Abstract:
A gas spring with a first gas chamber communicated with a second gas chamber through a calibrated orifice to control the rate of return of gas from the first chamber to the second chamber to thereby control the rate at which a piston rod of the cylinder returns to its extended position. Desirably, the rate of return of the piston rod to its extended position can be made slow enough to prevent damage to a die stamped part as the gas spring lifts the part from a lower die half. The gas spring composite shell is preferably formed of and contains materials which are highly thermally conductive and define the return passage to prevent the gas spring from becoming overheated and to permit an increased number of cycles to be completed in a given period of time. Desirably, the gas spring may be completely self contained and utilize only gas to resist the movement of the piston rod to its retracted position, provide a force to move the piston rod to its extended position, and to control the rate of return of the piston rod to its extended position.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to gas springs and more particularly to a gas spring having a delayed return stroke. 
     BACKGROUND OF THE INVENTION 
     A typical gas spring for die stamping applications is constructed with an actuating rod connected to a piston slidably received in a cylinder having a cavity which is precharged to a predetermined pressure with an inert gas such as nitrogen. When the rod and piston are forced into the cavity the gas therein is compressed and when the force applied to the rod is removed, the compressed gas within the cavity immediately forces the piston and rod toward its fully extended position. 
     In some die stamping applications, gas springs adjacent a lower die half may be used to dislodge the stamped part from a cavity of a lower die half. A problem develops on the return stroke of the upper die half when typical gas springs are used because they immediately and rapidly return to their fully extended position and thereby quickly dislodge and lift the die stamped part from the lower die half. At least with parts having a somewhat large surface area, the rapid return of the gas springs toward their extended positions can cause the die stamped part to buckle or flex and thereby adversely affect the quality of the stamped part. 
     To delay or control the return of the piston and rod to their extended positions, some prior gas springs have utilized mechanical or electronic controls on the gas springs. Such controls are undesirable and increase the cost and complexity of the gas springs. Another type of gas spring, such as that disclosed in U.S. Pat. No. 5,823,513 uses hydraulic fluid in one chamber, compressed gas in another chamber and a delay valve to cause a momentary dwell at the bottom of the gas spring stroke. This dwell is provided to prevent damage to the press among other reasons. A critical aspect of any delay cylinder, is its ability to withstand and/or dissipate the heat generated in use. 
     SUMMARY OF THE INVENTION 
     A gas spring with a first gas chamber communicated with a second gas chamber through a calibrated orifice to control the rate of return of gas from the first chamber to the second chamber to thereby control the rate at which a piston rod of the cylinder returns to its extended position. Desirably, the rate of return of the piston rod to its extended position can be made slow enough to prevent damage to a die stamped part as the gas spring lifts the part from a lower die half. The gas spring cylinder assembly contains components which are highly thermally conductive to prevent the gas spring from becoming overheated and to permit an increased number of cycles to be completed in a given period of time. Desirably, the gas spring may be completely self contained and utilize only gas to resist the movement of the piston rod to its retracted position, provide a force to move the piston rod to its extended position, and to control the rate of return of the piston rod to its extended position. 
