GB2126316A - Temperature compensated gas spring - Google Patents

Abstract

A piston cylinder gas spring uses a combination of fluids to achieve a spring force which is more nearly constant with change in temperature than it would be if a single fluid were used. A primary pressure source is located within the casing (in chambers 26, 34) and acts against the piston 24 to urge the rod 30 out of the casing. A secondary pressure source is located within the casing (in chambers 28, 36) and acts against the piston to urge the rod into the casing. The primary pressure created by the primary pressure source is greater than the secondary pressure created by the secondary pressure source, and the percent change of the secondary pressure with temperature variation is greater than the percent change of the primary pressure with the same temperature variation. The primary source is nitrogen. The secondary source is a two phase liquid/vapour system e.g. acetylene, freon, ammonia, hydrogen bromide, sulphur hexafluoride. <IMAGE>

Description

SPECIFICATION Temperature compensated gas spring This invention relates to gas springs and more particularly to gas springs which are automatically compensating so as to operate uniformly over a broad temperature range.

The springs used to support automobile trunk lids, hoods, and the like, especially the hatch-back trunk lid, are often ofthe gas spring variety. Agas spring is essentially a sealed cylinder containing a gas under high pressure and having a piston rod extending from one end of the cylinder. Typically, nitrogen gas having a pressure of approximately 1000 psi is used in the cylinder. The spring force results from the pressure ofthe gas acting on a cross sectional area equal to that ofthe rod within the cylinder and urging the rod outwardly. When the rod is pushed into the cylinder, as when the hatch-backtrunk lid is closed, the rod displaces a certain volume within the cylinder which was previously occupied bythe gas.Since the total volume within the cylinder is fixed, the remaining volume available to the gas decreases, resulting in an increase in the pressure of the gas. Thus, the force acting to move the rod outward increases. In conventional gas springs, a piston-like structure may be attached to the rod inside of the cylinder and used for damping and limiting the extent of motion of the rod. Sincethe gas pressure is normally equal on both sides ofthe piston, it produces little if any force on the rod.

Ideally, the pressure of the gas should be sufficient to move the piston rod outwardly from the cylinder and liftthe trunk lid or the like which is attached thereto. The gas pressure should also be low enough when the rod is completely extended and the trunk lid orthe like is raised to enable a person to easily move the rod into the cylinder when the trunk is being closed. A drawback arising from the use of a single gas in a gas spring is that the pressure of any gas in a fixed volume is related to thetemperature of the gas.

For an ideal gas, which nitrogen resembles, the pressure is directly proportionl to the absolute temperature of the gas. This dependence can cause considerable problems when such gas springs are used in automobiles that are exposed to ambient temperatures ranging from below 0 F to above 1 00 F.

When the ambient temperature is low the pressure ofthe gas inside the cylinder is low, resulting in insufficient force to urge the rod outwardlyto liftthe weight of the trunk lid. When the ambienttemperature is high the pressure ofthe gas inside the cylinder is high, resulting in a large force urging the rod out of the cylinder, a situation which may cause a trunk lid connected to the rod to raise very quickly and strike a person opening the trunk. Furthermore, when the ambient temperature is high, the gas pressure inside the cylinder is large when the rod is completely extended, making itdifficultto move the rod into the cylinderwhen it is desired to close the trunk lid.

Shock absorbers which automatically compensate for changes in ambient temperature are known in the art. See,forexample, U.S. Patent No. 2,944,639 to Blake U.S. Patent No. 3,107,752 to McCean, U.S. Patent No.3,301,410 to Seay, U.S. Patent No.

3,971,551 to Kendall et al., and U.S. Patent No.

3,944,197 to Dach icou rt. These devices generally provide an extra chamber orthe like within the shock absorber to accommodate the changing volume of the primary damping fluid as the ambienttemperature changes. Such devices are not appropriate for providing temperature compensation in a gas spring because they are concerned with keeping the fluid volume constant, ratherthanwith keeping an outwardly directed pressure constant.

It is an object ofthe present invention to provide a gas spring in which the sensitivity of its spring force to temperature variation is reduced to an acceptably low level.

The invention is a temperature compensated gas spring which includes a sealed casing, a slidable rod extending from the interiortothe exteriorofthe casing through one end thereof, and a piston mounted to the rod within the casing. A primary pressure source is located within the casing and acts againstthe piston to urge the rod out of the casing, and a secondary pressure source is located within the casing and acts against the piston to urge the rod into the casing. The primary pressure is greaterthan the secondary pressure, and the percent change of the secondary pressure with temperature is greater than the percent change ofthe primary pressure with temperature.

The primary pressure source is preferably a pressurized primary gas, such as nitrogen gas, whose pressure varies essentially proportionally with absolute temperatue and which remains in the gas phase overthetemperature range to which the gas spring is exposed. A desirabletemperature range is -30"C to 80"C.

The secondary pressure source is preferably the vapor pressure of a two-phase system in which the liquid and vapor phases are in equilibrium over the temperature range of -30"C to 80"C. Such vapor pressure varies approximatelyexponentiallywith absolutetemperature. Suitabletwo-phase systems include acetylene, ethane, FREON-i 2, FREON-i 3, FREON-114, propane, propadiene, perfluoropropane, dimethyl ether, N - butane, ammonia, hydrogen bromide and hydrogen iodide.The secondary pressure source may also be a two-phase system in which the liquid and vapor phases remain in equilibrium over a substantial portion ofthe temperature range of -300C to 800C, such as sulfur hexafluoride.

