Attain proper machine alignment by fixing soft foot.
A soft foot exists during machine alignment when not every foot of a machine’s supporting base sits flat, so when tightening the foot bolts, it distorts the machine case. This will make a machine difficult to align and a distorted case results in poor overall machine performance.
Proper laser alignment of machinery is an essential element for the reliability of a new or repaired machine (pump, gearbox, motor, etc.). One common hurdle to achieving correct laser alignment and smooth operation is a “soft foot” situation.
A soft foot exists when not every foot of a machine’s supporting base sits flat, so when tightening the foot bolts, it 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 angled, bent, rusted, or the base just isn’t flat.
A soft foot can and will make a machine difficult to align, and a distorted case will increase bearing load and create internal misalignment of the machine’s rotating and stationary elements, resulting in weak performance and heightened machine vibration.
Proper shimming can usually correct non-coplanar machine bases, and to some degree feet that are bent or angled. Feet with substantial corrosion damage must be re-machined to avoid the effects of a soft foot situation.
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 uncomplicated; correcting a bent or angled foot is more difficult.
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 number 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. 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.
Utilizing a Laser Tracker to Correct Soft Foot
Our laser tracker service is perfect for correcting soft foot of a machinery due to co planer issues. Whether it’s the machine’s base or the foundation being out of tolerance. Utilizing laser trackers for machine alignment we can measure the base of the machinery and determine which feet are short or bent and the distance to the center of the shaft. This information will allow us to set all the shims in place within one thousandth before the machine is even set in place. This is perfect for installing large gearboxes that should not have any internal distortion. So, when the critical lift of the gearbox is preformed and completed no further elevation shim changes will have to be made.
Final Words
Proper attention to detecting and correcting soft foot of repaired or even new machine installations will save time in achieving proper shaft alignment, prevent frame or casing distortion that reduces machine reliability, efficiency, and avoid increased vibration levels. Contact us to discuss how we can help you!