GB1595435A - Lenear force shock absorber - Google Patents

Description

(54) LINEAR FORCE SHOCK ABSORBER (71) We, RICHARD GEORGE DRESSELL, JR., of 14609 Stonehouse, Livonia, Michigan 48154, and ROBERT JOHN HEIDMAN, of 37780 Westwood Circle, Apartment 101, Westland, Michigan 48185, United States of America, both citzens of the United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to shock absorbers for linearly decelerating a machine part by imposing a relatively constant force to the part over the stroke of the device and more particularly to such a device wherein the constant decelerating force may be adjusted for use with machine parts having differing weights and velocities.

Shock absorbers which force fluid through a restricted orifice to convert the kinetic energy of a moving part into an increase in the thermal energy of the fluid are commonly used on machines. The smoothest deceleration of the moving part is obtained by absorbers which offer a constant resistive force to the motion over the total length of the deceleration.

One class of such devices employ a piston connected to the machine part and movable within a cylinder having one closed end. A series of exponentially spaced holes are formed along the length of the cylinder wall and the cylinder is supported .within a housing filled with fluid. As the piston is forced into the cylinder by motion of the machine part the fluid is forced through the orifices and the kinetic energy of the part is converted into thermal energy of the fluid.

As the piston moves down the cylinder it successively closes off the orifices so that the force imposed on the load is maintained relatively constant resulting in a linear deceleration of the moving part.

The force imposed on the part is a function of the configuration of the fluid orifices, and linear decelerators of this class have been designed wherein the orifice configuration may be varied to accommodate the device for use with parts having varying weight and kinetic energy. One approach to this problem has been to place a tubular sleeve over the cylinder and provide ports in the sleeve that co-operate with the ports in the cylinder to define the fluid orifice. The angular position of the sleeve on the cylinder may thus be adjusted to vary the orifice configuration and the resistance provided to a load.

According to one aspect of the invention a device for absorbing the kinetic energy of a moving member so as to decelerate the member, comprising a tubular cylinder having a plurality of holes formed in its wall which are spaced relative to one another along the longitudinal axis of the cylinder, a piston slidable within the cylinder, a sleeve having an interior wall surrounding the cylinder and having a number of holes formed in it equal to the number of holes formed through the cylinder and spaced relative to one another longitudinally along the length of the sleeve at the same spacing as the holes in the cylinder, and further having a plurality of annular grooves formed in the interior wall of the sleeve, each groove being in substantial alignment with one of the holes in the sleeve, and having a width at the surface of the interior wall of the sleeve which varies over the length of the groove, a volume exterior to said sleeve, the holes in said sleeve providing fluid communication with the grooves and the exterior volume, means for positioning the sleeve over the cylinder so that the grooves overlie the holes in the cylinder and a fluid orifice is formed by the interface of each of the holes in the cylinder and a respective overlying surface of the overlying groove, each of the grooves being of such configuration that at any point along the length of the groove, the crosssectional area of the groove is greater than the area of the fluid orifice formable by the interface of the hole and groove at that point, where the area of the orifice so formed is a function of the rotational position of the sleeve relative to the cylinder, whereby said grooves are at relatively low pressure relative to the pressure in said cylinder to thereby minimize leakage flow between said cylinder and said sleeve.

According to another aspect of the invention a device for absorbing the kinetic energy of a moving member so as to decelerate the member, comprising, a cylinder, a piston slidable within the cylinder, a plurality of ports formed in the cylinder wall spaced relative to one another along the longitudinal axis of the cylinder, a sleeve having an interior wall surrounding the cylinder, and a spiral groove formed in the interior wall of the sleeve, the groove having a pitch which is an equal divisor of the spacing between each pair of adjacent ports in the cylinder wall whereby the ports will all bear the same relationship to their adjacent sections of the groove. means positioning the sleeve over the cylinder so that groove sections overlie the ports in the cylinder and a fluid orifice is formed by the interface of each of the ports and the respective section of the groove overlying each of the ports, said means positioning the sleeve and the cylinder producing a change in the effective orifice area upon rotation of the cylinder relative to the sleeve by changing the positioned relationship between each port and its adjacent section of the groove in the same way, whereby each of said orifice areas are simultaneously adjusted by said relative rotation, and a volume exterior to said sleeve and means providing fluid communication with the groove and the exterior volume.

