FR2532386A1 - TEMPERATURE COMPENSATED GAS SPRING - Google Patents

Abstract

THIS GAS SPRING HAS A WATERPROOF ENCLOSURE 12, A SLIDING ROD 30 EXTENDING FROM THE INSIDE TO THE OUTSIDE OF THE ENCLOSED THROUGH ONE OF THE ENDS OF THE SAME, AND A PISTON 24 MOUNTED ON A ROD THE INSIDE OF THE SAME; IT INCLUDES A SOURCE OF PRIMARY PRESSURE LOCATED WITHIN THE SAME ENCLOSURE WHICH CREATES PRIMARY PRESSURE EXERCISING ON SAID PISTON TO PUSH SAID STEM OUTSIDE OF THE SAME AND A SECONDARY PRESSURE SOURCE LOCATED WITHIN THE SAME ENCLOSURE THAT CREATES A SECONDARY PRESSURE EXERCISING ON THE SAME PISTON TO PUSH THE SAID STEM INTO THE INSIDE OF THE ENCLOSURE, THE PRIMARY PRESSURE BEING SUPERIOR TO THE SAID SECONDARY PRESSURE AND THE PERCENTAGE CHANGE IN PERCENTAGE OF THE SECONDARY PRESSURE AS A FUNCTION OF THE TIME THAT THE CHANGE IN PERCENTAGE OF THE PRIMARY PRESSURE AS A FUNCTION OF THE TEMPERATURE.

Description

"GAS SPRING WITH TEMPERATURE COMPENSATION"

  The present invention relates to gas springs and more particularly to gas springs with automatic compensation, which allows them to operate uniformly over a wide range of temperatures. The springs used to support the trunk covers, hoods and the like, in particular the trunk lid of rear-door cars, are often gas springs. A gas spring consists essentially of a sealed cylinder containing a gas under high pressure and a piston rod which

  extends to the end of the cylinder Commonly, this is the-

  zote used in the cylinder at a pressure of approximately 68.94 105 Pa The expansion force is supplied pa Y the pressure of the gas which acts on the surface

  with a cross section equal to that of the rod inside

  of the cylinder, which pushes the rod outwards

  When the rod is inside the cylinder as when the trunk lid of the rear door is closed, the rod moves inside the cylinder a

  certain volume which was previously occupied by gas.

  Since the total volume inside the cylinder is fixed, the volume that the gas can occupy decreases, which

  which has the effect of increasing the pressure

  In this way, the force which forces the rod to move outwards increases In conventional gas springs, a piston type structure can be attached to the rod inside the cylinder and used to cushion and limit the stroke of the rod Since the gas pressure is normally the same on each side of the piston, it has little or no effect on the rod. Ideally, the gas pressure should be sufficient to drive the piston out of the cylinder and lift the trunk lid or other accessories to which it is attached. The gas pressure should also be sufficient, when the rod is fully extended and the cover or other device is raised, to allow a person to easily push the rod back into the cylinder and

  to close the cover The use of a single gas in the tank

  gas outlet has a disadvantage that the pressure

  of a gas in a fixed volume is proportional to the tem-

  gas temperature For an ideal gas to which nitrogen can be assimilated, the pressure is in direct proportion to the absolute temperature of the gas This dependence can raise considerable problems when these gas springs are used in cars exposed to ambient temperatures at - below -18 'C going up to more than 37.80 C.

  When the ambient temperature is low, the pressure

  gas in the cylinder is low, and provides

  therefore insufficient force to push the rod outwards

  laugh and lift the weight of the trunk lid When the ambient temperature is high, the gas pressure inside the cylinder is strong and it provides a significant force to push the rod outside the

  cylinder, which may eventually open

  extremely fast closing of the trunk lid connected to

  the rod which is likely to strike the person opening the cof-

  fre In addition, when the ambient temperature is high, the gas pressure inside the cylinder is high when the rod is in its most elongated position, this

  which makes it difficult for the rod to move in the cylinder

  dre when you want to close the trunk lid.

  In the prior art, amor-

  weavers that automatically compensate for changes in ambient temperature See for example US Patent No. 2,944,639 to Blake, US Patent No. 3,107,752 to Mc Cean, US Patent No. 3,301,410 to Seay, US Patent No. 3,971,551 to

  Kendall et al., And US Patent No. 3,944,197 to Dachicourt.

  These devices generally provide for an additional room.

  mental or other similar element inside the a-

  mortiser which makes it possible to accommodate the changing volume of the primary damping fluid as the ambient temperature changes. Such devices are not suitable for providing temperature compensation in a gas spring, since they are intended to maintain a constant volume of fluid rather than maintaining a constant pressure facing outward. The object of the present invention is to provide a gas spring in which the sensitivity of the expansion force with respect to temperature variations is

  limited to a low acceptable level.

  The invention consists of a compensating gas spring

  temperature station which comprises a sealed casing, a sliding rod which extends from the inside towards the outside of the casing through one of its ends, and a piston mounted on the rod inside the casing A primary pressure source is located inside the casing and acts on the piston to push the rod outside the casing, and a secondary pressure source is located inside of the casing and acts against the piston in order to bring the rod back into the casing The primary pressure is more

  strong as the secondary pressure and the change in

  centering of secondary pressure under the effect of tem-

  is more important than change in percent

  tage of the primary pressure under the effect of the temperature

ture.

  The primary pressure source is, preferably,

  of a primary gas under pressure, such as nitrogen, the pressure of which varies essentially in proportion to the absolute temperature and which remains in the gas phase over the whole range of temperatures to which the gas spring is subjected A range of temperature suitable ranges from -30 'C to 80 C. The secondary pressure source is preferably a vapor pressure of a two-phase system in which the gas and liquid phases are balanced over a range of temperatures varying from -30 'C to 80 C Such a vapor pressure varies approximately

  exponential with respect to absolute temperature The systems

  suitable two-phase temes use gases such as a-

  ketylene, ethane, chloro-fluoro-methane (FREON-12,

  FREON-13 and FREON-114) propane, propadiene, perfluo-

  ro-propane, dimethyl ether, N-butane, ammonia gas, hydrogen bromide and hydrogen iodide Source

  secondary pressure may also consist of a system

  two-phase system in which the gas and liquid phases

  remain in equilibrium over a substantial portion of the temperature range from -30 ° C to 80 ° C, such that

sulfur hexafluoride.

