fig5 shows the calculations of the oil film thicknesses in the same bearings as those used in the measurement of fig4 calculated by archard cowking equation shown to the top in fig9 the calculations were made with different radial clearances and different operating temperatures on both inner and outer race sides the lower the operating temperature the higher the base oil viscosity of the grease becomes to increase the theoretical oil film thickness incidentally the double sign in the equation refers to the inner and outer rings with its upper and lower signs respectively it is found from a comparison between fig4 and 5 that the surface roughness r a u03bcm of the raceway surfaces 1 a 2 a and the theoretical value of the oil film thickness u03bcm preferably have a ratio surface roughness r a oil film thickness of 0 3 or higher and more desirably of 0 6 or higher fig6 8 show other embodiments of the present invention any of which is intended to support a photoconductor drum fig6 shows a deep groove ball bearing in which inner and outer raceway surfaces 1 a and 2 a are formed into flat surfaces the radius of curvature u221e a bearing for supporting e g a photoconductor drum sometimes has only to receive purely radial light loads in such cases.