Other objectives, advantages and applications of the present invention will be made apparent by the following detailed description of a preferred embodiment of the invention. The description makes reference to the accompanying drawings in which: Fig. 1 is a side view of a shock absorber formed in accordance with a first embodiment of the present invention supported on a machine so as th exert decelerating force upon the machine member; Fig. 2 is a sectional view of the shock absorber taken along line 2 2 of Fig. 1; Fig. 3 is a perspective exploded view of the shock absorber; Fig. 4 is a side view of the pressure cylinder and metering sleeve of the shock absorber illustrating a first rotational relationship between the two; Fig. 5 is a side view of the pressure cylinder and metering sleeve of the shock absorber illustrating a second rotational relationship of the two; Fig. 6 is a view of the internal diameter of the meter sleeve as it would be seen ifit were laid out linearly, illustrating the configuration of the metering grooves; Fig. 7 is a sectional view through the pressure cylinder and metering sleeve along a groove, taken along line 77 of Fig. 4; Fig. 8 is an enlarged cross-sectional view through a groove in the metering sleeve; Fig. 9 is a schematic diagram illustrating the manner of attachment of an external accumulator to the cylinder; Fig. 10 is a sectional view through a second, alternative form of grooved sleeve and the mating cylinder; Fig. 11 is a sectional view of a third, alternative form of grooved sleeve and mating cylinder illustrating a first rotational relationship between the two; Fig. 12 is a sectional view of the form of grooved sleeve and mating cylinder of Fig.

11 illustrating a second rotational relationship between the two; Fig. 13 is a view of the internal diameter of the meter sleeve of Figs. 11 and 12 as it would be seen if laid out linearly; and Fig. 14 is a graphical representation of fluid pressure in the groove in the metering sleeve comparing prior art groove designs with the present groove design.

The preferred embodiment of the invention is constructed about a cylindrical outer tube 10. One end of the tube, which will be hereinafter termed the forward end, has a radially extending metal flange 12 fixed to its outer diameter for the purpose of attaching the shock absorber to a machine mounting surface, such as the wall 13 of Fig. 1.

The rear end of the tube 10 is closed off by a cylindrical cap 14. An O-ring 16 fitted in a groove of the outer perimeter of the cap 14 bears against the inner wall of the tube 10 to provide a fluid seal. A split steel retaining ring 18 fits in a groove in the inner wall of the tube 10 fin secure the cap within a tube.

A tubular metering cylinder 20 is formed integrally with the rear cap 14 and projects forwardly into the tube 10. The outer diameter of the cylinder 20 is substantially smaller than the inner diameter of the tube 10 so that a volume is formed between them.

The rear cap 14 and the cylinder 20 are normally secured against rotation within the tube by a set screw 22 which mates with a threaded hole in the wall of the tube, near the rear end, and bears against the outer diame terofthecap 14.

A pair of tabs or ears 24 project outwardly from the forward end of the tube 20. The tabs are not quite diametrically opposed but are displaced from one another by approximately 140 so that a wider spacing separates the two on one side than on their other side.

The tabs 24 extend into a pair of radial slots 26 formed in the rear end of a piston rod bearing retainer member 28 which seals the forward end of the tube 10. The retainer 28 is secured within the tube by a split retainer ring 30 fitted in a groove in the inner diameter of the tube 10, near its forward end.

An O-ring seal 32 fits in a groove in the outer diameter of the retainer 28.

A cylindrical sleeve bearing 34 is pressfitted within a central cavity in the retainer 28 and acts to slidingly support an elongated piston rod 36 that projects out of the forward end of the assembly. The retainer 28 has a forward cylindrical extension beyond the bearing 34 to accommodate a plastics rod seal 38. The rod seal bears against a shoulder formed in the retainer and is secured by a retainer member 40 which is in turn secured by a retaining ring 42 seated in a groove in the inner diameter of the retainer 28. A plastics rod wiper 44 is secured between the retaining ring 42 and a second retaining ring 46 fitted within another groove in the retainer 28.

