|Publication number||US3845690 A|
|Publication date||Nov 5, 1974|
|Filing date||Mar 16, 1973|
|Priority date||Mar 16, 1973|
|Publication number||US 3845690 A, US 3845690A, US-A-3845690, US3845690 A, US3845690A|
|Original Assignee||Us Army|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Referenced by (6), Classifications (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent 1 Ziegler DIGITAL FLUIDIC CIRCUIT WITH ADJUSTABLE TIME DELAY MEANS FOR OUTPUT CONTROL [4 1 Nov. 5, 1974 Primary ExaminerEdgar W. Geoghegan Assistant Examiner-A. M. Zupcic Attorney, Agent, or FirmEugene E. Stevens, III
 Inventor: William H. Ziegler, Waterford, NY.
 Assignee: The United States of America as  ABSTRACT represented PY the Secretary of the A fluidic circuit provided with a digital feedback cay Washmgton, pacity is utilized to sense the rotation of a shaft from  Filed: 16,1973 an initial zero position within a cylindrical housing therefor and actuate a piston in either direction to re- PP 341,801 store the initial angular relationship between the shaft and the housing. The circuit includes a pulse valve 52 us. Cl. 91/35, 91/368 which is triggered by the Same external force utilized 51 Int. Cl. Fb 21/02 to rotate the Shaft out of Zero Position to SuPPIY flow  Field of Search 91/35 38 461 390 304 of fluid over a predetermined interval of time at least 1 equal to the time required and the housing to the initial angular relationship therebetween. Once the rela-  References Cited tive rotation between the shaft and the housing is completed, the pressure against the opposite ends of UNITED STATES PATENTS the actuating piston is equalized and is thereafter I 3,215,044 11/1965 L ssan 91/390 maintained in this Condition when the Supply of fl from the pulse valve is terminated. 3Z641I876 2/1972 Wienke 9l/46l 7 Claims, 6 Drawing Figures i REGULATOR 32 I 30 PS1 32 42 v j suagpu F 46 1 46 27 F F 3Q] 48 -34 T39 48 44 65 is 65 3 2Q 50 4o 40 3 I A A 6i r63 T I 35 35 I 11' l i 59 J DIGITAL FLUIDIC CIRCUIT WITH ADJUSTABLE TIME DELAY MEANS FOR OUTPUT CONTROL BACKGROUND OF THE INVENTION This invention relates to a fluid control system responsive to the rotation of a shaft from an initial zero position within a mating housing therefor and is more particularly directed to a digital feedback circuit to which fluid is supplied over a period of time at least equal to that required to restore the initial angular relationship between the shaft and the housing.
ln a previous fluidic system for the linear actuation of a piston in response to the rotation of a shaft from an initial zero position within a mating housing therefor, analog or proportional amplifiers were utilized in order to provide a circuit of maximum simplicity. While such system is functionally adequate, the need for a continuous flow of fluid through the analog type amplifiers has, for all practical purposes, prevented the application of fluidic control to those devices which must be operated in an outdoor environment where a supply of fluid is not readily available.
SUMMARY OF THE lNVENTlON It is therefore an object of this invention to provide a fluid control system for actuating a piston to restore the initial angular relationship between a rotatable shaft and a similarly rotatable housing therefor in a manner which does not require any flow of fluid prior or subsequent to relative rotation between the shaft and housing.
Another object of this invention is to provide a fluid control system, as aforesaid, which incorporates a digital feedback circuit responsive to a metered flow of fluid from a self-contained portable supply.
Still another object of the present invention resides in the provision of a digital feedback circuit, as aforesaid, which includes an adjustable time delay valve for controlling the interval during which fluid is supplied to the actuating piston for restoring the initial angular relationship between the rotatable shaft and housing therefor.
A further object of the present invention is to provide a fluidic circuit of the digital feedback type which includes a locking valve for preventing movement of the actuating piston while the rotatable shaft is in the zero position thereof.
lt is also an object of this invention to provide a digital type of fluidic circuit, as aforesaid, which will instantly respond to any rotation of the shaft and will operate in a reliable and effective manner without the necessity for the continuous flow of fluid heretofore required to maintain the system in operational readiness.