     Objects, features and advantages of this invention include providing a gas spring which has a controlled rate of return to its extended position, does not use any hydraulic fluid or other liquid, is self contained, uses only compressed gas uses highly thermally conductive components to increase the dissipation of heat where it is created and to conduct it away from the gas spring, may have a relatively short cycle time, may be used with a surge tank, does not require any active electronic or manual control, and is of relatively simple design and economical manufacture and assembly, and has a long, useful life in service. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiment and best mode, appended claims and accompanying drawings in which: 
     FIG. 1 is a diagrammatic side view of a stamping die set having gas springs embodying the present invention; 
     FIG. 2 is a bottom view of a gas spring; 
     FIG. 3 is a cross-sectional view of the gas spring taken along line  3 — 3  of FIG.  2  and shown in its extended position; 
     FIG. 4 is a cross-sectional view of the gas spring of FIG. 3 in its retracted position; 
     FIGS.  5 - 9  are fragmentary, diagrammatic views of the press and a pair of gas springs of FIG. 2 shown in  5  different positions throughout a cycle to stamp a part; 
     FIG. 10 is a cross-sectional view of an alternate embodiment of a gas spring with a surge tank; 
     FIG. 11 is a cross-sectional view of a modified gas spring embodying the present invention; 
     FIG. 12 is a fragmentary sectional view of another modified gas spring embodying the present invention; 
     FIG. 13 is a fragmentary sectional view of another modified gas spring embodying the present invention; and 
     FIG. 14 is a fragmentary sectional view of another modified gas spring embodying the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings, FIG. 1 illustrates a plurality of gas springs  10 , 13  in a die stamping press  11  having an upper die half  12  carried by an upper platen  14  of the press  11  and movable towards a lower die half  16  fixed to a lower platen  18  of the press  11  to stamp and form a sheet metal blank  20  disposed between the die halves  12 ,  16 . Gas springs  13  are attached to the upper die half  12  and gas springs  10  are attached to the lower die half  16 , or the gas springs  10 , 13  may be carried by the platens  14 ,  18  of the press  11 . Desirably, each gas spring  10 , 13  has a piston rod  24  extending therefrom and preferably attached to upper and lower draw rings  26 ,  27  to engage, locate and hold the blank  20  to be stamped relative to the die halves  12 ,  16 . The draw rings  27  of the lower gas springs  10  may also lift the formed part from the lower die half to facilitate removing it and replacing it with a subsequent blank  20  to be formed. 
     As shown in FIGS. 2 and 3, the gas springs  10  preferably have an outer generally cylindrical shell  30  surrounding a cylinder body  32  in which a piston rod assembly  34  is reciprocated. The shell  30  is preferably formed of a material having high thermal conductivity, such as copper or aluminum to increase heat transfer away from the gas spring  10 . To further increase heat transfer away from the gas spring  10 , annular fins  36  may be formed about the upper end of the shell  30 . To further improve heat transfer from the gas spring  10 , the shell  30  may have a plurality of blind bores  38  which extend into adjoining blind bores  39  in base  72  with a heat pipe  40  in each bore  38 ,  39 . The heat pipes  40  are elongate, generally tubular rods formed of a material having high thermal conductivity, closed at both ends, containing a quantity of a working liquid at a controlled pressure and a central wick. When the liquid at one end of the heat pipe  40  reaches a certain temperature, it evaporates and rises in the heat pipe. The heat pipe  40  is designed and positioned such that a sufficient temperature difference exists between its ends to permit the evaporated working fluid to recondense at the other end to thereby dissipate heat in this phase transformation. The condensed working fluid returns through the wick to begin another cycle. A suitable heat pipe  40  is commercially available from Thermacore, Inc. of Lancaster, PA. 
     The cylinder body  32  preferably has a generally cylindrical side wall  42  welded to a base  44 . The side wall  42  and base  44  are preferably formed of a thermally conductive material, such as steel, which is also strong enough to withstand the pressure exerted on the cylinder body  32  by compressed gas within the gas spring  10  and the forces exerted by the retaining ring  48 . An annular groove  46  formed in the interior of the side wall  42  is constructed to receive a retaining ring  48  which retains the piston rod assembly  34  within the cylinder body  32 . A generally helical groove formed about the exterior  50  of the side wall  42  defines a fluid passage  52  between the cylinder body  32  and shell  30 . Spaced apart annular grooves  54 ,  56  formed outboard of opposed ends of the fluid passage  52  receive o-rings  58 ,  60  to provide a fluid tight seal between the shell  30  and cylinder body  32 . A restricted passage  62  having a calibrated flow area communicates the fluid passage  52  with a first gas chamber  64  of the cylinder body  32 . A bore  66  through the base  44  of the cylinder body  32  communicates a second gas chamber  68  with a passage  70  formed in a mounting plate  72  to which the shell  30  and cylinder body  32  are connected. 
