Soft foot
101
5.16 Soft foot
Soft foot is the condition of machine frame distortion. Any cause that results in
machine frame distortion when the machine is anchored to its foundation is a soft
foot. Some of the principal causes are:
› Non-coplanar machine mounting surfaces,
› Deformed machine frame or feet,
› External forces e.g. from connecting pipe or bracket,
› Improper shimming or soiled machine feet.
› Too many shims under a machine foot (a maximum of 3 should be used)
The consequences of forcibly tightening down the feet are deformed machine
frames, bent shafts and distorted bearings. This leads to high vibration and premature
failure.
Soft foot should be checked before aligning the shafts. This can be done quickly and
conveniently with the aid of the instrument’s soft foot function. With the sensor and
laser mounted on the shaft at any angular position, the system is able to sense any
machine movement when the machine bolts are loosened individually. By entering the
machine dimensions, the system is able to calculate, from shaft movement, by how
much each foot has moved as it is loosened.
Angular soft foot
The base of the foot is at
an angle to its foundation
and they are only partly in
contact. In this case, suspect
foot is checked with a feeler
gauge and corrected by
building a custom ‘shim
wedge’ or machining the
underside of the foot.
Parallel soft foot
One or more feet are
too short or too long.
This usually results in the
machine rocking on the
longer feet. This is corrected
by shimming the shorter
feet.
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