It has been found that the foregoing objects can be achieved by a fluidic circuit with a digital feedback arrangement adapted to sense any rotation of a shaft from an initial zero position relative to a cylindrical mating housing therefor and thereby trigger a time delay pulse valve for supplying operating pressure air for a predetermined interval of time to a pair of interface valves which impart linear actuation to a reciprocating piston. When the interior of the pulse valve is momentarily vented by a triggering device responsive to the same external force responsible for rotation of the shaft, the resulting change in pressure displaces a relatively rigid floating diaphragm within the valve to unblock an output port for the passage of the operating pressure air. Upon completion of the momentary venting of the pulse valve, the input flow of air across the interior end of the output port creates a static pressure drop which, in accordance with the predetermined setting of a needle valve communicating with the input port, gradually returns the floating diaphragm into position to block further passage of air through the output port.
The output flow of operating pressure air from the pulse valve is directed to each of the interface valves and to a 4-way locking valve communicating with the reciprocating piston to which the rotatable housing is mechanically linked for joint movement for the restoration thereof to the initial angular position relative to the shaft. The pulse valve output is additionally transmitted through pressure-reducing restrictions to provide a lower level of control pressure air for functioning a pair of OR/NOR logic elements. Each logic element is connected between an interface valve and one of a pair of opposed sensing orifices located in the cylindrical housing so as to be normally blocked by the rotatable shaft in the zero position thereof. When the shaft is rotated to unblock one of the sensing orifices, the backpressure in the line leading therefrom to the corresponding OR/NOR logic element is exhausted thereby actuating a switching arrangement therein to divert the flow of control pressure air to an exhaust port. The interface valve associated with this logic element responds to the resulting absence of control pressure flow thereto by exhausting the higher level of operating pressure flow therethrough to thereby reduce the pressure against the corresponding face of the reciprocating piston. Since the other sensing orifice continues to be blocked by the rotatable shaft, the logic element connected to this orifice responds to the resulting backpressure in the line leading thereto by directing the control pressure air to the associated interface valve which then functions to transmit a flow of operating pressure air against the other face of the reciprocating piston.
The resulting differential pressure on the piston imparts linear movement thereto in the direction required to actuate the housing for restoring the initial angular position thereof relative to the rotatable shaft. Once the open sensing orifice is again blocked by the periphery of the rotatable shaft, the logic element associated with this orifice functions to again direct control pressure air to the interface valve associated therewith for renewing the output flow of the operating pressure air against the corresponding face of the reciprocating piston until the differential pressure thereon is nullified and the linear movement thereof halted. Thereupon, the pulse valve closes in accordance with the predetermined delay set therein and terminates the output flow of operating pressure air to the 4-way valve to effect the closing thereof. The equilibrium pressure to which the reciprocating piston is thereafter subjected maintains such piston in a stationary position without the need for any flow of operating pressure air until the rotatable shaft is again actuated to unblock one of the sensing orifices.
BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of the invention, as well as other objects and advantages thereof, will be readily apparent from consideration of the following specification relating to the annexed drawings wherein:
FIG. 1 is a schematic circuit diagram showing the arrangement of the components thereof in the absence of any flow of air between actuations of the pulse valve;
FIG. 2 is a similar circuit diagram showing the flow pattern during the period in which the housing is being rotated subsequent to the rotation of the shaft to restore the initial angular relationship therebetween;
FIG. 3 is a vertical section through the pulse valve showing the triggering device in the closed position thereof;
FIG. 4 is a sectional view similar to that of FIG. 3 but showing the position of the floating diaphragm while the pulse valve is being vented in response to the activation of the triggering device;
FIG. 5 is a view similar to FIGS. 3 and 4 but showing the position of the diaphragm upon completion of the venting of the pulse valve and immediately prior to the closing of the output port therein; and
FIG. 6 is an exploded view of the pulse valve in perspective showing the interior configuration thereof.
