GB2520646A

GB2520646A - Regenerative hydraulic vibration damper

Info

Publication number: GB2520646A
Application number: GB1501801.3A
Authority: GB
Inventors: Richard Samuel Mcadam
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-02-03
Filing date: 2015-02-03
Publication date: 2015-05-27
Anticipated expiration: 2035-02-03
Also published as: GB2520646B; GB201501801D0

Abstract

A regenerative hydraulic vibration damper 1 is provided. The vibration damper 1 comprises: a hydraulic fluid retaining volume including a piston 6 and cylinder 4 and optionally one or more further tubes 12,14, the cylinder or further tubes have a plurality of axially spaced strips of piezoelectric material 32 extending circumferentially in or on a wall of the respective tube to generate electricity from shock waves in the hydraulic fluid. An electrical circuit (see Figures 5 & 7 ) rectifies, combines and outputs current from the strips of piezoelectric material 32 to the vehicle battery or other load. As the strips of piezoelectric material 32 are axially spaced, they may generate electricity with different phases, which is combined and smoothed via a low pass filter (42 in Figures 5 & 7) to produce a DC output. A rotatable shaft 28 connected to an electrical generator may be rotated by linear motion of the piston 6 within the cylinder 4 to thereby regenerate energy from shocks.

Description

1 REGENERATIVE HYDRAULIC VIBRATION DAMPER

3 Field of the invention

The present invention relates to the field of hydraulic vibration dampers, particularly vehicle 6 shock absorbers, with the ability to recover energy by generating electricity from vibrations 7 while they damp the vibrations.

9 Background to the invention

11 Hydraulic vibration dampers are found on a wide variety of vehicles. For example, cars and 12 trucks typically include hydraulic shock absorbers which contain a cylinder and a piston.

13 Wheels are attached to the base of the vehicle chassis through springs along with the shock 14 absorbers. Either the piston or more usually the cylinder of the shock absorber is coupled to the vehicle chassis and the other is coupled to one or more wheels. The piston slides within 16 the cylinder as the distance between the chassis and wheels varies and in the event of a 17 sudden shock the flow of hydraulic fluid is restricted by a valve and so vertical movement of 18 the vehicle on the springs is damped. Numerous variations to this basis principle are known 19 which provide different levels of damping depending on the position of the piston within the cylinder, the magnitude of the shock etc. Damping in this way is effective at improving the 21 ride of a vehicle but dissipates energy as heat. A considerable amount of energy can be lost 22 in this way, decreasing the fuel economy of vehicles.

1 It has been proposed to leclaim eneigy from the action of shock absorbers and to feed this 2 regenerated energy back to the electrical system of a vehicle, where it can be used to 3 provide motive force or to power any of the vehicle's other electrical systems, thereby 4 reducing overall energy consumption.

6 JP 2012202530 (KYB Co Ltd) discloses a shock absorber adapted to generate power from 7 eneigy which would otherwise be dissipated by shock absorbers, which uses a piezoelectric 8 element wrapped in a spiral around the outel surface of the cylinder. As the piessure within 9 the cylinder fluctuates due to expansion and contraction of the shock absorber, the force on the piezoelectric element leads to the generation of current, by the piezoelectric effect, and 11 this current is used to provide electiicity to the vehicle battery oi another electrical load.

13 This is potentially a useful way to regeneiate power which would otherwise be dissipated as 14 heat. However, the configuiation of piezoelectiic element disclosed in JP 2012202530 suffers inefficiencies alising from the variation in pressure along the length of the cylinder.

16 The potential difference generated in regions of the cylinder where pressure is relatively high 17 may lead to a deformation of the piezoelectric element in regions of the cylinder where 18 pressure is relatively low, which can reduce the efficiency of damping and electricity 19 generation. With this configuration there is a high probability that no useful charge will be generated.

22 The present invention seeks to address the limitations of the prior art and to provide an 23 energy efficient shock absorber, or other hydraulic vibration damper.

