A 15 kW pump fails its third coupling in two years. The mechanic swaps the spider, blames the cheap parts, and installs another. Nobody checked the alignment because the pump is small and the coupling is flexible. Flexible does not mean forgiving.
What misalignment actually does
When two shafts are offset or angled, the coupling flexes twice per revolution. That flex works the elastomer, loads the bearings on both machines, and bends the shafts back and forth. A jaw coupling rated for years will shred its spider in months under a millimeter of misalignment.
The bearings take the rest of the load. Misalignment pushes the shaft sideways against the bearing, which shows up as overheating and early failure. You replace the coupling and the bearings and never fix the root cause.
Straightedge alignment is a starting point, not a finish
Laying a straightedge across the coupling catches gross misalignment. It won’t catch a few tenths of a millimeter, which is enough to wear things out. Dial indicators on the rim and face get you close; a laser alignment tool gets you within spec in a fraction of the time.
For small pumps, even a cheap laser kit pays for itself after the second avoided coupling replacement.
Check it hot
Machines grow when they run. A pump handling hot fluid rises a fraction of a millimeter as it heats, which moves the alignment. Align cold to the manufacturer’s thermal growth numbers, or check alignment at operating temperature the first time.
Don’t trust the base. A welded skid that’s level empty can twist when the pipes go in and the anchors tighten. Pipe strain pulls the pump out of alignment even if the base was set right.
Soft foot
Before aligning, check soft foot: loosen each hold-down bolt one at a time and see if the machine moves. A foot that’s hanging in the air means the machine is sitting on three points, and tightening the bolts twists the casing. Shim the feet first, then align. Most bad alignments are actually soft foot problems.
