Imagine a triangular table with three legs. The legs are identical and the table
stands on a flat and level horizontal surface. If a slight rocking force were
applied on the table top the table would remain still. If one of the legs were
slightly shorter or longer, or if the surface were uneven, and the rocking force
were applied likewise the table would remain steady and not move.

Now, imagine a rectangular table with four legs. If one of the legs were slightly
shorter or longer, or if the surface were uneven, and a rocking force were
applied on the table, the table would rock back and forth. The table would
continue to rock, or "vibrate", as long as the rocking force were applied
repeatedly. This condition of the four-legged table is the simplest form of soft
foot.

In technical term, soft foot is a condition where the contact surface of the four
mounting feet of a machine - a pump, motor, turbine, compressor, or similar
equipment - are not in the same plane and are not in proper contact with their
mounting pads on the baseplate. However, as illustrated above with a three-
legged table, soft foot does not exist in a three-legged equipment such as in an
OH1 pump - small overhung pump with two simple mounting feet in opposites
sides of the casing and a third "leg" supporting the bearing bracket in its center.

Soft foot arises if any of the following condition occurs:

  • The equipment mounting feet are not flat and on the same plane.
  • The mounting pads on the baseplate are not flat and on same plane.
  • Both mounting feet and pads are not flat and on the same plane.
CENTRIFUGALPUMP.COM
Soft foot
Causes of soft foot

In spite of careful efforts to machine the feet of an equipment and its baseplate
mounting pads so that they are flat and level on the same plane, a soft foot is
can occur due to factors, such as:

  • The natural sag of the equipment.
  • Distortion of the baseplate during transit, or grouting on the foundation.
  • Thermal expansion, or contraction, of materials due to changes in
    temperature.
  • Eccentricity and tolerance build-up during machining and assembly.
  • Induced stresses from bolting, piping, and other connections.
  • Distortion of casing due to external nozzle loads, or excessive hold-down
    bolting stress.
  • Using instruments that are out of calibration, or errors in measurement.

Checking for soft foot

There are two different ways to check for soft foot but their basic principle is
similar:

  • Set the pump on its baseplate and place a dial indicator on one foot.
    Tighten the hold down bolt slowly and watch the dial indicator. If the
    pointer moved by [#1], or higher, while tightening the bolt the pump has
    soft foot.

  • Or, set the pump on its baseplate and bolt it down. Place a dial indicator
    on one foot and loosen the hold down bolt slowly, and watch the dial
    indicator. If the pointer moved by [#1], or more, while loosening the bolt
    the pump has soft foot.

(Question: Which of these two ways is preferable, and why?) [#2]

The amount of movement measured by the instrument indicates how much shim
is needed underneath the contact surface of the foot  to level the pump. Repeat
this procedure on each of the pump foot. The same procedure can also be
applied to check the driver for soft foot.

Effects of soft foot

The presence of soft foot should be corrected because it can cause equipment
misalignment, higher nozzle loads, higher pipe stress, and increased vibration.
Soft foot should be corrected by shimming. When shimming use laminated shims
that are corrosion resistant and use as few shims as possible by using shims
with varying thickness. Never try to correct soft foot by over-tightening the bolts -
it would induce harmful stress on the casing that can result in permanent casing
distortion, or deformation, if left uncorrected over a period of time.

The presence of soft foot condition may be indicated by high pump vibration
level at [#3] frequency in horizontal or vertical direction.


DASHBOARD:

Consider a situation where a pump has been shimmed and aligned. If the bolts
are loosened one at a time and the shims spring back such as to open a gap in
the four feet of 0.05", 0.10", 0.05", and 0.08", should all four feet be considered
soft foot, or only the ones with 0.10" and 0.08", treating the 0.05" as a datum
this being the lowest value? [#4]

The data or information referred to in [#1], [#2], [#3] and [#4] are available in the
full version of this article.


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