Microgel disersion for hydraulic apparatus and processes

A novel energy hydraulic media is disclosed comprising a microgel dispersion. The microgel dispersion is a Non-Newtonian fluid. This microgel dispersion is useful in devices that utilize a hydraulic fluid, such as hydraulic energy transmission systems and mechanical energy absorbing devices. Examples include hydraulic brake systems, liquid springs, and dynamic damping devices such as aircraft landing gear shock struts, car bumpers, and automobile shock absorbers. Methods of transmitting hydraulic energy and for dissipating kinetic energy using this microgel dispersion are also disclosed.

BACKGROUND
 This invention relates to a hydraulic media for hydraulic processes and
 apparatus. More particularly, the invention relates to a hydraulic media
 comprising a microgel dispersion. The microgel dispersion is a
 Non-Newtonian fluid. This microgel dispersion is useful in devices that
 utilize a hydraulic fluid, such as hydraulic energy transmission systems
 and mechanical energy absorbing devices. Examples include hydraulic brake
 systems, liquid springs, and dynamic damping devices such as aircraft
 landing gear shock struts, car bumpers, and automobile shock absorbers.
 Methods of transmitting hydraulic energy and for dissipating kinetic
 energy using this microgel dispersion are also disclosed.
 Hydraulic devices employ a fluid as the working medium, and many prior art
 devices employ organic fluids. In many applications, silicone fluids have
 replaced organic fluids because silicone fluids are more chemically
 stable, they can be employed at higher operating temperatures, and the
 viscosity of certain silicone fluids is less dependent upon temperature
 changes than prior organic fluids. A silicone hydraulic fluid having
 improved viscosity stability with respect to temperature is described in
 U.S. Pat. No. 2,398,187. Prior art hydraulic fluids tend to be difficult
 to seal, especially at higher pressures greater than 5000 psi.
 Elastomers having the ability to flow like a liquid have been employed as
 energy dissipation media and have proven to be quite leak resistant
 compared to hydraulic fluids, especially silicone fluid. Silicone
 elastomers have been employed due to their high thermal stability, low
 glass transition temperature, lack of crystallinity and high
 compressibility. An example of a silicone elastomer that flows like a
 fluid under high pressure is described in U.S. Pat. No. 3,843,601. This
 patent discloses the chemistry of an silicone elastomer that is easily
 deformed under pressure and breaks into soft particles under high shear.
 The particles have the property of flowing under pressure. The silicone
 elastomer is formed by preparing a vinyl-containing silicone fluid having
 a molecular weight of between 20,000 and 200,000 having predominantly
 dimethylsiloxane units with a small amount of methylvinyl siloxane units.
 The elastomer is unusual since there are between 0.074 and 0.74 free end
 groups on the elastomer per 100 silicon atoms. It was discovered, however,
 that prior art elastomers degrade under high pressure shear flow.
 Rendering a stable media is difficult and time consuming, for example by
 repeatedly shearing the media until the properties become stable.
 Furthermore, the material described in U.S. Pat. No. 3,843,601 would
 crystallize at -40.degree. C., rendering the material unsuitable for use
 in a landing gear shock strut application, which could operate at a
 temperature as low as -40.degree. C.
 Therefore, a material that is more resistant to leaking than prior art
 hydraulic fluids is desired. More particularly, a material having the
 ability to flow like a fluid is desired, and that remains relatively
 stable when repeatedly sheared.
 SUMMARY OF THE INVENTION
 The present invention is directed to a hydraulic media comprising a
 microgel dispersion. This novel microgel dispersion is leak resistant,
 flowable under pressure, has high compressibility, low thermal expansion,
 low hysteresis and properties that are relatively stable with time and
 usage as compared to prior hydraulic media. The microgel dispersion
 comprises from about 30 to about 80% of microgel particles dispersed in a
 liquid phase. The liquid phase of the microgel dispersion comprises from
 about 20 to about 70% of a low viscosity fluid.
 The energy dissipation media is useful in hydraulic energy transmission
 apparatus and mechanical energy absorbing devices and applications. In one
 embodiment, the microgel dispersion of the instant invention is used in
 aircraft landing gear shock struts. In another embodiment, the microgel
 dispersion of the instant invention is used in an automobile bumper. In
 yet another embodiment, the microgel dispersion of the instant invention
 is used in an automobile shock absorber.
 Additionally, a method for dissipating kinetic energy using the microgel
 dispersion is disclosed. Essentially, the method comprises applying a high
 shearing force to the microgel dispersion of the instant invention. The
 method of dissipating kinetic energy can comprise flowing the microgel
 dispersion through an orifice.
 For a better understanding of these and other aspects and objects of the
 invention, references should be made to the following detailed description
 taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION
 Referring to FIGS. 1-4, certain apparatus and processes according to
 various aspects of the invention are presented that are not drawn to
 scale, and wherein like components are numbered alike. Referring now to
 FIG. 1, a hydraulic transmission apparatus 10 is presented according to an
 aspect of the invention.
 Hydraulic transmission apparatus 10 employs the hydrostatic principle of
 operation and comprises a prime mover 12, a microgel dispersion 14, a
 hydraulic generator 16 connected to said prime mover 12 and containing a
 first portion 18 of said microgel dispersion 14, a hydraulic motor 20
 containing a second portion 22 of microgel dispersion 14, and a pipe 24
 connecting the hydraulic generator 16 to the hydraulic motor 20. The pipe
 24 is filled with the microgel dispersion 14 in fluid communication with
 the first 18 and second 22 portions of microgel dispersion 14. The
 hydraulic generator 16 is configured to convert mechanical energy from the
 prime mover 12 to hydraulic energy in the first portion 18 of microgel
 dispersion 14. The hydraulic motor 20 is configured to convert hydraulic
 energy from the second portion 22 of microgel dispersion 14 to mechanical
 energy at the hydraulic motor 20. The pipe 24 transmits the hydraulic
 energy from the hydraulic generator 16 to the hydraulic motor 20 through
 the microgel dispersion 14. In the example presented, the prime mover 12
 is a pedal, the hydraulic generator 16 is a first piston and cylinder
 assembly, and the hydraulic motor 20 is a second piston and cylinder
 assembly. Movement of the prime mover 12 induces movement of the hydraulic
 motor 20, as indicated by arrow 21.
