylinder is a single pressure vessel, the position of the inject/exhaust port can be located at any position on the vessel. This fact is 30 widely exploited in order to avail of suitable access when the cylinder is fitted to the machinery in particular applications.

Statements of Invention According to the invention there is provided cushioning device comprising: 14/05/2019 an outer bushing; an inner bushing; the bushings being at least partially axially movable relative to one another; a cushioning chamber between the bushings, the bushings having a flow path for pressure medium to and from the cushioning chamber; the bushings being adapted for movement between an extended configuration in which the bushings are axially spaced-apart a first distance to define an enlarged cushioning chamber, and a retracted configuration in which the bushings are spaced-apart a second distance which is shorter than the first distance and the volume of the cushioning chamber is thereby reduced; and a compression spring located in the cushioning chamber between the bushings to bias the bushings into the extended configuration in which the volume of the cushioning chamber is maximised.

In one case the outer bushing is a guide bushing which is adapted to travel axially within a cylinder barrel, and the inner bushing comprises a cushioning bushing at least a portion of which is adapted to travel axially within the guide bushing.

The cushioning bushing may comprise an orifice to control the flow of pressure medium into and 20 out of the cushioning chamber. The orifice may be a tapered orifice which is adapted to be progressively closed as the cushioning bushing is moved to decrease the volume of the cushioning chamber.

In one case the guide bushing comprises a flow path for flow of pressure medium into and out of 25 the cushioning chamber. The flow path may be defined by one or more axially extending grooves in the guide bushing.

The invention also provides a hydraulic cylinder comprising a cushioning device of the invention.

In one case the hydraulic cylinder comprises:a cylinder barrel; a piston rod having an internal end within the cylinder barrel and an external end external of the cylinder barrel; an end cap at one end of the cylinder barrel; 14/05/2019 a gland at the opposite end of the cylinder barrel, the piston rod extending from the cylinder barrel through the gland; the bushings being mounted to the internal end of the piston rod and one bushing being movable relative to the other bushing as the piston rod approaches the end of the stroke to 5 bring the piston rod to a gradual stop.

The piston rod may comprise a journal at the internal end of the piston rod and the bushings are mounted on the journal for movement between the extended and retracted configurations.

In one case the outer bushing is adapted to travel axially within the cylinder barrel and the inner bushing is adapted to travel axially within the outer bushing.

In one case the cylinder comprises an end stop at one end against which one of the bushings is engageable to cushion the stopping of the piston rod.

The cylinder may comprise a first end stop and a second end stop and wherein the outer bushing is engageable against the first end stop to cushion the stopping of the piston rod at one end of a stroke and the inner bushing is engageable against the second end stop to cushion the stopping of the piston rod at an opposite end of a stroke.

Brief Description of the Drawings The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is an isometric exploded view of a cushioning device according to the invention; Fig. 2 is a side elevational view of the cushioning device in one configuration; Fig. 3 is a cross sectional view of the cushioning device in the configuration of Fig, 2; Fig. 4 is a side elevational view of the cushioning device in another configuration; Fig. 5 is a cross sectional view of the cushioning device in the configuration of Fig. 4; 14/05/2019 Fig. 6 is an isometric view from an end of the cushioning device in the configuration of Fig. 4; Fig. 7 is an isometric view of the cushioning device and a piston rod of a hydraulic 5 cylinder; Fig. 8 is a cross sectional view of a hydraulic cylinder with the piston rod fully extended and the cushioning device in one end position; Fig. 9 is a cross sectional view of the hydraulic cylinder with the piston rod fully retracted and the cushioning device in another end position; Fig. 10 is an enlarged view of a detail of Fig. 9; and Fig. 11 is a cross sectional view of the hydraulic cylinder with the piston rod and the cushioning device in an intermediate position.

Detailed Description The cushioning system of the invention comprises a matched pair of cylindrical close fitting 20 bushings which can slide one within the other. The inner bushing has a tapered orifice machined on the outside surface. These two bushings are fitted together with an internal compression coil spring, which in the absence of end pressure on either bushing, forces the two bushings apart and causes them to remain in an ‘open’ position. In the presence of oil or other pressure medium, this action draws oil into the area between the two bushings and around the compression spring.

The pressure medium is now ‘trapped’ in the inner chamber between the two bushings and around the coil spring.

