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Thermal Rating of Gear Drives Balance Between Power Loss and Heat Dissipation
From the balance between the generated heat in a gear box and the dissipated heat from the gear case surface the mean value for the expected lubricant temperature can be evaluated. The maximum oil temperature in a splash lubricated enclosed gear drive limits the transmittable power. High oil temperatures influence wear, scuffing, micropitting and pitting load capacity of the gears as well as the gear oils’ service life. Experimental investigations of no-load and load dependent gear losses in cylindrical and bevel gears as a function of lubricant type and viscosity, load, speed and temperature are reported. The mean value for the coefficient of friction in a gear mesh is evaluated and compared to measurements in twin disk machines. A rating method for gear mesh power loss is derived. Investigations, using model and actual gear boxes, show the influence of radiation, free and forced convection as well as conduction to the foundation on the heat dissipation from the gear case surface. The influence of design, size, air velocity, surface finish of the housing, etc., is reported. The results of such investigations together with accepted rating methods for bearing and seal power loss are introduced into a calculation method for the evaluation of the oil temperature in the sump of a gear box. In sample calculations the possibilities as well as the limits of this thermal rating method is shown.
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