June 4, 2010
Soft Foot–What It Is and How to Minimize It
By Brad Case
Soft foot is a common issue when aligning rotating equipment. It is a major cause of repeatability problems in shaft alignment measurements. In addition to alignment quality and repeatability problems, it can be a cause of machinery vibration, reduce life in electric motors, and cause internal clearance problems in gearboxes and pumps.
But if the proper precautions are taken, soft foot can be minimized and controlled.
The term “soft foot” is the common term used for the improper contact between a machine casing, and the baseplate used to support it. It may be either an angular or parallel soft foot, but often it is a combination of the two.
It is often compared to a straight-backed wooden chair, where one leg, being shorter, does not contact the floor, causing a rocking motion in the chair when you are seated in it. While this is a good mental image, soft foot in machinery is a little more complex. While an angular soft foot might make contact with a baseplate or foundation, it does not make a UNIFORM amount of contact. Once base bolts are tightened, the foot tends to bend to conform to the baseplate to which it is mounted.
CAUSES OF SOFT FOOT CONDITIONS
- Twisted or warped machinery foundations or baseplates.
- Twisted, warped, or damaged machinery feet.
- Improper amount of shims under machine feet.
- Dirt, trash, or other unwanted materials under machine feet.
- Dents or other flaws in machine base or machine feet.
- Excessive tension on machine feet due to jack bolts warping the machine feet.
WHAT SOFT FOOT DOES TO YOUR ALIGNMENTS
As stated above, soft foot conditions adversely affect alignment quality and repeatability. Here’s how:
Regardless of whether you have:
- an angular or parallel soft foot,
- a bent or deformed shim,
- a bolt hole with a burr,
- a bent motor foot,
- a deformed machine base,
it can cause the same type of problem. Once you tighten the bolt where the soft foot is, you change the relative position of the shaft. In other words, the position of the shaft changes between tight and loose. If you have used laser alignment tools, you might have noticed that there is sometimes a foot that causes a noticeable change in the vertical or horizontal readings as you tighten a particular bolt. Often, this is the location of the soft foot condition.
Notice that the relative position of the shaft centerline changes due to changes in the position of the soft foot.
In addition, unless you tighten and loosen the bolts in sequence, the position of the shaft centerline in relation to the stationary machine can change. As an example, if you tighten the inboard left foot first one time, and the inboard right foot the second time, you may take measurements in different shaft centerline locations. Using the straight-backed chair analogy, not tightening in a known sequence causes the movable machine to “rock” into different positions.
STEPS TO MINIMIZE AND CONTROL SOFT FOOT
- Confirm that base plates and foundations are installed and leveled to specifications.
- Make sure that baseplates and machine feet are clean, deburred and free from dents in the areas to which machinery will be mounted.
- Use clean, flat shims. If you must cut thicker shims from steel stock, be sure they are clean, flat and deburred.
- Once the machine components are placed on the baseplate, rough align, and perform a gross soft foot check of both the movable and stationary machines.
- Leave all foot bolts loose
- Check one foot at a time, on at least three corners of each foot, with a 0.005” shim or feeler gauge. Correct by shimming with the minimum number of shims possible. If an angular soft foot condition exists, cut one shim in half, leaving the tab in place, to make it easier to reposition.
- Do not attempt to cut several shims to “step” the shim thicknesses under a particular foot. While this might seem like a good idea to increase the amount of contact area, it becomes much more complicated to reposition several shims each time.
- Once gross soft foot has been corrected, tighten each bolt, using a bolt torque pattern similar to the diagram below. The starting point can be any foot, as long as the same pattern is followed EACH TIME the feet are loosened and tightened.
- Once all bolts are tight, loosen one foot, and recheck for soft foot with a 0.002” shim or feeler gauge. Repeat the process of checking each foot at three corners, to identify angular soft foot. Shim to correct as needed. Then retighten the foot and move to the next one. Repeat this process until all feet have been checked and shimmed as needed.
- Tighten foot bolts using 3 passes and following the same bolt torque pattern on each pass. 1st pass tighten to hand tight. 2nd pass tighten the bolts, using a wrench, to around 50% tightness. 3rd pass, complete tightening the bolts. This will minimize the effect of any remaining soft foot.
- Additional soft foot checks can be done using laser alignment tools that have soft foot check capability or with dial indicators. But be mindful that soft foot can not only be in the machine foot – it can also be in the baseplate to which the machine is mounted. In other words, when you loosen the foot bolt, the machine foot may not move, but the base or support underneath it might. This can happen when the foot of the machine you are aligning is thicker than the base or riser onto which it sits. In the example below, if the motor feet are tightened, the base may bend upward to contact the thicker motor foot.
- If the base moves up to contact the motor foot, you may not be able to measure it with a laser alignment tool, or a dial indicator, but it might still cause a soft foot response in the movement of the motor. A shim or feeler gauge is preferred to determine and correct this type of soft foot.