Design Guidelines for Predicting Contact Loss in Revolute Joints of Planar Linkage Mechanisms

Contact loss at clearance joints of mechanisms gives rise to system degrading impacts when contact is remade. Traditional design tools such as the zero-clearance analysis are inadequate to predict contact loss. Theoretical analyses which have the ability to predict contact loss, such as the massless link model, are complex, computationally expensive and need further refining. This study employs the massless link model, which models the joint clearance of mechanisms as an extra degree of freedom, as a vehicle to study the phenomenon of contact loss. The equations of motion for a four-bar mechanism with a clearance joint are derived and the equations are uncoupled to yield an expression for the clearance joint force magnitude in terms of the system kinematic variables and the clearance link response. Hence it is shown that there exists a single zero-clearance dimensionless parameter which governs the contact loss condition at the clearance joint. In particular it is shown that contact loss will occur if this parameter is equal to or greater than unity. The condition for predicting contact maintained is less clearly defined, as it contains a quantity not easily evaluated. It is argued that the Earles and Wu empirical criterion (γ/R) for maintaining contact can be applied with reasonable confidence, leaving a region of uncertainty which it is considered will in general be relatively small. The empirical parameter γ/R is contained within the theoretical dimensionless parameter derived for predicting contact loss.