Summary of the invention

27 According to a first aspect of the present invention there is provided a hydraulic vibration 28 damper comprising a hydraulic fluid retaining volume, the hydraulic fluid retaining volume 29 being defined at least in pait by one or more tubes, a fiist said tube defining a cylinder having a piston slidably mounted therein, one or more said tubes having a plurality of strips 31 of piezoelectric material which are axially spaced and extend circumferentially in or on a wall 32 of the respective tube to thereby generate a potential difference between opposite faces of 33 the strips of piezoelectiic material in dependence on pressure valiations in the at least one 34 said tube arising from movement of the piston within the cylinder, and an electrical circuit extending between said faces of the strips of piezoelectric material and an output and 36 configured to receive, combine and output current from the plurality of strips of piezoelectiic 37 matelial.

2 As the strips of piezoelectric material are axially spaced, they may each generate current 3 individually, in response to compressive stresses, which can be combined by the electrical 4 circuit. They can therefore efficiently generate electricity from pressure waves which pass rapidly through the hydraulic fluid within the hydraulic fluid retaining volume as shocks are 6 damped. If they were not axially spaced, but instead continuous, then at different positions 7 along a tube, different and possibly opposite forces would be applied, leading to energy 8 being dissipated and substantial capacitive effects within the piezoelectric material.

The strips of piezoelectric material extend circumferentially. They may extend the whole way 11 around the circumference of the respective tube, however this is not essential. They may for 12 example extend around the majority of the circumference of the respective tube but with a 13 gap. It is advantageous that the strips of piezoelectric material extend circumferentially 14 because the pressure is typically uniform around the wall of a tube and so current should be generated in the same sense through each strip at any given time.

17 It may be that at least the cylinder has a wall with the said plurality of axially spaced strips of 18 piezoelectric material located within the wall of the cylinder, or on the inner or outer surface 19 of the wall of the cylinder. Nevertheless, it may be that one or more said tubes, other than the cylinder but in fluid communication with the cylinder, has a wall with a said plurality of 21 axially spaced strips of piezoelectric material within the wall, or on the inner or outer surface 22 ofthewall.

24 In particular, it may that the vibration damper comprises an outer chamber (which may be an outer annular cylinder), around the cylinder and in fluid communication with the cylinder.

26 The outer chamber will have both an outer wall and an inner wall (which may be separate to, 27 or part of the outer wall of the cylinder). It may be that the outer wall of the outer chamber 28 has a said plurality of axially spaced strips of piezoelectric material located within the outer 29 wall, or on the inner or outer surface of the outer wall. It may be that the inner wall of the outer cylinder has a said plurality of axially spaced strips of piezoelectric material located 31 within the inner wall, or on the inner or outer surface of the inner wall.

33 It is advantageous to provide a said plurality of axially spaced strips of piezoelectric material 34 between the cylinder and the outer chamber, to receive forces acting both radially inwards and radially outwards.

1 In general! it may be that one or more said tubes (which at least in part define the hydlaulic 2 fluid retaining volume) have a wall with a said plurality of axially spaced strips of piezoelectric 3 material located on the inner surface of the wall. It may be that one or more said tubes have 4 a wall with a said plurality of axially spaced strips of piezoelectric material located on the outer surface of the wall. It may be that one or more said tubes have a wall with a said 6 plulality of axially spaced strips of piezoelectiic material located within the wall.

8 The electronic circuit typically complises one or more rectifiers, which typically complise one 9 or more diodes, and may for example be bridge rectifiers, to rectify the current from individual piezoelectric strips. The electronic circuit may be configured to combine current 11 from different piezoelectric strips with different phases. For example, it may have a lust wire 12 connected to the positive output of a plulality of said rectifiers and a second wire connected 13 to the negative output of a plulality of said rectifieis, and each rectifier may have two inputs 14 connected to opposite faces of a piezoelectric strip.

16 The electronic circuit may comprise isolator elements to isolate strips of piezoelectric 17 material from the output of other strips of piezoelectric material to prevent a potential 18 difference generated by one strip of piezoelectric material from driving expansion and/or 19 contraction of another strip of piezoelectric material. The isolator elements may be diodes.

The isolator elements may be bridge rectifiers through which strips of piezoelectric material 21 are connected in parallel.

23 The inner or outer surface of one or more said tubes may comprise a multilayer strip, the 24 multilayer strip comprising a said strip of piezoelectric matelial and typically also at least one conductive layer. Each said multilayer strip may have a non-conductive coating layer, 26 typically on both sides.

28 The electronic circuit may comprise one or more filters, for example a low pass filter, to 29 smooth the output potential diffeience. This piovides a more smooth DC power supply to a vehicle battery or other load.