 Referring now to FIG. 2, an alternative embodiment of a hydraulic
 transmission apparatus 30 according to a further aspect of the invention
 is presented that employs the hydrokinetic principle of operation.
 Hydraulic transmission apparatus 30 employs the hydrostatic principle of
 operation and comprises a prime mover 31, a microgel dispersion 14, a
 hydraulic generator 36 connected to said prime mover 31 by a drive shaft
 33 and containing a first portion 38 of said microgel dispersion 14, a
 hydraulic motor 40 containing a second portion 42 of microgel dispersion
 14, and a pipe 44 connecting the hydraulic generator 46 to the hydraulic
 motor 40. The pipe 44 is filled with the microgel dispersion 14 in fluid
 communication with said first 38 and second 42 portions of microgel
 dispersion 14. The hydraulic generator 36 is configured to convert
 mechanical energy from the prime mover 31 to hydraulic energy in the first
 portion 38 of microgel dispersion 14. The hydraulic motor 30 is configured
 to convert hydraulic energy from the second portion 22 of microgel
 dispersion 14 to mechanical energy at the hydraulic motor 40. The pipe 44
 thereby transmits the hydraulic energy from the hydraulic generator 36 to
 the hydraulic motor 40 through the microgel dispersion 14. In the example
 presented, the prime mover 31 is an internal combustion engine or electric
 motor, the hydraulic generator 36 is a hydraulic pump, the hydraulic motor
 40 is a rotary hydraulic motor having an output shaft 45. A reservoir 46
 is provided connected to the hydraulic motor by a return line 47. The
 reservoir 46 is also connected to the hydraulic generator by a feed line
 49, thus allowing continuous feed of the microgel dispersion 14 to the
 hydraulic generator 36 and the hydraulic motor 40 in a closed-loop circuit
 through the reservoir 46. Rotary motion of imparted to the hydraulic
 generator 36 by the drive shaft induces rotary motion of the output shaft
 45, as indicated by arrow 41. Although specific examples of hydraulic
 generators and hydraulic motors have been presented in FIGS. 1 and 2, it
 is not intended to limit the invention to the specific examples presented
 since other types of hydraulic devices are evident to those having skill
 in the relevant art.
 Referring now to FIG. 3, a damping device 50 is presented that employs the
 microgel dispersion 14 according to a further aspect of the invention.
 Damping device 50 comprises a hollow cylinder 52 defining a first aperture
 54 at one end and second aperture 56 at an opposite end, and a damping rod
 58 extending through the hollow cylinder 52 through the first aperture 54
 and the second aperture 56. The hollow cylinder 52 defines a cavity filled
 with the microgel dispersion 14. The damping rod 58 comprises a flange 61
 that defines an orifice 62. Movement of the damping rod 58 as indicated by
 arrow 64 causes the microgel dispersion 14 to flow through the orifice 62
 which generates dynamic damping by developing a pressure difference across
 the flange 61. Depending upon the viscosity of the microgel dispersion 14,
 a substantial amount of viscous damping may also be generated.
 Referring now to FIG. 4, a spring 70 according to a further aspect of the
 invention is presented. The spring 70 comprises a hollow cylinder 72 that
 defines a first aperture 74 and a cavity filled with the microgel
 dispersion 14. A displacement rod 78 is disposed within the hollow
 cylinder 72 and protrudes through said first aperture 74. The displacement
 rod 78 comprises a flange 80 that defines an orifice 82. Movement of the
 displacement rod 78 into the cylinder 72 compresses the microgel
 dispersion 14 thereby generating a spring force on the displacement rod
 78. Movement of the displacement rod 78, as indicated by arrow 84, causes
 microgel dispersion 14 to flow through the orifice 82 which generates
 dynamic damping by developing a pressure difference across the flange 80.
 Depending upon the viscosity of the microgel dispersion 14, a substantial
 amount of viscous damping may also be generated. Damping may be minimized
 or eliminated by omitting the flange 80.
 The hydraulic media of the instant invention comprises a non-Newtonian
 fluid. More particularly, the energy dissipation media is a microgel
 dispersion. Preferably, the microgel dispersions of the instant invention
 have the following desired properties. First, the microgel dispersions are
 flowable under pressure. Second, they may be formulated to have relatively
 high compressibility which is comparable to other fluids used
 conventionally to dissipate kinetic energy. Third, the microgel
 dispersions of the instant invention have thermal expansion which is lower
 than the conventional energy dissipation fluids. Fourth, the microgel
 dispersions have low hysteresis. Fifth, the flow properties of the
 microgel dispersions are believed to be stable with time and usage.
 The microgel dispersions of the instant invention preferably comprise a
 fluid as the liquid phase of the dispersion and microgel particles. The
 microgel particles are dispersed within the fluid to form a viscous
 gel-like material. The microgel dispersion has a high solids content.
