Springing to life. (WIP).

Gas springs. Now there's a topic sure to excite. Gas springs are better known as the struts that hold up liftgates, hoods, and hatches on an increasing number of vehicles. They sit in a harsh environment--often subject to salt, road spray, and the elements since they're located outside of the access panel's sealing surface--and must work across a broad range of temperatures. Except many won't provide assist, or hold a liftgate open when the weather turns cold.

"These are gas-pressurized struts," says Doug Bauman, an engineer with ArvinMeritor's Light Vehicle Systems, "whose output force is determined by the pressure of the nitrogen gas sealed within the strut. As the temperature drops, the strut's ability to provide adequate lift assistance, or to hold a liftgate in place, is seriously diminished." Simply stated, muscle power must be brought into play. Any solution to this problem must cost effectively provide more nitrogen in cold weather--to increase both volume and pressure--and less in warm weather.

"We started by dividing the strut chamber in two," says Bauman, "and placing a one-way check valve between the two chambers. But we had to find a way to open and close the valve as the temperature changed in order to keep the gas pressure at its specified level." The answer is a bi-metallic valve that controls the opening and closing of a one-way check valve. As the temperature of the units passes through 40 F, the valve opens, releasing the pressurized nitrogen in the second chamber. When the temperature--and pressure inside the cylinder--rises, excess nitrogen returns to the secondary chamber. "Our temperature-compensated gas spring is patented, has been tested in conditions from -40[degrees]C to 80[degrees]C, and its powder-coated finish endured 600 hours in our salt spray chamber," boasts Bauman. The ultimate goal is 800 hours of salt spray resistance.

Currently accounting for 15% of production at the company's Marion, SC, plant, ArvinMeritor sees demand for temperature-compensated gas springs rising. Recommended for applications with opening panels greater than 70 lb.--for reference, the PT Cruiser's hatch weighs 75 lb., the CMC Yukon's 105 lb.--ArvinMeritor sees a growing market for this technology. Perhaps gas springs are exciting after all.