Soft Foot–What It Is and How to Minimize It

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.

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About the Author

Brad Case

Brad Case has been associated with VibrAlign since 1990, first as a manufacturer's representative, then joining the company as a direct employee in 2005. He has 29 years experience in aligning industrial machinery. Brad began his career in the automotive industry providing technical training, sales, and service for Murray Goldseal an aftermarket air conditioning component manufacturer. His background includes 25 plus years experience in sales, service and training for laser shaft alignment systems, couplings, gearing, gear reducers, and design, installation and service of centralized lubrication systems. Brad and his wife currently live near Waco, TX.
19 Comments
  1. As a part of best practices angular soft foot shall not be corrected by putting CUT shims. If we put cut shims between base plate and equipment (pump, motor, compressor,etc), there may be two line contact and not a desired area contact of that particular foot. The angular soft foot to be identified first, whether angularity is in base plate or equipment foot? If the base plates are not perfectly flat/level or angularity is more than acceptable limits.then all four plates to be machined, ground & blue matched. The level and flatness can be checked with straight edge and master level. Acceptance tolerance for level shall be 0.2mm per meter. Flatness can be checked by placing the straight edge on any two feet/plates, 0.05mm/0.002″ feeler gauge should not enter between the straight edge and feet/base.
    plate

    Thanks & regards,

    • Rajdhar – Thanks for your comment. You are correct! Especially if following API Specifications, however there are tens of thousands of pieces of rotating machinery in general industry that have bases that were not installed or maintained to the tight flatness specifications you mention. Also, those industries typically do not have redundant machines for the primary one so it is impractical to take a machine out of service for an unscheduled extended outage. So for them the best correction is to step shim for angular soft foot to minimize machine frame distortion and of course non-repeatable alignment readings.

      We do recommend for all industries to install bases to strict flatness tolerances, when possible. If all did we certainly would see much less soft foot on rotating machinery.

  2. ARUN

    how should check motor base soft foot?.

    • Arun – check out our You tube video on pre-alignment steps which shows how to check Obvious and Final Soft Foot.

  3. Safar Hassanvand

    Improper tensioning of machinery body bolting and/or not following the right sequence of casing bolts have been missed out in Brad,s report. In some cases, I have seen multi-stage compressors that by releasing and retightening of casing bolts, softfoot (if we could call it so) disappeard.

    • I disagree. Proper tightening sequence is important, as you said. And it is included in this article.

  4. alwyn marks

    is it possible to correct the softfoot with two holdon bolts? eg. (pillow block bearings, with 2 mounting bolts). if yes can somebody explain the method of doing

    • I would suggest to start by having all the hold down bolts tight. Loosen one foot bolt at a time, check under the foot with a .002″ or .003″ feeler gauge. If you find the foot has lifted more than .002″ to .003″ determine how much shim will be needed to fill the gap between the foot and base.

      Be sure to check the foot on both sides of the hold down bolt to insure there is not an angled soft foot. If so you will need to cut one leg off of the shim to fill the gap on just the one side of the foot.

      Thanks for your question.

      • Chris Crozier

        In regards to soft foot and shimming a motor to a base.

        If the amount of shims needed is 100″ I believe the least amount of shims should be used. As an example 2 x .040 + .020 with the thinner in between the thicker. is this correct?

      • Chris, yes for both. The rule of thumb is to have no more than 5 shims under the machine (motor) feet. Some facilities have a 4 shim rule. Of course you can consolidate the 3 shims you mentioned by using .100″ thick shims. Just be sure to mic any shim over .050″ thick as we have seen them vary by +/- .005″ to .008″.

      • Phil Nedved

        Use a dial indicator on the foot as loosening bolts. Tighten all bolts, and loosen one at a time measuring deflection with indi cator. Clean every nut, bolt, base plate, etc before you start. And dont use a grinder to do that! Machinist skills 101!

  5. Marc

    Brad,

    I have a doubt. I am doing pump alignment. The soft foot & pre-alignment without flange bolting is within tolerance .005mm. Then bolts were tighten, the offset & angular on the vertical position / movement is within tolerance (0.005) but the soft foot/shim value at far end of motor has increased to .008. Is this acceptable as long as the offset & angular value is alright or we have to adjsut the shims again.

    Thanks.

    • D Mike Minks

      Marc. soft foot check is the first thing you look for when initially installing a motor/pump configuration. The next is flange stresses. Most pump manufacturers have minimum standards but if those were not stated either in the installation specifications or in shipping documents, you should use the same standard – the alignment standard of 2 mils.

      This may require loosening of the expansion joints of the inlet and outlet piping. If not installed, expansion joints may need to be installed to eliminate flange stresses. If these stresses are left unattended, bearings and possibly pump internals may be out of tolerance and cause premature failure.

    • Phil Nedved

      When in doubt, readjust.

  6. naaz

    But in api stander half or cutting not allowed.then how can I remove angler softfoot?

    • Naaz,

      You are correct. API 610 also specifies flatness tolerances for bases, machine pads, and feet. All of which are well under the allowable soft foot tolerance, angled or otherwise.

      If you have a machine with an angled soft foot that is more than the acceptable tolerance (2 to 3 mils) and can’t cut a shim in half to correct than a course of action to correct the problem needs to be discussed with your engineering department as either the foot (or base) in question may no longer meet the API flatness tolerances.

      A short term solution maybe to temporarily allow a cut shim until a time when the machine feet/base can be checked for flatness. You may also check into having a special shim ground to a taper to match the angled soft foot.

  7. S D banerjee

    Is it essential to put shims between a gearbox and its bedplate to avoid vibrations due to soft foot?

    • It is essential to minimize soft foot, so if it exists between the bedplate and the gearbox, yes. Not only can it increase vibration, it can make alignment more difficult, and change clearances within the gearbox, due to case distortion. Many of the gearboxes I have installed need a slight amount of shimming due to irregularities in the baseplate.

  8. Muhammad Hunain

    I found this blog very helpful for me, i have some confusion regarding soft foot.

    thanks.

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