Predicting impact conditions due to bearing clearances in linkage mechanisms.

Abstract

Clearances at connections of mechanical systems may allow contact to be momentarily lost between the interconnected elements resulting in impact loading, the generation of noise and deterioration of the bearing surfaces. The work presented examines the dynamic behaviour of a planar mechanism having clearances-at two of its bearings and suggests criteria which may be applied at the design stage to ensure that contact loss does not occur. A test rig has been developed having both rotary and oscillatory inputs and operable as a single input 4/bar ljnkage or a double input 5/bar linkage mechanism. The rotary input is varied between 170 and 370 rev/min and the oscillatory input between 10 and 22 Hz at amplitudes of 1.8 and 2.6 mm. Acceleration impact levels are measured by means of small accelerometers embedded in the bearing housing. The bearings are nominally 25 mm diameter steel pins with sintered bronze bushes. Clearances in the range of 15 to 105 gm are examined. An-additional dynamic constraint to the system is provided by spring loading one of the links. The series of tests performed establish various conditions at which impact loading occurs. A mathematical model of the test rig is developed, the equations of motion being solved numerically. Following some earlier work a zero-clearance analysis of the system has been used to develop empirical relationships for describing the occurrence and magnitude of the impact loading. These relationships are applicable to the system when only one bearing has a finite clearance and when two bearings have finite clearances. The interactive effects of a two clearance bearing system are examined in detail and it is concluded that the empirical relationships developed may be generally applied.