Which of the two methods mentioned above by CENTRIFUGAL-PUMP.ORG is preferable, and why?
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, say, to 0.10, 0.12, 0.10, and 0.18, should all four feet be considered as "soft foot", or only the one with 0.18 (treating the 0.10 as the reference point, this being the lowest value)?
Imagine a triangular table with three legs. The legs are exactly the same and the table stands on a level and flat surface. If someone applied a soft rocking force on top of the table with his hand, the table would remain steady and not move. If one of the legs were slightly shorter, or if the surface were uneven, and a soft rocking force were applied similarly on the table, the table would still hold steady and not move.
Now, imagine a rectangular table with four legs. If one of the legs were slightly shorter, or if the surface were uneven, and a soft 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 is applied. This condition of the four-legged table is the simplest form of soft foot.
In technical term, soft foot is a condition whereby the mounting feet of a machine - a motor, pump, compressor, turbine, or similar equipment - are not in the same plane and are not in proper contact with their mounting pads on the baseplate. This condition arises if any of the following occurs:
The mounting feet are not on the same plane.
The feet are on same plane but the mounting pads are not.
Both the feet and the pads are not on the same plane.
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 likely to occur due to many factors, such as:
The natural sag of the equipment, or of the baseplate.
The expansion, or contraction, of materials due to changes in temperature.
The eccentricity and tolerance build-up during machining and assembly.
The induced stresses from bolting, piping, and other connections.
Using equipment that are out of calibration, or errors in equipment readings.
Checking for soft foot
There are two slightly different methods in checking for soft foot but their basic principle is the same:
Set the machine 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 [ * ] while tightening the bolt the machine has soft foot.
Or, set the machine on its baseplate and bolt it down. Place a dial indicator on one foot, loosen its hold down bolt slowly, and watch the dial indicator. If the pointer moved by [ * ] while loosening the bolt the machine has soft foot.
The amount of movement measured by the instrument indicates how much shim is needed underneath to level the machine. Repeat this procedure on each of the machine's foot.
Effects of soft foot
Soft foot should be corrected during the machinery installation before aligning the driver and the driven equipment on their baseplate. The correction can be done by inserting the right amount of shims between the "soft foot" and the mounting pads.
Never try to correct it by over-tightening the bolts - it will induce harmful stress on the machine and can result in permanent casing distortion if left uncorrected after a period of time.
Uncorrected soft foot will cause misalignment and will result in high equipment vibration that will show on a vibration spectrum as a peak at [ * ] frequency with predominant peak on [ * ] direction.
Both the driver and the driven equipment should be checked for soft foot.
[ * ] Some information are excluded in this article. Read more.