     The mounting plate  72  is preferably connected to the base  44  of the cylinder body  32  by one or more cap screws  74  received in threaded blind bores in the base and to the shell by cap screws  75 . The mounting plate  72  is constructed to be fixed directly to one of the die halves  12 ,  16  or platens  14 ,  18  of the press  11  preferably by cap screws  76  (FIG.  2 ). To permit compressed gas to be delivered into the gas spring  10 , a gas filler valve  78  is provided in an inlet  80  of the mounting plate passage  70  which in use is normally closed by a plug  82 . A branch passage  84  extends through the mounting plate  72  and into the shell  30  to communicate the fluid passage  52  with the passage  70  in the mounting plate  72 . Thus, the passage  70  in the mounting plate  72  communicates with the second gas chamber  68  within the cylinder body  32  and the fluid passage  52  defined between the cylinder body  32  and the shell  30 . An O-ring is provided between base  44  and plate  72  to provide a fluid tight seal between them. Another O-ring surrounds passage  84  between the shell  30  and mounting plate  72  to provide a fluid tight seal between them. 
     An annular bearing and seal assembly  86  is received within the cylinder body  32  and has a housing  88  with a reduced diameter upstream end  90  providing a generally radially outwardly extending shoulder  92  to engage the retaining ring  48  which retains the assembly  86  within the cylinder body  32 . The housing  88  has a groove  94  formed about its exterior and constructed to receive a seal ring such as an O-ring  96  to provide a fluid tight seal between the housing  88  and the cylinder body  32 . A back-up  97  is preferably provided to ensure the integrity of the seal under high pressures. Such a back-up may be needed for all static seals of the gas springs  10 . A throughbore  98  slidably receives the piston rod for reciprocation and defines an annular surface  100  sized to closely receive the piston rod  24  therethrough to prevent extrusion of the rod seal  104  against the piston rod  24  as it reciprocates. A counterbore  102  in the housing  88  receives a seal ring  104  to provide a fluid tight seal between the piston rod  24  and the housing  88 . A wiper  101  prevents bearing contamination. An inserted annular plastic bushing  103  guides the piston rod  24 . 
     The piston and rod assembly  34  is slidably received for reciprocation within the cylinder body  32  between an extended position as shown in FIG. 3 and a retracted position as shown in FIG.  4 . The piston  106  has a groove  108  formed therein constructed to receive an annular bearing  110  to guide the piston  106  for reciprocation within the cylinder body  32 . A second groove  112  formed in the piston  106  preferably receives a a low friction, low wearing slip ring  113  supported by an O-ring  114  to provide a fluid tight seal between the exterior of the piston  106  and the interior of the side wall  42 . A central passage  116  through the piston  106  receives a valve  118  which permits a substantially free flow of compressed gas from the second gas chamber  68  to the first gas chamber  64  and provides at least a partial restriction to the flow of gas from the first gas chamber  64  to the second gas chamber  68 . Preferably, the valve  118  is a check valve which substantially prevents fluid flow from the first gas chamber  64  to the second gas chamber  68 . The valve  118  has a valve head  120  yieldably biased onto a valve seat  122  such as by a spring  124 . 
     To connect the piston  106  and piston rod  24 , a split ring retainer  130  has a generally radially inwardly extending rib  132  constructed to be received in an annular groove  134  in the piston rod  24  and is fixed to the piston  106  by one or more cap screws  136  extending into threaded blind bores in the piston  106 . Travel of the piston rod assembly  34  to its extended position with the piston rod  24  extending out of the cylinder body  32  is restricted by engagement of the split ring retainer  130  with the housing  88  of the bearing and seal assembly  86 . 
     Operation 
     A specific, but not exclusive application of this gas spring is the double draw ring inverted stretch draw shown in FIGS.  1  and  5 — 9 . To form a sheet metal blank  20  received between the upper and lower die halves  12 ,  16 , the upper die half  12  is advanced by the upper press platen  14  towards the lower die half  16  to form the blank  20  between them. As shown in FIG. 5, gas springs  13 , 10  carried by the upper and lower die halves  12 ,  16  have draw rings  26 ,  27  thereon which engage the blank  20  to locate and hold it as the die halves  12 ,  16  form it. After engagement of the draw rings  26 ,  27  with the blank  20 , further advancement of the upper press platen  14  displaces the piston rod  24  of the gas spring  10  on the lower die half  16  until it “bottoms out” or reaches its fully retracted position, as shown in FIG.  6 . Still further advancement of the upper press platen  14 , as shown in FIG. 7, moves the piston rod  24  of the gas spring  13  of the upper platen  14  to its fully retracted position and causes the upper press platen  14  to engage and form the blank  20 . As shown in FIG. 8, as the upper press platen  14  is retracted, the piston rod  24  of the gas spring  13  on the upper press platen  14  returns to its extended position and eventually, its draw ring  26  disengages from the lower draw ring  27  to permit the gas spring  24  on the lower press platen  18  to return to its extended position (FIG.  9 ). Desirably, the gas springs  10  carried by the lower die half  16  engage the blank  20  after it has been formed to lift it from the lower die half  16  so that it may be removed from the press  11  and a subsequent blank to be formed inserted into the press  11 . 