 Preferably, the fluid used to form the liquid phase of the dispersion is a
 low viscosity fluid. Examples of the low viscosity fluids include silicone
 fluids, hydrocarbon fluids, and alcohols. Examples of suitable silicone
 fluids which can be used as the liquid phase which can be used in the
 microgel dispersion of the instant invention include polydimethylsiloxane
 fluids, polyphenylmethylsiloxane fluids as well as mixtures of these
 fluids. Examples of suitable alcohols include glycols, as well as mixtures
 of glycols with water. If glycol is employed the low temperature limit may
 be decreased to as low as -52.degree. C. by the addition of an appropriate
 amount of water. A corrosion inhibitor may be desirable if water is
 employed in the microgel dispersion. Fluids know in the art for use as
 hydraulic fluids may be employed, including mineral oil, synthetic
 hydrocarbon fluids, alkyl benzenes, and dibasic acid esters. Suitable
 fluids are described in the Encyclopedia of Chemical Technology, Volume 12
 (John Wiley 1980), under the title "Hydraulic Fluids."
 Generally, the fluid used as the liquid phase of the microgel dispersion
 has a viscosity of at least 10 centistokes at 25.degree. C. It is believed
 that a fluid having a viscosity of about 300,000 centistokes at 25.degree.
 C. can be used in the microgel dispersion of the instant invention.
 Preferably, the fluid has a viscosity in the range of about 100 to about
 150 centistokes. The viscosity of the liquid phase of the microgel
 dispersion can be increased depending upon the application of the final
 microgel dispersion. However, the fluid viscosity should not be low in
 order to minimize swelling of the microgel particles dispersed in the
 liquid phase. For example, the viscosity of the fluid in the liquid phase
 for use in a hydrostatic or hydrodynamic hydraulic transmission apparatus
 is approximately in the range of 10-10,000 centistokes. The viscosity of
 the liquid phase for an application wherein the microgel dispersion is
 forced to flow through an orifice is 10-1000 centistokes. The choice of
 the viscosity level of the low viscosity fluid is a matter of design
 choice well within the scope of one of ordinary skill in the art depending
 upon the final application of the microgel dispersion.
 Examples of commercially available silicone fluids that can be used to form
 the liquid phase of the microgel dispersions of the instant invention
 include polydimethylsiloxane ("PDMS") trimethylsiloxy terminated fluids,
 phenylmethylpolysiloxane fluids and polydiphenyldimethylsiloxane fluids.
 As the phenyl group replaces the methyl group in the polysiloxane, the
 lubricity, oxidation resistance, thermal stability and shear resistance
 are enhanced, enabling the alteration of the fluid to the desired end
 application. Examples of commercially available polydimethylsiloxane
 fluids are Masil SF5, available from BASF Industries, PS 043, available
 from United Chemical Technologies, Inc. and SF 96, SF 97, SF 81 and
 Viscasil.RTM. fluids, all available from General Electric. Examples of
 commercially available phenylmethylpolysiloxane fluids are Masil SF 1221,
 available from BASF Corporation and PS 160, available from United Chemical
 Technologies, Inc. Examples of commercially available
 diphenyldimethylsiloxane fluids are PS060.5, available from United
 Chemical Technologies, Inc. And SF 1154, available from General Electric.
 The exact grade and type of silicone fluid chosen is dependent upon the
 exact application and well within the purview of one of ordinary skill in
 the art.
 In addition to the fluid in the liquid phase, the microgel dispersion
 includes microgel particles. The microgel particles are preferably
 particles formed from a fully cured, high tear strength silicon rubber.
 This feature of the microgel particles is important since in a very high
 tear strength silicone rubber, excess strain energy brings about plastic
 deformation which facilitates in the dissipation of the kinetic energy
 applied to the dispersion. Generally, the tear strength of silicon rubbers
 should be at least 10 pli. Examples of suitable silicon rubbers that can
 be formed into the microgel particles include polydimethylsiloxane
 rubbers, polyphenylmethyl silicone rubbers and polydiphenyldimethyl
 silicone rubbers or mixtures thereof. The silicone rubber chosen depends
 upon the desired end application of the microgel dispersion. The choice of
 the particular silicone rubber is well within the skill of one of ordinary
 skill in the art.
 These rubbers can be cured with an organic peroxide as instructed by the
 manufacturer of the silicon rubber. Any organic peroxide which can be used
 to cure the silicon rubber which forms the microgel particles of the
 instant invention. Examples of suitable peroxides and amounts which could
 suitably be used include but are not limited to benzoyl peroxide (level:
 0.8 wt. %), bis (2,4-dichlorobenzoyl peroxide (level: 1.0 wt. %),
 dimethyl-2,5 di(t-butyl peroxide)hexane (level: 1.0 wt. %) and dicumyl
 peroxide (level: 1.0 wt. %). Generally, when silicone rubber is cured with
 peroxides, a two step process is used. In the first step, the rubber is
 cured for 10 minutes at 125.degree. C. In the second stage of the cure,
 the rubber is cured for 8 hours at 205.degree. C. An example of a
 commercially available peroxide used to cure the silicone rubber is PC
 020, available from United Chemical Technologies.
 Examples of commercially available silicon rubbers which can be used to
 form the microgel particles of the instant invention include SE 5211 U, a
 filled, peroxide curable low temperature methylphenylvinylsiloxane rubber,
 available from GE Silicones and SE 6635, a filled, peroxide curable low
 temperature phenylvinylsilicone compound also available from GE Silicones,
 and SE 54, a diphenylmethylvinyl silicone gum, available from GE
 Silicones.
 These silicone rubbers used to form the microgel particles can also be
 blended with additives or polymer enhancers or modifiers or plasticizers.
 Generally, any additive or enhancer or modifiers or plasticizers can be
 used so long as it does not affect the properties of the microgel
 particles themselves. Examples of additives enhancers or modifiers which
 can be used include but are not limited to silica, thixotropic agents,
 corrosion inhibitors, stabilizers, flame retardants, adjuvents, and
 colorants.
