What is 'soft foot' and why is it important to correct during alignment?

• A. Soft foot is when the belt is slipping and must be corrected to prevent wear.
• B. Soft foot is when machine feet do not sit flat on the baseplate, causing misalignment and bearing/load issues; correcting reduces vibration and wear.
• C. Soft foot is when the foundation is soft underfoot.
• D. Soft foot is when lubricant leaks.

Answer: (B)

Soft foot means one or more machine feet do not sit flat on the base or foundation. When you tighten the mounting bolts, those uneven contacts push the machine frame and tilt it. That tilt changes the relative positions of shafts and bearings, so the alignment you measure with the frame tilted isn’t the true, in-service alignment of the motor and driven equipment. If you don’t fix this first, the frame will settle or twist when fully bolted, and the alignment readings you rely on will be biased, leading to hidden stresses, higher vibration, and premature bearing or coupling wear.

Correcting soft foot during alignment ensures the machine sits level and square just as it does in operation. By contacting all feet fully—often using shims or face-work on the base or feet—you remove the tilt. With the frame level, the shaft alignment measurements reflect the actual operating condition, so any adjustments you make will stay stable once the equipment is running. This reduces vibration, bearing loads, and wear, and helps prevent misalignment from reappearing after installation.

Eliminate soft foot first, then verify again after tightening and applying the operating load. Other issues like belt slip or a merely soft foundation are not the same phenomenon and don’t address the need for a flat, stable mounting surface.