     To provide a more controlled return stroke of the piston rods  24  to more gently lift the formed blank  20  from the lower die half  16 , the gas springs  10  are constructed such that pressurized gas in the second gas chamber  68  flows freely through the valve  118  into the first gas chamber  64  as the piston rod  24  is moved to its retracted position and as the piston rod moves to its extended position, the flow of gas from the first gas chamber  64  is restricted by orifice  62  to control the rate of return of the piston rod  24  to its extended position. 
     To accomplish this, the valve  1   18  carried by the piston  106  is preferably a check valve which readily opens as the piston rod assembly  34  is moved to its retracted position to substantially freely permit the gas in the second gas chamber  68  to flow into the first gas chamber  64 . On the return stroke, the valve  118  closes to prevent the flow of gas through it from the first gas chamber  64  to the second gas chamber  68 . Thus, the gas in the first gas chamber  64  is compressed as the piston rod assembly  34  returns to its extended position and may only escape through the orifice  62  to control the gas flow rate out of the first gas chamber  64  and decrease the rate of return of the piston rod assembly  34  to its extended position. The gas which flows at a controlled rate out of the first gas chamber  64  through the calibrated orifice  62  flows into the fluid passage  52 , branch passage  84 , passage  70  through the mounting plate  72 , the bore  66  through the base  44  and into the second gas chamber  68  to decrease the pressure within the first gas chamber  64  and return the gas to the second gas chamber  68  to ensure that the piston rod assembly  34  returns to its extended position. 
     Notably, at or near the bottom of the stroke of the piston rod assembly  34  from its extended to its retracted position, when the gas in the second chamber  68  is not being further compressed, the pressure in the first and second gas chambers  64 ,  68  will become substantially equal and the valve  118  will close. At this time, a significant force differential exists across the piston  106 , due to the significant difference in surface area of the piston  106  acted on by gas in the first gas chamber  64  compared to the second gas chamber  68 . Thus, at least initially after the piston  106  reaches its fully retracted position, a significant force exists tending to return the piston rod assembly  34  to its extended position. As the piston rod assembly  34  moves toward its extended position, the volume of the second gas chamber  68  increases and the pressure therein decreases. In one embodiment, after less than 10% of the return stroke, the force tending to return the piston rod assembly  34  to its extended position decreases dramatically and thereafter, the net force on the piston rod assembly  34  may be just great enough to ensure that the assembly  34  returns to its fully extended position. Of course, the gas flow through the calibrated orifice  62  controls the pressure in both the first and second gas chambers  64 ,  68  and hence, the forces acting on the assembly  34 . 
     The compression of the gas and subsequent throttling through orifice  62  in the gas spring  10  generates significant heat, which if not adequately dissipated, will cause the temperature of the various seals within the gas spring  10  to exceed a maximum allowable temperature above which they deteriorate or degrade and cease to provide an adequate seal causing failure of the gas spring. Thus, a number of features are preferably designed into the gas spring  10  to increase dissipation of heat from the gas spring  10  to thereby reduce the maximum temperature of the gas spring in use and permit an increased cycle rate of the gas spring. 
