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Split Bearings

5 min read

Parent
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Plain Bearings (Bushings)
Simple sleeve bearings for moderate speeds and loads
Current
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Split Bearings
Adjustable two-piece bearings for heavy shafts

A split bearing is two bronze half-shells bolted together around a shaft with shims between the halves, so it can be fitted to a heavy shaft that cannot be slid through a one-piece bushing and tightened up as it wears. Removing shim thickness from the joint closes the clearance by about the same amount, up to the point where the bore goes noticeably oval and has to be bored or scraped back to round.

Part of Bearings and Bushings

A two-piece sleeve bearing with a removable cap, used on water-wheel, mill and other heavy shafts where adjustment and replacement have to be done in place.

A Split Design Lets You Replace A Bearing Without Removing The Shaft

A one-piece bushing has to be slid over the end of the shaft. On a water-wheel axle, a line shaft with pulleys already mounted, or a mill spindle with its stone, that is impossible without taking the whole machine apart. The split bearing solves it: a lower half supports the shaft, an upper half (the cap) closes over it, and bolts clamp the pair together. The carrier article notes that most water wheel, mill and heavy-machinery bearings use this design.

The second advantage is adjustment. A bronze sleeve wears, and clearance grows. A one-piece bushing can only be replaced. A split bearing carries thin shims between the two halves; taking one out lets the cap sit lower, which closes the gap again.

What You Need

  • Two matching half-shells of bronze, cast and then machined (Casting Bronze Bushings)
  • Mating faces machined flat so the halves seat evenly
  • A set of shim leaves of known thickness, in brass or other sheet metal you can measure with a micrometer
  • Cap bolts, and a housing that holds the lower half securely
  • A micrometer or feeler leaves to measure shaft and bore (Micrometers)

Method: Make And Adjust A Split Bearing

Step 1: Cast two half-shells from bearing bronze (Bronze Bushings).

Step 2: Machine the mating surfaces flat. Check them against a straight edge or a surface plate; if they do not seat flat, the cap will rock when tightened and the bore will close unevenly.

Step 3: Insert a shim pack and bore the pair as one. Put the shims between the halves, bolt them together, and bore the assembled bearing to the shaft diameter plus the clearance (see Boring and Finishing). Boring with the shims in place is what makes later shim removal bring the bore back to size. This is standard shop practice rather than a step in the carrier.

Step 4: Fit around the shaft and tighten the cap bolts evenly. Turn the shaft by hand; it should turn freely, with no catching.

Step 5: Adjust as it wears. When clearance has opened, remove one shim leaf from each side of the joint, equally, so the cap stays level. Retighten and test.

Step 6: Replace the half-shells when the shim pack is used up. See Wear Assessment and Replacement for how to tell.

Worked Example

A 50 mm shaft is bored with 0.10 mm diametral clearance (about 0.002 x the diameter). After a season the measured clearance has opened to 0.25 mm. Taking 0.15 mm of shim out of each side of the joint, say a 0.10 mm leaf and a 0.05 mm leaf from each, lowers the cap by 0.15 mm and brings the vertical clearance back to 0.25 - 0.15 = 0.10 mm.

The bore is now no longer a perfect circle: it has been pulled down by 0.15 mm in one direction only. That ovality is acceptable for a few small adjustments and is the signal to re-bore the pair with the remaining shims in once it accumulates. The wiki does not give an allowable ovality, so treat the first adjustment that causes a noticeable change in running as the limit.

Common Mistakes

  • Taking shims out of one side only. The cap tilts and the shaft binds at one end.
  • Over-tightening to close the gap. The oil film needs room; a shaft that will not turn freely has been closed too far.
  • Machining the mating faces carelessly. The joint plane sets how well the halves seat; a rocking cap gives an uneven bore.
  • Forgetting to re-oil after an adjustment. The clearance has changed and the oil path should be checked.

Safety

A heavy shaft can drop when the cap comes off

Before unbolting the cap, support the shaft and anything it carries, such as a water wheel, a flywheel or a millstone, so it cannot drop or roll. Block the drive so the machine cannot turn while your hands are at the joint, and name where the stop is. Never adjust a bearing on a shaft that is still coasting down.

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