In the preferred embodiment ofthe invention the sealed casing includes a cylindrical tube with a closed end wall at one end and an end wall having an opening to allow the rod to pass therethrough atthe other end. An innertube is coaxially disposed within the casing, and one end ofthe innertube is attached to the closed end wall. The piston is disposed within the innertube and dividestheinnertubeintoafirst inner volume between the piston and the closed end wall and a second innervolume in the remainder of the innertube. A baffle is located between the inner and outertubes to divide the casing volume, exterior of the innertube, into a first outervolume adjacent the closed end wall and a second outervolume adjacent the end wall with the opening.Afirstconduit means permits fluid flow between the first inner volumeandthefirstoutervolume,andasecond conduit means permitsfluid flow between the second inner volume and second outer volume. The primary pressure source is located in the first inner volume and first outer volume and the secondary pressure source is located in the second inner volume and the second outer volume.

Thefirst conduit means is preferably one or more holes through the inner tube beyond the extent of travel of the piston toward the closed end wall. In one embodiment the inner tube isshorterthan the cylindrical tube, and the inner tube is in fluid flow communication with the second outervolume to form the second conduit means. In a second embodiment, the innertube is the same length as the cylindrical tube, and both ends are attached to the end walls of the casing, and the second conduit means is one or more holes through the inner tube beyond the extentoftravel ofthe piston toward the end wall with the opening.

The gas spring may also include a stop which limits the travel ofthe piston. Afluid seal is provided between the piston and the innertube and between the rod and the wall through which it passes.

Figure 1 is an axial, sectional view of a first embodiment of a gas spring in accordance with the present invention; Figure 2 is a graph showing force as a function of temperature ofthe gas spring shown in Figure 1 wherein the primary pressure source is nitrogen gas and the secondary pressure source is ammonia liquid and vapor in equilibrium; Figure 3 is an axial, sectional view of a second embodiment of a gas spring in accordance with the present invention; Figure 4 is a log-log graph showing the phase diagram for FREON-12; Figure5 is a graph of net outward spring force versus temperature of the gas spring shown in Figure 3wherein the primary pressure source is nitrogen gas and the secondary pressure source is FREON-i 2; Figure 6 is a log-log graph showing the phase diagram forsulfur hexafluoride: and Figure 7 is a graph of net outward spring force versus temperature ofthe gas spring shown in Figure 3 wherein the primary pressure source is nitrogen gas and the secondary pressure source is sulfur hexafluoride.

One embodiment of a temperature compensated gas spring in accordance with the present invention is shown in Figure 1. The gas spring 10 includes a sealed casing 12 made of an outer sleeve ortube 14 with a closed end wall 16 mounted at one end and with a wall 18 mounted atthe other end. Wall 18 has an opening 20 thereth rough. The gas spring 10 includes an inner sleeve ortube 22 located inside of and coaxial with casing 12. One end of innertube 22 is attached to end wall 16, and the other end is axially spaced from the wall 18. A piston 24 is located within innertube 22 and divides the interior of innertube 22 into a first inner volume 26 between closed end wall 16 and piston 24and a second innervolume 28 on the side ofthe piston toward wall 18.An elongated rod 30 is attached to piston 24 and extends out of casing 12 through opening 20 in wall 18.

A baffle 32 is attached to and extends between the outside of innertube 22 and the inside ofoutertube 14 and divides the casing volume, exclusive of the innertube, into a first outervolume 34 adjacent wall 16 and a second outer volume 36 adjacentthewall 18.

The baffle is peferably an annular plate. The baffle 32 also provides support to innertube 22. One or more holes 38 are provided in inner tube 22 at a location beyond the extent oftravel of pistion 24 therein in the direction toward end wall 16. Holes 38 form a first conduit means which permits fluid flow between first innervolume 26 and first outervolume 34. Inthe embodiment shown in Figure 1 outervolurne 34. In the embodiment shown in Figure 1, inner tube 22 is open at the end closestto wall 18 and forms a-second conduit means which permits fluid flow between second inner volume 28 and second outervolume 36.

The gas spring 10 may include a stop 40 at the free end of innertube 22 which limits the travel of piston 24toward the wall 18 and retains the piston within inner tube 22. Stop 40 shown in Figure lisa washer-like plate attached to the end of innertube 22 which surrounds, but does not contact, rod 30 to leave an annular space 42 between rod 30 and stop 40 forfluid flow. The gas spring 10 includes a first seal 44 between piston 24 and the innersurface of inner tube 22 to prevent fluid flow between the first and second inner volumes 26,28 and includes a second seal 46 between rod 30 and wall 18to prevent fluid flow between the interior and exterior of casing 12.

The gas spring 10 also includes a rod 48 attached to the exteriorofclosed end 16 of casing 12 having an eye 50 at its end. Rod 30 has eye 52 at its free end exterior of casing 12. Eyes 50 and 52 allow gas spring 10 to be mechanically connected between two points, such as between the body and the trunk lid of an automobile. Preferably innertube 22, outertu be 14 and piston 24 are cylindrical in shape.

