Selecting for Zero Misalignment

Zero misalignment can be achieved by assembling both shafts in self-aligning bearings (See figure 21). In this way both shafts can float into concentric relationships, allowing the use of a solid coupling which simply supports the shaft in perfect alignment.

Difficulties arise when attempting to connect fixed axis shafts in this way, as the level of alignment is difficult to both achieve and maintain, due to settlement, creep, thermal expansion and contraction. The influence of these factors results in relative movement between the shafts and the alignment achieved in the factory may not be achievable ‘in the field’. Therefore, a flexible coupling is always the preferred option.

Before installing a solid coupling an interesting test is to try a flexible coupling first. With the machine at normal operating temperature measure the speed and/or the current drawn by the motor. The difference between these readings and those with the solid coupling indicate the losses generated by the additional friction at the bearings.

Fig.21 Shafts located by self-aligning bearings can float into perfect alignment for connection with a solid coupling.