Soft foot is a common issue when aligning rotating equipment. It is a major cause of repeatability problems in shaft alignment measurements. In addition to alignment quality and repeatability problems, it can be a cause of machinery vibration, reduce life in electric motors, and cause internal clearance problems in gearboxes and pumps. But if the proper precautions are taken, soft foot can be minimized and controlled.. The term “soft foot” is the common term used for the improper contact between a machine casing, and the baseplate used to support it. It may be either an angular or parallel soft foot, but often it is a combination of the two. It is often compared to a straight-backed wooden chair, where one leg, being shorter, does not contact the floor, causing a rocking motion in the chair when you are seated in it. While this is a good mental image, soft foot in machinery is a little more complex. While an angular soft foot might make contact with a baseplate or foundation, it does not make a UNIFORM amount of contact. Once base bolts are tightened, the foot tends to bend to conform to the baseplate to which it is mounted. CAUSES OF SOFT FOOT CONDITIONS
As stated above, soft foot conditions adversely affect alignment quality and repeatability. Here’s how: Regardless of whether you have:
Notice that the relative position of the shaft centerline changes due to changes in the position of the soft foot. In addition, unless you tighten and loosen the bolts in sequence, the position of the shaft centerline in relation to the stationary machine can change. As an example, if you tighten the inboard left foot first one time, and the inboard right foot the second time, you may take measurements in different shaft centerline locations. Using the straight-backed chair analogy, not tightening in a known sequence causes the movable machine to “rock” into different positions. STEPS TO MINIMIZE AND CONTROL SOFT FOOT
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AuthorWith unceasing appetite for knowledge - Karthik Kumarasamy Archives
August 2015
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