  In the preferred embodiment of the invention, the sealed envelope comprises a cylindrical tube having a bottom wall closed at one of its ends and a wall

  bottom provided with an opening which allows the rod to pass

  go through this end to the other end

  tee An inner tube is arranged coaxially inside the casing and one end of the inner tube is fixed to the closed bottom wall The piston is placed inside the inner tube and divides the inner tube into a first interior volume located between the piston and the closed bottom wall and a second interior volume in the remaining part of the interior tube A baffle is located between the interior and exterior tubes in order to divide the

  volume of the envelope outside the inner tube

  térieur in a first external volume adjacent to the closed bottom wall and a second external volume adjacent to the bottom wall provided with the opening A first means of

  passage allows the fluid to flow between the first vo-

  interior lume and the first exterior volume and a second

  medium-flow allows fluid to flow between the two

  tenth interior volume and second exterior volume La

  primary pressure source is located in the first volu-

  me interior and the first exterior volume and the source of

  secondary pressure is in the second volume

  the second exterior volume.

  Preferably, the first means of passage consists

  in one or more holes through the inner tube

  beyond the stroke of the piston towards the closed bottom wall.

  In one of the embodiments, the internal tube is shorter than the cylindrical tube and the internal tube is in communication by the flow of the fluid with the second external volume in order to form the second passage means In a second mode the inner tube has the same length as the cylindrical tube and the

  two ends are fixed to the bottom walls of the envelope

  pe; the second passage system consists of one or more

  holes through the inner tube beyond the

  stroke of the piston towards the bottom wall provided with the opening

re.

  The gas spring may also include a nozzle.

  tee limiting the piston stroke A sealing fluid is provided between the piston and the inner tube and between the

  rod and the wall through which it passes.

  The present invention will be better understood by the

  description which follows, with reference to the diagrammatic drawing

  that annexed in which: Figure 1 is an axial sectional view of a first embodiment of a gas spring according to the present invention; Figure 2 is a graph that shows the strength in

  as a function of the gas spring temperature illus-

  see Figure 1, in which the primary pressure source

  re is nitrogen and the secondary pressure source is

  ammonia gas in liquid and gaseous state in equilibrium

  free Figure 3 is an axial sectional view of a second

  embodiment of the gas spring according to the present in-

  vention; Figure 4 is a log-log graph of the phase diagram of FREON12 Figure 5 is a graph of the expansion force

  clear towards the outside compared to the temperature of the

  gas outlet illustrated in FIG. 3 in which the primary pressure source is nitrogen and the secondary pressure source of FREON-12 FIG. 6 is a log-log graph of the phase diagram of sulfur hexafluoride, and Figure 7 is a graph of the expansion force

  clear towards the outside compared to the temperature of the

  gas outlet illustrated in Figure 3, in which the primary pressure source is nitrogen and the pressure source

  secondary of sulfur hexafluoride.

  One of the preferred embodiments of the temperature compensated gas spring according to the present invention is illustrated in FIG. 1 The gas spring (10) comprises a sealed envelope (12) consisting of a sleeve or external tube (14 ) with a closed bottom wall (16) mounted at one end and a wall (18) mounted at the other end The wall (18) is provided with an opening (20) right through The spring to gas (10) has a sleeve or inner tube (22) located in a coaxial position inside the casing (12) One end of the inner tube (22) is fixed to the bottom wall (16) and

  the other end is axially spaced from the wall (18).

  A piston (24) is located inside the inner tube

  (22) and divides the interior of the inner tube (22) into a pre-

  mier internal volume (26) located between the closed bottom wall

  mée (16) and the piston (24) and in a second integrated volume

  laugher (28) located on the side of the piston towards the wall (18) An elongated rod (30) is fixed to the piston (24) and extends outside the envelope (12) through the opening (20)

of the wall (18).

  A baffle (32) is fixed to the inner tube (22) and extends between the outside of said tube (22) and the inside of the outer tube (14) and divides the volume of the envelope,

  excluding the inner tube, in a first external volume

  laughing (34) adjacent to the wall (16) and in a second volu-

  outside me (36) adjacent to the wall (18) The chicane

  preferably consists of an annular plate The chicane

  (32) also provides support for the inner tube (22).

  7. The inner tube (22) is provided with one or more orifices

  (38) located beyond the stroke of the piston (24) at the interior

  laughing of said tube in the direction of the bottom wall (16).

  The orifices (38) form a first passage system which allows the fluid to flow between the first internal volume (26) and the first external volume (34). In the embodiment illustrated in FIG. 1, the internal tube ( 22) is open at its end closest to the wall (18) and forms a second passage system which allows the fluid to flow between the second volume

  interior (28) and the second exterior volume (36).

  The gas spring (10) may include a stop (40) at the free end of the inner tube (22) which limits the

  stroke of the piston (24) towards the wall (18) and retains the piston

  tone inside the inner tube (22) The abutment (40) illustrated in FIG. 1 consists of a plate, of washer type, fixed to the end of the inner tube (22) which surrounds, without coming into contact with it, the rod (30) so as to leave an annular space (42) between the rod

  (30) and the stop (40) which allows the fluid to flow.

  The gas spring (10) comprises -a first sealing joint-

  it-é (44) between the piston (24) and the inner surface of the

  inner tube (22) to prevent fluid flow-

  between the first and second interior volumes (26; 28) and has a second seal (46) between the rod (30) and the wall (18) to prevent the flow of fluid between the interior and the outside of envelope

(12).

