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The Design of Self-Centring Seal-Less Hydraulic Pistons

This paper describes the design optimization of hydraulic pistons in which the only seal is the small clearance between the piston and the cylinder wall. The small leakage thus permitted is used, in conjunction with an appropriate piston geometry, to provide a pressure distribution which makes the piston self-centring. Metal-metal contact is avoided, even under considerable side loads. Similar design principles applied to the piston rod bearing make the rod self-centring in the bearing. Several possible advantages arise, e.g., the elimination of break-out friction problems common with conventional seals, the elimination of maintenance checks and seal replacements, use in environments which are hostile to conventional seals, and use with non-lubricating fluids. Three piston designs, the tapered piston, the stepped piston and the slot-controlled piston, are considered. The slot-controlled piston is shown to have performance characteristics which are superior to those of the other two designs.