US3587397A - Acceleration-deceleration pneumatic device - Google Patents

Abstract

A SINGLE PNEUMATIC CYLINDER IS DESCRIBED WHEREIN GAS PRESSURE APPLIED TO ONE FACE OF A PISTON ACCELERATES THE PISTON FOR A PORTION OF A STROKE, WHEREUPON THE PISTON REACHES A GAS RELIEF REGION WHICH PERMITS GAS TO FLOW TO THE OPPOSITE SIDE OF THE PISTON WHERE IT IS COMPRESSED TO DECELERATE THE PISTON ORIFICE MEANS ARE PROVIDED FOR LIMITING GAS FLOW FROM THE SECOND END OF THE CYLINDER FOR CONTROLLING DECELERATION TO BRING THE PISTON TO A STOP AT THE END OF THE STROKE WITHOUT SUBSTANTIAL IMPACT OR REBOUND AND WITHOUT CEASING THE FLOW OF PRESSURIZING GAS USED FOR ACCELERATING THE PISTON.

Description

United States Patent [72] Inventor Berge Hagopian 1,490,633 4/1924 Peters 91/399 Costa Mesa, Calif. 2,463,537 3/1949 Hoor et a1... 91/416 [21] App1.No. 763,688 2,569,504 10/1951 Thierry 91/399 [22] Filed Sept. 30, 1968 2,703,558 3/l955 Wilcox 9l/4l6 [45] Patented June 28, 1971 2,746,425 5/1956 Schafcr 91/416 [73! Assignee North American Rockwell Corporation Primary Examiner Paul E Muslousky Attorneys-William R. Lane. Allan Rotlhenberg and Richard [54] ACCELERATION-DECELERATION PNEUMATIC Seibel DEVICE 2 C1 2D Fi aims rawmg gs ABSTRACT: A single pneumatic cylinder is described U-S. ..i .v wherein gas res ure to one face of a piston ac 92/164, 92/253, 92/258 celerates the piston for a portion of a :stroke, whereupon the [51] ll". piston gaches a gas relief region which ermits gas to flow to 1 9/00 the opposite side of the piston where it is compressed to [50] Field of Search ..91/399, 416 decdcrate the piston O ifi means are provided f Hmiting (cursory); 92/253 (cursory), 253 y) gas flow from the second end of the cylinder for controlling deceleration to bring the piston to a stop at the end of the [56] References cued stroke without substantial impact or rebound and without UNITED STATES PATENTS ceasing the flow of pressurizing gas used for accelerating the 3,175,474 3/1965 Eickmann 92/253 piston.

14 43 Y 37 34 ll [3 42 ACCELlERATION-DECELERATION PNEUMATIC DEVICE BACKGROUND The invention described herein was made in performance of work under a NASA contract and is subject to the provisions of the National Aeronautics and Space Act of 1958, Public SUMMARY OF THE INVENTION Therefore, in the practice of this invention according to a preferred embodiment, there is provided a single cylinder having a piston slidably movable therein with gas relief means within the cylinder for permitting gas flow from one end of the cylinder to the other end thereof at a selected piston location. Means are provided for introducing pressurized gas for accelerating the piston and, after gas flows through the gas relief means, the piston compresses the gas in a chamber having having limited gas flow for deceleration of the piston without substantial rebound or impact shock.

Attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings where:

FIG. 1 comprises a perspective view of a pneumatic device as provided in practice of this invention according to a preferred embodiment; and

FIG. 2 comprises a longitudinal cross section of the device of FIG. 1.

Throughout the drawings like reference numerals refer to like parts.

DESCRIPTION FIG. 1 illustrates a pneumatic acceleration-deceleration device incorporating the principles of this invention. As illustrated in this embodiment there is provided a generally cylin drical housing having a rectangular flange 11 in which bolt holes 12 are provided for mounting the flange on a supporting structure. A rectangular boss 13 is provided on the housing and includes a threaded opening 14.

A first end of the housing 10 is closed by a cap 16 screwed into the housing and sealed thereto by a a conventional O-ring 17 as seen in the the cross section view of FIG. 2. The other end ofthe housing is closed by a cap 18 screwed into the hous ing by spanner holes (not shown) and sealed thereto by an O- ring 19. A gland 21 is seated in the housing for containing a dynamic seal 22 against the end cap 18 and against a piston rod 23 which is slidably movable within the housing. The end cap 18 and gland 21 not only capture the seal 22, but also provide bearing support for the piston rod 23. The dynamic seal 22 may, for example, be a spring loaded rubber seal such as is commercially available from Bal Seal Engineering Company, La Habra, California.

