Hydrodynamic Principles of Journal Bearing Design

Abstract

A survey of existing theories of the journal bearing without side leakage suggests that optimum conditions as regards friction and film thickness are obtained when the bearing is designed with such a clearance that the operating eccentricity, in the case of the half-bearing, is about 0·4. Reasons are given for assuming that a similar optimum eccentricity exists in bearings of ordinary width. In order to take rational account of side leakage, use is made of Kingsbury's experimental results obtained by electrical analogy. Coefficients are deduced embodying the effect of bearing width on friction and film thickness, and for a given total load it is found that the film thickness increases continuously with the width of bearing, but the frictional coefficient has a minimum value when a certain ratio exists between the width of a bearing and its diameter; in the case of the half-bearing, the ratio is about 3/2. For a specified minimum film thickness the necessary width Bo of the bearing can be found, and the rational procedure in design differs according as the journal diameter D is greater or less than . The critical diameter is jointly dependent on the least permissible clearance and the hydrodynamic data. A discussion of practicable clearances leads to a working expression for this critical diameter, which is conveniently written in the form , where W is the bearing load in pounds, and D1 the critical diameter in inches. The symbol Ds, defined as the specific diameter, is plotted in Fig. 9, p. 424. The following procedure is suggested in design. For the given speed, N r.p.m., the specific diameter D s is read from Fig. 9 and the critical diameter for a bearing load of W lb. is then for oil of normal viscosity. If the actual journal diameter D < D1 the bearing width should be and the diametral clearance This gives the lowest friction consistent with safe film thickness. If D > D1 the bearing width B should be and the diametral clearance This gives optimum frictional conditions and a margin of safety in film thickness. Comparative data for bearing arcs of 120 deg. and non-central loads are shown in Table 2, p. 412.