Soft foot is one of the most common faults found in rotating machinery. If not corrected, this fault can significantly hinder or even prevent proper alignment. Soft foot can lead to high levels of vibration. It may be perceived as looseness and exhibits a high 1X vibrational signature with a 180° phase shift between the baseplate and the concrete foundation. Soft foot can occur due to issues with the baseplate or improper leveling of the concrete foundation, as shown in the diagram. There are four types of soft foot, as shown below.
Parallel soft foot
Parallel soft foot, also known as short foot, occurs when the machine’s foot does not fully contact the entire base and creates a gap between the foot and the base. The lower part of the foot is parallel to the base plate. Tightening the holding bolts can lead to frame distortion as the soft foot is pulled down towards the base. This condition can be easily detected using a feeler gauge or dial indicator.
Angular soft foot
Angular soft foot can occur when one side of the machine’s foot contacts the base while the other side is lifted or bent, creating an angle between the base and the bottom of the foot. This type of soft foot can be corrected by using plastic shims to fill the angular gap and ensure even contact between the foot and the base.
A flexible foot exists when the gap between the foot and the base is already filled with corroded / soft pads. Until the retaining screws are tightened, the machine will appear to have soft foot problems. This type can be repaired by replacing the corroded / soft pads with integrated / one-time hardened ones.
Stress-induced soft foot
The condition of “Stress-induced soft foot” is one of the most challenging to detect and is caused by external forces acting on the machine. It can result from piping stresses, electrical connections, or significant misalignments. Forces induced by stress can occur at any stage of the alignment process, making it necessary to perform multiple checks to eliminate this type of soft foot.