Anyone who has performed shaft alignment very often knows about soft foot. Simply stated, soft foot is when one or more feet do not rest in a common plane.
If soft foot is not corrected, as hold down bolts are tightened, the motor can move in unintended ways, changing the shaft alignment, and possibly causing distortion of the motor frame. Ignoring soft foot can make alignments less accurate, less repeatable, and take more time than needed. Soft foot can also be a contributor to electrical and mechanical problems in machines.
In most cases, soft foot is thought of as an alignment problem. But soft foot issues in stationary machines can cause some serious problems as well.
• Pumps – A soft foot condition in a pump can cause the pump casing to distort as the pump hold down bolts are being tightened. Many mechanics have witness this occur when a pump turns freely on a work bench, but binds when bolted down. It can cause a rub in the impellor, and can internally misalign the bearings and seal(s).
• Gearboxes – Case distortion caused by soft foot can change the gear tooth clearances, internally misaligning shaft bearings.
• Reciprocating compressors – If the crankcases are not flat, the bearings can bind, connecting rods can drag, and valves can stick.
If one of these types of machines is being rebuilt, or going through a large-scale repair, and soft foot test is recommended. It is also recommended if vibration measurements detect a 2x vibration on the driven machine.
The same procedure used to correct soft foot under a motor is recommended.
Author: Stan Riddle, VibrAlign