The cushioning device is secured on the inner end of a piston rod and acts as a guide within the barrel and a stop at the out-going end of stroke. On the outward stroke, as the outer bushing 30 begins to make contact with the gland, the compression spring is compressed and the bushings are telescoped into each other, thereby compressing the trapped pressure medium and forcing the trapped medium out of the pressure chamber via the tapered orifice, thus slowing the piston and causing the cushioning effect. As the piston rod and cushioning device move away from the gland into the cylinder, the coil spring pushes the inner and outer bushings apart and draws in 14/05/2019 pressure medium between the two bushings and thereby charges it for the next end of stroke cushioning in either the outward or inward direction. When the piston rod moves in an inward direction and the inner bushing begins to make contact with the bottom end cap, the same cushioning effect takes place as for the gland end. Thus, the cushioning device is equally effective irrespective of which direction the piston is travelling, outwards or inwards.

The two grooves in the outside of the outer bushing provide an unencumbered flow path for the oil or other pressure medium and allow oil to flow from one side of the cushioning device to the other side. The whole cushioning device is securely mounted to the internal end of the piston 10 rod with a round wire circlip.

The cushioning device takes no more space than a guide/stop, which would be required in the absence of the cushioning device.

The cushioning device is used to bring a piston rod to a gradual stop as it approaches the end of its stroke within a cylinder, thereby reducing shock loads. The cushioning device is effective on both the “retraction” and also on the “extension” strokes.

In the drawings the reference numerals represent the following items: Closing cap 1 Cylinder barrel 2 Pressure vessel 3 Circlip 4 Internal cushion bushing 5 Compression spring 6 External guide bushing 7 Piston rod 11 Second seal 12 Circlip 13 Gland 14 Seal 15 Shoulder 16 Wiper seal 18 Cushion chamber 19 14/05/2019 Tapered orifice 25 Grooves 26 Journal 27 Shoulder 28 Weld 30 The cylinder shown is a typical displacement (plunger) type. A closing cap 1 is inserted into a cylinder barrel 2 and sealed with a weld 30. A piston rod 11 slides through a gland 14. The gland 14 has a seal 15 which provides a pressure-tight connection with the piston rod 11. The 10 gland 14 may also be fitted with a wiper seal 18. The wiper seal 18 prevents foreign matter entering the system. The gland 14 is fitted with a second seal in the form of an O-ring 12 which provides a pressure-tight joint between the cylinder barrel 2 and the gland 14. The gland 14 is mechanically retained within the cylinder barrel 2 by a circlip 13. It may also be retained by threading or other means. The internal end of piston rod 11 has a reduced diameter journal 27 15 extending as far as a shoulder 28. This journal 27 is fitted with an external guide bushing 7 and an internal cushion bushing 5. A compression spring 6 extends between the bushings 5, 7. The bushings 5, 7 and the spring 6 are retained on the piston rod 11 by means of a circlip 4. They could also be retained by other means, such as a threaded nut. A port is provided at any suitable location to allow the entry and exit of the pressure medium to the pressure vessel 3.

The pressure medium is injected via a directional control valve (not shown), into the pressure vessel 3 through the inlet port. The pressure medium can flow in both directions past the external guide bushing 7 via two oil grooves 26. The pressure vessel 3 comprises the combined volumes at each side of the guide bushing 7. After the vessel 3 is completely filled, any further 25 injection of pressure medium will cause the piston rod 11 to extend. A sustained delivery of pressure medium will cause the piston rod 11 to travel to the full extension of its stroke. Conversely, an external force applied to the outer end of piston rod 11 will cause the piston rod 11 to retract, assuming that the directional control valve is set to a “retract” position.

Referring to Fig. 8, the cushioning device is in an open and “charged” configuration, ready for its next cushioning demand. Fig. 11 shows a hydraulic cylinder with the piston rod 11 in midstroke. The cylinder is a single-acting displacement cylinder. The piston rod 11 may be extended by pressurised oil or other medium and may be retracted by an external force. In this 14/05/2019 position, the cushioning device 5 is in the open and “charged” position, ready to respond to a cushioning demand.

Referring to Fig. 9, the cushioning device is in a compressed configuration, having completed its 5 cushioning function as the piston rod 11 retracts. Fig. 9 shows the cylinder 2 with the piston rod in the fully retracted position. In this position, the cushioning device 5 is compressed having made contact with the shoulder 16 on completion of the cushioning function.

