This in turn may be due to differences when the machine was manufactured, a squishy footage with oil film etc.
between foot and base, a bent foot, or it may be induced by a pipe to which the machine is attached (e.g. a pipe on top of a pump), which prevents the machine from touching all its feet to its base.
Soft foot usually gives rise to a 1x harmonic of the rotation frequency in the spectrum when radial (horizontal or vertical) vibration measurements are taken.
If loosening one of them reduces vibration amplitude (which would be opposite to what one would expect if the machine was all right and we caused looseness by unscrewing of the bolt), then soft foot was present.
Soft foot can be detected by using a variety of methods, including: Once soft foot has been detected, it can be corrected by shimming the machine feet until all four feet are in solid contact with the foundation or baseplate.