| Special designs Special operating conditions may call for angular contact spherical plain bearings with sliding contact surface combinations steel-on-steel or steel/PTFE fabric and, consequently, such bearings are also produced by SKF. Bearings with sliding contact surface combination steel/PTFE fabric fig 3 ) should be used when maintenance-free operation is specified and where the bearing arrangement is such that the presence of any lubricant is not permitted. Steel-on-steel bearings fig u ) are preferred where operating temperatures, loads or load frequencies are high or where shock loads occur. To ensure correct operation, steel-on-steel bearings must be provided with an adequate supply of lubricant. Depending on the lubricant the sliding surface of the outer ring may be equipped with various types of lubrication grooves figs [a and |6 ). Inch-size steel-on-steel angular contact spherical plain bearings are also available to order. |