Thermally efficient shock absorber

A thermally efficient shock absorber comprising a strut housing having an open end and a closed end, a piston including a piston housing and a piston head slideable in the strut housing, and a preselected volume of fluid and gas located in the strut and piston housings. A compensating system is provided coupled to the fluid for maintaining a constant fluid-to-gas volume ratio irrespective of thermal variations and a valve is provided for isolating the compensating system from the fluid. In a particular embodiment of the invention, the compensating system includes a gas compensating chamber and a fluid compensating chamber having a movable separator piston therebetween, the fluid compensating chamber being coupled to the fluid in the strut and piston housings. The gas and fluid compensating chambers are contained within the strut housing separate from the gas and fluid in the strut and piston housings. A position indicator is coupled to the valve for causing the valve to isolate the compensating system from the fluid when the shock absorber is positioned to receive loads.

TECHNICAL FIELD 
The invention relates to the field of shock absorbers and, in particular, 
to a shock absorber in which the effects of temperature changes on shock 
absorber efficiency on landing are greatly reduced. 
BACKGROUND ART 
In the design of shock absorbers, a particular fluid/gas compression ratio 
for landing is chosen to provide optimization of the shock absorbing 
capabilities of the shock absorber. However, the shock absorbing 
capabilities of a conventional shock absorber are markedly affected by 
temperature changes. This is due to changes in the fluid/gas compression 
ratio caused by thermal variations. When the ambient temperature changes 
cause a change in the temperature of the shock absorber, the fluid inside 
the shock absorber expands or contracts according to a temperature 
increase or decrease, respectively. Since the fluid is essentially 
incompressible and there is a fixed total volume of fluid/gas inside the 
shock absorber, if the temperature changes the volume of the fluid 
increases or decreases, thereby causing the volume of the gas to decrease 
or increase, respectively. This temperature change thus alters the 
compression ratio of the shock absorber. If the temperature increases, the 
compression ratio increases and the shock absorber becomes "stiffer". 
Conversely, if the temperature decreases, the compression ratio decreases 
and the shock absorber becomes "softer". These changes can seriously 
impact the landing loads imposed on an airplane structure and the landing 
characteristics of the airplane, such as the maximum allowable landing 
weight of the airplane, which affects its load carrying capabilities, and 
the vertical descent speed at which the airplane can land, which restricts 
its flight patterns. 
Accordingly, it is a general object of the present invention to provide an 
improved shock absorber. 
It is another aspect of the present invention to provide a shock absorber 
which is thermally efficient. 
It is a further advantage of the present invention to provide a shock 
absorber in which the effects of temperature changes on shock absorber 
efficiency are greatly reduced. 
It is still another object of the present invention to provide a shock 
absorber in which the fluid/gas compression ratio is substantially 
unaffected by thermal variations. 
DISCLOSURE OF INVENTION 
A thermally efficient shock absorber is provided. The shock absorber 
comprises a strut housing having an open end and a closed end, a piston 
including a piston housing and a piston head slideable in the strut 
housing, and a preselected volume of fluid and gas located in the strut 
and piston housings. Compensating means is provided coupled to the fluid 
for maintaining a constant fluid-to-gas volume ratio irrespective of 
thermal variations, and valve means is provided for isolating the 
compensating means from the fluid. In a particular embodiment of the 
invention, the compensating means includes a gas compensating chamber and 
a fluid compensating chamber having a movable separator piston 
therebetween, the fluid compensating chamber being coupled to the fluid in 
the strut and piston housings. The gas and fluid compensating chambers are 
contained within the strut housing separate from the gas and fluid in the 
strut and piston housings. Position indicator means is coupled to the 
valve means for causing the valve means to isolate the compensating means 
from the fluid when the shock absorber is positioned to receive loads. 
The novel features which are believed to be characteristic of the 
invention, both as to its organization and its method of operation, 
together with further objects and advantages thereof, will be better 
understood from the following description in connection with the 
accompanying drawings in which a presently preferred embodiment of the 
invention is illustrated by way of example. It is to be expressly 
understood, however, that the drawings are for purposes of illustration 
and description only and are not intended as a definition of the limits of 
the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, a cross-sectional diagrammatic elevation view of a 
first embodiment of the present invention is illustrated. The shock 
absorber 10 has a strut housing 12 with a closed end 14 and an open end 
16. A piston 18 having a piston head 20 and a piston housing 22 is 
slideable within the strut housing 12. The piston housing 22 has a 
moveable separator 24 therein to separate the pressurized gas 26 in gas 
chamber 28 from the hydraulic fluid 30 in fluid chambers 32, 32' and 32". 
In operation, the hydraulic fluid 30 flows between chambers 32, 32' and 
32" via metering pin 34 and passageway 35 and the position of separator 24 
changes as external forces move the piston 18 within the strut housing 12. 
When the external forces are removed from the shock absorber 10, the 
piston 18 extends until the piston head 20 bottoms on stop 36 on the open 
end 16 of the strut housing 12. The separator 24 in turn moves up and 
bottoms against stop 38 in the piston housing 22 due to the pressure of 
the gas 26 in the gas chamber 28. 