DESCRIPTION OF A PREFERRED EMBODIMENT The digital feedback circuit of the present invention is particularly useful in a mechanical system wherein the initial relationship between two mating but individually rotatable components is changed by random displacement of one component from an initial zero position relative to the other component and consequently requires that the latter be correspondingly displaced to restore the initial relative position therebetween. As shown in FIG. 1, one component may be a rotatable shaft 12 responsive to an external force thereon, as indicated by arrows F-F, and the other component a rotatable cylindrical housing 14 surrounding shaft 12 in mating relation and provided with a diametrically spaced pair of sensing orifices 16 therethrough. In the initial zero position therebetween, both orifices 16 are simultaneously blocked by the exterior periphery of shaft 12. A portion of shaft 12 is segmentally slabbed, as indicated at 18, to cooperate with the interior periphery of housing 14 and define an exhaust opening 20 which communicates with one or the other of the two orifices 16 during rotation of shaft 12. While orifices 16 are shown in diametrical alignment in FIGS. 1 and 2, such arrangement is dictated by the particular slabbed contour of shaft 12. Actually, the sensing portion of shaft 12 may possess any contour and orifices 16 may be located at any point along the interior periphery of housing 14 as long as the rotation of shaft 12 will block one orifice 16 while exposing the other. The system also includes a piston 22 slidably housed in a cylinder 24 for reciprocal linear displacement in response to the flow of air against either face thereof. Piston 22 is provided with a rod portion 26 which projects from one end of cylinder 24 into pivotal connection with housing 14, as shown at 29, through suitable mechanical linkage, indicated by 28, capable of converting linear movement of piston 22 to rotation of housing 14 relative to shaft 12.
Each sensing orifice 16 is in communication with a conduit 30 connected to a digital logic element 32 which is, in turn, connected by a conduit 34 to a common pulse valve 36. Fluid, preferably in the form of air pressurized at a regulated 30 psi, is supplied to pulse valve 36 from a self-contained storage unit 38 through a conduit 39. The output flow of pulse valve 36 is equally divided between conduits 34 leading to logic elements 32, conduits 35 leading to a pair of interface valves 50, and a conduit 37 leading to a 4-way locking valve 54. While the supply pressure to each logic element 32 is substantially reduced, preferably to 2 psi, by a suitable restriction 40 in conduit 34, the corresponding supply pressure to interface valves 50 and locking valve 54 is maintained at 30 psi.
Each logic element 32 includes a restrictive bypass 42 for producing a switching flow of reduced pressure air which responds to the absence or presence of backpressure in conduit 30 by diverting the 2 psi supply pressure air between an exhaust leg 44 and an output leg 46 communicating with a conduit 48 leading to interface valve 50.
Conduit 48 is connected to a control port 51 disposed at one end of valve 50 in position to direct the flow of control pressure air against a flexible diaphragm 49 schematically designated as a spring. Valve 50 also includes a supply port 61 in alignment with an oppositely disposed output port 63 and an exhaust port on the same side as supply port 61. The passage of 30 psi operating pressure air through interface valve 50 is diverted between an exhaust or an output flow in accordance with the position of diaphragm 49 as will be hereinafter explained.
The 4-way locking valve 54 includes a pair of input or supply passages 55 in respective communication with conduits 53 leading from interface valves 50, a pair of output ports 56, and a pair of dead-end closures 57 alternating with ports 56. A spring-biased actuator 59 is incorporated in valve 54 in position to respond to the flow of 30 psi air from pulse valve 36 by shifting passages 55 into alignment with output ports 56. In the absence of any flow from pulse valve 36, actuator 59 returns passages 55 into alignment with dead-end closures 57 to prevent further flow of air into and out of cylinder 24.
As best shown in FIG. 3, pulse valve 36 consists of mating members, here shown as rectangular blocks 58 and 60, which are retained in assembled relation by a plurality of screws (not shown). Block 60 is provided with a central well 62 opening at the surface thereof adjacent the surface of block 58. Extending centrally upward from the bottom of well 62 is a cylindrical boss 64 provided with a beveled portion terminating in a planar end face 66. A cylindrical protrusion 68, equal in diameter to the interior of well 62, projects downwardly from block 58 in position to fit into the open end of well 62. The end face of cylindrical protrusion 68 contains a circular recess 70 having a diameter dimensionally between the diameters of well 62 and boss 64. Planar end face 66 of boss 64 is spaced from recess 70 to define a chamber 72 therebetween which communicates with an input port 74 in block 60. The outer end of port 74 is suitably threaded as at 73 to provide a connection for the end of conduit 39. Surrounding the base of cylindrical protrusion 68 is an annular groove 75 containing an O-ring 76 which serves to seal chamber 72 against leakage of air therefrom.
