When aligning machine trains, one of the most frustrating problems can be the problem caused by soft foot. If you have a soft foot problem within your machine train, you will struggle to get any sort of repeatability with your coupling alignment results.

A soft foot can be described as a machine foot that lifts from the machine’s base when the bolt is loosened. First step to solving this problem is to understand the underlying reasons that the soft foot is occurring.

The main reasons for soft foot are:

  1. The machine’s base is unstable. This can be a problem if the base is built on a steel framework that has become distorted or has been designed from materials that are not ridged enough and subsequently have a range of flexibility.
  2. The machine’s feet could also be distorted or they could be slightly tapered. The underside of the feet could be corroded or could have layers of paint, dirt or other waste materials clogging the space between the underside of the machine’s foot and the machine’s base.
  3. External forces may be exerted on the machine. Very often the machine is connected to piping or the machine is connected to other supporting structures that are causing excessive stress and strain on the machine train components.

Therefore, when we talk about soft foot, we tend to describe three varying types of the problem as discussed below.

Angular Soft Foot

Angular soft foot denotes a tapered mounting surface between the underside of the foot and the base of the machine. Remedy of this type of soft foot can be challenging. The taper can be measured using feelers gauges to understand the types of shimming required to eliminate the problem.

Squishy Soft Foot

As described above, when the underside of the machine’s foot and the base of the machine is covered in peeling paint, dirt or corrosion a squishy soft foot can appear. This can also be evident if too many shims are used underneath the foot to correct misalignment.

Parallel Soft Foot

As the name describes, parallel soft foot means that the underside of the machine’s foot is parallel to the machine’s base, however, the machine’s foot is either shorter than the rest of the machine’s feet or the machine’s base at this point may drop at this point or may not be level with the rest of the base.

Soft foot can severely affect the operating conditions of the machine train by inducing greater vibrations and leading to problems that will essentially cause the machine train to have poorer mean time between failures. Simple measures should be taken to avoid soft foot. The area between the underside of the machine feet and the machine base should be free from corrosion, dirt and flaking paint. The shims used should also be clean and where possible no more than three shims at a time should be inserted underneath each foot. Finally, external forces should be eliminated. Any supports or pipework connected to the machine train that are causing stress and strain on the machine train should be adjusted to remove the stresses.

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