At its forward end the piston rod 36 carries a button 48 secured by a screw 50 threaded in a hole at the end of the piston rod. The button acts to bear against a machine part to be controlled by the decelerator. A spiral spring 52 extends between the rear side of the button 48 and the retaining ring 30 and acts to return the piston rod to its normal extended position after the machine part is moved away from the unit.

A piston 54 is formed integrally with the rear end of the rod 36. A groove on the outer diameter of the rear end of the piston carries a piston ring 56 which bears against the inner diameter of the cylinder 20. The piston is formed with a central aperture 58 opening on its rear end and communicating at its forward end with a central cavity 60. That in turn communicates with a radially extending aperture 62. The concave surface between the larger aperture 58 and the smaller aperture 60 acts as a seat for a ball check valve 64.

A valve retainer 66 is supported rearwardly of the ball by a retaining ring 68. When the piston moves rearwardly under the influence of a force exerted on the button 48 by a machine part the passage 60 is sealed by the ball 64 and when the piston moves in the forward direction under the force of the return spring 52 a free flow path is established through the passages 58, 60 and 62.

The passage 62 communicates with the slots 26 in the rear end of the forward bearing retainer 28 and through those slots to an annular slot 70 formed in the outer diameter of the retainer. The slot 70 is filled with an annular accumulator pad 72 formed of a cellular plastics filled with nitrogrn to give it a high degree of resilience. A similar accumulator pad 74 is disposed within the inner diameter of the tube 10, forwardly of the rear cap 14, and surrounding the cylinder 20. This accumulator section also has fluid communi cation with the slot 26.

A sleeve 76 surrounds the outer diameter of the cylinder 20 and has its outer surface in contact with the inner diameter of the accumulator pad 74. The sleeve 76 is locked in position relative to the tube 10 by a pin 78 extending radially inward from the outer wall of the tube and passing through a hole formed in the accumulator pad 74.

Three fluid ports 80, 82, and 84 are formed radially through the wall of the cylinder 20.

The three ports are in longitudinal alignment with one another and the spacing between the ports 80 and 82 is greater than the spacing between the ports 82 and 84. More than three ports may be employed in alternative embodiments and generally the spacings are arranged at exponentially decreasing distances in the direction of the rear of the cylinder. The ports are circular in crosssection.

The ports co-operate with three annular grooves 86, 88 and 90 formed on the interior surface of the sleeve 76. These three grooves are spaced along the length of the sleeve 76 at the same spacing as the holes 80, 82 and 84 so that when the sleeve is disposed over the cylinder 20, the grooves lie over the holes.

This is illustrated in Figs. 4 and 5 wherein the grooves and ports are illustrated in hidden lines.

The grooves have a unique shape which is best illustrated in Figs. 6, 7 and 8. Only the groove 90 is illustrated in Fig. 8 but it is representative of all the grooves. The grooves each have one sidewall 92 extending normally to the interior diameter surface of the sleeve and terminating in a flat bottom 94 which extends parallel to the interior diameter surface and normally to the sidewall 92.

As is best seen in Fig. 7 the depth of the groove varies along the length of the groove.

At one point, denominated 96 in Fig. 7, the groove has nearly a zero depth, almost merging with the interior diameter of the sleeve 76. At a diametrically opposed point, indicated at 98, the groove has a maximum depth. The depth of the groove varies linearly along the groove between these two points.

The groove has a second sidewall 100 which is inclined with respect to the interior diameter surface of the sleeve 76 so that the groove is wider at the interior diameter than at the bottom 94. The groove bottom 94 has a constant width along the length of the groove and accordingly the width of the groove at the interior diameter varies linearly along the groove, as is best seen in Fig. 6. Each groove has a radial port 102 formed through its wall at the deepest point of the groove (98, for groove 90).