An output port 78 arranged to receive a suitable fitting 79 at the end of conduit 37 extends into block 60 through the center of boss 64 to terminate in a funnelshaped throat 80 opening into chamber 72. Directly opposite output port 78 is a similar port 82 which originates in recess 70 and continues through block 58 to terminate in a fitting 84 arranged to seat a vent valve 86 therein. The outer end of valve 86 is normally blocked by a spring-biased closure 87 adapted to be triggered by the same inertial force which rotates shaft 12 out of the initial zero position thereof. A control chamber 88 formed into the side of block 58 in parallel relation to input port 74 in block 60 is arranged to communicate with port 82 by means of a connecting duct 90. A similar duct 92 extends from control chamber 88 to connect with input port 74 in block 60. Chamber 88 is internally threaded as indicated at 94 in FIG. 6 to accept a valve body 96 which is, in turn, internally threaded to receive a correspondingly threaded stem 98 terminating in a needle-shaped end 100 disposed centrally adjacent to the end of duct 90. The opposite end of valve stem 98 projects from control chamber 88 to terminate in a knurled adjusting knob 102. Rotation of knob 102 serves to advance or retract needle end 100 of stem 98 relative to duct 90 for controlling the flow of air therethrough. A circular diaphragm 104 of a relatively rigid material, such as neoprene or the like, is disposed in chamber 72. The diameter of diaphragm 104 is smaller than that of well 62 but larger than that of circular recess 70 in cylindrical portion 68. As a result, diaphragm 104 is free to be displaced into contact with either boss 64 or protrusion 68 in response to changes in pressure thereagainst.
Operation of the system is initiated by the triggering of vent valve 86 substantially simultaneously with the rotation of shaft 12 out of the initial zero position thereof relative to housing 14. One approach to this requirement is to utilize an inertial mass or pendulum (not shown) suitably fastened to valve closure 87 in position to momentarily respond to the same external force, indicated by arrow F-F, which serves to rotate shaft 12 out of zero position. As soon as closure 87 in vent valve 86 is opened, as best shown in FIG. 4, the resulting escape of 30 psi air through port 82 produces a drop in pressure against the upper face of diaphragm 104 which permits the input air entering chamber 72 to lift diaphragm 104 off end face 66 of cylindrical boss 64 and into contact with cylindrical protrusion 68. In this position, diaphragm 104 blocks circular recess 70 against further escape of the input air through port 82 and, as a result, a major portion of the air flowing into input port 74 is directed into output port 78. At the same time, the remainder of the input air passes through duct 92 into control chamber 88 and past needle end 100 of valve stem 98 into duct 90. Once the compressed spring (not shown) within vent valve 86 returns closure 87 to block the escape of air from port 82, the flow of air therein begins to build pressure against the upper face of diaphragm 104. When the pressure thereagainst exceeds the pressure on the lower face of diaphragm 104, the latter begins to move downwardly in chamber 72, as best shown in FIG. 5, toward contact with end face 66 of boss 64 for blocking throat 80 in port 78. The rapidity with which diaphragm 104 is thus displaced is determined by the position of end 100 of valve stem 98 relative to the inlet end of duct 90.
In order to facilitate identification of the various elements in the schematic circuit of FIG. 2, the numerals for the portion in which the back-pressure is maintained will include the suffix A while the numerals for the portion without the back-pressure will include the suffix B." As shaft 12 is rotated out of the initial zero position thereof relative to housing 14 in response to force F-F thereon, pulse valve 36 is simultaneously actuated to send 30 psi air to interface valves 50A and 508, to actuator 59 of 4-way valve 54 through conduit 37, and through conduits 34A and 34B to respective logic elements 32A and 328. However, suitable restrictions 40A and 40B in conduits 34A and 34B reduce the 30 psi pressure to the 2 psi level required to function logic elements 32A and 32B. As the 2 psi control pressure air enters logic element 32A, a small portion thereof flows out into conduit 38A to create a backpressure therein as long as orifice 16A is blocked by shaft 12. This back-pressure operates, as best shown in FIG. 2., to return the lower pressure air passing through restrictive bypass 42A back toward the junction of exhaust leg 44A and output leg 46A to divert the flow of the 2 psi control pressure air through output leg 46A to conduit 48A leading to interface valve 50A. As a result, diaphragm 49A is flexed to complete a flow channel 67 between supply port 61A and output port 63A for passing the 30 psi operating pressure air into concluit 53A leading to 4-way valve 54. When pulse valve 36 began to provide an output flow of 30 psi operating pressure air into conduit 37, actuator 59 functioned to align passages 55A and 55B with output ports 56A and 56B. Thus, the output flow of air from interface valve 50A is directed through 4-way valve 54 and into housing 24 for action against one face of piston 22.