 For certain low temperature applications which require operability of the
 energy dissipating media in the range of approximately -40.degree. C.,
 such as the aircraft shock strut applications, the microgel particles may
 be formed from low temperature silicon rubbers. Examples of such low
 temperature silicone rubbers which can be used include polyphenylmethyl
 silicon rubbers and polydiphenyldimethyl silicon rubbers. For other
 applications, other silicone rubbers can be used provided that they meet
 the desired operating temperature ranges.
 The particles used in the microgel dispersion should have a particle size
 in the range of about 1 microns to about 200 microns. Provided that the
 particles fall within this range, the microgel particles can be dispersed
 into the fluid to form the microgel dispersion. However, the exact size of
 the microgel particles depends upon the final desired properties of the
 microgel dispersion and are well within the scope of one of ordinary skill
 in the art. Preferably, the particles are about 1 micron to about 100
 microns.
 The microgel particles of the instant invention can be formed through a
 variety of methods. For example, it is believed that the microgel
 particles can be formed through emulsion polymerization. Alternatively,
 the microgel particles can be formed from cured silicone rubber which is
 reduced in size by any mechanical size reduction method and means. During
 the size reduction process, the linkage forces within the material to be
 reduced must be surmounted at the fracture surfaces.
 Any mechanical size reduction means or method which can reduce the size of
 the particles to the desired micron size range set forth above can be used
 in the microgel dispersions of the instant invention. Mechanical size
 reduction means include grinders and crushers. An example of a suitable
 mechanical size reduction method to form the microgel particles of the
 instant invention is cryogenic grinding.
 In cryogenic grinding, the temperature of the silicone rubber is reduced
 below -150 C using liquid nitrogen. A suitable grinder for size reduction
 is a Brinkmann Centrifugal Grinding Mill, available from Brinkmann
 Instruments, Inc., Westbury, N.Y.
 Alternatively, the microgel particles can be made from a solid state
 extrusion pulverization process. In this process, the silicone rubber is
 extruded in a cured solid state at room temperature as opposed to the
 molten state and subjected to the simultaneous action of shear deformation
 and pressure at a temperature above the glass transition temperature of
 the rubber to be reduced in size. In carrying out this solid state
 extrusion pulverization process, an Instron capillary rheometer was used
 to extrude the cured silicone rubber through an orifice with a diameter of
 0.05 inches and at a shear rate of at least 10 sec.sup.-1 at room
 temperature, that means the cured rubber is subjected to simultaneous
 action of shear deformation and pressure at a temperature that is above
 its glass transition temperature (below -100 C). Thus, the cured rubber is
 reduced in size to obtain the preferred particle range.
 The microgel particles are then mixed with the silicon fluid to yield a
 microgel dispersion. The dispersion comprises from about 1% to about 99%
 solid microgel particles in the liquid phase. Preferably, the dispersion
 comprises from about 30% to about 80% solid microgel particles in a liquid
 phase. In this preferred embodiment, the liquid phase comprises from about
 20% to about 70% low viscosity fluids. Generally, the microgel dispersion
 of the instant invention will have a maximum viscosity of 60,000 poise at
 a shear rate of 100/sec. A lower viscosity may be desirable for an
 application wherein the microgel dispersion is forced to flow through an
 orifice. For a landing gear application, a viscosity on the order of
 10,000 poise at a shear rate of 100/sec is applicable. The final viscosity
 of the microgel dispersion can be varied by the amount of the fluid in the
 liquid phase. The rheological properties of this microgel dispersion are
 influenced by the concentration of the microgel particles in the
 dispersion.
 Additionally, additives can also be added to the microgel dispersion
 provided that they do not detract from the desired rheological properties
 of the microgel dispersion. Conventional additives known in the art as
 well as any other additive may be used. Examples of additives which can be
 used include but are not limited to silica, thixotropic agents, corrosion
 inhibitors, stabilizers, flame retardants, adjuvents, and colorants. A
 fluid having a relatively greater compressibility is preferably used in a
 microgel dispersion employed as the compressible medium in a spring.
 The microgel dispersions of the instant invention are useful for the
 dissipation of the kinetic energy. The microgel dispersions have a initial
 yield stress. Up to this yield stress point, the material remains as a gel
 and no flow occurs. It is believed that the microgel particles form
 networks in the quiescent state of the microgel dispersion to enhance the
 dispersion viscosity that is characterized by the transient networks. When
 high shear forces are applied, the transient networks of the microgel
 dispersion are sheared. The microgel dispersion is converted from a high
 viscosity gel state to a low viscosity fluid state. This conversion from
 the high viscosity gel state to the low viscous fluid state facilitates
 the dissipation of the shock/impact energy that is being imparted to the
 microgel dispersion. After the high shear forces are relieved, the
 microgel dispersion reforms its transient network to increase the
 viscosity of the microgel dispersion. The low viscosity at high shear rate
 facilitates the lubrication of the moving piston. The characteristics of
 high viscosity at quiescent static state minimizes the leaking of the
 hydraulic media in the device in which it is employed.
 This novel microgel dispersion can be used in various mechanical energy
 absorbing devices as well as devices which simultaneously absorb shock and
 dampen vibrations. Examples of possible uses of the microgel dispersion
 include aircraft shock struts, automobile bumpers, vibration dampers, bump
 stops for elevators, shock absorbers for collision energy and vibration
 energy.
 In one application of the microgel dispersion of the invention, the
 microgel dispersion is used as the energy dissipating media in an aircraft
 suspension system.
 The microgel dispersion can be used in a damped spring. The damped spring
 for an aircraft suspension comprises a hollow media housing having a media
 housing aperture; a displacement rod received within the media housing
 aperture, the displacement rod and the media housing defining a sealed
 cavity, the displacement rod terminating within the sealed cavity; and, a
 the microgel dispersion filling the cavity, the displacement rod
 telescoping into the media housing and compressing the microgel dispersion
 during landing of the aircraft thereby providing a vertical stopping force
 on the aircraft, wherein compression of the microgel dispersion is
 mechanically altered as the displacement rod telescopes into the media
 housing to emulate the compression of a gas.
