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What is soft foot and why is it so damaging to machinery?

Roughly two-thirds of all rotating machinery have a condition called “soft foot” that contributes to premature asset failure. But most facilities rarely, if ever, check for it.

The preponderance of this machinery ailment and the mystique around it prompted Ryan Best, Technical Sales Representative at the PRUFTECHNIK unit of Fluke, to present a webinar titled, “A hard look at soft foot: detecting machine frame distortion before it causes major issues.” His presentation made a convincing case for learning how to diagnose and correct soft foot.

Soft foot occurs when rotating machinery is set on its base, frame, or sole plate in a way that one or more of the “feet” are not making contact with the “foot points” on the frame. Think of a chair that has one leg shorter than the other three; Best often uses this analogy but points out that it’s a bit more complicated with machinery.

Asset professionals check for soft foot because it can result in serious misalignment and damage. More specifically, soft foot can shift the centerline of rotation, warp the machine, increase residual vibration, induce cyclical fatigue and cause fretting corrosion from repetitive impact.

The impacts of soft foot should sound familiar to machine vibration professionals who have dealt with these machinery issues: Bent shafts, increased load on the bearings, bearing misalignment, high vibration levels, seal failures, distortion, and increased power consumption, followed eventually by premature machine failure.

There are four types of soft foot to check for, Best says, but two are most common: parallel and angular.

  • Parallel soft foot indicates that the baseplate and machine foot are parallel to each other, but the feet are at different heights. When bolts are tightened, the machine’s frame distorts as it is pulled toward the base.
  • Angular soft foot occurs when the foundation and machine foot are at an angle with one another. Similar distortion occurs, but the configuration is more complex to diagnose.

Parallel vs. Angular soft foot
Figure 1. Parallel soft foot (left) versus angular soft foot (right).

In the webinar, Best also discusses “squishy foot” and “external force” soft foot instances and the circumstances where they commonly occur.

Overall, these are the top causes of soft foot:

  • Warped frame or base plate
  • Damaged, cracked foundation
  • Dirt, debris, or paint under the machine’s feet
  • Excessive shimming
  • Bent or damaged feet
  • Grout deterioration
  • Poorly machined feet or base
  • Pipe strain

All of that may sound logical within the lifespan of a rotating asset, yet the tricky part comes in identifying, measuring, and then diagnosing the problem in order to put the correct fix in place. Soft foot tolerance is only 0.003” (0.05mm), which requires measuring in the thousandths.

In his presentation, Best outlines his recommended toolkit for soft foot detection, from feeler gages to calipers, and describes the purpose and function of each tool. He also discusses how to use a laser system to inspect and diagnose soft foot on most systems. The laser system is more sophisticated than the manual tools but also easier to use.

As he says, most soft foot goes unaddressed, to the detriment of the machine. Correcting for soft foot can be time-consuming and frustrating, but very much worth it. Be patient and thorough; don’t ignore the basics such as cleaning base plates, feet, and shims. Successfully correcting the problem can greatly extend the life and health of machinery.

Best’s presentation concludes with some helpful best practices for correcting different kinds of soft foot. He provides rules for shims, bolt tightening, and variance, and extensive encouragement for getting the job done. Visit this webpage to watch the full presentation on demand.

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