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Abstract

A pneumatic counterbalance spring includes a hollow cylinder defining a first internal chamber divided into a compression compartment and an extension compartment by a piston movably mounted in the chamber. The compression compartment contains pressurized gas and a seal assembly mounted on the piston prevents the gas from escaping into the extension compartmint. A piston rod is slibably mounted in the open end of the cylinder and has its inner end connected to the piston. The piston rod includes a hollow portion defining a second internal chamber which is fluidly connected to the compression compartment by a flow passage. As a result, the compression compartment has an effective volume of A+B, where A represents the volume of the compression compartment and B represents the volume of the piston rod internal chamber. This design is advantageous in applications where a low spring rate is desired and where the length of the spring needs to be minimized. A gas permeable insert may be positioned in the flow passage to provide restricted rate flow of gas through the flow passage. Alternatively, a check valve may be provided for providing restricted rate flow of gas through the flow passage during the extension stroke and a tree, by-pass flow of gas through the flow passage during the compression stroke.

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First Claim