  The gas spring (10) also comprises a rod (48) fixed to the outside of the bottom wall (16) of the casing (12) which is provided with an eyelet (50) at its end. The rod (30) has an eyelet (52) at its free end outside the envelope (12) The eyelets (50) and (52) allow the gas spring (10) to be mechanically connected between two points, such as the bodywork and the trunk lid of an automobile Preferably,

  the inner tube (22), the outer tube (14) and the plunger

  ton (24) are cylindrical in shape.

  A primary pressure source is located in the

  first interior volume (26) and the first exterior volume

  laugher (34) and actuates the piston (24) which pushes the rod (30) towards the outside of the casing (12) A secondary pressure source is located in the second interior volume (28) and the second exterior volume ( 36) and

  actuates the piston (24) which pushes the rod (30) inside

  envelope laughter The net force acting on the pis-

  ton (24) comes from the difference between the forces generated

  from primary and secondary pressure sources.

  Since the gas spring (10) has to function as a spring with an outwardly directed expansion force, it is necessary that the primary pressure

  is stronger than the secondary pressure.

  Preferably, the primary pressure source

  will come from a primary gas under pressure, the pressure of which varies in proportion to the absolute temperature * and which remains

  in gaseous state for the whole range of temperatures

  which the gas spring is subjected to A primary gas of

  choice is nitrogen which behaves essentially in con-

  formity with the ideal gas law (PV = n RT) for a temperature range from -300 C to 80 C In the prior art, it is well known that no gas will have a performance exactly in accordance with the theoretical law some gas

  ideal Other gases may be used, among them ar-

  gon, helium, hydrogen, crypton and neon -

  The reduction of the temperature sensitivity in the gas spring (10) is obtained by the application of an inverted force on the piston (24) which is exerted by a secondary pressure source which tends to cancel the additional force exerted by the primary pressure source due to temperature increases La

  secondary pressure is chosen because of its behavior-

  very different from gaseous behavior basically

  perfect primary gas In one of the embodiments

  tion of the present invention, the secondary pressure source is the vapor pressure of a two-phase system in which the gas and liquid phases remain in equilibrium The vapor pressure of such a system

  two-phase varies approximately exponen-

  relative to the absolute temperature rather than in direct proportion The main requirement for

  any secondary pressure source you wish to use

  ser is that the percentage change in secondary pressure as a function of temperature is greater than the percentage change in primary pressure

depending on the temperature.

  There are many organic substances or

  inorganic which can serve as a primary pressure source

  mayor, including acetylene, ethane, le-chloro-fluoro-

  methane (FREON-12, FREON-13 and FREON-114), propane,

  propadiene, perfluoropropane, dimethyl ether, N-

  butane, ammonia gas, hydrogen bromide and hydrogen iodide The pressure of these vapors varies from 0 to

  , 34 x 105 Pa approximately at a temperature of approx.

  ron -30 WC and 6.89 x 105 Pa approximately-more than

  62 x 105 Pa at 70 WC In a two-phase system, the pressure

  Zion exerted by the vapor of a given substance will depend only on the temperature The best substance to use for a given application is determined by the conditions of design of the application such as expansion force, dimension of the spring, cost of materials production costs, seal life and degree

  sought temperature compensation.

  It is not absolutely necessary that the pressure

  secondary is produced by a two-phase system.

  As described in detail later in this document, with respect to Example 3, sulfur hexafluoride can be used as a secondary pressure source. Above a critical temperature, sulfur hexafluoride cannot exist. as a two-phase system and it only exists in the form of vapor without the presence of a liquid phase However, temperature compensation e

  even intervenes above the critical temperature by-

  what the percentage change in pressure from

  sulfur hexafluoride vapor (i.e.

  secondary) as a function of temperature will be -5 anyway stronger than the percentage change

  the pressure of an ideal gas as a function of the temperature

  cross out. Since a substance will remain in a two-phase system with its gaseous and liquid phases in equilibrium only for certain ranges of volumes

  specific, this requires that a certain volume of the

  tance is available in the gas spring In general, the two gas and liquid phases should always be present so that the vapor pressure can only depend on the temperature As the spring compresses, i.e. -display as the piston (24) moves towards the wall (16), the volume available for the two-phase system increases If, at the start, there is an insufficient quantity of liquid phase of the

  substance such an increase in the total volume available

  fuel could convert all the liquid to vapor The pressure of this vapor will generally vary; as a function of temperature in a manner analogous to

  that of other gases and thus will provide little or no time

  compensation temperature However, if too much substance is used, a problem may arise when the spring relaxes, as it reduces

  similarly the volume available for the substance.

  This reduction in volume could lead to

  sation of the entire gas phase, which would force any

  the substance to be converted to the liquid phase This prevents

  would actually cost the piston to move further.

  To avoid any kind of possible problems with the two-phase system used as a pressure source

  secondary, the following conditions must be met

  tees: 1) the minimum quantity of substance necessary is that which is just sufficient to give a system

  two-phase when the gas spring is fully com- posed

  awarded, that is to say when the volume available is the highest, at the highest temperature to which the gas spring can be subjected, and 2) the volume available for the substance must be large enough so that the vapor does not condense entirely into liquid

  when the gas spring is fully extended, i.e.

  say when the available level is at its minimum level.

  The highest temperature to which the gas spring can be exposed is also a factor.

  limiting of this second condition.

  The concentric tube system illustrated in

  Figure 1 has particular advantages for a res-

  gas outlet A sufficient additional external volume of the internal tube (22) is available in the first external volume (34) for the primary gas so that it is not excessively compressed when the rod

  (30) is fully retracted inside the res-

  gas outlet Otherwise, excessive pressures

  resulting ves could cause expansive forces

  excessive and undesirable likewise, an external volume

  additional laughter of the inner tube (22) is available in the second outer volume (36) for the source of

  secondary pressure so that it is not excessive

  strongly compressed when the rod (30) is fully extended. All components of the gas spring (10), with the exception of the seals (44) and (46) are made of a metal having sufficient strength to tolerate confined gas pressures The use of an inner tube (22), an outer tube (14) and a piston (24) of cylindrical shape has particular advantages.