The piston rod 23 has a threaded end 24 extending from the housing for connection to an object to be actuated by the pneumatic device. Nearer the other end of the piston rod 23 is a flange 26 having a small clearance from an inside cylindrical bore 27 in the housing 10. A piston 28 is seated against the flange 26 by a retainer 29 which is secured to the piston rod 23 by a nut 31 threaded thereon. A hexagonal portion 32 on the end of the piston rod 23 permits tightening of the nut 31 and any connection to the threaded portion 24.

A lip 33 on the flange 26 overlaps an end of the piston 28 to hold in place a circular rubber seal 34 having a generally U- shaped cross section. A plurality of radially extending holes 36 are provided through the lip 33 to permit gas to freely enter the seal in the open part of the U. Similarly, a lip 37 is provided on the retainer 29 for holding captive a similar U-shaped cross section seal 38 facing in a direction opposite that of the seal 34. A plurality of radially extending holes 39 are provided through the lip 37 to permit gas to freely enter the open portion of the U-shaped seal. Seals 34 and 38 are commercially available under the trademark Omniseal," from Airoquip Corporation, Burbank, California.

In operation, gas pressure within the open part of the U- shaped section of the seal causes expansion thereof so that good sealing is obtained between the piston 28 and the cylindrical bore 27 in the housing. The pair of oppositely facing seals 34 and 38, respectively, are employed so that the piston operates equally well with higher pressure gas on the side facing the cap 16 as compared with the side facing the cap 18 or vice versa. The significance of this will be appreciated hereinafter when it is recognized that one seal 38 is operable during acceleration of the piston and the other seal 34 is operable during deceleration of the piston. The seals 34 and 38 are not only tightly sealed by gas pressure therein, but also are retained captive by the lips 33 and 37, respectively, so that they do not blow out" when the piston is out of contact with a surrounding wall. The piston 28 is also statically sealed to the piston rod 23 by a conventional O-ring 41.

Within the housing 10 there is a cylindrical bore 27 as hereinabove described. The bore 27 continues as a single cylindrical bore 27' on the opposite side of an enlarged portion 42 in the bore of the housing, intermediate the ends thereof. The enlarged portion 42 includes a pair of oppositely facing conical tapered regions 43 extending between the single, constant diameter cylindrical bore 27, 27' and the enlarged portion 42. In a preferred embodiment the angle between the conical portions 43 and the cylindrical bore 27, 27 is about 15 in order obtain optimum operation ofthe seals 34 and 38 without cutting thereof. Proper gas flow around the piston length of stroke is provided in a minimum distance during operation by the gently sloping, conical surfaces 43 and the enlarged portion 42. The seals 34 and 38 are gradually and uniformly pressed into position in the piston 28 by the surfaces 43 as it traverses the gas relief portion 42 and a large number of cycles of operation of the piston with this arrangement demonstrates that no appreciable seal wear is obtained.

At one end of the housing 10 a plenum 44 is provided communicating with the gas inlet port 114 for introducing highpressure gas to one side of the piston for causing movement of the piston and piston rod.

An orifice 46 is provided through the end cap 15 and one or more holes 47 are provided through the gland 21 so that the orifice 46 is in good fluid communication with the interior of the housing. The gland 21 may be keyed to the cap 18 so that the orifice 46 and a hole 47 are aligned upon assembly of the device; however, alignment is unnecessary since a chamfered corner on the gland provides a gas flow path to the orifice. The orifice is provided for bleeding gas out of the cylindrical bore 27' at a controlled rate. This rate is readily predetermined by changing the orifice diameter (area) and a plurality of caps having different orifice diameters are readily employed for varying the orifice size to provide controlled deceleration in a selected situation. Rather than a plurality ofcaps, it will be apparent that orifice inserts may be provided in the cap or in the side of the housing for obtaining various orifice sizes. Similarly, an orifice plus a needle valve or the like may be employed for control of gas flow.

