Align shafts and bearing housings to within 0.1 mm per metre of span, checking with a straight edge across the housings, a taut string line along long spans, and a dial indicator on the shaft turned by hand to show runout. Misaligned bearings wear fast and run hot, and the usual signs are heat or bright wear at one end of a bearing rather than along its whole length.
Part of Bearings and Bushings
Getting the bearing seats along a shaft onto one straight line, and parallel to the shaft they drive.
Misalignment Loads A Bearing At One End
A shaft that runs through several bearings is only as straight as the line joining them. If one housing sits a fraction high, the shaft bends or tilts, and each bearing carries its load on one end instead of along its length. The film breaks there first, the end heats, and the bearing wears into a taper. The carrier gives the target: parallel to within 0.1 mm per metre of span, with angular misalignment under 0.5 degrees for ball bearings and up to 2 degrees for self-aligning bushings. The two quantities are not equivalent; see the worked comparison in Fitting and Alignment.
What You Need
- A straight edge long enough to span two housings (Straight Edges and Surface Plates)
- A fine string or wire, pulled taut, for long spans
- A dial indicator with a stand, or a fixed pointer and feeler leaves
- Shims to raise housings, and a way to loosen and retighten the holding bolts
- The drive locked out so the shaft turns only by hand
Method: Three Checks
Step 1: Straight edge across the housings. Lay it across two neighbouring housings and look or feel for light or a feeler gap. This finds a housing that sits high or low.
Step 2: String line for long spans. Stretch a taut string parallel to the shaft at a fixed offset. Measure from the string to the shaft at each bearing with the same gauge. Differences between the readings are misalignment. Pull the string as tight as it will go, because a slack string sags and fakes an error; the sag is not given in this wiki, so verify by checking that the readings agree when you restring it.
Step 3: Dial indicator on the shaft. Set the indicator on the shaft journal and turn the shaft by hand. The swing of the needle shows runout. A shaft that is bent, or turning off the centre of its bearings, makes the needle move by twice the offset of its centre from the axis it turns on. That is a general geometric fact, not a figure from the carrier.
Step 4: Correct by shimming or moving the housing, then recheck all three.
Step 5: Run and feel. After a short run, stop and check each bearing for heat at one end.
Worked Example
Two bearings are 3 m apart. A string line gives 40.00 mm to the shaft at the first bearing and 40.25 mm at the second. The difference is 0.25 mm over 3 m, so the slope is 0.25 / 3 = 0.083 mm per metre, inside the 0.1 mm per metre target.
At 40.40 mm the difference would be 0.40 / 3 = 0.133 mm per metre, outside it. Shimming the second housing by 0.40 mm, or the first by 0.40 mm the other way, brings it back. Shim thickness is read with a micrometer, not guessed.
Common Mistakes
- Using a straight edge that is not straight. Check it against the method in Testing Flatness first.
- Letting the string sag. The reading then reflects the string, not the shaft.
- Aligning cold and ignoring thermal growth. On a long shaft, expansion moves things; see the end-float note in Thrust Bearings.
- Correcting one bearing at a time without rechecking. Moving one changes the others.
Safety
A line shaft stores enough energy to drag you in
Lock out the drive before putting a hand, string or indicator near the shaft, and turn it by hand only. Name the stop, who can reach it, and how long the line coasts down. Belts and pulleys on the shaft are nip points. See Power Transmission for the line shaft itself.
What’s Next
- Learn next: Wear Assessment and Replacement - reading what the bearing tells you after a season
- Combine with: Interference Fits - the seat the housing sits on