EXAMPLE 1 -

  An example of a gas spring (10) according to the embodiment shown in Figure 1 is shown below in which nitrogen is used as the primary pressure source and ammonia gas as the two-phase system for the source. of secondary pressure The relationship between the various parameters incorporated in the gas spring (10) can be described algebraically by means of the following variables: Ag = area of the piston on which the nitrogen pressure is exerted (in cm Av = area of the piston on which the ammonia gas pressure is exerted (in cm 2 Dp = diameter of the piston (in cm) Dr = diameter of the rod (in cm) F = force of the gas spring (in kg) Pg = pressure of nitrogen (Pascal) Pv = ammonia gas pressure (Pascal) Po = nitrogen pressure at 200 C (Pascal) T = temperature (in OC)

  The net outward expansion force F -

  is determined by subtracting the force of the ammonia gas acting on the piston from the force of the nitrogen acting

  on the piston Ignoring the force of the atmospheric pressure

  spherical on the stem (30) and to calculate "F", we use the following equation (1) F = Ag Pg Av Pv

  The nitrogen pressure is relatively well represented

  felt by the following equation (2) Pg (T + 273) Po

  For this example, assume that the force of expan-

  sought "F" is 45.359 kg at extreme temperatures of -30 WC and 70 WC The gas pressure of the gas

  ammonia in a two-phase system can be determined

  born by consulting well-known standardized manuals, such as the "Chemical Engineers Handbook", edited by John H.

  Perry (McGraw-Hill, 1950, 3 rd edition) A -30 WC, la pres-

  gaseous sion of the ammonia gas is 1.379 x 10 Pa and at WC, the pressure is 32.75 x 10 Pa By incorporating

  these figures, the force sought F = 45.359 kg and the equa-

  tion 2 to equation 1, we obtain the following equations (3) 100 = Ag (30 + 273) Po 20 Av (4) 100 Ag (70 + 273> PO 475 Av

293 4

  By simultaneously solving equations 3 and 4, we obtain the following results (5) Av 0.5941 cm 2 (6) Ag Po = 55.701 kg If the diameter of the rod chosen Dr is

  7.9375 mm, a typical gas springs value super

  the piston Av on which the gaseous ammonia gas must exert its pressure, is

2 2 2 _ 2

  (7) Av = vr / 4 (Dg Dr) = çr / 4 (Dg (7.9375) 2 = 0.5941 cm 2 The solution of equation 7 gives as result Dg = 1.152 cm on the basis of which we determine that Ag = 1.089 cm Using equation 6 above, we obtain the necessary filling pressure, Po, of nitrogen at

20 WCs, i.e. 49.48 x 105 Pa.

  By loading the first interior volume and the first exterior volume with nitrogen at 49.48 x 105 Pa at 20 C, choosing a rod of 7.9375 mm in diameter and a piston diameter of 1.1775 cm and by loading the second interior volume and the second exterior volume with an amount of ammonia gas such that the gas and liquid phases remain balanced, the gas spring will have a nominal force outwards of 45.35 kg

  at extreme temperatures of -30 WC and 70 WC Le bompor-

  This gas spring at other temperatures can be determined by calculating Pg at other temperatures using equation 2 above, determining the values of Pv at various temperatures and calculating the forces using from equation 1 above On the Table

  I below, a list of these corresponding data is found.

  corresponding to the desired temperature ranges.

TABLE I

Expansion force Temp. o C

20

70

  Pg Pa 41.60 43.25 44.96 46.67 48.38, 09 51.79 53.50, 21 56.92 58.64 1, 2, 3, 4, 6, 8, 11,,,, 32, We will find, at Pv Pg Ag) 5 Pa Kg

378 46,115

067 48.018

49,920

778 51.823

201 53,725

612 55.628

713 57.531

847 59,433

256 61,336

493 63,238

727 65.096

Figure 2, a Pv Av F Kg Kg

0.816 45.359

1,270 46,749

* 1,859 48,063

2.721 49.105

3,764 49,965

, 216 50.418

, 753 50.418

9,603 49,830

12.276 49.059

, 447 47.791

19,796 45,359

free representation

  phique of the above results where the curve "A" represents the outward force exerted on the piston (Pg Ag), the curve "B" represents the inward force exerted on the piston ( Pv Av) and the curve "C" represents the net force of expansion of the gas spring of Example 1, all as a function of the temperature This gas spring has a maximum force of 50.845 Kg approximately between 20 C and 30 C and a minimum force of 45.359 Kg at extreme temperatures The temperature compensation of the

  gas spring of Example 1 can be compared with the utility

  readout of nitrogen alone, comparing minimum forces

  and maximum expansion developed, the force at 20 C cons-

  titling the standard The deviation is about 10% for the gas spring of Example 1, while the deviation is

  approximately 34% for the gas spring which uses

  se nitrogen only It can thus be seen that the gas spring of the present invention considerably reduces the variation of the expansion force as a function of the temperature, if we compare it to a gas spring where we do not use than nitrogen. Pressure In Figure 3 an illustration of a

  second embodiment of a compensating gas spring

  temperature station according to the present invention The res-

  gas spell (60) illustrated in Figure 3 has many elements which are identical to those of the gas spring (10)

  illustrated in Figure 1 and discussed above.

  Consequently, the similar elements in the two figures bear the same reference numbers and only the differences between the two modes of discussion will be discussed.

production.

  In the gas spring (60) illustrated in Figure 3, the inner tube (62) extends over its entire length between the bottom walls (16) and (18) and is fixed to the two bottom walls The second volume interior (64) is located inside the interior tube (62) between the piston (24) and the wall (18) and the second exterior volume (66) is located outside the interior tube between the baffle (32 ) and the wall (18) One or more orifices (68) are located in the tube (62) at a location located beyond the

  stroke of the piston (24) in the direction of the wall (18) deter-

  undermined by the stop (40) The orifices (68) form a system

  passage between the second interior volume (64)

  and the second external volume (66).

