Second Paper: Hydraulic Lock with Single-Land Pistons

Abstract

Experiments are described in which the break-out force associated with hydraulic lock on single-land pistons is measured. The test pistons used were of 0.5-in diameter × 1 in long, with diametral clearances in their bores in the range 2-20 × 10-4 in. The pistons were subjected to axial pressure drops in the range 0-5000 lbf/in2. The finish of the pistons and bores was closely controlled as regards surface finish, roundness and parallelism. It was found in all cases that a relatively small inherent lateral force acted on the pistons. The lateral force-pressure profile was found to increase linearly with clearance. Beyond a particular pressure for each clearance used, the lateral force necessary to generate hydraulic lock disappeared. Leakage flow rates were measured and used to help assess the attitude of a test piston in its bore. An hypothesis is given which explains the presence of hydraulic lock with single-land piston configurations. The hypothesis relates the hydraulic couple due to small residual tilts of pistons in their bores, and the mechanical friction in the reaction bearing required to support the piston against axial pressure forces. Experiments designed to test the hypothesis give positive support to it.