The maximum constriction in the flow path formed by one of the ports 80, 82 or 84, and its associated groove 86, 88 or 90, respectively, occurs at the interface between the port surface at the outer diameter of the cylinder 20, and the groove surface at the interior diameter of the sleeve 76. The width of this restriction is a function of the rotational position of the sleeve 76 relative to the cylinder 20. Fig. 4 illustrates the sleeve in such a relationship with respect to the cylinder that the entire surfaces of the ports in the cylinder are uncovered by the groove.

In this position. the effective orifice area is a maximum. Fig. 5 illustrates the sleeve rotated through 180" so that the minimum depth portions of the groove overlie the ports in the cylinder. In this position the ports are practically closed off. In between these two extreme positions the minimum restriction in the fluid flow path occurs on a line drawn between the circumference of one of the ports in the cylinder and the opposed edge of the groove. This orifice is of effectively zero length.

The radial orientation of the cylinder 20, relative to the sleeve 76, which is fixed within the outer tube 10 by the pin 78, may be adjusted by means of an externally knurled cylindrical adjustment member 112 that may be attached to the rear end 14 by a split ring 114. A pin 116 passes through the member 112 to align it in a hole in the end cap 14.

Alternatively, the same adjustment member may be attached to the fromt end of the bearing retainer 28 through use of ring groove 118 and a pin socket 121. The member 112 maybe removed to prevent undesired readjustment of the orifice size.

The tube 10 is equipped with a port 106 in its side wall, which communicates with the interior volume adjacent the accumulator pad 72. This port may be plugged so as to restrain the fluid flow within the tube 10 or it may be used to connect the interior volume to an external accumulator 108, via a conduit 110, as illustrated in Fig. 9. This arrangement allows for a more efficient dissipation of the heat induced in the hydraulic fluid as a result of the energy absorbed from the decelerating machine member.

In use, the shock absorber is attached to a machine part 13, by bolts 120 which pass through the flange 12, as illustrated in Fig. 1.

The piston is positioned to receive the impact of a moving part 122 and exert a linear decelerating force on the part. During the rearward motion of the piston fluid is forced through the ports 80, 82 and 84 in the cylinder through a metering orifice created by the interaction of the outer diameter of these ports with the adjacent section of the grooves 86, 88 and 90, respectively in the sleeve 76. The ports are successively closed off as the piston moves down the cylinder maintaining the decelerating force relatively constant. The fluid then moves along the groove and passes out of the holes 102 and compresses the accumulator material 72 and 74 or passes out through the port 106 to an external accumulator. When the machine part 122 moves away from the shock absorber, the spring 52 returns the piston to its forward position and the valve in the piston formed by the ball 64 moves against the valve retainer 66 so that fluid can freely flow from the accumulator back into the interior volume of the cylinder.

The asymmetrical ears 24 on the forward end of the tube 20 allow the piston 54 to be inserted in the cylinder without the use of a piston ring holder, in the manner previously described. The asymmetrical ears mate with the asymmetrical slots 26 in the bearing retainer 28 to ensure a unique alignment to the assembly.

Fig. 10 illustrates a second, alternative form of the metering sleeve, generally indicated at 130. The sleeve, shown in position with respect to cylinder 20 and the three metering ports, 80, 82 and 84, has a rectangular spiral groove 132 formed on its interior diameter. The lead of the spiral is an integral divisor of the spacings between each adjacent pair of metering ports. That is, the distance between the centres of the metering ports 80 and 82, and the distance between the centres of the metering ports 82 and 84. If additional metering ports are provided in the cylinder 20 at exponentially related distances, the pitch of the spiral 132 should also be an integral divisor of the distance between that additional metering port and the adjacent metering port of the series.

With this constraint placed upon the spacing between the metering ports of the cylinder, it may not be possible to space the metering ports at exactly exponentially related distances, but the number of ports and their positions can be adjusted to provide satisfactory operation with this constraint.

Alternatively the ports need not be in line, but could be displaced radially with respect to one another, so as to align with the spiral groove, or the ports could have different sizes, but be non-exponentially spaced, to achieve the same dynamic effect as exponential spacing.

