There are three common errors made in shaft alignment—coupling backlash, soft foot and not tightening and loosening moveable machine hold down bolts in a proper sequence. Of these, soft foot can be the most problematic. But coupling backlash runs a close second.
Simply stated, backlash is angular movement in any mechanical system between mating parts. Coupling backlash is common, and to a point, desirable in many types of couplings. However, the amount of coupling backlash required for efficient coupling backlash is minimal. Often, excessive coupling backlash is caused by a worn coupling insert.
Coupling inserts should be inspected prior to performing a precision shaft alignment. If the coupling insert is excessively worn, it should be replaced. Signs of wear include excessive tooth wear on toothed neoprene inserts, as well as excessive “twisting” of the insert. On spider-type couplings, look for excessive compression of the “spider.” On grid-type couplings, inspect for excessive wear of the “spring” type inserts, as well as wear of the grids. On shim-pack couplings, look for wear of the rubber bushings, as well as breakage of the shim packs. Less likely, but just as important, causes of backlash can be improper hub to shaft fits, or excessive keyway wear. Occasionally, backlash can be caused by loose foot bolts or other bolted components.
Backlash can cause erratic shaft alignment values in both dial indicator and laser alignment readings. A backlash of greater than 2° of angular movement should be considered excessive, and should be reduced to less than 2° before alignment begins.
Some methods of controlling and minimizing backlash include:
•Replacing the worn or defective components in the system which contributes to excessive backlash, such as worn couplings or inserts.
•Minimizing the effects of backlash by rotating shafts to maintain torque at a consistent level and direction. This can be done by rotating the shafts to be measured in a consistent direction, such as clockwise, or counter-clockwise.
•Utilizing temporary mechanical means, such as duct tape or mechanic’s wire, to temporarily override the coupling’s ability to experience backlash.
Remember, most mating rotational systems have a slight degree of backlash, which is both harmless, and desirable for efficient operation. But excessive backlash can decrease the accuracy of your alignment. Keep it to a minimum.
VibrAlign sells measurement systems for aligning and positioning machines and machine components. We are also a recognized leader in vibration analysis, balancing, alignment, and training services.