Understanding Soft Foot

Definition: What is Soft Foot?

Soft foot is a common condition in rotating machinery where one or more of the machine’s mounting feet do not make full, flat contact with the baseplate or foundation it is bolted to. When the mounting bolts are tightened, the machine’s frame or casing is forced to bend or distort to conform to the uneven mounting surface. This distortion can induce significant internal stresses, leading to a host of reliability problems.

A simple analogy is a four-legged chair that rocks because one leg is shorter than the others. When a person sits on it, the chair’s frame is twisted until all four legs touch the ground. A machine with a soft foot condition experiences a similar type of frame distortion every time it is bolted down.

Types of Soft Foot

There are four main types of soft foot:

1. Parallel Soft Foot: This is the most common type, where one machine foot is shorter than the others, creating a parallel air gap between the foot and the baseplate.
2. Angular Soft Foot: This occurs when a machine foot is bent or the baseplate is sloped, causing the foot to make contact at an angle. When bolted down, the foot is forced to bend to become parallel with the base.

– Squishy Foot (Induced Soft Foot): This happens when there is excessive corrosion, debris, or too many shims (more than 4-5) under a machine foot. When the bolt is tightened, the material compresses, creating a soft foot condition.

– External Force Soft Foot: This is caused by external forces, such as pipe strain, conduit, or bracing, pulling or pushing on the machine’s frame and lifting one of its feet off the base.

Why is Soft Foot a Major Problem?

The frame distortion caused by soft foot is a root cause of numerous machinery problems:

• Increased Vibration: The internal stresses and distortion create a pre-loaded condition that can significantly increase vibration, often showing up at the running speed (1X) and its harmonics (2X, 3X, etc.), making it easy to mistake for unbalance or misalignment.
• Alignment Difficulties: It is impossible to achieve a precision alignment if a soft foot condition exists. As bolts are tightened and loosened during the alignment process, the machine’s frame will move in unpredictable ways, making repeatable alignment readings impossible. Correcting soft foot is a mandatory prerequisite for precision alignment.
• Bearing and Seal Failure: The distortion of the machine casing leads to distortion of the bearing bores. This puts uneven loads on the bearings, leading to premature wear and failure. It can also cause seals to open up, leading to leaks.

– Shaft and Coupling Stress: The distortion can lead to internal misalignment of the bearings, which can bend the shaft and put excessive stress on couplings.

How to Detect and Correct Soft Foot

Soft foot is typically detected and corrected during a precision alignment job using dial indicators or laser alignment systems.

1. Initial Check: With all bolts loosened, a feeler gauge can be used to check for obvious gaps under each foot.
2. Systematic Measurement: A more precise method involves using a dial indicator at each foot. The indicator is zeroed with the bolt tight. The bolt is then loosened, and the amount of movement (spring) in the machine foot is measured. A movement of more than 0.05 mm (0.002 inches) typically indicates a soft foot that needs correction.
3. Correction: The correction involves adding high-quality, pre-cut stainless steel shims under the soft foot to fill the measured gap. The process is repeated for all feet until the movement on loosening each bolt is within tolerance.

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