Analysis of Controlled Clearance Seals

Abstract

An analytic approach, based on thin-wall theory, to analysis of the "controlled clearance" method bf restricting the gap between a shaft and a surrounding cylinder is presented. A modified form of thin-wall theory allows this analysis to be extended to seals with outside to inside diameter ratios up to 2,3. The effects of (a) the supply pressure, the jacket pressure, the pressure dependence of the viscosity of the oil and the geometry of the system on (b) the deformation of the cylinder, the pressure profile in the gap between the shaft and the cylinder and the flow of oil are illustrated. A path using this information is given for the design of a controlled clearance seal. It is shown that when the jacket pressure and the supply pressure are equal (reentrant cylinder condition) the supply pressure needed to close the gap is infinite. The difference between this result and that for a cylinder with a built-in end, for which the supply pressure for closure is finite, is discussed. Experimental tests are described and the results are shown to agree with computed figures based on the theory. This work is of particular relevance to hydraulically actuated standards of pressure and force, where "simple" piston-cylinder assemblies suffer excessive deformations at pressures beyond 50 MPa.

Design of a pressure balance with a small and calculable change of area with pressure

“…The model, theory and calculation methods are based on the thin-walled cylinder method [2,3], extended to thick-walled cylinders using FEA to calculate the cylinder deflections.…”

“…The height of the step is chosen so that there is a large increase in the flow conductance outside the stepped region and . The second difference is that the ends of the cylinder extend a distance x beyond the gap region, isolating the gap from the piston and cylinder end conditions and simplifying the FEA calculation [2]. In addition, the gap width is made independent of axial forces by applying s to the end face of the cylinder as well as the piston [3].…”

“…Recently, numerical methods to calculate have been developed, based either on elastic theory or on finite element analysis (FEA) solutions [1][2][3][4]. These methods have been used to analyse the behaviour of standard pressure balances and to compare the calculated coefficients with experimentally determined values [5,6].…”

A novel hydraulic force standard machine

“…The best value of the ratio of these pressures depends on the thickness of the cylinder wall, its radius, its length, the strength of the material it is made from and the initial gap between the cylinder and the piston. An analytical basis for the design of controlled-clearance systems has been developed [10][11][12] by treating these as thinwalled devices and then by extending their behaviour to more realistic thick-walled devices. Experimental tests using a thick-walled cylinder confirm the results of this theoretical work [11].…”

“…An analytical basis for the design of controlled-clearance systems has been developed [10][11][12] by treating these as thinwalled devices and then by extending their behaviour to more realistic thick-walled devices. Experimental tests using a thick-walled cylinder confirm the results of this theoretical work [11]. Based on this research, a controlled-clearance system was designed with a practical clearance between piston and cylinder (dimension b in figure 1), with equal internal and external pressures (applied pressure P S = P J in figure 1), and with low oil flows.…”