A pump bearing that failed every 3 months. The pump was a centrifugal, 15 kW, coupled to an electric motor by a jaws coupling. The bearing was a 6308 on the pump shaft. The customer replaced the bearing each time. It always failed on the drive side. The motor was new. The pump was new. The coupling was new. The issue was alignment. The motor and pump shafts were misaligned by 0.5 mm parallel and 1 degree angular. This is about coupling alignment and how misalignment destroys bearings.

The misalignment types

There are three types of misalignment:

  • Parallel (offset): the two shafts are parallel but offset. The coupling must flex sideways. The jaws coupling can tolerate 0.1 mm offset. At 0.5 mm, the elastomer flexes 5x beyond its design. It wears out in months. The side load on the bearing doubles.
  • Angular: the two shafts meet at an angle. The coupling must flex in a rocking motion. A jaws coupling tolerates 0.5 degrees. At 1 degree, the elastomer wears rapidly. The bearing sees a cyclic radial load at 1x RPM.
  • Combined: both parallel and angular. This is the most common. The coupling wears fastest.

What was changed

1. Re-aligned the shafts. A dial indicator was mounted on the pump shaft. The motor feet were shimmed until the indicator read under 0.05 mm parallel and 0.05 mm angular. The alignment took 2 hours. The bearing life extended from 3 months to 5 years. The alignment cost nothing — just time and shims.

2. Switched to a disc coupling. For precision alignment, a disc coupling (laminated steel) tolerates less misalignment (0.05 mm parallel, 0.1 degree angular) but has higher torsional stiffness. For a pump at 2900 RPM, the alignment must be within 0.05 mm. The disc coupling doesn’t forgive misalignment — it forces correct alignment. The jaws coupling was masking the misalignment (and wearing out).

3. Checked the soft foot. During alignment, a “soft foot” was found: one motor foot was 0.2 mm off the base plate. The motor was twisted when bolted down. The alignment kept drifting. The foot was machined flat. The alignment held. The soft foot is a common alignment problem — the frame is distorted, not the shaft position.

The alignment procedure

A proper alignment follows this sequence:

  1. Check soft foot: loosen each motor bolt one at a time, check for movement with a feeler gauge. Shim any foot that rocks.
  2. Rough align: use a straightedge across the coupling halves. Get within 0.5 mm.
  3. Fine align: mount a dial indicator on one coupling half. Rotate the shaft. Adjust the motor shims until the indicator reads 0.05 mm in both directions.
  4. Check again after tightening: the bolt torque can shift the motor. Re-check the indicator after final torque.
  5. Re-check after warm-up: the pump and motor grow at different rates. Re-align when the machine is at operating temperature.

Alignment tolerance table

Coupling type Max parallel Max angular Speed limit
Jaws (elastomer) 0.15 mm 0.5° 3000 RPM
Grid (spring) 0.10 mm 0.25° 4000 RPM
Disc (laminated) 0.05 mm 0.1° 10,000 RPM
Gear (lubricated) 0.08 mm 0.2° 5000 RPM

The alignment rule: dial indicator alignment within 0.05 mm for any coupling running over 1500 RPM. The failing bearing wasn’t defective — the motor and pump were offset by 0.5 mm. Check soft foot, align cold and hot, and recheck after bolt torque. A jaws coupling tolerates misalignment but at the cost of bearing life. Better to align correctly.