A primary pressure source is located in first inner volume 26 and first outervolume 34 and acts against piston 24to urge rod 30 outwardly of casing 12. A secondary pressure source is located in second inner volume 28 and second outervolume36 and acts against piston 24to urge the rod into the casing. The net force acting on piston 24 results from the difference between the forces from the primary and secondary pressure sources. Since gas spring 10 is to function as a spring with an outwardly directed spring force, it is necessarythatthe primary pressure be greater than the secondary pressure.

Preferabiy, the primary pressure source will be a pressurized primary gas whose pressure varies proportionally with absolute temperature and which will remain gaseous overthetemperature range to which the gas spring is exposed. A preferred primary gas is nitrogen gas which behaves essentially accord ing to the ideal gas law (PV = nRT) overthe temperature range of -30 C to 80"C. It will be recognized in the artthat no gas will perform exactly in accordance with the theoretical ideal gas law.

Other gases which may be used include argon, helium, hydrogen, krypton and neon.

The reduction oftemperature sensitivity in gas spring 10 is accomplished by providing a reverse force on piston 24 from the secondary pressure source which tends to cancel out the extra force from theprimarypressuresourcedueto increases in temperature. The secondary pressure is chosen to behave quite differently from the essentially perfect gas behaviour ofthe primary gas. In one aspect of the invention, the secondary pressure source is the vapor pressure of a two-phase system in which the liquid and vapor phasesare in equilibrium. Thevapor pressure of such a two-phase system varies approx imatelyexponentiallywith absolute temperature rather than directly proportionally.The main requirement of any secondary pressure source selected is thatthe percent change of secondary pressure with temperature be greaterthan the percent change of the prima ry pressu re with tem pe ratu re.

There are many organic and inorganic substances that can serve as asecondary pressure source, including acetylene, ethane, FREON-12, FREON-13, FREON-114, propane, propadiene, perfluoropropane, dimethyl ether, N - butane, ammonia, hydrogen bromide, and hydrogen iodide. The vapor pressure of these substances rangefrom about 0 to 150 pounds per square inch (psi) at a temperature of about -30"C to about 100 psi to over 900 psi at 70"C. In a two-phase system, for a given substance the pressure exerted by its vapor will depend only on temperature.The best substance to use in a given application is determined bydesign requirements forthe application, such as spring force, spring size, material cost, manufacturing cost, seal lifetime, and degree of temperature compensation desired.

It is not abolutely necessary th atthe secondary pressure be generated by a two-phase system. As described in detail hereinafter in connection with Example 3, sulfur hexafluoride can be used as the secondary pressure source. Above a critical tempera ture, sulfur hexafluoride cannot exist as a two-phase system, but exists solely as a vapor with no liquid phase present. However, temperature compensation is achieved even above the critical temperature because the percent change ofthe sulfurcompensation is achieved even above the critical temperature because the percent change of the sulfur hexafluoride vapor pressure (i.e. the secondary pressure) with temperature will still be greaterthan the percent change of a perfect gas pressure with temperature.

Since a substance will remain in a two-phase system with its vapor and liquid phases in equilib rium only for certain ranges of specific volume, a requirementisplacedonthevolumeavailableforthe substance in the gas spring. In general it is desired that both the liquid and vapor phases always be present so that the vapor pressure will depend only on temperature. As the spring is compressed, i.e., as the piston 24 is moved toward wall 16, the volume available forthe two-phase system is increased. If initially there is an insufficient amount of the liquid phase ofthe substance, such an increase ofthe total available volume could cause all ofthe liquid to convertto vapor.The pressue of this vapor will in general vary with the temperature in a fashion similar to other gases and thus provide little if any temperature compensation. However, if too much ofthe substance is used, a problemmariseswhenthe spring is allowed to expand, thus reducing the volume available for the substance. This reduction in volume could cause all of the vapor phase to condense, forcing the substance entirely into the liquid phase.Thiswouldeffectivelypreventthe piston from moving any further.

To avoid these possible problems with a two-phase system used as a secondary pressure source, the following requirements must be met: 1. The minimum amount ofthe substance neces saryisthatwhich is just sufficient to provide a two-phase system when the gas spring is fully compressed i.e., when the available volume is the greatest, at the highesttemperature to which the gas spring may be exposed, and 2. The volume availableforthe substance should be sufficiently large so that the vapor does not entirely condense into liquid when the gas spring is fully extended i.e. when the available volume is the smallest. The limiting environmentforthis second requirement is also the highesttemperature to which the gas spring may be exposed.

The concentric tube arrangement shown in Figure 1 is particularly advantageous in a gas spring. A sufficient additional volume exterior of inner tube 22 is provided in first outer volume 34forthe primary gas so that it is not excessively compressed when rod 30 is completely retracted within the gas spring.

Otherwise, the resulting excessive pressures could cause undesirable and excessive spring forces. Likewise, an additional volume exterior of innertube 22 is provided in second outer volume 36 forthe secon- dary pressure source so that it is not excessively compressed when rod 30 is completely extended.

All of the components of gas spring 10, with the exception of seals 44 and 46 are made of a metal with sufficient strength to withstand the pressures of the confined gases. The use of a cylindrical inner tube 22, outertube 14, and piston 24 is particularly advantageous.

