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What is soft foot?
One of the most overlooked machine problems, soft foot describes a
machine’s foot that deflects a small amount when the hold-down bolt is
tightened.
Imagine a person sitting on a three-legged
bar stool that has one leg shorter than the other two. If that person
moves his weight slightly, the stool tips in the direction of the short
leg and is very uncomfortable--even dangerous. The solution is simply to
pack sufficient shims under the short leg until it gets back on an even
setting. This situation is also very common with machinery seatings.
Check each hold-down foot of the driver and
driven units for a soft foot condition. If a soft foot condition exceeds
+/- 0.002", corrections must be made to ensure that the unit sits
squarely on the bedplate with no stress.
Soft foot problems come in many different
forms, some more obvious than others. These are:
Parallel air gap: One leg is too
short or one baseplate-mounting pad is not level with the other three.
This can also be caused when the shims under one foot are the wrong size.
Bent foot: The
bottom of one foot is not parallel with the base.
Squishy foot:
Dirt, grease, paints or rust, bent or burred shims, or too many shims are
found under the foot. Feeler gauges should be used to measure the
clearance all around the foot’s three exposed edges.
Induced soft
foot: Two soft-feet are located on the same side or same end of a
machine, and the feeler gauge indicates a gap that is parallel or nearly
parallel.
Gaps without
soft foot: There is a visual gap under the foot before tightening, and
there is none after tightening. A laser check might indicate a relatively
small soft foot, but feeler gauges will indicate a much larger gap.
Installing shims is not the answer to this problem. The most logical thing
to do is to machine the baseplate or the machine housing.
There are also external forces that can
cause soft foot. Some of these are:
- Overhung
machines or attachments
- Belt, gear, or
chain loads
- Hoses or
stressed conduit
- Structural
bracing attached to the machine
- Jack bolts left
tight against the machine base
- Poorly finished
foundations
- Piping that
does not meet zero-cold-spring tolerances at flange connections
Each machine foot location on both the
driver and driven units must be individually checked for the existence of
soft foot. If a soft foot movement is observed to be greater than +/-
0.002", it must be immediately corrected. The checkout procedure is
as follows
- Visually check
all of the machine’s feet for breakage, cracks, or bent feet. If
these defects exist, they must be corrected before soft foot checks
are made.
- All shims that
are rusted, dirty, and have been burred at the edges must be replaced
with correctly dimensioned, pre-cut stainless steel shims.
- Using a feeler
gauge, or taper gauges, determine the gap between the bottom of the
foot and the bedplate. Ensure that the gauges do not touch only at one
point. In many cases, machine feet can suffer from a compound bend,
which produces angular irregularities and creates a tapered gap. If
the foot cannot be straightened, tapered off shims must be fitted to
correct the condition. Always perform a three-point check at all of
the three edges of the feet when investigating soft foot conditions.
- Placing a
magnetic based dial indicator stand on a stationary location (usually
the bedplate), and ensuring the indicator dial stem button is
contacting the machine foot surface makes this check. The dial
indicator face is calibrated in thousandths of an inch increments, and
when the stem is pressed in toward the dial, the needle rotates in a
clockwise, or positive (+) direction.
All measurements taken in that mode will be
preceded with a + sign. The indicator dial stem is depressed onto the
shaft surface enough to rotate the needle (pointer) approximately one
revolution. It is then set to 0.000". The hold-down bolt’s nut is
slackened, and the deflection of the needle is carefully recorded. The
hold-down bolt’s nut is again tightened to the specified torque value.
This procedure is carried out at all of the machine’s feet, and all
readings are recorded for comparison and determination of any soft foot
conditions.
The machine foot, or feet, which exhibits
the largest movement, and the greatest measurement, is in need of being
shimmed to correct the discrepancy. If the measurement was 0.008", a
shim thickness of 0.004" should be inserted under that foot as an
initial start in correcting the soft foot. The bolt would be torque
tightened, the dial indicator would again be set up and adjusted to
0.000" before slackening off the bolt, and the new measurement would
be recorded. This procedure would be repeated until the soft foot
condition disappears. A final all-round check is again made before signing
off the job to ascertain the accuracy of the correction. All hold-down
bolts must be left tight before closing this inspection.
The amount of shims needed for correction
of soft foot and alignment must be kept to a minimum. If, for example,
0.100" was the required depth of shims, you should install one
0.090" shim and maybe one or two other shims to make up the
difference. Each successive shim layer is a potential soft foot cause if
air or liquid is trapped under each shim.
Soft foot is one
of the most overlooked machinery problems. It will distort heavy machine
casings badly enough to cause the machines to fail through shaft
misalignment, bearing failures, and mechanical and gear wear. The problem
is often found to have originated in the initial installation, which was
assumed to be done correctly because the manufacturer's representative set
it. Always check for soft foot before making any shaft alignment
adjustments. You will never regret the little extra time spent on this
important task.
Reliability
Direct, Inc. supplies a variety of Laser
& Dial Indicator Alignment systems as well as precision pre-cut shims. |
