Bearings: Technology and needs
A brief status report on bearing technology and present and near-term future problems that warrant research support is presented. For rolling element bearings a material with improved fracture toughness, life data in the low Lambda region, a comprehensive failure theory verified by life data and incorporated into dynamic analyses, and an improved corrosion resistant alloy are perceived as important needs. For hydrodynamic bearings better definition of cavitation boundaries and pressure distributions for squeeze film dampers, and geometry optimization for minimum power loss in turbulent film bearings are needed. For gas film bearings, foil bearing geometries that form more nearly optimum film shapes for maximum load capacity, and more effective surface protective coatings for high temperature operation are needed.
Dynamic characteristics of a high-speed rotor with radial and axial foil-bearing supports
An asymmetric rotor (19N; 4.3 lb), supported radially and axially by compliant bearings (foil bearings), is subjected to severe excitation by rotating unbalance (43 μm.N; 6100 μin.oz) in the “pitching” mode, at speeds to 50,000 rpm. The resilient, air-lubricated bearings provide very effective damping, so that regions of resonance and instability can be traversed with impunity, with amplitudes and limit-trajectories remaining within acceptable bounds. A novel journal bearing is introduced, in which a resilient support is furnished by the outer turn of the coiled foil-element, initially bent to form an open polygon. The experimental apparatus and procedure are described, and the response of the rotor and flexible support system are copiously documented by oscilloscope records of motion.
The lubrication of rollers
When lubricated rollers are run together they are separated by a hydrodynamically formed oil film. The thickness of this film has been measured by a capacitance method up to loads of 1000 Lb. per inch of face (1.76 x 103 dyn cm -1) for conditions of pure rolling and for conditions of rolling with sliding such as exist at the contacts of gear teeth. It has been found, at low loads, that the film thickness varies inversely with load and proportionately with speed, as simple hydrodynamic theory suggests, but that the actual thickness is approximately one-half of the theoretical thickness. At higher loads, of greater practical importance, the film thickness is shown to be of the order 1u (-4x10-5 in) which is greater than that predicted by simple theory. Experimental evidence is presented that this failure of the simple theory is due to the increased viscosity of the oil under pressure and to the deformation of the surfaces by the load. The film thickness, at practical loads, has been found to vary little with load, less than proportionately with speed but to vary greatly with the temperatures of the surfaces. Estimates of the temperature reached by the oil in its passage through the conjunction of the surfaces have been made from measurements of the electrical resistivity of the oil. This temperature, which depends upon load and the peripheral speeds and which may exceed 250° C, appears to have little influence upon the film thickness. From this it is argued that the film thickness is largely determined by the conditions, on the entry side of the conjunction, ahead of the region in which the viscous losses and heating of the oil become intense.