Ford Motor Company has recently received a patent for a gas strut design that is less sensitive to temperature effects. It is especially useful as a strut for hoods, deck-lids and other applications that expose the strut to wide temperature variations. Traditional struts use a single gas that becomes affected by the ideal gas law (PV=nRT). This causes a temperature dependence on the force generated by a gas strut manifesting itself in slow or partial opening in cold weather. The problem is further exacerbated by higher seal friction. Ford designed a strut that is less dependent on temperature by using two chambers having different gasses.

The Ford design employs a traditional strut, but with two important differences. First, it uses carbon dioxide (CO2) as the "non-ideal" gas. The gas is inexpensive, non-toxic, and chemically inert. The figures below illustrate the performance of the two-gas system and the construction of the strut. The two-gas system results in extremely even force curve over a wide temperature range (to within ±2.6% over a -40 to +80 degrees C).

A great advantage of using CO2, is the ability to handle this material as a solid ("dry ice") at room temperature and pressure. This provides for a simple manufacturing process. The ideal gas (e.g., nitrogen) is introduced at high pressure into the first chamber of the tube and sealed by a sliding seal on the movable rod of the strut. The insertion of CO2, into the second chamber can then be done under ambient conditions. A measured mass of dry ice is simply dropped into the second chamber and a crimp seal made. The dry ice sublimes to provide the desired high-pressure gas.

A sliding seal on the movable rod is required to separate the two gases. This presents a potential reliability problem, since if the two gases mix the temperature compensation characteristic is lost. A conventional seal will have difficulty providing the necessary long-term sealing, since the two gases are at comparable pressures and indeed the pressure difference between them can reverse depending upon temperature. Our patent overcomes this problem with a double seal, each of the seals providing a seal between a high-pressure gas and atmospheric pressure. This high-pressure difference across the seal ensures reliable sealing, providing a situation similar to the seal between the high-pressure gas and atmospheric pressure of a conventional gas strut. Further, if any leakage does occur, the leaked gas vents to the atmosphere rather than to the other chamber.

The device, in a simpler form, can also be packaged as a separate temperature-compensation module and used with conventional struts. Our design also includes a simple means of providing variable damping, specifically increased damping at the end of the stroke.

 

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