     Among the features designed to dissipate heat, the helical fluid passage  52  increases the surface area of contact between the heated compressed gas and both the cylinder body  32  and especially the exterior shell  30  which is formed of a material having high thermal conductivity to conduct heat away from the gas in the fluid passage  52 . To increase the heat dissipated from the shell  30 , the cooling fins  36  are provided adjacent its upper end and the shell  30  may be received within a pocket to expose its exterior surface to ambient air so that at least some heat may be removed by convection, to the air surrounding the shell  30 . The mounting plate  72  is also formed of a material having high thermal conductivity to remove heat from the cylinder body  32  and shell  30  by conduction. Further, the mounting plate  72  is bolted directly to the lower die half  16  or lower platen  18  of the press  11  which acts as a heat sink to greatly improve the conduction of heat away from the gas spring  10 . Still further, the heat pipes  40  received within the bores  38  in the shell  30  take advantage of the dissipation of heat which occurs during the phase change of the fluid within the heat pipes  40  as it is evaporated by the heat within the shell  30  at one end and condensed at the other end back to liquid form. Each of these features is designed to remove heat from the gas spring  10  to limit its maximum temperature and to increase the cycle rate of the gas spring  10 . 
     Second Embodiment 
     To improve the cooling of a gas spring  10 ′, as shown in FIG. 10, a reservoir or surge tank  150  is provided to cool compressed gas therein which is exchangeable with the compressed gas in the gas spring  10 ′ to supplement and increase the cooling of the gas spring. The gas spring  10 ′ itself may be constructed substantially the same as in the first embodiment and hence, to the extent that it is the same as the first embodiment, it will not be described further. 
     To control the flow of compressed gas between the gas spring  10 ′ and surge tank  150 , a flow control valve  152  may be received in the passage  70  of the mounting plate  72  instead of the gas filler valve  78  of the previous embodiment. The flow control valve  152  preferably permits a relatively free flow of gas from the gas spring  10 ′ to the surge tank  150  and permits a restricted flow of gas from the surge tank  150  back to the gas spring  10 ′. To accomplish this, as shown in FIG. 10, the valve  152  may have a valve head  154  yieldably biased onto a valve seat  156  by a spring  157  with a small orifice  158  through the valve head  154  to permit fluid flow therethrough even when the valve head  154  is engaged with the valve seat  156 . Gas flow from the gas spring  10 ′ to the surge tank  152  displaces the valve head  154  from the valve seat  156  and the gas may flow relatively freely past the valve  152 . Gas flow in the opposite direction, from the surge tank  150  to the gas spring  10 ′, causes the valve head  154  to bear on the valve seat  156  such that fluid flow in this direction occurs only through the orifice  158  and thus, at a controlled rate. 
     The surge tank  150  preferably has a generally tubular sidewall  160  welded to a lower end cap  162  to define an open ended cylinder. An internal groove  164  in the sidewall  160  receives a retaining ring  166  in assembly. The lower end cap  162  has a through passage  168  communicating with the passage  70  of the gas spring mounting plate  72  through the valve  152  and a conduit  170 . An upper end cap  172  is releasably retained in the sidewall  160  by the retaining ring  166  and has an annular groove  174  with a seal ring  176  therein providing a fluid tight seal between the upper end cap  172  and sidewall  160 . A gas chamber  178  is defined between the upper end cap  172 , sidewall  160  and lower end cap  162  and is in communication with the passage  168  and constructed to contain a supply of compressed gas interchangeable with the gas in the gas spring  10 ′ to reduce the pressure increase and to enhance the cooling of the gas spring  10 ′. 
     A copper or aluminum heat sink  180  is preferably attached to the upper end of the surge tank  150  and has a radial array of fins  182  exposed to ambient air to improve the dissipation of heat from the surge tank  150 . The heat sink  180  and upper end cap  172  preferably have central throughbores  184 ,  186 , respectively, in which an elongate heat pipe  188  is press-fit. The heat pipe  188  is preferably of similar construction as the heat pipes  40  of the gas spring  10  and contains a fluid at a controlled pressure designed to evaporate above a predetermined temperature, with the evaporated fluid moving in the heat pipe  188  towards the heat sink  130  and thereafter condensing when the temperature of the evaporated fluid drops below the predetermined temperature to dissipate heat due to the phase change of the fluid. Thus, the heat released as the fluid recondenses is dissipated into the heat sink  180  to remove heat from the surge tank  150 . The recondensed fluid returns in the heat pipe  188  to begin the process again through a wick structure (not shown) within the interior of the heat pipe  188 . 