32 The electronic circuit may comprise a control circuit configured to determine the potential 33 difference applied to each of the plurality of piezoelectric strips (in the absence of variations 34 in potential alising from forces acting of the piezoelectric strips from vibrations) in dependence on a contiol signal to vary the shape of the plurality of piezoelectric ships and 36 thereby vary the iesponse (e.g. stiffness) of the hydlaulic vibration dampel to vibrations 37 and/ol shocks.

2 The hydraulic vibration damper may be a shock absorber.

4 The invention also extends in a second aspect to a vehicle comprising a chassis and a plurality of ground engaging wheels, and a vibration damper according to the first aspect of 6 the invention with the piston of the vibration dampel coupled to the chassis (or one or more 7 wheels) and the cylinder of the vibration damper coupled to one or more wheels (or the 8 chassis, respectively), to theleby damp vibiations between the chassis and one or more 9 wheels. The vehicle may comprises a battery and/or a DC electrical power rail in electrical communication with the said output of the hydraulic vibration damper electrical circuit, to 11 receive electrical energy obtained from the strips of piezoelectiic matelial fiom vibrations 12 between the chassis and the one oi moie wheels.

14 The invention extends in a thud aspect to a hydraulic vibiation damper complising a cylinder, a piston slidably mounted in the cylindel, and a lotatable shaft, the piston being coupled to 16 the rotatable shaft (by a coupling) such that linear movement of the piston within the cylinder 17 is coupled to rotation of the rotatable shaft, and a power take off device to receive energy 18 from the rotation of the rotatable shaft in response to linear movement of the piston within the 19 cylinder.

21 The rotatable shaft may extend through the piston. The rotatable shaft may extend along the 22 axis of the cylinder. The rotatable shaft may be telescopic. Thus, the length of the rotatable 23 shaft can vary as the piston slides linearly within the cylinder.

The coupling may comprise one or more clutch bealings. The coupling may comprise two 26 clutch bealings. The two clutch bearings may allow rotation of the lotatable shaft in the same 27 direction of rotation (and not in the opposite direction of rotation) so that the rotatable shaft is 28 rotatable in one direction of rotation (and not in the other direction of rotation) as the piston 29 moves linearly in either direction in the cylindel. The rotatable shaft may comprise one or more spirals (e.g. spiral grooves or splines). The couplings may comprise one oi more spilal 31 clutch bearings in contact with respective spirals.

33 In an embodiment, the rotatable shaft complises two spirals (formation grooves oi splines), 34 which extend alound the rotatable shaft in opposite dilections, and two clutch bearings, each of which is coupled to a respective spiral foimation, to be diiven by or to drive the lespective 36 spiral foimation, and which allow the rotation of the lotatable shaft in one and the same 37 dilection of rotation (and not in the other dilection of rotation).

2 The power take-off device is typically an electrical generator. The electrical generator 3 typically comprises a stator and a rotor. Typically the rotor of an electrical generator is 4 coupled and rotates with the rotatable shaft and the stator is connected to the vehicle chassis. Other types of power take-off device are possible, for example hydraulic pumps.

7 The invention also extends in a fourth aspect to vehicle comprising a chassis and a plurality 8 of ground engaging wheels, and the vibration damper according to the third aspect of the 9 invention with the piston of that vibration damper coupled to the chassis (or one or more wheels) and the cylinder of the vibration damper coupled to one or more wheels (or the 11 chassis, respectively), to thereby damp vibrations between the chassis and one or more 12 wheels. The vehicle may comprise a battery and/or a DC electrical power rail in electrical 13 communication with the said output of the hydraulic vibration damper electrical circuit, to 14 receive electrical energy obtained from the rotation of the rotatable shaft due to movement of the piston within the cylinder arising from the vibrations between the chassis and one or 16 more wheels.

