Jeff Martin
Technical Education Committee Member
Independent Electric Machinery Co.
Have you ever been out at a restaurant or sports bar to meet up with some friends and colleagues to have an adult beverage or two when suddenly someone leans on the table ever so slightly and liquid spills everywhere? What just happened? The small space between the table leg and the floor has just created a big consequence. While it’s not too difficult to solve if you have some napkins and a few coasters, it does, however, illustrate the results of a soft foot and how a very small space can lead to a much larger consequence.
Soft foot is often not taken seriously enough, perhaps because the consequences are not always immediately visible. Most repair facilities have a smooth flat surface to test motors. The motors are checked for soft feet prior to running and shims are utilized to correct the soft foot. A permanent mounting position at the customer's site may not be as smooth and flat as your repair shop's is. This could lead to a big consequence.
So, what is soft foot all about? Soft foot refers to a condition where the motor’s mounting feet are not all sitting flat and evenly on the base or mounting surface. When the motor hold-down bolts are tightened, the distorted foot pulls the motor frame into stress, which misaligns internal components. There are three main types of soft foot.
- Parallel soft foot, which is when all feet are flat but not at the same height.
- Angular soft foot, which is when one or more of the feet are angled or warped and leads to contact on only one of the sides.
- Squishy soft foot, which is when contaminants like paint, rust or some kind of debris get between the mounting surface of the motor, the shims and the feet.
When a small amount of soft foot exists, it can have a big consequence. Some of the more common results from having soft foot include shaft misalignment, increased vibration, excessive bearing wear, premature failure and distorted airgap. Ultimately, soft foot reduces equipment lifespan, reliability and efficiency. Big consequences often come without visible signs until significant damage occurs.
With all the new technology available today, there are many ways to detect and identify soft feet and even the “old school” ways still work well. Dial indicators, laser alignment tools and feeler gauges are a few of the most common ways to measure and identify the soft foot causes and guide you to repair them. Once soft foot is identified, corrective steps must be taken to restore a stable, flat mounting surface.
Here are a few recommended solutions:
- Clean and inspect all mounting surfaces to remove rust, paint and debris. Failure to check and clean cured varnish from the bottom of the motor feet is a common problem in our industry.
- Use clean, precision shims to eliminate gaps under the feet (0.002”). Avoid stacking more than three shims when possible.
- Machine or grind warped feet or uneven bases when shimming cannot resolve the issue.
- Check for induced stresses from piping or conduit and relieve strain if present.
- Torque bolts evenly to manufacturer specifications in a crisscross pattern.
- Re-check alignment after all corrections — soft foot and misalignment often coexist.
- Conduct an Off-Line-To-Running (OLTR) analysis to detect any possible frame distortion under load and when the motor is at normal operating temperature. (Thermal growth can cause issues and does need to be taken into consideration.)
Soft foot, while it may appear minor, can cause big consequences like major mechanical and electrical issues, if left uncorrected. Precision installation with satisfactory soft foot correction techniques is essential to eliminate soft foot and ensure the reliability, efficiency and longevity of electric motors. By integrating regular soft foot checks into alignment and condition monitoring programs, maintenance teams can significantly reduce unplanned downtime and extend motor life.
So, the next time the waitress puts you and your friends at a table with a soft foot or two, just remember that the small space that is between the floor and the table leg can lead to a big consequence. Grab a few coasters and make the necessary onsite repairs. Then, raise your glass with your colleagues for a cheer to successfully repairing a small space from becoming a big consequence!
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