  Apart from the structural differences indicated above, the embodiment of the gas spring (60) illustrated in FIG. 3 works exactly the same.

  so that the gas spring (10) illustrated in Figure 1.

  A primary pressure source is located in the first interior volume (26) and in the first exterior volume (34) and a secondary pressure source is located in the second interior volume (64) and in the second exterior volume (66), as described above for the

figure 1.

  The relationship between the various parameters incorporates

  res in the gas spring (60) can be described algebraically

  using the following variables: Ag = surface of the piston on which the pressure of the primary gas is exerted (cm?) Ar = surface of the rod on which the atmospheric pressure is exerted (cm 2) Av = surface of the piston on which exerts the secondary pressure (cm Dp = diameter of the piston (cm) Dr = diameter of the rod (cm) Dt = inside diameter of the external tube (cm) F = net force towards the outside of the gas spring (kg ) L = length of the inner and outer tubes (cm) Ls = distance between the stop (40) and the wall (18) (cm) Lt = length of the gas spring between the eyelet (50) and the eyelet (52 ) which varies as a function of X (cm) Lv = distance between the baffle (32) and the wall (18) (cm) M = mass of the material of the secondary pressure source (kg) Pa = atmospheric pressure (Pa) Pg = primary gas pressure (Fa) Po = primary gas pressure at 20 'C (Pa) Pv = pressure of the secondary pressure source (Pa) S = stroke or maximum design value of

compression of the spring X (cm).

T = temperature (OC)

  Vg = volume of primary gas which varies depending on

tion of X (cm 3)

  Vv = volume of pressure source material

  secondary sion which also varies as a function of X (cm 3) Wc = thickness of the inner tube (cm) Wp = thickness of the piston (cm) Ws = thickness of the baffle (cm) X = spring compression value measured

  from the piston stop (cm).

  The characteristics of the pre-

  fairy illustrated in Figure 3 can be described by initial values chosen 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 ratio of the two parameters Pg and Pv as a function of temperature For Pg, it is usually sufficient to use the ideal gas law For Pv, the dependence of the vapor pressure as a function of temperature can be obtained, for a particular substance, in

current manuals.

  The following equations relate to parameters

  remaining compared to the initial parameters listed above

  above-: (8) Ag = "Dp 2 (9) Ar = ri Dr 2

4

  (10) * Lt (minimum) = Lt (maximum) S (11) Lt = Lt (maximum) X (12) AV = Ag Ar (13) Vg = "/ 4 (Dt 2 _ (Dp + 2 Wc) 2 ) (L Lv Ws) ± Ag (L Wp Ls X) (14) Vv = i / 4 (Dt 2 (Dp + 2 Wc) 2 Lv + Av (X + Ls) The pressure of the primary gas is a function of X, via volume Vg, temperature T and

  filling pressure Po Assuming that the filling

  gas puckering is done when the rod is extended

  (i.e. when X = 0) and the gas obeys essen-

  tial to the ideal gas law, its pressure is deduced from the following equation: (15) Pg Po (T + 273) Vg (maximum) 293 Vg The equation for the force of expansion as a function of compression and the temperature is given by

(16) F = Pg Ag Pv Av Pa Ar.

EXAMPLE 2

  In this example, a gas spring according to the second embodiment with the use of a source of

  two-phase secondary pressure has an expansion length

  sion reaching 127 cm and a stroke of 50.8 cm At a time

  temperature of 20 WC, it will have a net expansion force F -

  68.04 Kg in the extended position and 72.575 kg in compression The primary gas is nitrogen and the substance

  used as a secondary pressure source is FREON-

  12 which is permanently maintained in gaseous phases and

  liquid for the whole range of operating temperatures.

  The temperature compensation ranges from 30 WC to 80 WC.

  The values of the parameters initially retained are the following Dp 1, 506 cm Dr = 0.7937 cm Dt = 5.716 cm L = 66.04 cm Ls = 8.163 cm Lv = 0.454 cm Lt (max) = 127 cm M = 18.14 g Pa = 1.0128 X 10 Pa Po = 41.643 X 105 Pa S = 50.8 cm Wc = 0.1587 cm Wp = 0.635 cm Ws = 0.1587 cm Equations 8, 9 and 12 above then give the following values:

2 2 2

  Ag = 1.787 cm, Ar = 0.4967 cm and Av = 12.903 cm

  The amount of FREON-12 to be introduced into the reservoir

  gas outlet (60) in this example is therefore 18.14 g A quantity less than approximately 13.137 g would cause all the liquid to be converted into vapor when the spring is fully compressed if the temperature were to reach 800 C An amount greater than 22 , 65 g would prevent the spring from fully extending if the temperature were to reach a temperature as high as 80 C, because any

  the vapor would be compressed in the liquid state.

  gure 4 illustrates a phase diagram of FREON-12 in the-

  which vapor pressure is plotted against a volume

  me specific There are twelve curves which represent the temperatures for which the spring is able to function The curve "D" in dotted lines indicates the zone inside which the liquid and the vapor are in equilibrium and, consequently, where the this example is supposed to work Table II below gives a list of the quantities used to calculate the specific volume of FREON-12 in borderline cases where the gas spring is

  fully elongated or fully compressed.

TABLE II

Gas spring Gas spring.

  completely fully extended compressed

3 3

  Volume Vv (Eq (14)) 20.975 cm 86.523 cm Mass M of FREON-12 18.14 g 18, 14 g Specific volume = Vv / Mass 1,157.58 cm 4,775 cm 3 per Kg per Kg The operational area corresponding to 18, 14 g of FREON-12 is indicated by the curve "E" in dotted lines in FIG. 4 For an interesting temperature range, the FREON-12 remains clearly inside the phase

gaseous-liquid.