18 Descrirtion of the Drawings An example embodiment of the present invention will now be illustrated with reference to the 21 following Figures in which: 23 Figure 1 is a cross-section through a shock absorber according to the invention; Figure 2A is a perspective view of the reserve tube; 27 Figure 2B is a perspective view of the electrical connections on the inner surface of the 28 reserve tube; Figure 2C is a cross section through the reserve tube showing axially spaced strips of 31 piezoelectric material on the inner surface; 33 Figure 3A is a perspective view of the outer pressure tube; Figure 3B is a perspective view of the inner pressure tube; 37 Figure 3C is an exploded view of the inner pressure tube within the outer pressure tube; 2 Figure 3D is a cross section through the inner and outer pressure tube when fitted together; 4 Figure 4 is a cross section through a multilayers strip; 6 Figure 5 is a cilcuit diagram of the electiical circuit; 8 Figule 6 is a perspective view of a spiral guide clutch bearing; and Figure 7 is a circuit diagram of an alternative electrical circuit.

12 Where coiresponding features appeais multiple times, only some instances have been 13 numbeled for clarity.

Detailed Description of an Example Embodiment

17 With reference to Figure 1, a shock absorber shown generally as 1 has an inner pressure 18 tube 2 (innermost tube) which defines a cylinder 4 within which is slidably mounted a piston 6 19 having check valves 8. An annular outer chamber 10 which is around and coaxial with the cylinder is defined by reserve tube 12 (outermost tube) and an outer pressure tube 14 21 (intermediate tube) which fits tightly around the inner pressure tube. A continuous body of 22 hydraulic fluid extends between the cylinder and the outer chamber (which functions as a 23 reservoir) via bottom pressure tube valve 16 and base cup 18. Accordingly a hydraulic fluid 24 retaining volume is defined at least in part by the inner wall of the inner pressure tube, the outel wall of the outer pressure tube, and the inner wall of the reserve tube, as well as by the 26 intelior surface of base cup and by steel piston guide 20 at the top of the cylinder. The upper 27 end of the outer chamber opens into the head space of the cylinder. (References to upper, 28 lower, base, top etc. are references to the shock absorber in the position shown in Figure 1 29 in an ornamentation where the piston sliding linearly in the cylinder to shorten the length of the shock absoibel is referred to as the piston moving down, and in practice it may be 31 employed in any suitable orientation).

33 The piston is mounted on a piston lod 22 and slides within an aperture 24 at one end of the 34 cylinder, around which extends an oil seal 26. The piston and piston rod slide on a rotatable shaft 28 having spiral grooves 30A, 30B extending around the rotatable shaft in opposite 36 senses and which cooperate with a clutch 31 comprising two clutch bearings 60 (one of 37 which is shown in Figure 6) which have outer rings 64 mounted to the piston rod and inner 1 rings 66. The inner rings have teeth 62k 62B which slide in respective grooves 30A, 3DB 2 and which are formed generally as a spiral (one in each sense) to cooperate with grooves 3 30A, 30B. The inner rings can rotate within the outer rings in one direction of rotation but not 4 the other. The inner rings of both clutch bearings are rotatable in the same direction of rotation but each is slidingly coupled to a different one of the spiral grooves. The spiral 6 grooves and clutches cause the lotatable shaft 28 to iotate as the piston leciprocates, 7 converting linear motion of the piston into rotation of the lotatable shaft.

9 The piston rod terminates with a connector 50 and the rotatable shaft extends to a rotor 52 which rotates with the rotatable shaft and which is fitted within the stator 54 of an electrical 11 geneiatoi. The iotoi 52 has a plurality of magnets 56 and the stator, which is rigidly 12 connected to the vehicle chassis, has wire coils 58. The lotatable shaft is telescopic, with a 13 first portion (which is connected to the rotor) which slides lelatively to (typically within) a 14 second portion which has the spilal grooves, so that the rotatable shaft may lengthen and shoiten, as appropriate! as the piston moves within the cylinder 17 The check valves in the piston and the bottom pressure tube valve provide low resistance to 18 fluid flow into the space between the piston and the bottom pressure tube valve (cylinder 19 working volume) and high resistance to the front of hydraulic fluid out of the space between the piston and the pressure tube valve. Accordingly, the valves resist the flow of hydraulic 21 fluid out of the cylinder when the cylinder is compressed but provide low resistance to flow of 22 hydraulic fluid into the cylinder when the cylinder is extended.

24 In use, the cylindel is mounted to the chassis of a vehicle by way of a connection to the bottom cup or reserve tube and the piston lod is connected to a wheel support which 26 supports the axle of one oi more wheels through the piston rod connector 50. The wheel 27 support is also connected to the chassis through one or more springs or other resilient 28 members (not shown). The shock absorbers thereby damp vibrations between the chassis 29 and the wheels, including dissipating the energy of one off shocks, foi example as the vehicle passes ovel a bump or dip in a road surface.

