Proper alignment of direct-coupled machinery is an essential element for the reliability of a new or repaired machine (motor, pump, gear case, etc.). One common impediment to achieving proper alignment and smooth operation is a "soft foot" condition.
A soft foot exists when not all of a machine's feet sit flat on the supporting base, so that tightening the foot bolts distorts the machine case. A common occurrence with four-footed electric motors, this condition may result if machine feet aren't coplanar, individual feet are bent, angled or corroded (see Figure 1), or the baseplate isn't flat.
A soft foot can a make a machine difficult to align, and a distorted case can increase bearing load and create internal misalignment of the machine's rotating and stationary elements, resulting in poor performance and increased vibration.
Proper shimming can usually correct non-coplanar feet, and to some extent feet that are bent or angled (see Figure 2 for limits). Feet with significant corrosion damage must be re-machined to avoid the consequences of a soft foot condition.
Detecting soft foot
There are several methods for detecting a soft foot condition and suitable tests should be performed during every machine installation. Some of these tests employ shaft alignment tools, making it convenient to incorporate them into the shaft alignment procedure. Probably the best test for soft foot, though, is to insert feeler gauges between each foot and the base foot pad before tightening the feet. This can be done without setting up shaft alignment instruments or indicators and is a recommended practice whenever a machine case is set in place.
When using shaft alignment instrument or dial indicator setups to detect soft foot, the usual approach is to first tighten all feet bolts and then loosen only one of them while observing the movement on the shaft alignment indicators. The bolt is then re-tighten and each remaining foot is tested in the same way.
Dial indicators and some laser systems only indicate in a single plane (vertical or horizontal), so the vertical plane is the preferred plane to monitor for soft foot tests. Laser systems that indicate in both planes simultaneously are slightly better at detecting movement when the foot bolt is loosened.
Some alignment technicians prefer to not re-tighten the bolt, but instead successively loosen each bolt and observe the total indicated movement. They may then reverse the procedure, successively tightening each bolt while observing the indicated movement until all are secured. This extended procedure gives a good indication of how a soft foot condition might affect the alignment process.
Correct the soft foot condition
While this procedure of using shaft-mounted alignment indicators may detect the presence of a soft foot with reasonable accuracy, it should not be used when attempting to correct the condition. Rather, manual testing with a dial test indicator mounted at each foot provides a much more effective and efficient means of correcting soft foot (see Figure 3).
With all four bolts tight, the dial test indicator is mounted to the foundation and set to indicate upward movement of the foot as one foot bolt is loosened. An upward movement of more than 0.002-in. (0.05 mm) indicates a soft foot condition that should be corrected. The foot bolt is then re-tightened, and the test is repeated for each remaining foot bolt.
How to correct soft foot
The most common soft foot conditions are two diagonal soft feet or a single soft foot. These usually indicate two different types of soft foot. Diagonal soft feet tend to indicate a short foot-i.e., feet are flat but not coplanar. A single soft foot often results from a bent or angled foot. Correcting a short foot is pretty straightforward; correcting a bent or angled foot is more complex.
To correct a short foot (diagonal feet are soft), tighten only the two diagonal feet that were not soft, leaving the soft feet bolts loose. Remove all shims from those two feet. Use feeler gauges between the soft feet and the base foot pad to determine the amount of shims required for each of the two soft feet. Place the required shims and retest for soft foot.
When testing indicates a single soft foot, the foot is likely bent or angled. Loosen only that foot bolt and remove all shims. Use feeler gauges to determine the amount of shims required to correct the soft foot, carefully profiling the gap by measuring in from each side as well as from the front and back of the foot to the bolt hole.
General recommendations for shims
When correcting a soft foot that results from a non-coplanar foot or feet, each shim should cover at least 80 percent of the foot's area. Best practice is to place no more than five shims between a machine case foot and the base plate or foundation (excluding those used to correct angled foot conditions).
In addition, no more than one shim should be less than 0.003-in. (0.08 mm) thick, and the sum of the three thinnest shims should be 0.010-in. (0.25 mm) or greater. It's also important to accurately measure the thickness of any shims that are more than 0.020-in. (0.51 mm) thick, and to verify the thickness of the entire shim stack.
For a bent or angled foot, step or stagger a maximum of five shims in a single direction to match the angle of the gap variation (see Figure 4). For best results, do not use shims to correct a total gap variation greater than 0.015-in. (0.38 mm). Bent or angled feet with a larger gap variation should be straightened or re-machined to correct the condition.
Proper attention to detecting and correcting soft foot conditions on new or repaired machine installations will save time in achieving proper shaft alignment, prevent frame or casing distortion that reduces machine reliability and efficiency, and avoid increased vibration levels.
Eugene Vogel is a pump and vibration specialist at the Electrical Apparatus Service Association (EASA), a CFE Media content partner. EASA is an international trade association of about 1,870 firms in nearly 70 countries that sell and service electromechanical apparatus.