Logic element 328 functions in the same way until shaft 12 has been rotated sufficiently to unblock orifice 16B whereupon the back-pressure in conduit 30B is exhausted into opening 20 of housing 14 thereby permitting the lower pressure air in restrictive bypass 428 to flow out of logic element 323 and into conduit 308 to also exhaust through opening 20. The resulting drop in pressure functions to switch the input flow of 2 psi control pressure air into logic element 328 from output leg 46B to exhaust leg 443. The resulting absence of any flow of the 2 psi control pressure air into interface valve 50B permits diaphragm 498 to relax and thereby complete a flow channel 69 between output port 638 and exhaust port 658 for permitting a flow of air from the corresponding face of piston .22 through conduit 52B and then through passage 558 in 4-way valve 54 to exhaust from interface valve 50B. During the creation of flow channel 69, supply port 61B is deadended thereby effectively blocking further passage of air therethrough. The resulting decrease in air pressure against the corresponding face of piston 22 produces a differential pressure thereon which imparts linear movement thereto and consequently actuates linkage 28 to restore housing 14 to the initial angular relationship with shaft 12.
Once orifice 16B is again blocked by shaft 12, back pressure is built up in conduit 30B thereby causing the air passing through restrictive bypass 428 to switch the flow of 2 psi control pressure air from exhaust leg 448 to output leg 468 in the manner explained hereinbefore thereby bringing the air pressure against the corresponding face of piston 22 up to the same level as that against the opposite face thereof. At this time, the predetermined time delay action of diaphragm 104 in pulse valve 36 effects the closure thereof and halts further flow of operating pressure air against springbiased actuator 59 in 4-way valve 54. The resulting movement of actuator 59 switches passages 55 into alignment with dead-end closures 57 thereby creating a closed system which locks piston 22 in a stationary position until pulse valve 36 is again energized by the same external force FF applied to shaft 12.
This is an extremely desirable arrangement where a rotatable shaft is incorporated in apparatus which must be operated under circumstances where a continuous supply of air is not readily available. By limiting the air flow in the control circuit to at least the interval in which the initial angular relationship between shaft 12 and housing 14 is being restored, it is possible to supply the required air from a portable container of a size which can be easily replenished and which can be readily transported along with the apparatus to be operated thereby.
The foregoing disclosure and description of the invention is illustrative only. Various changes may be made within the scope of the appended claims without departing from the spirit of the invention.
1. In a mechanical system having first movable means responsive to an external force thereon, second movable means initially disposed in zero relationship with said first means, and piston means responsive to the movement of said first means for actuating said second means to restore said zero relationship thereof with said first means, the improvement of a fluidic digital feedback circuit for actuating said piston means, comprising,
a pulse valve responsive to the initial movement of said first means for supplying an output flow of fluid over a predetermined interval at least equal to the period of time required to restore said zero relationship between said first and second means,
a pair of sensing orifices spaced in said second means in position to be blocked by said first means against the flow of fluid therethrough when said first and second means are in said zero relationship, said orifices being also positioned so that movement of said first means out of said zero relationship with said second means unblocks one of said orifices for the flow of fluid therethrough while continuing to block the other of said orifices,
a pair of interface valves in fluid communication with said pulse valve and in respective fluid communication with the opposite ends of said piston means,
a pair of OR/NOR logic elements disposed in respective fluid communication with said pulse valve, said sensing orifices and said interface valves, and
means in each of said logic elements responsive to the unblocking of the one of said sensing orifices in communication therewith for actuating said corresponding interface valve to exhaust the fluid supplied to one end of said piston means while the other of said interface valves continues to supply fluid from said pulse valve against the other end of said piston means to create a differential pressure thereon for imparting linear movement thereto.
2. The circuit defined in claim 1 including,
restrictive means operative on the flow of fluid from said pulse valve to reduce the pressure of the fluid supplied to sad logic elements.
3. The circuit defined in claim 1 including, a locking ,valve in fluid communication with both of said interface valves, and
actuator means in said locking valve for effecting the closing thereof in response to the termination of the output flow from said pulse valve whereupon the pressure against the opposite ends of the piston means is maintained in equilibrium.