32 The inner surface of the reserve tube, the outer surface of the outer pressure tube and the 33 outel surface of the inner piessure tube each have a plulality of strips of piezoelectric 34 matelial 32. The snips of piezoelectric material are spaced apait axially (i.e. spaced apart along the length of the respective tubes) and each extends circumferentially around the 36 respective tube.

1 An electrical circuit, shown in Figure 5 is configured to rectify, combine and output current 2 from the strips of piezoelectric material to the battery of the vehicle in which the shock 3 absorber is mounted, or to another load (such as a specific circuit or other load within the 4 vehicle) to thereby regenerate energy from vibrations between the vehicle chassis and wheels.

7 Figule 2A is a perspective view of the reserve tube. Figule 2B illustrates the outline of ships 8 of piezoelectric material 32 on the inner surface of the reseive tube, budge iectifiers 34 9 electrically connected between opposite faces 36A, 3GB of individual strips of piezoelectric material, and positive 38 and negative wires 40. One of the AC inputs of each bridge rectifier 11 is connected to the inward surface of the strip of piezoelectric mateuial and the other is 12 connected to the outward surface.

14 Figuies 3A through 3C are perspective views of the innel 2 and outer 14 piessure tubes, each of which has a plurality of axially spaced strips of piezoelectric material extending 16 around their outside, and Figure 3D is a cross-section through the outer pressure tube fitted 17 tightly around and coaxial with the inner pressure tube. Bridge rectifiers are connected 18 between the faces of the strips of piezoelectric material on the outer pressure tube. The inner 19 pressure tube instead has pins 42 which extend through vias to additional bridge rectifiers on the outer surface of the outer pressure tube, simply as it is more efficient for manufacturing 21 purposes to fit the bridge rectifiers in a single layer. However, the bridge rectifiers may be 22 provided at any appropriate location.

24 With reference to Figure 4, the strips of piezoelectric material 32 can be conveniently formed as multilayer strips 45 with a strip of piezoelectuic mateuial in the middle, such as 26 polyvinylidene fluoride, with conductive layers 46, foi example formed from a metal such as 27 copper or silver, on either side 36A, 36B of the piezoelectric material, and non-conducting 28 layers 48, which may for example be formed from mylar, making up the outermost layer on 29 either surface of the multilayer strips. These ships can be manufactuied in lengths, cut to size and conveniently attached to the respective tubes. They may be made by printing 31 techniques, including screen printing and 3D printing.

33 Figure 5 is a circuit diagram. Each piezoelectric strip is connected in parallel between 34 positive 38 and negative 40 wires through a lespective bridge rectifier 34 and the positive and negative wires extend to a low pass filter 42 which provides a smoothed DC output 44 to 36 a battely oi load.

1 In use, the piston slides within the inner tube (cylinder) as the vehicle wheels move relative 2 to the chassis. Hydraulic fluid flows between the cylinder and the outer chamber as the 3 relative distance between the wheels and chassis varies. During steady motion the average 4 position of the pistons within the cylinder is determined by the mass of the vehicle and the characteristics of the springs through which the wheels are mounted to the vehicle chassis.

6 Whenever the wheels are moved closer to the chassis (for example by the vehicle crossing a 7 bump), hydraulic fluid flows from the cylinder to the outer chamber through the base valve, 8 which throttles this flow, as well as upward through the piston check valve which also 9 throttles this flow, damping the motion. When the chassis rebounds and the wheels move away from the chassis, the piston slides in the opposite direction through the cylinder and 11 this motion is permitted by the base valve and the piston check valve.

13 As a result of shocks which compress the shock absorber, and the resulting forces exerted 14 by the piston, the movement of the piston generates pressure waves which pass down through the cylinder, through the base cup and upwards along the outer chamber. These 16 pressure waves generate corresponding waves of compressive force acting on the strips of 17 piezoelectric material. As a result, a potential difference is generated across the thickness of 18 each strip of piezoelectric material and a current is generated and fed into the electrical 19 circuit through the respective bridge rectifier. The bridge rectifiers rectify the current so that current flows in the same sense from each strip of piezoelectric material to the positive and 21 negative wires, thereby combining the current from each strip of piezoelectric material.

