4. Clearance: If bearing clearance is too great then the inertia forces lead to 'bearing knock' This impulsive loading results in pressure above normal and break down of the hydrodynamic layer. Figure 2.29 illustrates these points graphically for a rotating journal type of bearing.
Referring to the graph in figure 2.29, hydrodynamic lubrication should exist in the main, bottom end and guide bearings. The top end and crosshead bearings will have a variable condition, for example, when the unit is at TDC the relative velocity between crosshead guide and the bearing surface is zero and the pressure from combustion is building. The swing of the connecting rod is however building the relative speed of the crosshead bearing surface across the crankpin journal as the forces of combustion are taking effect. Reflecting upon this point will enable the student to appreciate the