It should be noted that in addition to gas flowing from the interior of the housing through the orifice 46 there is also some gas leakage past the dynamic seal 22 around the piston rod and the seal 34 on the piston. It is found in some situations that this virtually unavoidable gas flow at the dynamic seals is sufficient to provide an acceptable deceleration. A dynamic test of a particular sealing arrangement indicates the equivalent orifice area of the seal and the orifice is sized accordingly to bring the gas pressure on the compression side of the piston to substantially the same pressure as the gas on the driving side of the piston only at the end of the stroke so that the piston stops without substantial impact or rebound, the energy of the moving mass is irreversibly transferred to the gas compressed beneath the piston.

In operation the piston is positioned in the housing in the location illustrated in FIG. 2 either by hand or by some pneumatic mechanism (not shown) for providing a return stroke. When a power stroke is desired pressurized gas is rapidly admitted to the plenum 44 to bear on the face of the piston and cause acceleration of the piston rod along the first part of the stroke while the piston is in sealing engagement with the cylindrical bore 27. When the piston reaches the enlarged portion 42 of the bore in the housing, gas is free to pass around the piston through the enlarged portion from the gas plenum 44 to the portion of the cylindrical bore 27' on the other side of the piston thereby equalizing pressure on both sides of the piston.

Inertia of the moving piston and any mass attached to the piston rod cause the piston to travel rapidly through the enlarged portion 42 and bring the piston into sealing engagement with the cylindrical bore 27 to again form a substantially closed chamber behind the piston. The seals 34 and 38 on the piston may expand as the piston reaches the enlarged portion 42. The lips 33 and 37 keep the seals from being dislodged, however. The conical sloping portions 43 gradually and uniformly compress the seals and bring them into sealing engagement with the bore 27'. It should be noted that at the instant the piston seals against the cylindrical bore 27 the pressure on both sides of the piston is substantially identical.

As the piston continues to travel toward the closed end of the housing the gas in the chamber ahead of the piston is compressed, thereby raising the pressure above that on the driving side of the piston and generating a decelerating force against the piston to bring it and any mass attached to the piston rod to a stop. If the chamber in the housing behind the piston were completely closed so that no gas could escape therefrom the increasing gas pressure would stop motion of the piston, however the energy stored in the gas would actually cause a reversal of motion to the cause the piston to rebound with an acceleration substantially the same as the deceleration. In order to prevent any substantial rebound, the orifice 46 is provided so that there is a means for bleeding gas out of the substantially closed chamber at a controlled rate, both for preventing rebound by venting the compressed gas and providing a controlled deceleration of the piston. It will, of course, be recog nized that if the orifice is too large insufficient compression of a gas behind the piston is obtained and the piston may have substantial impact load so it bottoms against the gland 2]. Proper orifice sizing is therefore important to achieve an appropriate deceleration without impact load at the end of the stroke or rebound of the piston.

The orifice area for controlled deceleration can be approximated on the basis of the kinetic energy of the moving piston and any object connected thereto at the beginning of the com pression or deceleration portion of the stroke, the stroke length over which deceleration is desired (i.e., the maximum deceleration), and the working pressure of the gas. The kinetic energy is determined by the mass of the piston and any object connected thereto and its velocity (K.E.=%Mv or kinetic energy equals one-half mass times velocity squared). The velocity is dependent on the gas pressure, piston area, stroke length and any resisting forces (including the mass of the piston and any object connected thereto)(v=as or velocity squared equals two times acceleration times stroke; and a=PA/M or acceleration equals driving pressure times piston area over mass). The work done on the gas compressed by the piston must balance the kinetic energy of the moving mass at the end ofthe stroke. Assuming an adiabatic compression, the work is dependent on the final and initial energy contents of the gas (Work=(P,V,-P,V,)l l--k or work is equal to the final pressure times volume less the initial pressure times volume, all over l-k where k=l .4 for adiabatic compression). The initial pressure, volume and work are known, and assuming that there is no gas leakage, the final pressure and volume are found for full energy absorption in the compressed gas. This pressure, P is used to find an average pressure behind the piston which approximates the numerical average of the starting (and ending) pressure and the maximum pressure P,. The weight of gas behind the piston is known from the volume and density and the time of the compression stroke is known from the deceleration and stroke, therefore the average mass flow rate is found. If the gas pressure outside the orifice is less that 0.53 times the pressure inside the orifice the velocity of gas through the orifice is a known terminal value (e.g., 1,020 ft./sec. for air) and the required orifice area is found from the mass of gas that must flow through the orifice during the time of the stroke.