     The surge tank may also contain a generally cylindrical heat collector  190  formed of a highly thermally conductive material, such as aluminum or copper, which preferably is generally cellular or foamed and has a plurality of cavities which may be permeated by the compressed gas to increase the heat transfer from the gas to the heat collector  190 . A press-fit, brazed or soldered portion connects the heat collector  190  to the heat pipe  188  with the increased temperature of the heat collector  190  transferred to the heat pipe  188  which in turn transfers the heat to the heat sink  180 . A brazed on ring  191  or formed shoulder retains the heat pipe below a seal  193  that contains the pressure in chamber  178 . 
     During the return stroke of the gas spring  10 ′ as its piston rod assembly  34  returns to its extended position, the volume of the second gas chamber  68  increases and gas in the first gas chamber  64  may return to the second gas chamber  68  through the orifice  62  and fluid passage  52 , and cooler gas from the surge tank  150  may also return to the gas spring  10 ′ through the interconnecting conduit  170  and the passage  70  in the mounting plate  72 . The cooler gas from the surge tank  150  supplements the cooling of the gas spring  10 ′ to reduce its temperature in use and permit an increased cycle rate of the gas spring  10 ′. 
     Third Embodiment 
     As shown in FIG. 11, a gas spring  200  according to a third embodiment of the present invention has a piston rod assembly  34 ′ with a modified piston  202  having a calibrated passage  204  therethrough to provide a controlled flow of gas between the first gas chamber  64  and second gas chamber  68 . The valve  118  preferably functions the same way as in the first embodiment to permit the flow of gas from the second gas chamber  68  to the first gas chamber  64  and prevent the reverse flow from the first gas chamber  64  to the second gas chamber  68 . In this embodiment, the passage  52  and branch passage  84  are not needed. Other than these exceptions, the gas spring  200  is preferably formed substantially the same as the first embodiment gas spring  10  and thus, similar parts have been given the same reference numbers and will not be described again. 
     As the piston rod assembly  34 ′ moves from its extended position to its retracted position, the valve  118  opens and gas in the second gas chamber  68  may flow relatively freely into the first gas chamber  64 . On the return stroke, as the piston rod assembly  34 ′ returns to its extended position, the valve  118  prevents the flow of gas through it from the first gas chamber  64  to the second gas chamber  68  and such flow occurs only through passage  204 . The relatively small flow area through passage  204  provides a restricted or controlled flow of gas out of the first gas chamber to control the rate of travel of the piston rod assembly  34 ′ toward its extended position. 
     Fourth Embodiment 
     As shown in FIG. 12, a gas spring  300  may have a modified piston rod assembly  302  received in a cylinder body  303  and having an annular piston  304  connected to a piston rod  306  by a split retaining ring  308  received in a groove  310  of piston rod  306  and further retained by a small retaining ring  309 . The piston rod assembly  302  is retained in the cylinder body by engagement of the piston  304  with a seal and bearing assembly (not shown) such as the assembly  86  shown in the previous embodiments. The piston preferably carries a bearing  311  to guide the piston as it is reciprocated in body  303  and O-ring  312 , low friction slip ring  313  and O-ring  314  to provide a seal between the piston  304  and both the piston rod  306  and body  303 . 
     A blind bore  316  in the piston rod  306  communicates with a transverse passage  318  extending through the piston rod  306  and opening to a first gas chamber  320 . A counterbore  322  opens to bore  316  and a second gas chamber  324 . 
     A valve  326  received in counterbore  322  has a valve head  328  yieldably biased onto a valve seat  330 , such as by a spring  332 , to control fluid flow through the valve  326 . A passage  334  through the valve head  328  permits a controlled fluid flow through the valve  326  even when the valve head  328  is engaged with the valve seat  330 . 
     When the piston rod assembly  302  moves from an extended position to a retracted position, the volume of the second gas chamber  324  decreases and the valve head  328  is displaced from the valve seat  330  so that gas flows relatively freely from the second gas chamber  324  through the valve  326  and into the first gas chamber  320 . On the return stroke, as the piston rod assembly  302  moves back toward its extended position, the volume of the first gas chamber  320  decreases, the valve head  328  is moved into engagement with the valve seat  330  and the flow of gas from the first gas chamber  320  to the second gas chamber  324  occurs only through the passage  334  through the valve head  328  at a restricted rate controlled by the flow area of the passage  334 . 