  Table III shows an illustration of

  the force of the gas spring corresponding to the full range

  te of spring compressions and temperatures for

  which it was designed The force values come from equation 16 above as well as from equation 15, as well as the vapor pressure values of FREON-12 which are

  from the phase diagram in Figure 4.

TABLE III

Compress Length

X gas spring Lt (cm) (cm)

116.84

106.68

  96.52 86.36 76.21 0.00, 16, 32, 48, 64, 80 Expansion force

-30 -20 -10 O 10

61.15 62.96 64.78 66.14 67.04

61.60 63.87 65.23 67.04 67.95

57.98 64.32 66.14 67.49 68.85

63.42 65.23 67.04 68.40 69.76

63.87 66.14 67.95 69.31 70.67

64.78 67.04 68.85 70.21 71.57

at various temperatures

30 40 50

67.95 68.40 67.95 67.04

68.85 69.31 68.85 67.95

69.76 70.21 69.76 69.31

, 67 71.12 70.67 70.21

71.57 72.02 72.02 71.12

72.48 72.93 72.93 72.02

  (C), 68 66.59 67.49 68.85 69.76, 67

80

63.87 60.25

64.78 61.60

, 68 57.98

67.04 63.42

67.95 64.78

68.85 65.68

  o ru Un w ri Co oe Figure 5 is a plot of the results in Table III for total compression and total elongation and gives a comparison of the results for a gas spring which did not have temperature compensation The curve "F" represents the expansion force over temperature for temperature compensated gas spring

  Example 2 when the spring is fully compressed

  mé The curve "G" indicates the same data but when the spring is fully extended The curves "H" and

  "I" give the force of expansion with respect to the temperature

  ture for a gas spring without compensation that is

  fully compressed and fully elongated, respectively

is lying.

  Table III, as well as Figure 5, indicates

  that there is compensation for changes in time

  The degree of temperature compensation can

  be quantified by calculating the percentage change re-

  resulting from the expansion force as a function of the temperature

  based on the data in Table III and comparing

  said variation with that expected from an application

  known without compensation In Table IV, you will find the results of temperature compensation On a

  overall basis, the deviation of the gas spring (60) from Example-

  ple 2 driven by temperature is less than 12

per 100.

TABLE IV

  PERCENTAGE VARIATION IN EXPANSION FORCE DUE TO CHANGES IN TEMPERATURE

  Spring length Compression Force (Kg) Minimum force Maximum force Variation in gas Lt (cm) X (cm) according to con male F min male F max ception Kg (Kg) Fmax Fmin (K 00

T = 20 C F (X, 20)

F (X, 20)

12.70 0.00 67.95 60.25 68.40 12%

  11.68 10.16 68.85 61.61 69.31 11%, 66 20.32 69.76 62.51 70.21%

9.65 30.48 70.67 63.42 71.12 11%

8.63 40.64 71.57 63.87 72.02 11%

7.62 50.80 72.48 64.78 72.93 11%

  N ct The corresponding variation for an uncompensated gas spring is much greater, namely 3.5%

  approximately We can estimate it by supposing that the gas

  carries as a perfect gas Consequently, for a given volume Vg, the pressure and therefore the force of expansion, is

  proportional to the absolute temperature For the purpose of

  parison with the example above, a variation in temperature

  deletion from -30 C to 90 C (243 K to 353 K) would cause the

next laugh:

F (80 C) F (-30 C)

o x 100 =

F (20 C)

  P (80 WC) p (-30 C) 100 x 100 = P (20 o C) P (353 K) P (243 K) x 100 =

P (293 K)

3530 K 243 OK

-23 x 100 = 37.5%

293K

EXAMPLE 3

  In this example, as in Example 2, the res-

  gas spell must have a length increased to 127 cm and a stroke of 50, 80 cm At a temperature of 20 C, it will have

  a net expansion force of 67.95 kg in the extended position

  72.48 Kg in compressed position Primary gas

  is nitrogen and the material constituting the source of

  secondary sion is sulfur hexafluoride (SF 6) The temperature compensation range varies from -30 C to C although the critical temperature of SF 6 is 45.55 C,

  above which it can only exist in ga-

zeuse.

  The values of the parameters initially retained are as follows: Dp = 1.995-cm Dr = 0.793 cm Dt = 6.35 cm L = 66.04 cm Ls = 7.62 cm Lv = 23.28 cm Lt (Max) = 127 cm M = 210 g Pa = 1.012 x 105 Pa Po = 64.042 x 105 Pa S = 50.80 cm Wc = 0.161 cm Wp = 0.635 cm Ws = 0.158 cm Equations 8, 9 and 12 give the following values as result: Ag = 1.316 cm 2 Ar = 0.496 cm 2 and Av = 0.825 cm 2 Figure 6 is a phase diagram of

  sulfur hexafluoride in which the pressure under

  me of steam is plotted according to a specific volume.

  There are twelve curves in solid lines which represent the

  temperatures at which the gas spring is supposed to function-

  ner The dotted curve "J" indicates the area inside

  of which the liquid and the vapor are in equilibrium For this example, 210 g of sulfur hexafluoride should be introduced into the gas spring. In Table V, a list of the quantities used is shown.

  to calculate the specific volume of sulfur hexafluoride

  fre in borderline cases where the gas spring is complete-

  elongated and fully compressed.

TABLE V

  Completely elongated gas spring Volume Vv (Eq (14)) 695.95 cm Mass M of SF 6 210.37 g Specific volume = Vv / Mass 330.8 cm per Kg Fully compressed gas spring 737.65 cm 210.37 g 350.7 cm per Kg The operational area corresponding to 210.37 g

  sulfur hexafluoride is indicated by the dot line

  "K" curve tillers in Figure 6 The range of specific volumes is very small in this example In addition, these

  specific volumes Apply at temperatures above

  above the critical point where it is impossible that the hexa-

  sulfur fluoride may be present in the liquid state.

