เครื่องคำนวณ Soft Foot

Soft foot is a condition where one or more machine feet do not make complete, flat contact with the baseplate or foundation. When the hold-down bolt at that foot is tightened, it pulls the machine frame down, causing internal distortion that misaligns bearings and seals.

The term comes from the analogy of a table with one short leg — it rocks and is unstable.

Soft foot is critical because it:

• Distorts the machine frame — causing internal bearing misalignment
• Makes alignment non-repeatable — readings change each time bolts are tightened
• Increases vibration — due to internal stresses and bearing preload
• Causes premature bearing failure — from uneven load distribution
• Wastes alignment time — if not corrected first, alignment will never converge

Always check and correct soft foot before performing shaft alignment.

Follow this procedure:

1. Tighten all hold-down bolts to normal torque
2. Mount a dial indicator on the machine frame near the first foot
3. Zero the indicator
4. Loosen the bolt at that foot (about 1/4 turn)
5. Record the maximum indicator movement (positive = gap)
6. Re-tighten the bolt
7. Move to the next foot and repeat

Alternatively, modern laser alignment systems have a built-in soft foot check function that measures all feet quickly.

There are three main types:

• Parallel (gap) soft foot — A uniform gap exists under the foot, typically due to missing, incorrect, or worn shims. Corrected by adding flat shims equal to the measured gap.
• Angular soft foot — The foot has a wedge-shaped gap and rocks on one edge. Caused by non-parallel machining of the foot or baseplate. Corrected with tapered or stepped shims, or by machining the foot.
• Springing (reactive/induced) soft foot — No gap exists, but external forces (pipe strain, conduit stress, dowel pin binding) distort the machine when the bolt is tightened. Cannot be corrected with shims — must address the external force.

Parallel soft foot: Add flat shims equal to the dial indicator reading under the affected foot. Use quality pre-cut stainless steel shims. Avoid using more than 3–4 shims in a stack.

Angular soft foot: Use tapered shims, stepped shims, or machine the foot surface to achieve full contact. Sometimes a thin coat of epoxy or polymer can fill small angular gaps.

Springing soft foot: Identify and relieve the external force. Common causes include pipe strain, electrical conduit stress, tight dowel pins, and foundation settling. Do not add shims for springing soft foot — it will make the problem worse.

After correction, always re-check all feet to verify the fix.