EXAMPLE 1 Thefollowing is an example of a gas spring 10 in accordance with the embodiment shown in Figure 1 using nitrogen gas asthe primary pressuresource and using ammonia as the two-phase system forthe secondary pressure source. The relationship between the various parameters involved in gas spring 10 can be described algebraically using the following variables: : Ag = area ofthe piston on which the nitrogen gas pressure acts (in.2) Av = area ofthe piston on which the ammonia vaporpressureacts (in.2) Dp = diameterofthe piston (in.2) Dr = diameterofthe rod (in.) F = force of the gas spring (Ib.) Pg = pressureofthe nitrogen gas (psi) Pv = pressure ofthe ammonia vapor (psi) Po = nitrogen gas pressure at20 C (psi) T = temperature ( C) The net outward spring force, F, is determined by subtracting the force acting on the piston due to the ammonia vapor from the force acting on the piston due to the nitrogen gas.The equation for calculating F, ingnoring the force of atmospheric pressure on the rod 30, is: (1) F=AgPg-AvPv The pressure ofthe nitrogen gas is reasonably well represented by: (T + 273) Po (2) Ps= 293 Forthis example it is assumed that the desired spring force F is 100 Ib. atthetemperature extremes of -30"C and 70"C. The vapor pressure of ammonia in a two-phase system can be determined from stan dard and well known handbooks such as Chemical Engineers Handbook, edited by John H. Perry (McGraw-Hill, 1950, 3d. Edition). At -300C the vapor pressure of ammonia is 20 psi, and at 70"C the vapor pressure is 475 psi.By inserting these values, the desired F = 100 Ib., and equation 2 into equation 1,the following equations are obtained: Ag (-30 + 273) Po (3) 100= - 20 Av 293 Ag (70 + 273) Po (4) 100= = - -475Av 293 Solving equations 3 and 4 simultaneously yields: (5) Av=0.0921 in.2 (6) AgPo=122.80lb.

If the rod diameter, Dr, is chosen to be 5/16", a value typical for gas springs, the area of the piston, Av, on which the ammonia vapor pressure acts, is: (7) Av = n/4 (Dg2 - Dr2) = rut/4 (Dg2 - (5/16)2) = 0.0921 in.2 The solution of equation 7 results in Dg = 0.4636 in., from which it is determined that Ag = 0.1688 in.2.

Using equation 6 above, the necessary fill pressure, Po, for the nitrogen gas at 20"C is 727 psi.

By loading the first inner volume and first outer volume with nitrogen gas at 727 psi at 20"C, selecting the rod diameter to be 5/16" and the piston diameter to be 0.4636 in., and loading the second innervolume and second outervolumewith anamountofammo- nia such thatthe liquid and vapor phaseswill remain in equilibrium, the gas spring will have a nominal outward force of 100 Ib. at the temperature extremes of -30 C and 70"C. The behaviour ofthis gas spring at othertemperatures can be determined by calculating Pg at othertemperatures using equation 2 above, determining the values of Pv at various temperatures, and calculating the forces using equation 1 above.

Table 1 below lists this data overthe desired temperature range.

TABLE 1 Pressure Spring Force Temp. PgAg PvAv F (C) (psi) (psi) (lib) (Ib) (lib) -30 603.9 20 101.8 1.8 100.0 -20 627.8 30 106.0 2.8 103.2 -10 652.6 45 110.2 4.1 106.1 0 677.4 65 114.4 6.0 108.4 10 702.2 90 118.6 8.3 110.3 20 727.0 125 122.8 11.5 111.3 30 751.8 170 127.0 15.7 111.3 40 776.6 230 131.2 21.2 110.0 50 801.4 294 135.4 27.1 108.3 60 826.2 370 139.6 34.1 105.5 70 851.1 475 143.7 43.7 100.0 The above results are shown graphically in Figure 2 where curve "A" represents the outward force acting on the piston (PgAg), curve "B" represents the inward force acting on the piston (PvAv), and curve "C" represents the net spring force ofthe gas spring of Example 1, all as a function oftemperature. This gas spring has a maximum force of about 111.3 lb.

between 20" and 30"C, and a minimum force of 100 Ib.

at the temperature extremes. The temperature compensation ofthe gas spring of Example 1 cam be compared with the use of nitrogen gas alone by comparing the maximum and minimum spring forces developed with the force at 20"C being the standard. The deviation is about 10% forthe gas spring of Example 1 while the deviation is about 34% for a gas spring using nitrogen gas alone. It can be appreciated thatthe gas spring ofthe present invention considerably reduces the variation of spring force with temperature as compared with the use of nitrogen gas alone.

Asecond embodimentof atemperature compensated gas spring in accordance with the present invention is shown in Figure 3. The gas spring 60 shown in Figure 3 has many elements which are identical with elements in the gas spring 10 shown in Figure 1 and discussed above. Accordingly, like elements in both figures are referred to be like reference numbers, and onlythe differences between the two embodiments will be discussed.

