Non-conventional lip seal mountings for pneumatic cylinders

Sliding lip seals are used in pneumatic cylinders to prevent leakage past the piston and rod. Though they guarantee excellent sealing, the friction forces at contact between lip and counterpart can be relatively high. This results in energy losses and problems with wear on components in relative motion, as well as difficulties in controlling actuator positioning. Standards for compressed air cleanliness restrict the use of greases and lubricants, especially in applications such as the food processing and pharmaceutical industries. A number of studies are thus under way in order to find effective alternatives to the use of common lubricants. These studies address both seal configuration [1-4], and the use of innovative materials [5-7]. Other approaches employ systems permitting small amounts of air leakage that operate as lubricated supports, variable-profile pistons [8], or rings with special micromachined surfaces [9-10]. Such solutions are not always economical, both because they require precision tolerances and geometries, and because they use special seals, often produced ad hoc. This paper discusses the possibility of using commercial lip seals with a non-conventional mounting on the piston in order to obtain a simple controlled-leakage system that reduces friction forces in pneumatic cylinders economically and effectively. The non-conventional mounting consists of positioning the lip seal in the direction opposite to that used in a conventional installation, so that the contact pressure reduces as air pressure in the chamber rises. In this way, an air gap is created at the sliding interface which allows air in the pressurized chamber to escape past the seal lip, thus reducing the contact force on the barrel. The piston is provided with a hole which exhausts the leakage flow to the atmosphere. To evaluate the validity of this approach, a number of preliminary tests were carried out in [11] to measure flow rate on a type of spring-loaded lip seal consisting of graphite-filled PTFE for barrel diameters of 50 mm. In particular, the tests determined air leakage behavior for different mounting tolerances. This paper presents further tests conducted on the same samples examined in [11] and on another PTFE and graphite-filled PTFE configuration, measuring leakage flow rates and friction forces. Leakage flow rate measurements were carried out with single and double seal installations. The results obtained are discussed together with the advantages of the proposed solution compared to the conventional mountings in ordinary use.