Soft foot, also known as machine frame distortion, is a common problem that affects rotating machinery. It can result in a range of mechanical and quality problems, including compromised alignment, high vibration levels, accelerated wear and tear, increased power consumption, premature failure of bearings and seals, metal fatigue and distortion. While soft foot generally isn’t regarded as a critical condition and is often overlooked, the consequences of ignoring the symptoms can be quite serious.

What Is Soft Foot?

In simple terms, soft foot can be likened to a chair that has one leg shorter than the other three. This unevenness and lack of uniform contact with the floor causes a rocking motion when someone sits in the chair.

With machines, soft foot results when one or more of the feet are shorter, longer or angled differently than the others, or if the feet are damaged, warped or twisted. It can also be caused by accumulated dirt or debris under the machine’s feet, dents in the machine base or excessive tension on the feet. When the base bolts are tightened, this non-uniformity puts undue stress on the machine, as the foot is forced to bend or twist to conform to the base plate.

Diagnosing Soft Foot

How can you determine whether your machinery is suffering from soft foot? The condition has different levels of severity, but your machine may have soft foot if you notice high vibration levels.

Soft foot creates a load on the rotating shaft, which causes shaft deflection. This in turn can result in discernible rocking of the machine or vibration when the shaft turns. However, the condition doesn’t always lead to an increase in overall machine vibration, so it’s recommended that vibration data be collected for an accurate diagnosis.

If the calculated movement of the machine’s foot when it is either tightened or loosened is at least 0.002 inches (0.0508 millimeters) to 0.003 inches (0.0762 millimeters) while the other feet are bolted tightly in place, then the machine is deemed to have soft foot.

When soft foot is present, there is often a significant 2x alternate current line frequency peak. When this peak is observed, it is a good indicator that the problem is soft foot rather than one of imbalance or misalignment. Phase analysis can also be used to determine soft foot by working out the relative motion between two different parts of the machinery, such as the machine frame and the foundation.

If you want to know whether your machine suffers from soft foot, it’s important to understand what causes the condition. Generally, it’s a combination of the following:

  • Twisted, bent or warped machinery frames or base plates
  • A damaged or uneven foundation
  • Too many shims under a foot
  • Bent or deformed shim
  • Twisted, warped, dented or damaged feet
  • Dirt or debris under the feet

Soft foot can severely affect a machine’s operation. Therefore, it is essential to include checks for this condition as part of the pre-alignment process for your equipment. If you suspect your machinery may be at risk for the condition, then it’s wise to investigate further using a good laser alignment tool.

About the Author

Jason De Silveira is the managing director at Nexxis, which offers a wide range of laser alignment tools that can be used to diagnose soft foot and a host of other alignment problems.