Line the hub’s axle hole with a press-fit cast iron box, a section of steel pipe driven in as a sleeve, or a plug of very hard wood such as lignum vitae, ironwood or dense oak, bored to match the axle and greased often. The liner takes the wear so that the elm hub does not, and a worn bore is repaired by replacing the liner, not the hub.
Part of Wheelwright Craft
The axle box is the replaceable lining in the hub bore that rides on the axle arm and takes the sliding wear between wheel and axle.
A Wearing Surface You Can Replace
The hub bore rides on the axle arm and rubs on it every turn of the wheel. An unlined elm bore wears oversize in a season of heavy use, and a wheel with an oversize bore wobbles. A liner moves that wear to a part you can swap. The parent article lists three options.
| Liner | Fit | Strength | Weakness |
|---|---|---|---|
| Cast iron box | press-fit into the bore | hard, long-wearing | cast iron breaks if struck; salvage only |
| Steel pipe sleeve | driven into the bore | easy to find | can rust; must be a true cylinder |
| Hardwood liner | bored plug, driven in | easy to replace | needs frequent grease |
How Hard The Bearing Works
Bearing pressure is the wheel load divided by the projected area of the bearing, the bore diameter times its length. Suppose a wheel carries 250 kg (an assumed load for a cart carrying 500 kg), in a bore 6 cm in diameter and 20 cm long.
- Load = 250 kg × 9.81 m/s² = 2,453 N
- Projected area = 0.06 m × 0.20 m = 0.012 m²
- Pressure = 2,453 N / 0.012 m² = about 204,000 Pa = 204 kPa
Different bearing materials tolerate different pressures; Axle and Bearing Design gives the allowance for each. Doubling the bore length halves the pressure, which is why long hubs bear better.
What You Need
- A liner: iron box, steel pipe, or a hardwood plug
- A bore sized to the liner, reamed or bored true
- A heavy mallet and a driving block of hardwood to drive the liner without damaging it
- Grease (Axle Grease)
Method: Fit And Check
Step 1: Match the sizes Measure the axle arm, and size the liner’s inside diameter to leave a small running clearance, enough for grease to film the surface but not enough to rock. Size the liner’s outside diameter to the hub bore for a tight drive fit.
Step 2: Drive the liner in Drive it in square with a hardwood block between the mallet and the liner. A liner driven in crooked wears one end and splits the hub.
Step 3: Check the fit on the arm Slide the wheel on the axle arm. It should turn freely by hand, without slop when you rock the rim.
Step 4: Grease it Wipe grease into the bore and onto the arm. For a wood liner, grease frequently, as the parent article says.
Common Mistakes
- A liner that is a loose fit in the hub — it turns with the axle and wears the hub bore instead.
- A liner with a thick wall in a thin hub — driving it can split the hub; keep wall thickness modest.
- Mixing arm and liner materials carelessly — dry wood on wood or iron on iron seizes; see Lubrication & Wear.
- Forgetting the taper — if the arm tapers, the liner must match it.
Safety
Struck iron and splitting wood
Cast iron can shatter when hit and send chips flying. Drive it only through a wooden block, wear eye protection, and never strike a cracked box.
What’s Next
- Learn next: Lubrication & Wear — keeping the bearing alive
- Combine with: Wooden Axle Construction — the arm the liner rides on