  This example is designed so that the volume specified

  be limited in its variations which cannot be substantial between the fully extended position and the

  fully compressed position This limitation is ob-

  held by locating the baffle (32) closer to the bottom wall (16) than in Example 2 Observe the difference in

  Lv of examples 2 and 3 This characteristic depends on

  dre the pressure of almost unique sulfur hexafluoride-

  temperature and very little of the parameter X de-

  placement It is important to note that even above

  400 C o the operational zone leaves the gas phase-li-

  that the properties of sulfur hexafluoride are

  such as the percentage change in its pressure vis-

  opposite the temperature is higher than that of gas

primary, nitrogen.

  In Table VI below, we see an illustration

  gas spring force for a full range

  spring compressions and temperatures for which

  the gas spring of example 3 is designed The force values come from equation 16 above, as well as from equation 15, and the vapor pressure values of sulfur hexafluoride from the diagram come from

phase diagram of figure 6.

TABLE VI

Gas spring length Lt (cm)

Compress-

  Zion X (cm) Expansion force at various temperatures (C)

-30 -20 -10

0 10 20 30 40 50 60 70 80

  0.00 66.59 67.95 68.85 69.31 68.85 67.95 66.59 65.68 66.14 67.04 67.49 68, 40

  116.84 10.16 67.49 68.85 69.76 69.76 69.76 68.85 67.49 66.59 67.04 67.95 68.85 69.76

co 0%

  106.67 20.32 67.95 69.76 70.67 70.67 70.67 69.76 68.40 67.49 68.40 69.31 '70, 21 71.12

  96.52 30.48 68.85 70.21 71.12 71.57 71.57 70.67 69.31 68.85 69.76 70.67 71.57 72.48

  86.36 40.64 69.76 71.12 72.02 72.48 72.48 71; 57 70.21 69.76 70.67 71.57 72.48 73.84

  76.20 50.80 70.67 72.02 72.93 73.38 73.38 72.48 71.57 71.12 72.02 72.93 73.84 75.20

  d) L Ml c "w ro Figure 7 represents a plot of the results of Table VI with regard to the total compression and the total elongation and gives a comparison of the results with those of an uncompensated gas spring The curve" L "represents an expansion force with respect to the temperature for the temperature compensated gas spring

  of Example 3 in which the spring is completely com-

  award winning The curve "M" gives the same data but for the fully elongated spring The curves "N" and "O" give an indication of the force of expansion compared to the

  temperature for an uncompensated gas spring which is com-

  fully compressed and fully extended respectively.

  Figure 7, as well as Table VI, show

  clearly that there is compensation for temperature changes

  even when the system is operating above the

  critical point of the secondary pressure source substance

  re during a certain part of the time As in the example-

  ple of FREON-12, the degree of temperature compensation

  re for the example of sulfur hexafluoride can be

  quantified by calculating the corresponding variation in

  centering of the expansion force with respect to the temperature

  erasure using the data in Table VI and doing so

  sant a comparison with the results of a design which does not include a compensation system Table VII (next page) gives the results with a compensation system On a global level, the deviation due to temperature is below 6 for 100 and is still more favorable F if we compare it with the deviation of 37.5 percent of the uncompensated gas spring discussed more

top about Example 2.

  Once the description of the modes of realization has been completed

  sation of the invention, it should be remembered that it can be carried out in other modes in the

  limits of the attached claims.

TABLE VII

  Gas spring length Lt (cm) 127.0

116.84

106.68

  96.52 86.36 76.20 Compression X (cm) o, 16, 32, 48, 64, 80 Strength of

design

tion (Kg)

  20 C _

F (X, 20)

67.95 68.85 69.76, 67 71.57 72.48

Minimum force Maximum force

male F min male F max.

  (Kg) (Kg), 68 66.59 67.49 68.85 69.76, 67 69.31 69.76 71.12 72.48 73.84, 20% change = Fmax Fmin F_ _ x 100

F (X, 20)

* 0 / No r> i Un (ru

Claims (29)

