Fundamental Concepts Associated with Hydraulic Seals for High Bandwidth Actuation
Arthur Bullock, 2010
This thesis is concerned with issues relating to the development of an active sealing system for hydraulic actuators where the sealing elements can be radially extended and retracted to vary the friction and leakage characteristics. In order to determine the feasibility of the active sealing concept it is necessary to establish that varying the seal geometry may achieve useful improvements in the friction-leakage trade-off and that a practical method of achieving this seal extension can be realised. Experimental and simulation approaches for seal friction prediction have been developed and active seal prototypes produced to demonstrate the concept.
Experiments were carried out to measure the constant velocity friction for single-lip and double-lip seals over a range of sliding speeds and sealed pressures with special consideration applied to the instroke-outstroke direction dependence. Additional experiments were performed with sinusoid motion to provide an indication of the transient friction characteristics. Friction was shown to increase towards the end of the outstroke cycle and decrease once the instroke motion began.
Tribology simulations were produced based on the results of a FEA simulation of the rod-seal contact pressure. Empirical friction-load relationships and novel contact mechanics approaches for high loads were considered. Simulations based on the Reynolds equation including standard inverse EHL theory and the GW-average Reynolds lubrication are also presented. Experimental agreement could be improved if loading is assumed to transfer to the fluid to maintain a fluid film. A hysteresis friction model was also developed in attempt to improve the prediction of speed dependent friction.
Two active seal prototypes were produced, each with an adjustable external pressure supplied to the outer circumference of the sealing element. Constant velocity friction measurements for different external pressures and the transient response following step changes in this pressure are presented.