A new gearmotor on a mixing tank was noisy from day one. Vibration reading at the motor bearing: 4.5 mm/s, well above the 2.8 mm/s warning line. The shaft coupling was brand new and aligned within 0.05 mm by laser. The customer’s engineer had checked everything the textbook says to check: coupling alignment, balance, bearings. All good. The gearmotor still shook.
The problem was the feet.
What soft foot is
Soft foot is when one of the four mounting feet doesn’t sit flush on its baseplate. The baseplate is slightly warped, or a foot is machined slightly proud, or there’s a burr or a paint blob under one corner. Tighten all four bolts and the frame twists. The frame distortion pulls the bearing housings out of alignment with each other. The rotor no longer sits concentric in the stator, the bearing preload changes, and the machine runs rough. On the outside, the coupling looks aligned. Inside, the machine is fighting itself.
The vibration showed up at 1x running speed, which the engineer read as unbalance. The balance was fine. It was the frame twist translating into a rotating force.
How to measure it
Soft foot measurement takes ten minutes with a dial indicator and a helper, or a few minutes with a laser alignment tool that has the soft foot routine built in:
- Mount a dial indicator on the machine foot, plunger vertical, resting on the baseplate.
- Loosen one bolt at a time. Read the indicator movement.
- Repeat for each foot. Any foot that moves more than about 0.05 mm when its bolt is loosened is soft.
- Shim the soft foot with stainless shims (never more than three stacked), re-tighten, and re-check.
On this gearmotor, the right-rear foot moved 0.4 mm when its bolt was loosened. The baseplate had a dip there from a weld that had cooled and pulled the plate. 0.4 mm of soft foot was distorting the whole frame.
The fix and the result
We shimmed the foot with 0.4 mm of stainless shims, re-tightened in the proper sequence (opposite corners, then the other pair), and re-measured. The indicator showed zero movement on all four feet. The vibration dropped from 4.5 to 1.3 mm/s — under the 1.8 mm/s good line. The noise dropped with it. The gearmotor has run three years without bearing trouble.
Note the torque sequence. If you tighten one foot fully, then the next, the frame walks. The sequence is: snug all four, then tighten opposite corners alternately, final torque all four, then re-check. A foot that reads soft before final torque may read fine after, and the distortion just moves somewhere else. Always re-check after final torque.
The two kinds of soft foot
There’s parallel soft foot — the foot sits above the plate uniformly, fixed with a simple shim. Then there’s angular soft foot — the foot contacts at one edge only. Angular soft foot doesn’t show on a single vertical indicator; you need two indicators (one vertical, one horizontal) or a rocking check. If a shim of one thickness fixes the vertical reading but the machine still vibrates, suspect angular soft foot and check with two indicators.
One more cause that shows up often: the baseplate itself. A baseplate that’s welded up in a hurry, or a chock under the plate that shifted, makes the plate flex when bolts are tightened. The machine can’t be shimmed flat if the plate itself moves. Grind the plate, or re-level it, before shimming the machine. Shim against a moving plate is a losing game.
The lesson
When a newly installed machine runs rough and everything checks out, check the feet before you start balancing things. Soft foot is invisible to the coupling alignment laser and to the balance analyzer. It shows up as mystery vibration and eats bearings. It costs ten minutes to measure and a couple of shims to fix. Every new installation should have a soft foot check in the startup procedure, right after the coupling alignment and before the first full-speed run.
The aligned, balanced, brand-new gearmotor was vibrating at 4.5 mm/s because one foot sat 0.4 mm above the plate. Shim it, and it dropped to 1.3. Soft foot is the misalignment that coupling lasers can’t see. Check the feet before you chase the vibration.