Because of the relationship between the pitch of the spiral groove 132 and the spacing of the ports, the ports have substantially identical relationships with the portions of the groove adjacent to or overlying each of them. In Fig. 10 the grooves are shown as having a width equal to the diameters of the ports and lying fully over the ports, so as to allow a maximum flow through the ports. As the cylinder 20 is rotated relative to the sleeves the grooves will effectively be shifted longitudinally, relative to the ports, decreasing the effective areas of the orifices. This motion can continue until ports are completely shut-off.

This form of groove is simple to fabricate and provides excellent adjustability.

Holes 134 are formed in the groove wall at regular intervals and provide a flow path to the exterior of the sleeve.

A third, alternative groove design is illustrated in Fig. 11, 12 and 13.

In Fig. 13 a cylindrical sleeve 140 is shown developed to illustrate a groove 142 having a width that varies linearly over the inner diameter of the sleeve. A radial port 144 is formed through the sleeve 140 at the widest point of the groove 142.

The grooved sleeve of Fig. 13 is shown in Fig. 11 abutting a cylinder 146 in a first rotational relationship that provides maximum flow through a metering port 148 into the groove 142. The groove is formed with one sidewall 150 extending normally to the interior diameter of the sleeve and in alignment with one side of the metering port 148. The sidewall 150 terminates at its inner end in a flat bottom 152 which extends parallel to the inner diameter surface and normally to the sidewall 150. The flat bottom 152 is generally of a width greater than the diameter of the metering port 148. A second sidewall 154 extends normally to the flat bottom 152 and parallel to the first sidewall 150 over a portion of the depth of the groove.

A knife-edge projection 156 adjoins the second sidewall 154 and juts toward the first sidewall 150 to define an orifice 158. The projection 156 varies linearly in width over the inner diameter of the sleeve 140. At one extreme, as represented by Fig. 11, the projection terminates immediately adjacent the right side of the metering port 148 to define an orifice 158 of maximum area.

At the other extreme, a represented by Fig.

12, the sleeve has been rotated 180 with respect to the cylinder 146, and the knifeedge projection 156 terminates adjacent the left side of the metering port 148 to define an orifice 158 of minimum area. The orifice area can thus be varied through an infinite number of settings between the extremes of Figs.

11 and 12 by rotating the sleeve 140 with respect to the cylinder 146.

The orifice 158 defined by the groove configuration of Figs. It 13 has virtually zero length to provide a minimal length-towidth ratio. This feature tends to create a highly turbulent flow through the orifice, making the flow independent of fluid viscosity, and thus, of fluid temperature.

In addition, at any point over the inner diameter of the sleeve 140 the groove 142 has a greater cross-sectional area than the area of the orifice 158. This feature assures that fluid in the groove 142 will be relatively "depressurized". By reducing fluid pressure in the groove, the likelihood of leakage flow from the groove into the tolerance between the sleeve 140 and cylinder 146 will be minimized.

Fig. 14 illustrates a comparison between the present device and prior art devices in this respect. The solid lines represent the pressure of the fluid in the groove of a shock absorber formed in accordance with the present design; the dashed lines represent the same for a typical prior art groove design. In the illustration, a is defined as the angle at which the sleeve is rotated with respect to the cylinder away from a minimum orifice setting. As 8 is increased, the angular distance from the orifice to the exhaust port is, of course, decreased proportionately.

In the case of 0 = 0 , the fluid pressure in a groove of the present design drops suddenly in an almost step-like fashion. The fluid pressure in the prior art groove design decreases much more gradually over the length of the groove. The relative "pressurization" of the prior art groove heightens the likelihood of fluid leakage from the groove into the tolerance space between the sleeve and cylinder.

In the case of 0 = 60', the situation is similar to before, but with the pressure drop being less marked. However, the fluid pressure in the prior art groove is significantly higher at any angular position over the groove length. This again points out the advantage of the present groove design.

