Bearing Alignment: Shaft Alignment and Bearing Seating in Housings

Why Alignment Matters

Every rotating machine transfers power through a shaft. When two shafts—such as a motor driving a pump—are not in line with each other, the coupling, bearings, and seals absorb the difference on every revolution. The result is accelerated bearing wear, seal leakage, vibration, elevated temperatures, and premature failure. Proper shaft alignment and correct bearing seating in housings are among the most cost-effective maintenance tasks in any industrial or marine facility.

Types of Misalignment

Parallel misalignment (also called offset) occurs when the two shaft centerlines are parallel but offset from each other—either horizontally, vertically, or both. You can see it as a gap between coupling hubs when you lay a straight-edge across them.

Angular misalignment occurs when the shaft centerlines meet at an angle rather than running true. Even if the coupling faces look close at the top, they may be wide apart at the bottom.

Real machines almost always have a combination of both types. The goal is to reduce each to the tolerance specified by the equipment manufacturer or, when no spec is given, to a general industry guideline such as 0.002 in. parallel offset and 0.002 in. per inch of coupling diameter for angular error.

Personal Protective Equipment and Safety

Before any work on a shaft alignment or bearing installation:

• Lockout/tagout (LOTO) the machine following your facility's written LOTO procedure before you remove guards, touch couplings, or rotate shafts manually. Verify zero energy with a meter or by attempting a start.
• Wear safety glasses at all times in the lab.
• Wear gloves when handling shim stock—edges are sharp.
• Keep hands clear of rotating parts during any powered test run.
• Secure the dial indicator and magnetic base firmly before rotating the shaft; a loose indicator can become a projectile.

Step 1 — Rough Alignment with a Straight-Edge and Feeler Gauge

With the motor and pump bolted loosely to the baseplate, lay a precision straight-edge across the outside diameter (OD) of both coupling hubs at the 12 o'clock and 3 o'clock positions. A gap between the straight-edge and one hub indicates offset. Insert feeler gauge blades into the gap to measure it. Shim the low machine's feet (typically the motor, because the pump is the fixed machine) until the gap is 0.010 in. or less at both positions.

Check that all four motor feet touch the baseplate without rocking—a foot that lifts when one bolt is loosened indicates soft foot, a condition that must be corrected before you proceed. Soft foot causes the frame to distort and makes precise alignment impossible.

Step 2 — Precision Alignment with a Dial Indicator (Rim-and-Face Method)

Clamp a dial indicator on a magnetic base attached to one coupling half. Position the indicator tip on the rim (OD) of the opposite coupling half to measure parallel offset. Zero the indicator at 12 o'clock, rotate the shaft to 3, 6, and 9 o'clock, and record the readings. The total indicator runout (TIR) at the rim divided by 2 equals the parallel offset.

Next, move the indicator tip to the face (flat end) of the opposite coupling half to measure angular misalignment. Record readings at the same four clock positions. The difference between the 12 o'clock and 6 o'clock face readings, divided by the coupling diameter in inches, gives angular misalignment in inches per inch.

Use the shim math formula to calculate the required shim change at each motor foot, add or remove shims, retighten bolts, and repeat the measurement cycle until both values are within tolerance.

Step 3 — Housing Squareness Check

A bearing seated in a housing that is not square to the shaft experiences a constant side load that mimics angular misalignment even in a perfectly aligned machine. To check squareness:

1. Install the pillow-block housings on the shaft with bearing outer rings seated but housing bolts only hand-tight.

2. Mount the dial indicator on the shaft (or an adjacent fixture) and sweep the tip across the face of the bearing outer ring through 360°.

3. The TIR reading represents how much the housing bore is cocked relative to the shaft. Acceptable TIR is 0.001 in. or less.

4. If TIR is out of tolerance, check for debris under the housing base or a warped mounting surface before re-seating.

5. Torque housing bolts in a cross pattern to the specified value with a calibrated torque wrench, then recheck TIR. Bolt torque can shift the housing slightly, so the final reading after torquing is the one that counts.

Reading the Numbers

Dial indicators read in thousandths of an inch (0.001 in.). A full revolution of the needle equals 0.100 in. on most standard indicators. Always preload the indicator slightly (about 0.020 in.) before zeroing so it can read both positive and negative movement without lifting off the surface.

Common Mistakes

• Forgetting to check and correct soft foot before attempting precision alignment.
• Rotating only the indicator, not the coupled shafts together—the rim-and-face method requires rotating both shafts as a unit.
• Adding too many shim layers; use the fewest, thickest shims possible to maintain a stable stack.
• Re-torquing housing bolts without rechecking TIR—torque always distorts readings slightly.