Coupling Misalignment

Misalignment, or the variance between the intended position and attitude of two shafts, is normally the result of manufacturing tolerances, and quantifying misalignment is crucial when seeking to specify the correct coupling. As the misalignment increases, the transmissible torque and life expectancy of the coupling reduce exponentially. Therefore, understanding the nature and origins of misalignment is important to you as a design engineer. Figure 11-14 Misalignment

The main types of misalignment are angular, radial, and axial displacement.

Factors that influence misalignment include thermal imbalances, wear, settlement and creep, and the influence of the last of these can, without correct maintenance, increase during the life of the coupling.

When determining alignment, measurements should always be taken when the system is cold and again when it is at operating temperature. Consideration should also be given to the class of tolerance being used in the assembly of the individual items. For example, the output shaft of a reduction gearbox with a die-cast housing with unmachined mounting faces and clearance holes for location purposes has a greater possibility for misalignment, than a face-mounted servo motor with machined registers.

Predicting Misalignment

By prediction we really mean verifying the worst-case misalignment in any given situation, so as to be certain that the correction capability of the coupling is adequate. In essence shaft misalignment has three components: parallel; angular; and radial – each being three-dimensional. The following explanation and the accompanying graphics should help in clarifying this situation.

In simple terms a shaft with angular error describes a cone when it is rotated, and while mating shafts can converge and intersect on the critical plane it is unlikely. This gives rise to radial error, which is at its maximum when the axes are tangentially opposed on the sphere diameter.