In accordance with the present invention, the strut housing 12 is extended 
beyond the closed end 14 to provide for a fluid compensating chamber 40 
and a gas compensating chamber 42 separated by moveable separator piston 
44. The gas 46 in the gas compensating chamber 42 is kept at a lower 
pressure than the gas 26 in the gas chamber 28 so that the moveable 
separator 24 will always stay bottomed against stop 38 when external 
forces are removed. The hydraulic fluid 48 in the fluid compensating 
chamber 40 communicates with the hydraulic fluid 30 in the fluid chambers 
32, 32', 32" via passageway 50 in the closed end 14, passageway 51, and 
the hollow metering pin 34. A spring-loaded valve 52 is positioned in the 
passageway 50 and is held open when external forces are removed, as may be 
done, for example, by the position of a jury brace 54 acting on rod 56 
when a landing gear (not shown) which incorporates shock absorber 10 is in 
the retracted or stowed position. 
It can be thus seen that when the valve 52 is open, hydraulic fluid 30, 48 
is free to flow in either direction between fluid chambers 32, 32', 32" 
and fluid compensating chamber 40. If the fluid 30, 48 expands due to an 
increase in temperature, the excess fluid 30 in fluid chambers 32, 32', 
32" will flow into the fluid compensating chamber 40 causing separator 
piston 44 to move and compress gas 46 in the gas compensating chamber 42. 
Conversely, if the fluid 30, 48 contracts due to a decrease in 
temperature, the fluid 48 will flow from the fluid compensating chamber 40 
into the fluid chambers 32, 32', 32". This provides a constant volume of 
gas 26 in the gas chamber 28 and a constant volume of fluid 30 in the 
fluid chambers 32, 32', 32" and therefore a fixed or constant compression 
ratio. When external forces are about to be applied to the shock absorber 
10, such as during an airplane landing when the landing gear is extended 
for landing, the jury brace 54 is moved by the landing gear to cause the 
rod 56 to allow the valve 52 to close and isolate the fluid compensating 
chamber 40 from the chambers 32, 32', 32". 
Because of the above configuration, the shock absorber fluid/gas volumes 
and pressure are always in the proper ratio for the most efficient shock 
absorption regardless of temperature. In FIG. 2, adiabatic load versus 
stroke curves are illustrated showing the advantages of the present 
invention. At a dynamic limit load of 130,000 lbs. at 107.degree. F., only 
12.9 inches of stroke can be utilized from the 16 inches of stroke 
available without the use of the compensating chambers. With the use of 
the compensating chambers, 14.4 inches of stroke become available. This 
increase in available stroke of 1.5 inches yields superior shock absorber 
performance and more energy absorption capability. In addition, the 
effects of temperature changes are greatly reduced yielding consistent 
operating characteristics under different temperature conditions. Thus a 
significant decrease in the maximum load that the shock absorber and the 
aircraft feels can be achieved because of the added stroke available by 
essentially holding the adiabatic load versus stroke curve in one area 
regardless of ambient and shock absorber temperatures. FIG. 2 also 
illustrates compensated and uncompensated dynamic landing load versus 
stroke curves and demonstrates the increased energy absorption available 
and lower loads with the present invention due to the extra available 
stroke. 
A second embodiment of the invention is illustrated in FIGS. 3, 4 and 5. 
The shock absorber 58 has, as in the first embodiment, a strut housing 60 
with a closed end 62 and an open end 64. Piston 66 has a piston head 68 
and a piston housing 70 slideable within the strut housing 60. The piston 
housing 70 has a moveable separator 72 therein to separate the pressurized 
gas 74 in gas chamber 76 from the hydraulic fluid 78 in fluid chambers 80, 
80', 80". Fluid compensating chamber 82 and gas compensating chamber 84 
are separated by moveable separator piston 86. The gas 88 in gas 
compensating chamber 84 is expanded or compressed by the moveable 
separator piston 86 as the fluid 90 and the fluid 78 expands or contracts 
due to thermal variations. In this embodiment, the fluids 78 and 90 
communicate via passageway 92 in the closed end 62 and the hollow metering 
pin 94. The passageway 92 comprises channels 96 and 98 in the closed end 
62. Channel 98 has a spring-loaded valve 100 positioned therein having 
passageways 102 to allow the flow of fluid from channel 96 to channel 98 
and to metering pin 94 when the valve 100 is in the open position. As 
before, valve 100 is held open when external forces are removed by the 
position of a jury brace, not shown, acting on cable 104, which causes arm 
106 to pivot and push the rod portion 108 to unseat the valve 100. The 
position of the edge 110 of the moveable separation piston 86 when the 
ambient and shock absorber fluid temperature is -30.degree. F., 70.degree. 
F. and 107.degree. F. are shown by numerals 112, 114 and 116, 
respectively. 
Having described the invention, it is obvious that numerous modifications 
and departures may be made by those skilled in the art. Thus, the 
invention is to be construed as being limited only by the spirit and scope 
of the appended claims. 
INDUSTRIAL APPLICABILITY 
The thermally efficient shock absorber is useful in reducing the impact of 
landing loads on an aircraft structure.