4. The circuit defined in claim I wherein said pulse valve comprises,
an input port,
an output port in fluid communication with said input port,
a floating diaphragm for closing said output port in the absence of a flow of fluid into said input port,
means responsive to initial displacement of said first movable out of zero position for venting the interior of said pulse valve whereby said diaphragm is actuated to open said output port, and
an adjusting valve in fluid communication with said input port and said venting means for controlling the flow of fluid against the face of said diaphragm remote from said output port to provide a predetermined delay in the return thereof to the position for closing said output port.
5. In a mechanical system having rotatable shaft means responsive to an external force thereon, rotatable housing means surrounding said shaft means to establish an initial zero relationship therebetween, and piston means mechanically linked to said housing means and responsive to the rotation of said shaft means for rotating said housing means to restore said zero relationship thereof with said shaft means, the improvement of a fluidic digital feedback circuit for actuating said piston means, comprising,
a pulse valve having internal time delay means responsive to the initial rotation of said shaft means out of said zero relationship with said housing means for supplying an output flow of fluid over a predetermined interval at least equal to the period of time required to restore said zero relationship between said shaft means and said housing means,
pair of diametrically opposed sensing orifices extending through said housing means in position to be simultaneously blocked by said shaft means against the flow of fluid therethrough prior to the movement thereof out of said zero relationship with said housing means, said sensing orifices being also positioned so that one of said orifices is unblocked when said shaft means is rotated out of said zero relationship with said housing means while the other of said orifices continues to be blocked by said shaft means;
a pair of interface valves in fluid communication with said pulse valve and in respective fluid communication with the opposite ends of said piston means whereby said output flow of fluid from said pulse valve is supplied to one end of said piston means whenever said shaft means is rotated to unblock one of said sensing orifices,
a pair of OR/NOR logic elements disposed in respective fluid communication with said pulse valve, said sensing orifices and said interface valves,
means in each of said logic elements for exhausting the flow of fluid therefrom in response to the unblocking of the one of said sensing orifices in communication therewith, and,
means in each of said interface valves responsive to the absence of fluid flow from said corresponding logic element for exhausting the fluid supplied to one end of said piston means while the other of said interface valves continues to supply fluid from said pulse valve against the other end of said piston 10 and an adjusting valve in fluid communication with said input port and said vent port for controlling the flow of fluid against the face of said diaphragm adjacent said vent port to provide a predetermined delay in the return thereof to said first position. 7. The circuit defined in claim 5 including, a 4-way valve in fluid communication with said pulse valve and with the opposite ends of said piston means, and
control means responsive to the initiation of said output flow from said pulse valve for opening said 4-way valve whereby said exhaust flow through one of said interface valves is directed away from said corresponding one end of said. piston means while the flow of fluid through the other of said interface valves is directed against said other end of said piston means, said control means being also responsive to the termination of said output flow from said pulse valve for closing said 4-way valve to retain said piston means against linear movement.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US3215044 *||Jul 24, 1962||Nov 2, 1965||Frederic Lissau||Hydraulic positioning servo system|
|US3215045 *||Oct 15, 1962||Nov 2, 1965||Frederic Lissau||Hydraulic positioning servo system|
|US3524634 *||Oct 14, 1968||Aug 18, 1970||Hoesch Ag||Pneumatic shock absorber arrangement|
|US3641876 *||Nov 12, 1969||Feb 15, 1972||American Hoist & Derrick Co||Two-speed hydraulic control system|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US4510638 *||Jun 2, 1982||Apr 16, 1985||Alten K||Transfer bridge for ramps|
|US4723474 *||Feb 5, 1986||Feb 9, 1988||Smith International, Inc.||Pneumatic stepping actuator positioner|
|US5802944 *||Apr 11, 1997||Sep 8, 1998||Livernois Research And Development Company||Gas cylinder with internal time delay|
|US6491114 *||Oct 3, 2000||Dec 10, 2002||Npk Construction Equipment, Inc.||Slow start control for a hydraulic hammer|
|US7673941 *||Aug 4, 2008||Mar 9, 2010||Humanscale Corporation||Delayed gas spring chair|
|US20080284218 *||Aug 4, 2008||Nov 20, 2008||Robert King||Delayed Gas Spring Chair|
|U.S. Classification||91/35, 91/368|
|International Classification||F15B13/00, F15B13/16|