In the gas spring 60 shown in figure 3, the inner tube 62 extends completely between end walls 16 and 18 and is attached to both ends walls. The second inner volume 64 is located within innertube 62 between piston 24 and wall 18, and second outer volume 66 is located exteriorofthe inner tube between baffle 32 and wall 18. One or more holes 68 are located in innertube 62 ata location beyond the extent of travel of piston 24toward wall 18, as determined by stop 40. Holes 68 form a conduit means between second inner volume 64 and second outer volume 66.

Otherthan the above mentioned structural differences the embodiment ofthe gas spring 60 shown in Figure 3 operates exactly the same as the gas spring 10 shown in Figure 1.A primary pressure source is located in the first inner volume 26 and first outer volume 34 and a secondary pressure source is located in second inner volume and second outer volume 66, as described above in connection with Figure 1.

The relationship between the various parameters involved in gas spring 60 can be described algebraically using the following variables: Ag = area ofthe piston on which the primary gas pressure acts (in.2) Ar = area of the rod on which atmospheric pressure acts (in.2) Av = area of the piston on which the secondary pressure acts (in.2) Dp = diameterofthe piston (in.) Dr = diameter ofthe rod (in.) Dt = inner diameter ofthe outer tube (in.) F = net outward force ofthe gas spring (lb.) L = length of the inner and outer tubes (in.) Ls = distance between stop 40 and the wall 18 (in.) Lt = length of gas spring between eye 50 and eye 52, which varies with X (in.) Lv = distance between baffle 32 and the wall 18 (in.) M = mass ofthe secondary pressure source material (Ib.) Pa = atmospheric pressure (psi) Pg = pressure ofthe primary gas (psi) Po = pressure ofthe primary gas at 200C (psi) Pv = pressure of the secondary pressure source (psi) S = stroke, or maximum design value forthe spring compression X (in.) T = temperature ("C) Vg = volume of the primary gas which varies with X (in.3) Vv = volume of the secondary pressure source material, also varying with X (in.3) Wc = thickness ofthe inner tube (in.) Wp = thickness of the piston (in.) Ws = thickness of the baffle (in.) X = amount of spring compression, measured from the stop to the piston (in.) The characteristics of the preferred embodiment, shown in Figures 3, can be described by selecting initial values for the following parameters: Dp, Dr, Dt, L, Ls, Lv, Lt (maximum), Pa Po, S, Wc, Wp, and Ws. It is also necessary to know the dependence of both Pg and Pv on temperature. For Pg it is usually sufficient to use the perfect gas law. For Pv, the dependence of vapor pressure on temperature for the particular substance selected can be obtained from well known handbooks.

The following equations relate to the remaining parameters to the initial ones listed above.

(8) Ag = 7rDp 4 3,Dr2 (9) Ar = 4 (10) Lt (minimum) = Lt (maximum) - S (11) Lt=Lt(maximum)-X (12) Av=Ag-Ar (13) Vg = 7r/4 (Dt2(Dp + 2Wc)2) (LLvWs) + Ag (LWpLs - X) (14) Vv=T/4(Dt-(Dp+2Wc)2)Lv+Av(X+Ls) The pressure ofthe primary gas is a function of X, through thevolumeVg, ofthetemperatureT, and of the fill pressure Po. Underthe assumption that gas filling is done with the rod extended (i.e.X = 0), and that the gas essentially obeys the ideal gas law, its pressure is given by: (15) Pg = Po(T + 273) Vg (maximum) 293 Vg The equation for the spring force as a function of compression and temperatures is given by: (16) F= PgAgPvPaAr EXAMPLE2 In this example, a gas spring in accordance with the second embodiment using a two-phase secondary pressure source is chosen to have an extended length of 50 inches and a stroke of 20 inches. At a temperature of 20"C it will have a net spring force F of 150 Ib. when extended and 160 Ib. when compressed.

The primary gas is nitrogen and the secondary pressure source substance is FREON-12 which is maintained such that there is always both the vapor and the liquid phases over the operating temperature range. The temperature compensation is to extend from -30 Cto 80 C.

The values ofthe initially selected parameters are: Dp = 0.593 in.

Dr = 0.3125 in.

Dt=2.25in.

L = 26.0 in.

Ls=3.214in.

Lv = 0.179 in.

Lt (max) = 50.0 in.

M =0.0400 Ib.

Pa = 14.7 psi Po = 604.4 psi s=20.Oin.

Wc = 0.0625 in.

Wp = 0.25 in.

Ws = 0.0625 in.

Equations 8,9 and 12 above, then yield the values: Ag = 0.277 in.2, Ar = 0.077 in.2, and Av = 0.200 in.2.

The amountof FREON-12to be inserted in gas spring 60 in this example is 0.0400 Ib. An amount less than about 0.029 Ib. would cause all of the liquid to convert two vapor when the spring is fully compressed if the temperature was as high as 80"C. An amount more than about 0.0500 Ib. would prevent the spring from extending fully ifthetemperature was as high as 80"C because all ofthe vaporwould be compressed into the liquid state. Figure 4 is a phase diagram of FREON-12, where vapor pressure is plotted as a function ofspecificvolume. There are twelve curves representing the temperatures over which the spring isto beabletofunction.The dotted line curve shows the region inside of which the liquid and vapor are in equilibrium together, and accordingly, where this example is supposed to operate. Table II below lists the quantities used to compute the specific volumeofthe FREON-12 inthe limiting caseswhere the gas spring is fully extended and fully compressed.