23 As the pressure waves pass through the cylinder and outer chamber, the potential difference 24 across each strip of piezoelectric material, and therefore the resulting current, varies with time. As waves will typically pass adjacent strips at slightly different times, there is a phase 26 difference between the current from each piezoelectric strip and some piezoelectric strips 27 may be expanding while others are being compressed, at least during some points during 28 waves of compression and then expansion. However, due to the bridge rectifiers, each 29 piezoelectric strip contributes current with the same sense. The low pass filter smoothes the current and provides a DC current to the output.

32 As the strips of piezoelectric are axially spaced, they can efficiently generate current despite 33 the phase difference between compression peaks along the length of the cylinder and the 34 outer chamber. Without this axial spacing, there would be greater energy losses with axial current flow and capacitive effects. The use of bridge rectifiers assists by enabling the 36 current contribution from each piezoelectric strip to be combined despite phase differences 37 and the diodes within the bridge rectifiers have the additional advantage that current from 1 one piezoelectric strip does not drive expansion or compression of another piezoelectric 2 strip, reducing the efficiency of energy generation.

4 Separately, rotation of the rotatable shaft within the piston, as a result of the linear motion of the piston being coupled to rotation of the rotatable shaft relative to the cylinder by the spiral 6 grooves and clutch rings, causes the rotor of the electrical generator to rotate relative to the 7 stator. As the piston slides down the cylinder the inner ring of one of the clutch bearings 8 rotates and as it slides up the cylinder, the inner ring of the other clutch bearing rotates.

9 Hence the rotatable shaft rotates in the same direction of rotation in response to movement of the piston in either direction. This is more energy efficient than rotating in different 11 directions of rotation as the direction of sliding of the piston moves, particularly for small 12 vibrations. This generates a further DC current electrical signal in the coils of the stator which 13 can again be fed to the positive and negative wires and so regenerate further electrical 14 energy from vibrations applied to the shock absorber.

16 In the Figures, the piezoelectric strips on the inner surface of the outer pressure tube are 17 shown as extending around and overlaying the piezoelectric strips on the outer surface of the 18 inner pressure tube. In practice, they could be offset from each other and may have different 19 breadths and spacings.

21 Although it is most efficient for each strip of piezoelectric material to be connected through 22 diode circuits so that the current generated by one strip of piezoelectric material does not 23 drive unwanted and counterproductive compression or expansion of another strip of 24 piezoelectric material which receives pressure waves with a different phase or amplitude, it is also possible to use the strips of piezoelectric material to change the response of the 26 shock absorber to shocks by detecting the presence of a shock wave in the shock absorber 27 from the current generated by strips of piezoelectric material and applying a potential 28 difference to strips of piezoelectric material to cause the piezoelectric material to selectively 29 expand, pushing back on the hydraulic fluid and adjusting the apparent stiffness of the shock absorber, or by applying a predetermined potential difference (which may vary from one strip 31 to another, or be varied in response to a control signal) to each strip of piezoelectric material.

32 Typically, in that case, application of a potential difference across a strip of piezoelectric 33 material causes it to buckle. The strips of piezoelectric material which push back in this way 34 may be in, or on the inner or outer surface of, a wall of the reserve tube. This can be implemented using a circuit such as that shown in Figure 7 where an actuator controller 70 36 provides positive 72 and negative 74 power supply rails and a switch control unit 78, 37 controlled by a CPU 80 (for example, a microprocessor or microcontroller executing a 1 computer program stored on a tangible computer readable data storage medium), which 2 controls switches 76 to switch the piezoelectric strips between being connected to the 3 positive and negative power supply rails and being connected to the positive and negative 4 rails of the current combining and filtering circuit. It is optional whether the potential differences are regulated in this way specifically in response to detection of a shock wave, or 6 continuously, for example.

8 The invention is applicable to other designs of shock absorbers, for example twin tube shock 9 absorbers with gas bags located in the outer chamber. Bags of pressurised gas (or other resilient bodies) in the reserve tube help to keep hydraulic fluid pressurised and reduce 11 foaming. Furthermore, although the invention has been illustrated with reference to shock 12 absorbers used to damp vibrations between the chassis and wheels of a vehicle, the 13 invention is equally applicable to hydraulic vibration dampers for use in other contexts.