It will be noted that in the compressed configuration the internal cushion bushing 5 and the 10 external guide bushing 7 have been compressed together, due to contact having been made with the shoulder 16 of the closing cap and the shoulder 28 of the piston rod 11. The compression spring 6 is also compressed.

Referring to Fig. 8, the cushioning device is also closed having completed its cushioning 15 function as the rod 11 extends. Fig. 8 shows the cylinder 2 with the piston rod 11 in the fully extended position. The cushioning device 5 is again in the compressed position.

It will be noted that the internal cushion bushing 5 and the external guide bushing 7 have been compressed together. However, in this case the function occurred by the external guide bushing 20 7 making contact with the end of gland 14 while the internal cushion bushing was in contact with the circlip 4. The compression spring 6 is also compressed.

Figs. 1 to 7 illustrate the internal cushion bushing 5. This bushing 5 has a tapered orifice 25 machined on one side.

Fig. 1 to 7 illustrates the external guide bushing 7. This bushing 7 has two grooves 26 machined on the outside diameter.

As the vessel 3 is filled with pressure medium, so also is the cushion chamber 19 filled with the 30 medium. The medium is allowed to enter the cushion chamber 19 via the tapered orifice 25. A small clearance between the external guide bushing 7 and the internal cushion bushings allows both of the bushings 5, 7 to slide axially within each other. 14/05/2019 The compression spring 6 serves to push the two bushings 5, 7 apart in an axial direction when the piston rod 11 is in any position other than end of stroke. The relative axial movement of the two bushings 5, 7 is however limited on the one hand by a shoulder 28 on the piston rod 11 and on the other hand by a circlip 4 which is fixed in a groove 8 cut into the journal 27. This action, as well as the pressure exerted on the medium, ensures the cushion chamber 19 is filled with the pressure medium.

As soon as the piston rod 11 approaches the end of its stroke in either direction, the external guide bushing 7 and the internal cushion bushing 5 are compressed together. This compressive action is resisted due to the fact that the cushion chamber 19 is full of pressure medium. The medium can escape from the cushion chamber 19 to the main vessel 3 via the tapered orifice 25 on the internal cushion bushing 5. The rate at which the medium can escape or “bleed-off’ is controlled by the sizing of the tapered orifice 25. The controlled bleed-off rate reduces the velocity of the piston rod 11 before the final stop is reached. The deceleration forces are absorbed by an increase in medium pressure, which is trapped within the cushion chamber 19. This pressure is released at a restricted rate via the tapered orifice 25.

The dimensioning of the tapered orifice 25 is selected to allow for the required cushioning rate, which may be needed to control varying external loads, which may be attached to the piston rod 11. The tapered orifice 25 has the added effect of providing a reducing flow capacity as the two bushings 5, 7 progress further into each other. Thus, the cushioning effect can be said to be progressive.

As the piston rod 11 returns away from the end position, the compression spring 6 again pushes the two bushings 5, Ί axially apart and draws the pressure medium into the cushion chamber 19 thus charging it for the next cushioning demand.

The invention provides a cushioning device which features in both directions; that is to say it works equally well when the piston rod 11 is reaching its fully extended position, or alternatively when it is reaching its fully retracted position.

In addition to the cushioning effect, the mechanism also acts as a guide for the piston rod 11 within the cylinder 2 and also acts as a final mechanical stop on the outward stroke. 14/05/2019 The mechanism occupies no more space than a simple guiding bush/mechanical stop. This means that unlike other cushioning devices, this invention does not shorten the available stroke of the piston rod 11 as compared to a comparable cylinder without any cushioning on either the inward or outward stroke.

The invention provides a solution for the cushioning of displacement cylinders in both directions of travel. The following advantages render the invention particularly suitable for displacement cylinders: • the device will function without the need of a piston head. · the device will function without the need of an exhaust port in the cylinder. • the device is a two-way system which works in both directions. • the device is a single package which remains together throughout the stroke. • no duplication needed at each end of the cylinder. • the “spike” pressure is contained within the cushioning device and therefore has no 15 negative effect on seal life expectancy. • due to the fact that the cushioning device is independent of any inlet or exhaust port, the location of the inlet port can be chosen to suit desired access for particular applications.

In the invention a pair of co-operating bushings 5, 7, together with a compression spring 6, is mounted on a reduced section at the internal end of the piston rod II. These parallel bushings 5, 7 are machined to form a close fit between them, allowing them to move in an axial motion within themselves. The fit is virtually oil tight. The bushings 5, 7 are hollowed out in such a manner that creates a space for the compression spring 6 plus an extra amount of free space. The volume of this free space reduces the more the bushings 5, 7 are pushed into each other and increases as they extend.

