Over the years we have fielded several technical support calls from maintenance personnel having repeatability and soft foot issues (during an alignment) all of which had a common issue; other influences. Meaning not the typical looseness, coupling backlash, soft foot issues.
During the “heat of the battle” to get the precision shaft alignment job finished it is easy to overlook simple items that can cause unexpected results, non-repeatability, and soft foot issues. Here are the issues and causes from the mentioned support calls.
The Issue: While using a chain hoist to lift a motor for the vertical shim change a customer was experiencing “weird” soft foot issues after the hold down bolts were tightened.
The Cause: The chain hoist was not completely slacked off.
The Fix: Easy to overlook and easy to correct by ensuring the chain hoist tension is released fully.
Vertical Jack (lifting) Bolts
The Issue: Similar to the customer using a chain hoist to lift the motor.
The Cause: The vertical jack bolts were difficult to turn with a wrench due to rust and dirt build up, even when not in contact with the base, plus it was difficult to see if the jack bolts were completely backed off the base. Two jack bolts were still contacting the base on diagonally opposed feet.
The Fix: If the vertical jack bolt threads are rusted, dirty, etc. its best to clean them first and look with a flash light if needed to ensure they are not contacting the base.
Horizontal Jack Bolts:
The Issue: After completing a Verti-Zontal Correction the vertical position of the motor was still high by a fair amount and out of tolerance. The customer questioned the laser’s repeatability.
The Cause: Before tightening the hold down bolts the horizontal jack bolts were tightened to “maintain” the horizontal position. The jack bolts had dug into the motor feet holding it up when the hold down bolts were tightened.
The Fix: After loosening the jack bolts and re-measuring, the vertical and horizontal alignment was in tolerance. Horizontal jack bolts can just barely touch the motor feet before tightening the hold down bolts, however do not tighten them with a wrench.
As with any alignment issue, when the precision shaft alignment job is not going to plan remember:
Soft foot, non-repeatable readings, unexpected results etc. all happen for a reason. Use the alignment tool on your shoulders to troubleshoot why and don’t assume anything when checking out the possible causes, even the simple obvious ones.