TABLE II Gas Spring Gas Spring Fully Fully Extended Compressed Volume Vv (Eq.(14)) 1.28 cu. in. 5.28 cu. in.

Mass M ofFreon-12 0.0400 Ib. 0.0400 Ib.

Specific Volume = Vv/Mass 32.0 cu. in. 132 cu. in.

perlb. perlb.

The operating region corresponding to 0.0400 lb. of FREON-12 is shown by the dotted line curve "E" in Figure 4. For the temperature range of interest the FREON-12 remains clearly within the liquid-vapor phase.

Table III below presents the gas spring force over the full range of spring compression and temperature forwhich it was designed. The force values come from equation 16 above, togetherwith equation 15, and the FREON-12 vapor pressure values are from the phase diagram of Figure 4.

TABLE Ill Gas spring Compression Length Lt X (in.) (in.) -30 -20 -70 0 10 20 30 40 50 60 70 80 50 0.00 135 139 143 146 148 150 151 150 148 145 141 133 46 4.00 136 141 144 148 150 152 153 152 150 147 143 136 42 8.00 138 142 146 149 152 154 155 154 153 149 145 138 38 12.00 140 144 148 151 154 156 157 156 155 152 148 140 34 16.00 141 146 150 153 156 158 159 159 157 154 150 143 30 20.00 143 148 152 155 158 160 161 161 159 156 152 145 Figure 5 is a plot ofthe results of Table Ill forfull compression and full extension and comparesthe results with a gas spring that had no temperature compensation.Curve "F" representthespringforce versus temperature forthe temperature compensated gas spring of Example 2 when the spring is completely compressed Curve "G" shows the same data when the spring is completely extended. Curves "H" and "I" showthespring forceversus temperature for an uncompensated gas spring which is completely compressed and completely extended, respectively.

Both Figure 5 and Table II show that there is compensation forchanges in temperature. The degree of temperature compensation can be quantified by computing the resulting percent variation of the spring force with temperature using the data from Table III and comparing it with that expected for an uncompensated design. Table IV givesthetempera- ture-compensated results. On afull swing basis, the deviation dueto temperature for gas spring 60 in Example 2 is under 12 percent.

TABLE IV Percent Variation in Spring Force Due to Temperature Gas Spring Compression Design Force Minimum Maximum Percent Length Lt X (in.) (lb.) Cct T=20 C Force Force Variation = (in.) F (X, 20) Fmin (Ib.) Fmax (lb.) Fmax-Fmin x 100 F(X,20) 50 0.00 150 133 151 12% 46 4.00 152 136 153 11% 42 8.00 154 138 155 11% 38 12.00 156 140 157 11% 34 16.00 158 141 159 11% 30 20.00 160 143 161 11% The corresponding variation for an uncompen sated gas spring is much larger, namely about37.5% .

It can be estimated by assuming that the gas behaves like a perfect gas. Accordingly, for a given volume Vg, the pressure, and thus the spring force, is proportion al to the absolute temperature. For comparison to the above example, a temperature swing of -30 Cto 80 C (243 K to 353 K) would causethefollowing variation: F(80 C) - F(30 C) x 100 = F(20 C) P(80 C)- P(-30 C) x100 = P(20 C) p353 K)- P(243 K) X100 = P(293 K) 353 K-243 K x100 = 37.5% 293 K EXAMPLE3 In this example, as in Example 2,the gas spring is to have an extended length of 50 inches and a stroke of 20 inches.At a temperature of 20"C it will have a net spring force of 1 501b. in the extended position, and 1601b. in the compressed position. The primary gas is nitrogen, and the secondary pressure source material is sulfur hexaflouride (SF6). The temperature compensation range is -30"C to 80"C despite the fact that the critical temperature for SF6 is 45.55"C, above which it can only exist as a gas.

The values ofthe initially selected parameters are: Dp = 0.510in.

Dr = 0.3125in.

Dt = 2.50in.

L = 26.0in.

Ls = 3.00in.

Lv = 9.166in.

Lt (Max) = 50.0in.

M = 0.4641b.

Pa = 14.7psi Po = 929.5psi S = 20.0in.

Wc = 0.0625in.

Wp = 0.25in.

Ws = 0.0625in.

Equations 8,9, and 12 above yield the following values: Ag = 0.204in.2, Ar = 0.077in.2, and Av = 0.128in.2.

Figure 6 is a phase diagram of sulfur hexafluoride, where vapor pressure is plotted as a function of specific vqlume. There are twelve solid curves representing the temperatures over which the gas spring is to function. The dotted line curve "J" shows the region inside of which the liquid and vapor are in equilibrium. For this example, 0.46441b. ofsulfur hexafluoride is to be inserted into the gas spring.

Table V below lists the quantities used to compute the specific volume ofthe sulfur hexafluoride in the two limiting cases where the gas spring is fully extended and fully compressed.

TABLE V Gas spring Gas Spring Fully Extended Fully Compressed VolumeVv (Eq. (14)) 42.47 cu. in. 45.02 cu. in.

MassMofSF6 0.4644 Ib. 0.4644 Ib.

SpecificColume=Vv/Mass 91.45 cu. in. 96.95 cu. in.

per Ib. per Ib.