     The controlled discharge of gas from the first gas chamber  320  provides a controlled rate of return of the piston rod assembly  302  in generally the same manner as described for gas spring  10 . The heat generated in use of this relatively simple gas spring  300  may severely limit its cycle rate unless some external cooling source, such as a circulating liquid coolant, is provided or other cooling or heat dissipation device(s) provided. The piston rod assembly  302  may be more compact than assembly  34  of gas spring  10 . The piston rod assembly  302  may be fitted with a valve such as valve  118  of gas spring  10  to be used within a gas spring otherwise constructed as in the first embodiment gas spring  10  if desired. 
     Fifth Embodiment 
     As shown in FIG. 13, a fifth embodiment gas spring  400  has a check valve  402  in a passage  404  formed in its piston rod  406  and open to the first gas chamber  64 . A piston  408  is formed from a ring and has an inner circumferential groove  410  which receives a seal  412  against the piston rod  406  and an outer circumferential groove  414  which receives a slip ring  415  and seal  416  against the cylinder body  32 . The piston  408  is retained on the piston rod  406  by a retaining ring  418  carried by the piston rod  406  and a circumferential shoulder  420  of the piston rod  406 . A split retainer  422  partially received in a groove  424  in the piston rod has a bearing  425  to guide the piston rod movement and retains the piston rod  406  and piston  408  in the cylinder body by engagement with a bearing and seal assembly  68 . 
     The remainder of the gas spring  400  is preferably constructed in the same manner as the first embodiment gas spring  10 , with like parts given the same reference numbers. Hence, the construction and operation of the gas spring  400  will not be further described. 
     Desirably, the gas spring  400  may be easier to manufacture than the gas spring  10  as the piston  408  is of relatively simple design. Also, the passage  404 , shoulder  420 , and groove  424  may be readily formed in the piston rod  406 . 
     Sixth Embodiment 
     As shown in FIG. 14, a sixth embodiment gas spring  500  has a piston rod  502  and check valve  504  arrangement which is preferably the same as the piston rod  406  and check valve  402  of the fifth embodiment gas spring  400 . A piston  506  has outer slots  508 ,  510  which carry a guide  512  and a slip ring  513  and seal  514 , respectively. The piston  506  surrounds the piston rod  502  and is retained between a piston rod shoulder  516  and a retaining ring  518 . A sidewall  520  of the piston  506  surrounds and abuts a retaining ring  522  received on the piston rod  502  to retain the piston rod  502  and piston  506  in the cylinder body  32  as in the previous embodiments. 
     In this embodiment, orifice  62  is closed or sealed when the piston rod  502  is in its extended position, such as by the piston guide  512  as shown in FIG. 14, to maintain some pressurized gas in the first gas chamber  64 . The pressurized gas in the first gas chamber  64  resists initial opening of the check valve  504  as the piston rod  502  is displaced toward its retracted position to reduce the impact strike or initial force on the gas spring. After a short interval of travel of the piston rod  502  toward its retracted position, the pressure in the second gas chamber  68  will increase to open the check valve  504 . Once the check valve  504  is open, the gas spring  500  will function in the same manner as the first embodiment gas spring  10 . 
     The cylinder body  32 , shell  30 , bearing and seal assembly  68 , and mounting plate  72  are preferably constructed as in the first embodiment gas spring  10 . Hence, the construction of the gas spring  500  will not be further described. 
     In either embodiment, the gas spring  10 ,  10 ′,  200 ,  300 ,  400 ,  500  provides a controlled transfer of compressed gas between its first and second gas chambers  64 ,  68  or  320 ,  324  to provide a controlled rate of return of the piston rod assembly  34 ,  34 ′ from its retracted to its extended position. Notably, no electronic or manual controls are needed nor is hydraulic fluid or other liquid used to provide a delayed return. Rather, the gas spring  10 ,  10 ′,  200 ,  300 ,  400 ,  500  may be self-contained and uses only compressed gas to control the rate of return of the piston and rod assembly  34 ,  34 ′. Desirably, the gas spring  10 ,  10 ′,  200 ,  300 ,  400 ,  500  is provided with numerous heat transfer features to improve the dissipation of heat from the gas spring  10 ,  10 ′,  200 ,  300 ,  400 ,  500  to improve its efficiency, prevent it from overheating and increase its maximum cycle rate.