 Referring now to FIGS. 9-11, various further embodiments of the invention
 are presented in an aircraft shock strut application. Referring now to
 FIG. 9, a cross-sectional view of a shock strut 500 at zero stroke is
 presented according to an aspect of the invention that may be employed in
 the suspension system of flight vehicles. In FIG. 9, shock strut 500 is
 fully extended, and has a fully extended length indicated as D. The shock
 strut 500 may be attached to an airframe 32 (shown in phantom) by a
 knuckle 501 and, by another knuckle 503, may be attached to a wheel and
 brake assembly or a wheel truck 58 (shown in phantom) that carries a
 plurality of wheel and brake assemblies. The shock strut 500 comprises a
 hollow outer housing 502 having an outer housing aperture 504, and a
 hollow media housing 506 received within the outer housing 502 and
 protruding from the outer housing 502 through the outer housing aperture
 504. The outer housing 502 and media housing 506 carry shear loads induced
 by side-loading of the shock strut 500 during taxi. According to a
 preferred embodiment, the outer housing 502 and media housing 506 are both
 cylindrical. The media housing 506 has a media housing aperture 508. A
 first displacement rod 510 is fixed to the outer housing 502 and is
 received within the media housing 506 through the media housing aperture
 508. The media housing 506 and the first displacement rod 510 define a
 sealed cavity 512 (boundaries indicated by heavier weight lines). A media
 housing seal 520 may be provided in the media housing aperture 508 between
 the media housing 506 and the first displacement rod 510. A damping head
 514 is disposed within the cavity 512 fixed to the first displacement rod
 510, and defines an orifice 516. According to a preferred embodiment, the
 first displacement rod 510 is hollow, and the damping head 514 defines the
 orifice in combination with a tapered metering rod 522 that passes through
 the damping head 514 inside the first displacement rod 510. The metering
 rod 522 is fixed to the media housing 506. Other orifice configurations
 are possible and contemplated within the practice of the invention, for
 example a fluted damping rod. A microgel dispersion 14 fills the cavity
 512. As used herein, the term "fills" means that the microgel dispersion
 14 fully occupies cavity 512, and excludes the intentional provision of a
 space occupied by gas or liquid within the cavity 512. The cavity 512 is
 filled by forcing the microgel dispersion 14 into the cavity with the
 shock strut 500 fully extended (as shown) and charging the microgel
 dispersion to an initial pressure Pi. During landing, the media housing
 506 and the outer housing 502/displacement rod 510 telescope toward each
 and further compress the microgel dispersion 14 to pressures greater than
 the initial pressure Pi. This movement also forces some of the microgel
 dispersion 14 to pass through the orifice 516. One or more extension,
 retraction and/or locking mechanisms 60 may be attached to the shock strut
 500, along with a torque linkage 62 that prevents rotation of the media
 housing 506 relative to the outer housing 502. Various other linkages and
 attachments may be provided as required for the specific application. A
 vent hole 548 may be provided to keep the pressure in an outer cavity 550
 between the outer housing 502 and media housing 506 at essentially
 atmospheric pressure.
 The microgel dispersion 14 is at the initial pressure Pi when the stroke is
 zero, before any vertical load is placed on the shock strut 500. The
 pressurized microgel dispersion 14 forces the media housing 506 away from
 the outer housing 502 when a vertical load is removed from the shock strut
 500, after take-off for example, and the initial pressure ensures that the
 shock strut 500 will return to its zero stroke position. A ledge 524 may
 be fixed to the outer housing 504, and a lower bearing 552 may be disposed
 adjacent to the ledge 524. A spacer sleeve 526 may be inserted inside the
 media housing 506 resting against the lower bearing 552. An upper
 bearing/stop 554 is fixed to the media housing 506. The top of the spacer
 sleeve 526 engages the upper bearing/stop 554 at zero stroke and prevents
 the outer housing 504 and media housing 506 from separating any further. A
 maximum stroke stop 556 may be formed in the media housing 506 that
 engages the outer housing 502 at the maximum stroke. The two bearings 552
 and 554 resist beam shear loads on the media housing 506 and outer housing
 502 induced by side loads, and enhance the shear load carrying
 characteristics of the shock strut 500. The outer housing 502 and media
 housing 506 are each shown as single pieces for the sake of clarity. In
 practice, the various components comprising shock struts according to the
 invention are preferably formed from high strength steel, and assembled
 from multiple pieces, according to methods well known in the aircraft
 landing gear art.
 The first displacement rod 510 compresses the microgel dispersion 14 to a
 pressure greater than the initial pressure Pi by decreasing the volume of
 the microgel dispersion 14, from its initial volume at zero stroke, as the
 outer housing 504 and media housing 506 are stroked toward each other.
 Stroking these two components toward each other forces the displacement
 rod 510 into the cavity 512, and the volume of the microgel dispersion 14
 is decreased from its initial volume by the distance (the stroke) the
 first displacement rod 510 is forced into the cavity multiplied by the
 cross-sectional area 528 of the first displacement rod 510. The microgel
 dispersion 14 resists this motion with a force corresponding to the
 pressure of the microgel dispersion multiplied by the cross-sectional area
 528. Thus, the microgel dispersion 14 acts as a spring.
 In addition to generating a spring force, the microgel dispersion 14 also
 generates a damping force when the shock strut 500 is stroked in either
 direction. In the example presented in FIG. 5, a damping head seal 530 is
 provided between the damping head 514 and the media housing 506. The
 damping head 514 translates through the microgel dispersion 14 as the
 outer housing 504 and media housing 506 are stroked toward each other,
 which forces the microgel dispersion 14 to pass through the orifice 516
 and develop a pressure differential across the damping head 514. One or
 more passages 534 may be provided in fluid communication with the orifice
 516 in order to permit the microgel dispersion 14 to pass from one side of
 the damping head 514 to the other through the orifice 516. Other orifice
 configurations are contemplated in the practice of the invention, for
 example an orifice without a metering rod 522. The damping force
 corresponds to the pressure differential multiplied by the cross-sectional
 area 532 of the damping head 514. The damping force and the spring force
 combine in summation to provide a predetermined vertical stopping force on
 the aircraft during landing, and a predetermined suspension force after
 landing for suspending the aircraft during taxi.