WHAT WE CLAIM IS: 1. A device for absorbing the kinetic energy of a moving member so as to decelerate the member, comprising a tubular cylinder having a plurality of holes formed in its wall which are spaced relative to one another along the longitudinal axis of the cylinder, a piston slidable within the cylinder, a sleeve having an interior wall surrounding the cylinder and having a number of holes formed in it equal to the number of holes formed through the cylinder and spaced relative to one another longitudinally along the length of the sleeve at the same spacing as the holes in the cylinder, and further having a plurality of annular grooves formed in the interior wall of the sleeve, each groove being in substantial alignment with one of the holes in the sleeve, and having a width at the surface of the interior wall of the sleeve which varies over the length of the groove, a volume exterior to said sleeve, the holes in said sleeve providing fluid communication with the grooves and the exterior volume, means for positioning the sleeve over the cylinder so that the grooves overlie the holes in the cylinder and a fluid orifice is formed by the interface of each of the holes in the cylinder and a respective overlying surface of the overlying groove, each of the grooves being of such configuration that at any point along the length of the groove, the crosssectional area of the groove is greater than the area of the fluid orifice formable by the interface of the hole and groove at that point, where the area of the orifice so formed is a function of the rotational position of the sleeve relative to the cylinder, whereby said grooves are at relatively low pressure relative to the pressure in said cylinder to thereby minimize leakage flow between said cylinder and said sleeve.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   width that varies linearly over the inner diameter of the sleeve. A radial port 144 is formed through the sleeve 140 at the widest point of the groove 142.

   The grooved sleeve of Fig. 13 is shown in Fig. 11 abutting a cylinder 146 in a first rotational relationship that provides maximum flow through a metering port 148 into the groove 142. The groove is formed with one sidewall 150 extending normally to the interior diameter of the sleeve and in alignment with one side of the metering port 148. The sidewall 150 terminates at its inner end in a flat bottom 152 which extends parallel to the inner diameter surface and normally to the sidewall 150. The flat bottom 152 is generally of a width greater than the diameter of the metering port 148. A second sidewall 154 extends normally to the flat bottom 152 and parallel to the first sidewall 150 over a portion of the depth of the groove.

   A knife-edge projection 156 adjoins the second sidewall 154 and juts toward the first sidewall 150 to define an orifice 158. The projection 156 varies linearly in width over the inner diameter of the sleeve 140. At one extreme, as represented by Fig. 11, the projection terminates immediately adjacent the right side of the metering port 148 to define an orifice 158 of maximum area.

   At the other extreme, a represented by Fig.

   12, the sleeve has been rotated 180 with respect to the cylinder 146, and the knifeedge projection 156 terminates adjacent the left side of the metering port 148 to define an orifice 158 of minimum area. The orifice area can thus be varied through an infinite number of settings between the extremes of Figs.

   11 and 12 by rotating the sleeve 140 with respect to the cylinder 146.

   The orifice 158 defined by the groove configuration of Figs. It 13 has virtually zero length to provide a minimal length-towidth ratio. This feature tends to create a highly turbulent flow through the orifice, making the flow independent of fluid viscosity, and thus, of fluid temperature.

   In addition, at any point over the inner diameter of the sleeve 140 the groove 142 has a greater cross-sectional area than the area of the orifice 158. This feature assures that fluid in the groove 142 will be relatively "depressurized". By reducing fluid pressure in the groove, the likelihood of leakage flow from the groove into the tolerance between the sleeve 140 and cylinder 146 will be minimized.

   Fig. 14 illustrates a comparison between the present device and prior art devices in this respect. The solid lines represent the pressure of the fluid in the groove of a shock absorber formed in accordance with the present design; the dashed lines represent the same for a typical prior art groove design. In the illustration, a is defined as the angle at which the sleeve is rotated with respect to the cylinder away from a minimum orifice setting. As 8 is increased, the angular distance from the orifice to the exhaust port is, of course, decreased proportionately.

   In the case of 0 = 0 , the fluid pressure in a groove of the present design drops suddenly in an almost step-like fashion. The fluid pressure in the prior art groove design decreases much more gradually over the length of the groove. The relative "pressurization" of the prior art groove heightens the likelihood of fluid leakage from the groove into the tolerance space between the sleeve and cylinder.