The inner bushing 5 has a machined exit path through which the pressure medium is allowed to escape in a controlled manner. This pathway is machined at a sloping angle relative axis of the bush itself. The sloping angle provides a variable area, which forms a variable orifice.

Pressure medium is allowed to enter the free space when the piston is in any position other than end-of-stroke. This free space forms the cushion chamber. The cushion chamber, in this invention, is contained between the said bushings 5, 7 as they travel together as a pair. 14/05/2019 The fact that the “spike” pressure is contained within this chamber also prevents the “spike” pressure reaching the main seal and reducing the service life of the said seal.

Because the bushings travel as a pair, the bushings are available to be actuated by striking from either end as appropriate. This eliminates the need to duplicate at each end.

When the device reaches end-of-stroke, the bushings are compressed. The compressed medium is forced from the cushion chamber as the piston rod continues to the dead stop.

In the invention the expelled pressure medium is returned to the main body of oil in the pressure vessel. Not alone is no exhaust port needed but also there is complete flexibility in positioning the inlet port as discussed above.

The invention is not limited to the embodiments hereinbefore described, with reference to the accompanying drawings, which may be varied in construction and detail.

Claims (12)

Claims

1. A cushioning device comprising: an outer bushing; an inner bushing; the bushings being at least partially axially movable relative to one another; a cushioning chamber between the bushings, the bushings having a flow path for pressure medium to and from the cushioning chamber; the bushings being adapted for movement between an extended configuration in which the bushings are axially spaced-apart a first distance to define an enlarged cushioning chamber, and a retracted configuration in which the bushings are spaced-apart a second distance which is shorter than the first distance and the volume of the cushioning chamber is thereby reduced; and a compression spring located in the cushioning chamber between the bushings to bias the bushings into the extended configuration in which the volume of the cushioning chamber is maximised.

2. A cushioning device as claimed in claim 1 wherein the outer busing is a guide bushing which is adapted to travel axially within a cylinder barrel, and the inner bushing comprises a cushioning bushing at least a portion of which is adapted to travel axially within the guide bushing.

3. A cushioning device as claimed in claim 2 wherein the cushioning bushing comprises an orifice to control the flow of pressure medium into and out of the cushioning chamber.

4. A cushioning device as claimed in claim 3 wherein the orifice is a tapered orifice which is adapted to be progressively closed as the cushioning bushing is moved to decrease the volume of the cushioning chamber.

5. A cushioning device as claimed in any of claims 2 to 4 wherein the guide bushing comprises a flow path for flow of pressure medium into and out of the cushioning chamber.

6. A cushioning device as claimed in claim 5 wherein the flow path is defined by one or more axially extending grooves in the guide bushing. 14/05/2019

7. A hydraulic cylinder comprising a cushioning device as claimed in any of claims 1 to 6.

8. A hydraulic cylinder as claimed in claim 7 comprising:- a cylinder barrel; 5 a piston rod having an internal end within the cylinder barrel and an external end external of the cylinder barrel; an end cap at one end of the cylinder barrel; a gland at the opposite end of the cylinder barrel, the piston rod extending from the cylinder barrel through the gland; 10 the bushings being mounted to the internal end of the piston rod and one bushing being movable relative to the other bushing as the piston rod approaches the end of the stroke to bring the piston rod to a gradual stop.

9. A hydraulic cylinder as claimed in claim 8 wherein the piston rod comprises a journal at 15 the internal end of the piston rod and the bushings are mounted on the journal for movement between the extended and retracted configurations.

10. A hydraulic cylinder as claimed in claim 8 or 9 wherein the outer bushing is adapted to travel axially within the cylinder barrel and the inner bushing is adapted to travel axially 20 within the outer bushing.

11. A hydraulic cylinder as claimed in any of claims 8 to 10 wherein the cylinder comprises an end stop at one end against which one of the bushings is engageable to cushion the stopping of the piston rod.

12. A hydraulic cylinder as claimed in claim 11 wherein the cylinder comprises a first end stop and a second end stop and wherein the outer bushing is engageable against the first end stop to cushion the stopping of the piston rod at one end of a stroke and the inner bushing is engageable against the second end stop to cushion the stopping of the piston rod at an 30 opposite end of a stroke.