 Referring now to FIG. 10, a shock strut 600 for an aircraft is presented
 wherein compression of the microgel dispersion provides a load stroke of
 increasing curvature with a static position at the static stroke Ss, which
 may be 80% of the total stroke needed for landing (Sm). Shock strut 600 is
 similar to shock strut 500, except shock strut 600 comprises a second
 displacement rod 610 in addition to the first displacement rod 510. The
 second displacement rod 610 is preferably tubular and encircles the first
 displacement rod 510. A second displacement rod seal 620 is disposed
 between the first displacement rod 510 and the second displacement rod
 610. In this example, the media housing seal 520 is disposed in the
 aperture 504 between the media housing 506 and the second displacement rod
 620. Only the first displacement rod 510 is forced into the cavity 512
 until the outer housing 502 contacts the top of the second displacement
 rod 610 when the stroke equals a transition stroke St. At strokes less
 than St, the change in the volume of cavity 512 is the cross-sectional
 area 528 multiplied by the stroke S. When the stroke exceeds the
 transition stroke St, the outer housing 502 forces the second displacement
 rod 610 into the cavity 512 with the first displacement rod 510, thereby
 providing an increased cross-sectional area 628 that includes both the
 first and second displacement rods 510 and 610. Thus, for strokes greater
 than the transition stroke St, the change in the volume of cavity 512 is
 the cross-sectional area 628 multiplied by the stroke S. The first
 displacement rod 510 alone compresses the microgel dispersion 14 when the
 stroke is less than the transition stroke St, and the first displacement
 rod 510 and the second displacement rod 610 together compress the microgel
 dispersion 14 when the stroke is greater than the transition stroke St.
 Providing three or more displacement rods is also contemplated as may be
 provided to obtain a desired compression characteristic. Further
 embodiments suitable for mechanically altering the compression of the
 microgel dispersion are presented in co-pending patent application Ser.
 No. 08/947,412 entitled "Dry Media Suspension System for Aircraft" which
 is fully incorporated herein by reference.
 Referring now to FIG. 11, a cross-sectional view of a shock strut 900 is
 presented having a thermal compensator, according to a further aspect of
 the invention. An aircraft suspension system is subjected to temperature
 excursions during use. Such temperature excursions depend on the
 environment in which the suspension system is employed. For most systems,
 the temperature excursion will not likely exceed -70.degree. C. to
 55.degree. C. (-94.degree. F. to 131.degree. F.), and may be on the order
 of -23.degree. C. to 38.degree. C. (-10.degree. F. to 100.degree. F.). It
 is not intended to limit the invention to a particular temperature range,
 although functionality of the suspension system is necessary throughout
 the temperature range under which operation is required. When the aircraft
 is parked, thermal expansion and contraction of the microgel dispersion 14
 during temperature excursions increases and decreases the length D, which
 causes the aircraft to rise and fall. More importantly, thermal
 contraction of the microgel dispersion reduces the maximum available
 stroke Sm, which may render the shock strut susceptible to bottoming at
 colder temperatures. The thermal compensator according to the invention at
 least partially and/or fully mitigates these effects.
 Still referring to FIG. 9, shock strut 900 comprises the outer housing 502
 and a media housing 906 received within the outer housing 502, the
 microgel dispersion 14 filling a sealed cavity 912 defined within the
 media housing 906 and the outer housing 502. A damping orifice 516 is
 defined within the cavity 912. The microgel dispersion 14 fills the cavity
 912. As previously described in relation to other embodiments, the media
 housing 906 and the outer housing 502/displacement rod 510 telescope
 toward each other a stroke distance during landing and compress the
 microgel dispersion 14 and force the microgel dispersion 14 to pass
 through the orifice 516, thereby providing a predetermined vertical
 stopping force on the airframe 32 during landing and a predetermined
 suspension force during taxi. A temperature compensator 966 is
 incorporated into the shock strut 900 and subjected to the suspension
 force while at least partially counteracting thermal expansion and
 contraction of the microgel dispersion 14 over a predetermined temperature
 range. According to the invention, the temperature compensator may take
 various forms that have sufficient strength to resist mechanical failure,
 and that develop sufficient force to act against the microgel dispersion
 14 in order to counteract thermal contraction, while being subjected to
 the suspension force.
 In shock strut 900, the temperature compensator 966 is disposed within the
 cavity 912, and comprises an expandable and contractible bag 968
 containing a fluid 970 that changes phase over the predetermined
 temperature range. The bag 968 is preferably made out of an elastomeric
 material that may be fiber reinforced. The change in phase causes the
 volume of the fluid 970 to increase and at least partially compensate for
 the decrease in volume in the microgel dispersion 14 due to thermal
 contraction. According to a preferred embodiment, the fluid 970 is water
 if the predetermined temperature range includes the freezing point of
 water. Alternatively, a mixture of water and a freezing point suppressant,
 such as glycol or alcohol, may be employed provided that the objectives of
 temperature compensation are met. More than one bag 968 may be provided.