The operating region corresponding to 0.46441b. of sulfur hexafluoride is shown by the dotted line curve "K" in Figure 6. The range of specific volume is quite narrow in this example. Furthermore, these specific volumes extend to temperatures above the critical point where it is impossible forthe sulfur hexaf- luoride to exist in a liquid state. This example is designed such as to limit the specific volume from changing substantially from the extended to the compressed configuration. This limitation is accomplished by locating the baffle 32 closed to end wall 16 than in Example 2. Note the difference in Lvin Examples 2 and 3.Thisfeature has the effect of making the pressure ofthesulfurhexafluoride depend almost entirely on the temperature and very little on the displacement parameter X. It is important to note that even above 40"C, where the operating region leaves the liquid-vapor phase, the properties of sulfur hexafluoride are such that the percent change of its pressure with temperature is greater than that ofthe primary gas, nitrogen.

Table Vl below presents the gas spring force over the full range of spring compression and temperature for which the gas spring of Example 3 is designed.

The force values come from equation 16 above, together with equation 15 and the sulfur hexafluoride vapor pressure values from the phase diagram of Figure 6.

TABLE Vl Gas Spring Compression Spring Force at Various Temperatures ( C) Length, Lt X(in.) -30 -20 -10 0 10 20 30 40 50 60 70 80 50 0.00 147 150 152 153 152 150 147 145 146 148 149 151 46 4.00 149 152 154 154 154 152 149 147 148 150 152 154 42 8.00 150 154 156 156 156 154 151 149 151 153 155 157 38 12.00 152 155 157 158 158 156 153 152 154 156 158 160 34 16.00 154 157 159 160 160 158 155 154 156 158 160 163 30 20.00 156 159 161 162 162 160 158 157 159 161 163 166 Figure 7 is a plot ofthe results of Table Vl for full compression and full extension and compares the results with an uncompensated gas spring. Curve "L" represents the spring force versus temperature for the temperature compensated gas spring of Example 3 wherein the spring is completely extended. Curve "M" shows the same data where the spring is completely extended. Curves "N" and "0" showthe spring force versus temperature for an uncompensated gas spring which is completely compressed and completely extended, respectively.

Both Figure 7 andTableVl show clearly thatthere is compensation for changes in temperature, even when operating above the critical pointofthe secondary pressure source substance for the part of the time. As with the FREO N-12 exam ple, the degree oftemperature compensation for the sulfur hexafluoride example can be quantified by computing the resulting percent variation ofthe spring force with temperature using the data from Table Vl and comparing it with that expected for an uncompensated design. Table VII gives the temperature-compensated results. On a full swing basis, the deviation due to temperature is under 6 percent, a value that is only half as much asforthe FREON-12 example and compares even more favourablywith the 37.5% deviation ofthe uncompensated gas spring as discussed above in connection with Example 2.

TABLE VII Gas Spring Compression Design Force Minimum Maximum Percent Variation = Length, Lt. X (in.) (lib.) (Wfi 203C Force Force Fmax -Fmin x 100 (in.) F (X, 20) Fmin(lb) Fmax(lb) F(X, 20) 50 0.00 150 145 153 5% 46 4.00 152 147 154 5% 42 8.00 154 149 157 5% 38 12.00 156 152 160 5% 34 16.00 158 154 163 6% 30 20.00 160 156 166 6%

Claims (35)

1. A gas spring including a sealed casing, a slidable rod extending from the interiortothe exterior of said casing through one end thereof, and a piston mounted on the rod within said casing, a primary pressure source located within said casing and creating a primary pressure acting against said piston to urge said rod out of said casing, and a secondary pressure source located within said casing and creating a secondary pressure acting against said piston to urge said rod into said casing, wherein said primary pressure is greaterthan second secondary pressure, and wherein the percent change of said secondary pressure with temperature is greater than the percent change of said primary pressure with temperature.

2. A gas spring as claimed in Claim 1,wherein said primary pressure varies essentially proportional lywith absolute temperature.

3. A gas spring as claimed in Claim 2, wherein said primary pressure source is a pressurized primary gas which remains in the gas phase overthe temperature range to which the gas spring is exposed.

4. A gas spring as claimed in Claim 3, wherein said temperature range is -30 Cto 80"C.

5. A gas spring as claimed in Claim 4, wherein said pressurized primary gas is nitrogen.

6. A gas spring as claimed in Claim 1 ,wherein said secondary pressure varies approximately ex ponentiallywith absolute temperature.

7. Agasspring as claimed in Claim 1 or6,wherein said secondary pressure source is the vapor pressure of a two-phase system in which the liquid and vapor phases are in equilibrium.

8. A gas spring as claimed in Claim 7, wherein the liquid and vapor phases ofthe two-phase system remain in equilibrium overthetemperature range of -30"C to 80or.

9. A gas spring as claimed in Claim 8, wherein the two-phase system is selected from the group consisting of acetylene, ethane, FREON-i 2, FREON-73, FREON-114, propane, propadiene, perfluoropropane, dimethyl ether, N-butane, ammonia, hydrogen bro midland hydrogen iodide.