It is apparent that the determination of an orifice area as set forth is only approximate and that a precise finding would require a solution of integral equations accounting for the changing mass flow through the orifice with changing gas density due to changing pressure, and the changing volume of the trapped gas. it appears that an orifice with an effective area about two-thirds of that found by the above technique is effective. It should be noted that an approximate determination of orifice area is quite satisfactory since some leakage is virtually certain around the dynamic seals and the actual orifice area for a particular application is best found empirically, commencing with an orifice area estimated as pointed out herein.

In a specific application of an acceleration-deceleration device it was desired to rapidly move a l50pound mass through a distance of about 2% inches with a friction force of about 720 pounds during the first three-fourths inch of travel. For this purpose four pneumatic acceleration-deceleration devices were employed to provide symmetrical loading. Each device had a total stroke of 2% inches a piston diameter .of 0.738 inch. An operating pressure of 750 p.s.i.g. was used to drive the piston through the first three-fourths inch of stroke and this pressure was applied at the beginning of the stroke and maintained until the end of the full 2% inch stroke. The velocity at the end of the power stroke was about 3.8 feet per second. The piston (and mass attached thereto) traveled about three-eighths inch through the enlarged portion of the cylinder bore between the positions where one piston seal left engagement with the cylinder wall and the other seal engaged the continuation of the cylinder wall. The enlarged portion had a diameter of seven-eighths inch which permitted unrestricted gas flow. During this travel little change in velocity occurred and gas flowed freely around the piston to equalize pressure on both sides thereof at about 750 p.s.i.g, The total deceleration stroke was about 1% inch long, compressing gas behind the piston and forcing it out through the relief orifice to bring the piston to a stop without substantial impact or rebound. A maximum acceleration of about 10 gs was mea sured on the power and deceleration portions of the stroke. An orifice with an effective area of a little less than 0.0006 square inch including the leakage around the dynamic rod seal and piston seal provided excellent results with high reliability and reproducibility.

It will be apparent that many modifications and variations can be made in a pneumatic acceleration-deceleration device without departing from the principles of this invention. Thus, for example, in addition to the above suggested variations in the orifice, the piston arrangement could be modified, the piston rod could extend from the opposite end of the housing to provide a pulling" rather than pushing motion, other mounting arrangements can be provided, and the like.

lclaim:

1. A pneumatic acceleration-deceleration device comprisin a housing having a cylindrical bore;

a piston slidably movable in said housing;

a dynamic seal for sealing said piston to said cylindrical bore for substantially preventing gas flow therearound, while said seal is seated on said cylindrical bore;

gas relief means intermediate the ends of said bore for dividing said bore into a first cylindrical region towards one end and a second cylindrical region towards the other end, whereby gas can flow from said one end to said other end when said piston is positioned adjacent said gas relief 5 means;

means for introducing pressurized gas into said one end for accelerating said piston toward said other end;

orifice means for providing limited gas flow from said other end ofsaid cylindrical bore whereby gas in said other end of said cylindrical bore is compressed by said driven piston for gradually decelerating said piston without sub stantial rebound or impact;

said gas relief means comprising a first conical tapered enmeans on said piston for preventing said dynamic seal from being dislodged when said piston is positioned adjacent said gas relief means;

said dynamic seal comprising a first sealing member arranged to prevent gas flow from said one end to said other end of said cylindrical bore when the pressure in the one end exceeds the pressure in the other end; and a second sealing member arranged to prevent gas flow from said other end to said cylindrical bore when the pressure in said other end exceeds the pressure in said one end;

said housing comprising a cylindrical housing body; and an end cap for said housing body; and

said orifice means being in said end cap so that interchanging of end caps can effect a change in effective orifice area.

2. A pneumatic acceleration-deceleration device as defined in claim 1 wherein said piston comprises:

a piston rod extending through said end cap;

a flange on said piston rod;

a lip on said flange;

a retainer removable secured on said rod;

a lip on said retainer, the lip on said flange and the lip on said retainer facing towards each other; and

a piston member between said flange and said retainer, said piston member cooperating with said lips for preventing said first and second sealing, members from being dislodged when said piston is adjacent said gas relief means.

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