Soft foot and the dynamic soft foot test
Soft foot twists a motor frame when the bolts are tightened. It shows as high directional vibration near 2 times line frequency. A running test, loosening one bolt at a time, finds the foot and gives a before-and-after number.
What soft foot is
Soft foot is when one or more feet of a machine do not sit flat on the base. Tightening the bolts then pulls the frame out of shape. In a motor, that distorts the stator, changes the air gap and loads the bearings unevenly. It also shifts the shaft, so an alignment done before the soft foot was corrected is wrong.
Types
| Type | What is wrong | How it shows |
|---|---|---|
| Parallel | One foot is too short or too long | A rocking gap under the foot |
| Angular | The foot surface or base is not parallel | A wedge-shaped gap |
| Induced | Pipe strain, a bent frame or a base that is not flat | The gap changes as the pipes are connected |
| Spring | Dirt, paint or a corroded shim under a foot | The gap closes and opens as you tighten |
What the spectrum may show
A common sign is elevated directional vibration, often more noticeable horizontally. In a 2-pole motor, 2X running speed is the same as 2 times line frequency, and in a 4-pole motor 4X running speed is. On a 50 Hz supply both lines fall at about 100 Hz. Mechanical and electrical causes overlap, so a dynamic soft foot test helps separate them.
The dynamic soft foot test
- Get permission from the site first. Keep clear of rotating parts and make sure the guards stay in place.
- Put a sensor on the motor in the direction with the high reading and watch the live display.
- Loosen one mounting bolt slightly. Leave the others tight.
- Watch for a change in amplitude. A large drop means that foot is distorting the frame.
- Re-tighten, then repeat on the next foot.
A real result
On one motor, loosening a single bolt dropped the reading from 6.2 mm/s RMS to 1.2 mm/s RMS. That is more than a fivefold fall from a single bolt.
After finding it
- Stop the machine and shim the foot. Use a feeler gauge or a dial indicator to measure the gap with the bolt loose.
- Tighten the bolts one at a time with a dial indicator on the foot. The foot should lift less than 0.05 mm (0.002 in), a common rule of thumb.
- Redo the shaft alignment, then run the dynamic test again.