CLAIMS -   1 Gas spring comprising a sealed envelope (12), a sliding rod (30) extending from the inside to the outside of said envelope through one of the ends of said envelope, and a piston (24 ) my-   tee on a rod inside said envelope, character-   laughed at in that it includes a primary pressure source   located inside said envelope which creates a pres-   primary pressure exerted on said piston to push the   said rod outside of said envelope and a source   secondary pressure located inside said   velope which creates a secondary pressure exerted on said piston to push said rod towards the inside of   said envelope, said primary pressure being greater than   re at said secondary pressure and change in for   centering of said secondary pressure as a function of the time   being more striking than the change in percentage   tage of said primary pressure as a function of the temperature cross out.   2 gas spring according to claim 1, charac-   characterized in that said primary pressure varies essentially-   in proportion to the absolute temperature.   3 gas spring according to claim 2, charac-   terized in that said primary pressure source is a primary pressurized gas which remains in the gas phase for the range of temperatures to which the gas spring.   4 gas spring according to claim 3, charac-   characterized in that said temperature range varies from 30 WC to 80 WC.   Gas spring according to claim 4, charac-   terized in that the primary gas under pressure is of a- zote.   6 Gas spring according to claim 1, charac-   terized in that said secondary pressure varies so   approximately exponential as a function of temperature absolute ture.   7 gas spring according to claims 1 or 6,   characterized in that said secondary pressure source is the vapor pressure of a two-phase system in   which the gas and liquid phases are in equilibrium.   8 Gas spring according to claim 7, characterized in that the gaseous and liquid phases of a system   with two phases remain in equilibrium for a temporary   peratures ranging from -300 C to 80 'C.   9 gas spring according to claim 8, charac-   terized in that the two-phase system is chosen from   a group consisting of acetylene, ethane, chloro-   fluoro-methane (FREON-12, FREON-13 and FREON-114), propa-   ne, propadiene, perfluoropropane, dimethyl ether, N-butane, ammonia gas, hydrogen bromide and hydrogen iodide.   Gas spring according to claim 8, charac-   terized in that the two-phase system is ammo- gas niac.   11 Gas spring according to claim 8, charac-   terized in that the two-phase system is FREON-12.   12 Gas spring according to claim 7, charac-   that the gaseous and liquid phases of the system   with two phases remain in equilibrium for a subs-   tantielle from the temperature range from -30 'C to 800 C.   13 Gas spring according to claim 12, charac-   terized in that the two-phase system is hexafluo- sulfur rure.   14 Gas spring according to claim 1, charac-   terized in that said primary pressure source is   pressurized nitrogen, said secondary pressure source   daire is the vapor pressure of a two-phase system   its in which the gas and liquid phases are in equi-   free and said two-phase system is taken from a group   including ammonia gas, chloro-fluoro-methane (FREON- 12) and sulfur hexafluoride.   Gas spring with temperature compensation, characterized in that it comprises   a) a sealed envelope comprising an outer tube   laughing with a bottom wall closed to an ex-   end and a bottom wall with an opening   re right through at the other end; b) an inner tube disposed inside the outer tube, one end of which is fixed to said closed bottom wall c) a sliding piston disposed inside said inner tube which divides said inner tube into a first interior volume between the piston and   said closed bottom wall and a second volume   inside me in the remaining part of the tube lower-   d) a rod fixed to the piston which extends through the opening of said wall provided with an opening right through e) a baffle fixed between said inner tube and said outer tube which divides the volume of the envelope to the outside of said inner tube into a first outer volume adjacent to said   closed bottom wall and a second external volume   laughing adjacent to said bottom wall provided with an opening right through; f) a first passage means connecting said first   internal volume and said first external volume laughing   g) a second passage means connecting said two   x interior volume and said second exterior volume;   h) a primary pressure source in said pre-   better internal and external volumes which creates a primary pressure exerted on said piston to push said rod outside of said   envelope through said opening of the   said wall provided with an opening right through, and i) a source of secondary pressure in said second interior and exterior volumes which creates a secondary pressure exerted on said piston to push said rod inside said envelope, in which said primary pressure is greater than said secondary pressure and wherein the percentage change of said secondary pressure as a function of temperature is more significant than the percentage change of said primary pressure as a function of temperature.   16 Gas spring according to claim 15,   characterized in that said first passage means consists   at least one orifice through said inner tube beyond the stroke of said piston towards said closed bottom wall, said inner tube extending towards said wall   bottom provided with an opening from side to side, but stops   both just before reaching it and in which said inner tube is open between said second inner volume   and said second external volume in order to form a two- xth means of connection.   17 Gas spring according to claim 15, ca-   characterized in that said first passage means consists   te in an orifice at least through said inner tube beyond the stroke of said piston towards said bottom wall   closed, said inner tube extending to said pa-   king with an opening through and through   fixed and wherein said second passage means consists   te in an orifice at least through said inner tube beyond the stroke of said piston moving away from said closed back wall.   18 Gas spring according to claims 16 or   17, characterized in that it includes a stop which limits the stroke of said piston moving away from said bottom wall closed.   19 Gas spring according to claim 18,   characterized in that it has a first seal between said piston and said inner tube which prevents the fluid from flowing beyond said piston and a second seal between said rod and the periphery of the opening in said wall provided with an opening right through which prevents the fluid from flowing out of said envelope.   Gas spring according to any one of the re-   vendications 15, 16 or 17, characterized in that said primary pressure varies essentially in proportion to the absolute temperature.   21 Gas spring according to claim 20,   characterized in that said primary pressure source is a primary pressurized gas which remains in the gaseous state over the whole range of temperatures to which it is subjected the gas spring.   22 Gas spring according to claim 21,   characterized in that said primary gas under pressure remains   re in the gaseous state over a temperature range from -300 C to 800 C.   23 Gas spring according to claim 22,   characterized in that said primary gas under pressure is nitrogen.   24 Gas spring according to claim 15,   characterized in that said secondary pressure varies approximately   ximonually exponentially as a function of time absolute temperature.   Gas spring according to any one of the re-   vendications 15, 16 or 17, characterized in that said   secondary pressure source consists of the pres-   steam in a two-phase system in which the   gas and liquid phases are in equilibrium.   26 Gas spring according to claim 25,   characterized in that the gaseous and liquid phases of said two-phase system remain in equilibrium over the entire   temperature range from -300 C to 80 WC.   27 Gas spring according to claim 26,   characterized in that said two-phase system is chosen   from a group comprising acetylene, ethane, chloro-   fluoro-methane (FREON-12, FREON-13 and FREON-114), the pro-   pane, propadiene, perfluoropropane, dimethyl ether, N-butane, ammonia gas, hydrogen bromide and hydrogen iodide.   28 Gas spring according to claim 26,   characterized in that said two-phase system is ammonia gas.   29 Gas spring according to claim 26,   characterized in that said two-phase system is of the chloro-fluoro-methane (FREON-12).   Gas spring according to claim 25,   characterized in that the gaseous and liquid phases of said   two-phase system remain in equilibrium for a part   substantial part of the temperature range from -30 C to 80 C.   31 Gas spring according to claim 30,   characterized in that said two-phase system is constituted   killed by sulfur hexafluoride.   32 Gas spring according to any of the   Vendications 15, 16 or 17, characterized in that said primary pressure source is nitrogen under pressure, said secondary pressure source is the vapor pressure of a two-phase system, the gas and liquid phases remaining in equilibrium and said two system   phases being chosen from a group composed of ammo- gas   niac, chloro-fluoro-methane (FREON-12) and hexafluoru- re sulfur.   33 Gas spring according to any of the   vendications 15, 16 or 17, characterized in that said tu-   be inner, said outer tube and said piston are   cylindrical in their cross section.