 The ratio of freezing point suppressant to water may be varied in each bag
 in order to vary the temperature at which each bag freezes, thus creating
 a range of temperatures over which the compensation occurs. According to a
 further preferred embodiment, the cavity 912 comprises a sub-cavity 972 in
 fluid communication with the rest of the cavity 912, and one or more
 temperature compensators 966 are disposed within the sub-cavity 972. The
 sub-cavity 972 may be defined by a perforated plate 962 to which the
 metering rod 522 is fixed. Further embodiments of temperature compensators
 that may be employed in the practice of the invention are described in
 co-pending application Ser. No. 08/947,265 entitled "Dry Media Suspension
 System for Aircraft With Temperature Compensation," the contents of which
 are fully incorporate herein by reference. In certain embodiments,
 mechanically altered compression of the microgel dispersion is combined
 with thermal compensation. Damped springs not having an outer housing are
 employed in certain aircraft landing systems, and the principles described
 herein may be applied equally as well in such systems. Existing seal
 technology may be employed for the various seals described herein,
 including seal technology employed for liquid springs.
 Referring now to FIGS. 12-14, embodiments that employ the microgel
 dispersion of the instant invention are presented according to further
 aspects of the invention. Referring specifically to FIG. 12, a side view
 of a motor vehicle door 1000 is presented having a side-impact beam 1002
 (shown dashed). A cross-sectional view that is representative of the
 cross-sectional construction of the side-impact beam 1002 is presented in
 FIG. 14 and comprises an outer shell 1004 that defines a cavity 1006 that
 is partially or fully filled with the microgel dispersion 14. The cavity
 1006 is divided into a multitude of sub-cavities 1008, 1010, 1012, 1014,
 1016, 1018, and 1020 by perforated dividers 1022, 1024, 1026, 1028, 1030,
 and 1032. The outer shell 1004 and perforated dividers 1022, 1024, 1026,
 1028, 1030, and 1032 may be formed from metal and/or plastics, and/or
 fiber reinforced plastics. A side impact into the door 1000 causes
 deformation of the side impact beam 1002 which, in turn, causes the
 microgel dispersion 14 to flow through the perforated dividers from one
 sub-cavity the next, thereby dispersing kinetic energy. A rear view of a
 motor vehicle bumper 1100 according to a further aspect of the invention
 is presented in FIG. 13 that has a cross-sectional construction similar to
 that presented in FIG. 14, and operates in the same manner to dissipate
 kinetic energy from a rear impact.
 EXAMPLES
 Example 1
 Example 1 sets forth the composition of three different peroxide cured low
 temperature silicone rubbers which can be formed into the microgel
 particles used in the microgel dispersion of the instant invention. The
 first silicone rubber is formed from SE 54, a peroxide curable
 diphenylmethylvinyl silicone gum rubber available from General Electric
 Silicones. The second silicone rubber is formed from SE 5211U which is
 available from GE Silicones. SE 5211U is a filled
 methylphenylvinylsiloxane. Based upon its Material Safety Data Sheet
 ("MSDS"), SE 5211U is believed to comprise about 10-30 weight % of a
 tetramer treated fumed silica, about 5-10 weight % of a
 dimethyldiphenylsiloxane copolymer and about 60-80 weight % of a
 diphenylmethylvinyl siloxane gum. The third low temperature silicone
 rubber which can be used in the instant invention is SE 6635, a peroxide
 curable low temperature silicone which is also available from GE
 Silicones. SE 6635 is a phenyl vinyl silicone compound which is believed
 to be formed from about 10-30 weight % of tetramer treated fumed silica,
 about 1-5 weight % of a silanol stopped polydimethylsiloxane, about 1-5
 weight % of silicones and siloxanes which are dimethylmethoxy terminated
 and about 60-80 weight % of diphenylmethylvinyl siloxane gum according to
 its MSDS. These silicone rubbers are cured according to the manufacturers
 instructions using benzoyl peroxide PC 020, available from United Chemical
 Technologies, Inc.

Ex. C Ex. D Ex. E
 Microgel particles 100 25 10
 from SE 54
 PDMS trimethyl siloxy 25
 terminated fluid from
 United Chemical,
 100,000 cSt
 Masil SF 1221, 10
 phenylmethylpolysiloxane
 fluid from BASF Corp.
 Compressibility, % 3.97 4.58 4.42
 @ 10,000 psi
 Compressibility, % 7.47 8.13 8.01
 @ 20,000 psi
 Compressibility, % 10.2 10.89 10.70
 @ 30,000 psi
 The results set forth in the Table above show that either low viscosity
 silicone fluid or a high viscosity silicone fluid can be used as the
 liquid phase in the microgel dispersion to achieve similar compressibility
 results. Nonetheless, the hysteresis and the ability to flow of the
 microgel dispersion still need to be taken into account for the desired
 particular end application of the microgel dispersion. This generally
 requires the usage of a low viscosity fluid as defined herein.
 FIG. 1 shows a graph of flow pressure versus shear rate of the flow of the
 microgel dispersions of the this example as well as the dry media as
 described in U.S. patent application Ser. Nos. 08/728,340, 08/731,099, and
 08/728,352. The results show that the pressure required for flow can be
 tailored using the microgel dispersion of the instant invention.
 Additionally, the results show that less pressure is required to force the
 microgel dispersions of the instant invention through an orifice as
 compared to the other media described in the above-mentioned applications.
 Example 3
 Example 3 illustrates the compressibility of the microgel dispersions of
 the instant invention. The microgel particles in this example were formed
 from 200 parts of SE 5211U, peroxide curable low temperature silicone
 available from GE Silicones and 2.0 parts of PC 020 peroxide, available
 from United Chemical Technologies, Inc. The silicone rubber was cured
 according to the manufacturer's instructions and reduced in size to
 particles approximately 1 to about 200 microns using the cryogenic
 grinding method with a Brinkmann Centrifugal grinding mill described
 above. The examples set forth in the Table below show the compressibility
 as a function of the weight percent of the microgel particles in the
 microgel dispersions.