   In the case of 0 = 60', the situation is similar to before, but with the pressure drop being less marked. However, the fluid pressure in the prior art groove is significantly higher at any angular position over the groove length. This again points out the advantage of the present groove design.

   WHAT WE CLAIM IS: 1. A device for absorbing the kinetic energy of a moving member so as to decelerate the member, comprising a tubular cylinder having a plurality of holes formed in its wall which are spaced relative to one another along the longitudinal axis of the cylinder, a piston slidable within the cylinder, a sleeve having an interior wall surrounding the cylinder and having a number of holes formed in it equal to the number of holes formed through the cylinder and spaced relative to one another longitudinally along the length of the sleeve at the same spacing as the holes in the cylinder, and further having a plurality of annular grooves formed in the interior wall of the sleeve, each groove being in substantial alignment with one of the holes in the sleeve, and having a width at the surface of the interior wall of the sleeve which varies over the length of the groove, a volume exterior to said sleeve, the holes in said sleeve providing fluid communication with the grooves and the exterior volume, means for positioning the sleeve over the cylinder so that the grooves overlie the holes in the cylinder and a fluid orifice is formed by the interface of each of the holes in the cylinder and a respective overlying surface of the overlying groove, each of the grooves being of such configuration that at any point along the length of the groove, the crosssectional area of the groove is greater than the area of the fluid orifice formable by the interface of the hole and groove at that point, where the area of the orifice so formed is a function of the rotational position of the sleeve relative to the cylinder, whereby said grooves are at relatively low pressure relative to the pressure in said cylinder to thereby minimize leakage flow between said cylinder and said sleeve.
2. 2. The device of claim I, wherein the

   width of the grooves at the interior wall of the sleeve varies linearly over the length of the groove.
3. 3. The device of claim I, wherein each groove has a depth which varies gradually over its length and a bottom of constant width, one sidewall of each groove being formed substantially normally to the axis of the sleeve and the other sidewall being formed at an inclination relative to the axis of the sleeve so that the width of the groove at the bottom of the groove exceeds the width of the groove at the surface of the interior wall of the sleeve.
4. 4. The device of claim 3, wherein the other sidewall of the grooves of the sleeve have a constant inclination so that the intersection of the other sidewall with the surface of the interior wall of the sleeve varies in departure from the one sidewall of each groove along the axis of the sleeve, as a function of the depth of the groove.
5. 5. The device of claim 1, wherein the grooves are formed around the full circumference of the sleeve.
6. 6. The device of claim 1, wherein the one end of the cylinder is closed off so that motion of the piston toward the closed end of the cylinder forces fluid contained within the cylinder through said orifices to the volume exterior of the sleeve.
7. 7. The device of claim 6 including a fluid passage through said piston and unidirectional valve means supported in the passage to allow the relatively unrestricted flow of fluid through the piston from the volume exterior to the sleeve, and to prevent flow of fluid in the reverse direction.
8. 8. A device for absorbing the kinetic energy of a moving member so as to decelerate the member, comprising, a cylinder, a piston slidable within the cylinder, a plurality of ports formed in the cylinder wall spaced relative to one another along the longitudinal axis of the cylinder, a sleeve having an interior wall surrounding the cylinder, and a spiral groove formed in the interior wall of the sleeve, the groove having a pitch which is an equal divisor of the spacing between each pair of adjacent ports in the cylinder wall whereby the ports will all bear the same relationship to their adjacent sections of the groove, means positioning the sleeve over the cylinder so that groove sections overlie the ports in the cylinder and a fluid orifice is formed by the interface of each of the ports and the respective section of the groove overlying each of the ports, said means positioning the sleeve and the cylinder producing a change in the effective orifice area upon rotation of the cylinder relative to the sleeve by changing the positioned relationship between each port and its adjacent section of the groove in the same way, whereby each of said orifice area are simultaneously adjusted by said relative rotation, and a volume exterior to said sleeve and means providing fluid communication with the groove and the exterior volume.
9. 9. A device for absorbing the kinetic energy of a moving member so as to decelerate the member substantially as herein described and illustrated with reference to the drawings.