Soft foot

Correct Soft Foot. Soft foot exists when one of the four machinery feet is not level with the base. When the base bolts are tightened with soft foot, the effect can distort and misalign the pump casing. 

To check for soft foot, place a dial indicator onto the machinery foot, and loosening the base bolt. If the indicator moves more than 0.002 inches, the foot is soft and it should be corrected. Go through the same procedure on the remaining feet one at a time. 

To correct soft foot, place shims under the foot in the thickness corresponding to the movement of the dial indicator. 

Before starting the alignment procedure, check for soft-foot and correct the condition. 

Soft-foot is the condition when all four of a machine s feet do not support the weight of the machine. It is important to determine if this condition is present prior to performing shaft alignment on a piece of machinery. Not correcting soft-foot prior to alignment is a major cause of frustration and lost time during the aligning procedure. 

As in the chair example, when a machine with soft-foot is placed on its base, it will rest on three of its support feet unless the base and the bottoms of all of the feet are perfectly machined. Further, because the feet of the machine are actually square pads and not true points, it is possible that the machine can rest on only two support feet, ones that are diagonally opposite each other. In this case, the machine has two soft-feet. 

Possible sources of soft-foot are shown in Figure 54.12. 

Placing a piece of machinery in service with uncorrected soft-foot may result in the following ... 

Figure 54.12 Diagrams of possible soft-foot causes (Ref. 3) 1. Loose foot, 2. Cocked foot, 3. Bad shims, 4. Debris under foot, 5. Irregular base surface, 6. Cocked foot...

Dial-indicator readings taken as part of the alignment procedure can be different each time the hold-down nuts are tightened, loosened, and retightened. This can be extremely frustrating because each attempted correction can cause a soft-foot condition in another location. 

If the nuts do not loosen, metal fatigue may occur at the source of soft-foot. Cracks can develop in the support base/frame and, in extreme cases, the soft-foot may actually break off. 

Attempt to pass a thin feeler gauge underneath each of the four feet. Any foot that is not solidly resting on the base is a soft-foot. (A foot is considered soft if the feeler gage passes beneath most of it and only contacts a small point or one edge.)... 

If the feeler gage passes beneath a foot, install the necessary shims beneath that foot to make the initial soft-foot correction. 

The following procedure describes the final soft-foot correction ... 

Move the dial indicator and holder to the next foot to be checked and repeat the process. Note The nuts on all of the other feet must remain securely tightened when a foot is being checked for a soft-foot condition. 

Once soft-foot is removed, it is important to use the correct tightening procedure for the hold-down nuts. This helps ensure that any unequal stresses that cause the machine to shift during the tightening procedure remain the same throughout the entire alignment process. The following procedure should be followed ... 

After eliminating soft-foot, loosen all hold-down nuts. 

Prior to taking alignment measurements, however, remember that it is necessary to remove any soft-foot that is... 

---