Ex. G Ex. H Ex. I Ex. J
 wt. % microgel 100 40 50 70
 Compressibility, % 3.58 4.15 4.18 4.06
 @ 10,000 psi
 Compressibility, % 6.85 7.53 7.50 7.34
 @ 20,000 psi
 Compressibility, % 9.38 10.25 10.25 9.92
 @ 30,000 psi
 FIG. 2 shows the apparent shear viscosity (in poise) of these samples as a
 function of the shear rate. SF 1221 silicone fluid was used in this
 example. The results show that the apparent shear viscosity decreases at
 high shear rates.
 Example 4
 In this example, microgel particles were made from 200 parts of SE 6635,
 peroxide curable phenyl-vinyl silicone compound, available from GE
 Silicones. 2.0 parts of PC 020 peroxide, available from United Chemical
 Technologies was added to the silicone rubber to cure the rubber. The
 silicone rubber was cured according to the manufacturer's instruction. The
 silicone rubber was then reduced in size to form the microgel particles by
 cryogenic grinding to approximately 1 to about 200 microns. These
 particles were then mixed into Masil SF 1221 phenylmethylpolysiloxane
 fluids from BASF Corporation. The amount of solid particles dispersed into
 the liquid phase of the fluid was varied. The Table set forth below lists
 the compressibility of various compositions.

Ex. K Ex. L Ex. M
 Microgel Particles 10 15 18
 Masil SF 1221 10 10 7.5
 phenylmethylpoly-soloxane,
 BASF Corp.
 wt. % microgel 50 60 70
 Compressibility, % @ 10,000 4.42 4.22 4.12
 psi
 Compressibility, % @ 20,000 7.91 7.93 7.48
 psi
 Compressibility, % @ 30,000 10.61 10.56 10.11
 psi
 This example shows that the compressibility of the microgel dispersion can
 be tailored to achieve the desired compressibility by varying the
 concentration of the particles in the microgel dispersion.
 FIG. 3 shows the apparent shear viscosity of Examples K-M as a function of
 the shear rate.
 Example 5
 This example shows the room temperature compressibility of 60% solid
 microgel dispersion as a function of the silicone fluid used to form the
 microgel dispersion. The silicone fluids used in this example include a
 low viscosity polyphenylsiloxane fluid or a low viscosity
 polydimethylsiloxane fluid or a mixture of the two fluids. The low
 viscosity polyphenylsiloxane used was Masil SF 1221 from BASF Corporation.
 The low viscosity polydimethylsiloxane fluid used in this example was
 Masil SF 5, also available from BASF Corporation. In addition, the
 microgel particles in this example were formed from SE 5211 U and cured
 with 1 wt. % of PC 020 benzoyl peroxide. The particles were either cured
 to the first stage cure or to the second stage cure. The silicone rubber
 was reduced in size from about 1 to about 200 microns by cryogenic
 grinding method using a Brinkmann centrifugal grinding mill as described
 above.

Ex. S Ex. T
 SE 5211U, second stage cured 15 15
 microgel particles
 Masil SF 1221, BASF Corp. 10 0
 Masil SF 5, BASF Corp. 0 10
 wt. % microgel 60 60
 Compressibility, % @ 10,000 psi 4.21 4.45
 Compressibility, % @ 20,000 psi 7.59 8.03
 Compressibility, % @ 30,000 psi 10.20 10.74
 This example also illustrates that it is irrelevant whether the silicone
 rubber used in the microgel particles is partially cured or fully cured
 indicating that the degree of cure does not affect the ability of the
 dispersion to function as an energy dissipation medium. The amount of the
 solid microgel particles in the dispersion as well as the viscosity of the
 liquid phase are key. The amount of the solid microgel particles
 incorporated into the microgel dispersion relate directly to the ability
 of the microgel dispersion to dissipate energy.
 Example 6
 This example shows a composition according to the instant invention that
 can be used for aircraft landing gear strut application. In this example,
 200 parts of SE 5211U filled methylphenylvinyl siloxane were fully cured
 with 2.0 parts of PC 020 peroxide according to the manufacturer's
 instructions. The silicone rubber was reduced in size to approximately 1
 to 200 microns by the cryogenic grinding method using a Brinkmann
 Centrifugal grinding mill. These microgel particles were then mixed with
 Masil SF 1221 phenylmethylpolysiloxane fluid as the liquid phase to yield
 a dispersion comprising 60% by weight of the microgel particles and 40% by
 weight of the silicone fluid. The following results were obtained for
 hysteresis loss and compressibility. Hysteresis loss is an average
 calculated as indicated in FIG. 15, with curve 90 representing instroke of
 the plunger (or displacement rod), curve 92 representing outstroke of the
 plunger, and curve 94 representing the average of curves 90 and 92 with
 respect to load, in a spring according to the invention with no damping
 orifice.

Hysteresis Loss 0-30 ksi
 @ 130 F. 7.7%
 @ 70 F. 5.9%
 @ -40 F. 10.9%
 Compressibility, 0-30 ksi
 @ 130 F. 11.97%
 @ 70 F. 10.69%
 @ -40 F. 8.16%
 Those skilled in the art to which the invention pertains may make
 modifications and other embodiments employing the principles of this
 invention without departing from its spirit or essential characteristics,
 particularly upon considering the foregoing teachings. The described
 embodiments are to be considered in all respects only as illustrative and
 not restrictive and the scope of the invention, is therefore, indicated by
 the appended claims rather than by the foregoing description.
 Consequently, while the invention has been described with reference to
 particular embodiments, modifications of structure, sequence, materials
 and the like would be apparent to those skilled in the art, yet still fall
 within the scope of the invention.