Soft Foot
Encyclopedia
Soft Foot is a common term used for machine frame distortion. The distortion is caused when one or more feet of a machine differ in height from the others. 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. It can be detected in this way.
Similar harmonic 1x is caused by Misalignment and Looseness. Usually not only will it resemble them, but it also causes misalignment, imbalance and other problems. In fact, it may be loose, not strictly a Soft Foot. Therefore a way to tell the difference is needed.
To differentiate Soft Foot from Misalignment and Looseness, even if it causes Misalignment when present, one way is to loosen each foot bolt at a time, while the other three are tight. If loosening one of them reduces vibration amplitude (which would be opposite to what one would expect if the machine was OK and we caused looseness by unscrewing of the bolt), then Soft Foot was present.
Another way to differentiate Soft Foot from Looseness are Phase Measurements between the machine foot and the base/ground. 180 degrees difference indicates Looseness.
Soft Foot usually gives rise to a 1x Harmonic of the rotation frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
in the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
when Radial (Horizontal or Vertical) vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
measurements are taken. It can be detected in this way.
Similar harmonic 1x is caused by Misalignment and Looseness. Usually not only will it resemble them, but it also causes misalignment, imbalance and other problems. In fact, it may be loose, not strictly a Soft Foot. Therefore a way to tell the difference is needed.
To differentiate Soft Foot from Misalignment and Looseness, even if it causes Misalignment when present, one way is to loosen each foot bolt at a time, while the other three are tight. If loosening one of them reduces vibration amplitude (which would be opposite to what one would expect if the machine was OK and we caused looseness by unscrewing of the bolt), then Soft Foot was present.
Another way to differentiate Soft Foot from Looseness are Phase Measurements between the machine foot and the base/ground. 180 degrees difference indicates Looseness.
External links
- http://www.ludeca.com/casestudy/SoftFootandVibration_reliableplant0306.pdf Detecting Soft Foot with Vibration Analysis