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Wood Bearings

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Parent
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Plain Bearings (Bushings)
Simple sleeve bearings for moderate speeds and loads
Current
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Wood Bearings
Lignum vitae, elm, and other self-lubricating woods

A wood bearing is a stopgap for slow, light shafts when bronze is unavailable: a block of lignum vitae, elm, maple or beech, soaked for 24 hours in hot tallow or lard oil before it goes in, and re-oiled often. Wood on oiled iron drags at 0.05-0.15 of its load against 0.02-0.06 for bronze on steel, and wood insulates, so any speed or load limit assumes the fat stays below its 40-50°C softening range.

Part of Bearings and Bushings

Bearing blocks and sleeves made from dense hardwoods, with the oil soak that lets them run, and the loads and speeds that suit them.

Dense Oily Woods Work Where Bronze Is Not Available

The earliest bearing material was wood, and it still has a place. Lignum vitae is self-lubricating and extremely durable; it was used in ship stern tubes into the 20th century, and Bearings and Shafts lists water-lubricated mills and ships as its home ground. Elm has good shock resistance and takes up grease well. Maple and beech are dense and fine-grained enough for light duty. Oak is harder than elm and serves as a general-purpose wood. Lignum vitae is a tropical timber; in most places you will be working with the others.

The honest limits are in the friction table of Bearing Materials: wood on wood is 0.25-0.50 dry and 0.10-0.25 lubricated; wood on iron is 0.20-0.40 dry and 0.05-0.15 lubricated. That is two to three times the drag of bronze on steel. The wood also wears into the shape of the shaft until the play is enough to wobble, and the same article puts the right use as light loads, slow speeds, and when metal is not available.

What You Need

  • A dense, fine-grained hardwood block, free of splits and knots
  • Hot tallow or lard oil in a vessel you can hold it hot in (Lubricant Types)
  • A way to cut the cradle and, for a block bearing, a slot on top for periodic greasing
  • Spare blocks cut in advance: worn wood bearings are easily replaced

Method: Make And Soak A Wood Block

Step 1: Shape the block to cradle the shaft. A slot in the top lets you grease it without stopping to dismantle anything.

Step 2: Bore or carve to fit the shaft. Use the same clearance logic as a bronze bushing: roughly 0.002 x the shaft diameter across the bore, to leave the oil film room. Wood moves with moisture, so test the fit again after the soak.

Step 3: Soak in hot tallow or lard oil for 24 hours. The wood absorbs fat and holds it as reserve lubrication.

Step 4: Install and relubricate frequently. The wood supplies a reserve, not a supply. Re-oil on a written schedule (Lubrication and Failures).

Step 5: Watch the play. When the shaft shakes visibly in the block, replace the block.

Worked Example

Take the 50 mm shaft carrying 2,000 N at 100 rpm from Friction Fundamentals. Surface speed is 0.262 m/s.

  • Bronze on steel, lubricated (0.02-0.06): 40-120 N of friction, 10.5-31.4 W
  • Wood on iron, lubricated (0.05-0.15): 100-300 N of friction, 26.2-78.6 W (100 x 0.262 and 300 x 0.262)

The wood bearing makes two and a half times the heat and, because wood is a poor conductor of heat (the same wiki advises wooden boxes to insulate instruments), it sheds that heat more slowly. Tallow softens at 40-50°C. Together that is why wood bearings suit slow shafts: at 300 rpm the wood’s heat would be three times the figure above, and the fat that is supposed to protect it would be liquid and gone.

Common Mistakes

  • Skipping the soak. A dry wooden bearing chars in minutes (Lubrication).
  • Asking a wood bearing to do a bronze bearing’s job. It is a temporary arrangement for light and slow service.
  • Not keeping spare blocks. The bearing is cheap to replace only if the replacement already exists.
  • Quoting a load and speed for wood without stating the oiling. The numbers depend entirely on the soak and the schedule.

Safety

Hot fat burns and ignites

Tallow and lard oil heated for a 24-hour soak can flash and burn skin badly. Heat them gently in a heavy pot away from naked flame, never leave them unattended, never add water, and keep a lid or damp cloth ready to smother a fire. Wood bearings on a running shaft also char if starved of oil, so know where the machine’s stop is before the first test run.

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