10. A gas spring as claimed in Claim 8, wherein the two-phase system is ammonia.

11. A gas spring of Claim 8, wherein thetwo- phase system is FREON-i 2.

12. A gas spring as claimed in Claim 7, wherein the liquid andvaporphasesofthetwo-phase system remain in equilibrium over a substantial portion of the temperature range of -30"C to 800C.

13. A gas spring as claimed in Claim 12, wherein the two-phase system is sulfur hexafluoride.

14. A gas spring as claimed in Claim 1, wherein said primary pressure source is pressurised nitrogen gas, said secondary pressure source is the vapor pressure of a two-phase system in which the liquid and vapor phases are in equilibrium, and said two-phase system is selected from the group consisting of ammonia, FREON-12, and sulfur hexafluoride.

15. Atemperature compensated gas spring comprising: a) a sealed casing including an outertubewith closed end wall atone end and a wall having an openingtherethrough atthe other end; b) an innertube disposed within the outertube and having one end attached to said closed end wall; c) a slidable piston disposed within said inner tube and dividing said innertube into a first inner volume between the piston and said closed end wall and a second innervolume in the remainder of the innertube; d) a rod secured to the piston and extending through the opening in said wall having an opening therethrough; e) a baffle attached between said inner tube and said outertube and dividing the casing volume exterior of said innertube into a first outer volume adjacent said wall having an opening therethrough;; f) first conduit means connecting said first inner volume and said first outer volume; g) second conduit means connecting said inner volume and said second outer volume; h) a primary pressure source in said first inner and first outer volumes creating a primary pressure acting against said piston to urge said rod out of said casing through said opening in said wall having an openingtherethrough; and i) a secondary pressure source in said second inner and second outer volumes creating a secondary pressure acting against said piston to urge said rod into said casing; wherein said primary pressure is greaterthan said secondary pressure, and wherein the percent change of said secondary pressure with temperature is greaterthan the percent change of said primary pressure with temperature.

16. Agas spring as claimed in Claim 15, wherein said firstconduitmeans is at leastone holethrough said innertube beyond the extentoftravel of said piston toward said closed end wall, wherein said inner tube extends toward but stops short of said wall having an opening therethrough, and wherein said inner tube is open between said second innervolume and second outervolumeto form second connecting means.

17. A gas spring as claimed in Claim 15, wherein said first conduit means is at least one hole through said innertube beyond the extent of travel of said piston toward said closed wall, wherein said inner tube extends to and is attached to said wall having an opening therethrough, and wherein said second conduit means is at least one hole through said inner tube beyond the extent of travel of said piston away from said closed end wall.

18. A gas spring as claimed in Claims 16 or 17 including a stop which limits the travel of said piston away from said closed end wall.

19. A gas spring as claimed in Claim 18 including afirstseal between said piston and said inner tube which prevents fluid flow passed said piston and a second seal between said rod and the periphery of the opening in said wall having an opening therethrough which prevents fluid flow out of said casing.

20. Agasspring as claimed in Claims 15, or 17 wherein said primary pressurevaries essentially proportionally with absolute temperature.

21. A gas spring as claimed in Claim 20 wherein said primary pressure source is a pressurized primary gas which remains gaseous overthetemperature range to which the gas spring is exposed.

22. A gas spring as claimed in Claim 21 wherein said pressurized primary gas remains gaseous over the temperature range of -30"C to 80"C.

23. A gas spring as claimed in Claim 22 wherein said pressurized primary gas is nitrogen gas.

24. Agas spring as claimed in Claim 15 wherein said secondary pressure varies approximately ex ponentially with absolute temperature.

25. A gas spring as claimed in Claim 15,16 or 17 wherein said secondary pressure source is the vapor pressure of a two-phase system in which the liquid and vapor phases are in equilibrium.

26. Agasspring as claimed in Claim 25 wherein the liquid and vapor phases of said two-phase system remain in equilibrium overthetemperature range of -30'Cto 80'C.

27. A gas spring as claimed in Claim 26 wherein said two-phase system is selected from the group consisting of acetylene, ethane, FREON-12, FREON13, FREON-114, propane, propadiene, perfluoropropane, dimethyl ether, N-butane, ammonia, hydrogen bromide, and hydrogen iodide.

28. A gas spring as claimed in Claim 26, wherein said two-phase system is ammonia.

29. A gas spring as claimed in Claim 26, wherein said two-phase system is FREON-12.

30. A gas spring as claimed in Claim 25, wherein the liquid and vapor phases of said two-phase system remain in equilibrium over a substantial portion of the temperature range of -30'C to 80"C.

31. A gas spring as claimed in Claim 30, wherein said two-phase system is sulfur hexafluoride.

32. A gas spring as claimed in Claim 15,16 or 17, wherein said primary pressure source is pressurized nitrogen gas, said secondary pressure source is the vapor pressure of a two-phase system in which the liquid and vapor phases are in equilibrium, and said two-phase system is selected from the group consisting of ammonia, FREON-i 2, and sulfur hexafluoride.

33. A gas spring as claimed in Claim 15, 16 or 17, wherein said innertube, said outer tube, and said piston are cylindrical in cross section.

34. A gas spring substantially as described with reference to any one ofthe Example shown in Figures 1 and 2 and Figure 3 ofthe accompanying drawings.

35. A gas spring substantially as described with reference to any one of the embodiments shown in the accompanying drawings.