Axle and Bearing Design

Part of Roads and Transport

Engineering the axle and bearing system that carries load and allows wheels to rotate with minimal friction.

Why This Matters

The axle and its bearings are the most mechanically stressed components of any wheeled vehicle. A wagon loaded to 2,000 kg has 500 kg pressing down on each wheel hub. That load must be supported while the wheel rotates thousands of times per kilometer. The bearing surface — where the axle contacts the hub — must carry this load while allowing free rotation. If it seizes up or wears out, the vehicle stops; if it fails catastrophically, the wheel collapses.

Getting axle and bearing design right is therefore not just a comfort issue — it is a reliability issue. A well-designed axle runs for years with simple maintenance. A poorly designed one requires constant attention and fails at the worst times. Understanding the principles allows you to design, build, and maintain axles that give long, reliable service.

Axle Geometry

Straight axle: The simplest form — a single shaft runs the full width of the vehicle, both wheels fixed to it, the axle turning with the wheels. Simple to make but causes problems on turns (inside wheel must travel a shorter distance than the outside).

Dead axle with rotating hubs: The axle is fixed (does not rotate); the wheel hub rotates on the axle. This is the standard for wagons. Each wheel rotates independently, solving the turning problem.

Dish: Traditional wooden wheels were “dished” — the spokes were not perpendicular to the axle but angled outward slightly. This causes the wheel to act like a cone, so the bottom of the wheel is vertical when the axle carries weight and the wheel splays outward slightly. Dishing provides lateral stiffness.

Camber: The axle was often made with a slight downward slope toward the ends (or the hubs set lower). Combined with dish, this ensured the wheel was vertical when loaded.

Taper: Traditional wooden axle arms tapered slightly — smaller at the tip than at the wheel seat. This meant the wheel bore pressed harder on the larger section, and as the bearing wore and the wheel moved outward, it remained supported.

Bearing Types for Draft Vehicles

Type	Materials	Friction	Lifespan	Notes
Plain (sliding)	Wood on wood	High	Short	Original; adequate with constant lubrication
Plain (metal lined)	Iron hub on iron axle	Medium	Good	Standard until late 19th century
Bronze bush	Bronze on iron/steel	Low	Long	Excellent for heavy loads
Ball bearing	Hardened steel	Very low	Very long	Best; requires precision manufacture
Roller bearing	Hardened steel	Low	Very long	High capacity; good for axle loads

For a rebuilding civilization without precision manufacturing, plain bearings with bronze bushes are the best achievable option. Bronze (copper-tin alloy) has a natural lubricity that makes it ideal for sliding contact against iron or steel.

Designing a Plain Bearing Axle

Axle arm dimensions:

The axle arm (the portion that runs through the hub) must be sized for the load:

Bearing pressure (kPa) = Load (N) / (Diameter × Length of bearing surface)

For wood/iron bearing: maximum pressure ~300–500 kPa For bronze bearing: maximum pressure ~1,000–2,000 kPa For steel-on-steel with good lubrication: maximum pressure ~2,000–4,000 kPa

Example calculation: Load per wheel = 500 kg = 4,900 N Using bronze bush, target pressure = 1,500 kPa Area needed = 4,900 / 1,500 = 3.27 cm²

With a 40 mm diameter axle arm (circumferential contact area = diameter × length): Length needed = 3.27 cm² / 4 cm = 0.82 cm → round up to 10 cm for safety factor

A 40 mm × 100 mm bronze bush on a 40 mm diameter steel axle arm handles 500 kg per wheel with good margin.

Making Bronze Bushes

Bronze bushes can be cast and machined:

Composition: 88–90% copper, 8–10% tin, 2% lead (lead aids lubricity). Adjust for available materials — even impure bronze works better than iron on iron.

Casting: Pour molten bronze into a sand mold with a central core to create the cylindrical form. Allow to cool slowly.

Machining: Turn the outer diameter to fit the hub bore (press fit, typically 0.02–0.05 mm interference). Bore the inner diameter to fit the axle arm (running clearance, 0.05–0.10 mm).

Oil grooves: Machine a helical or longitudinal groove on the inner bore surface — this distributes lubricant around the bearing surface.

Lubrication

Without lubrication, a metal bearing seizes within minutes under load. Lubrication is not optional — it is the entire point of the bearing design.

Traditional axle grease: Tallow (rendered animal fat) mixed with lime or wood ash (to increase viscosity at temperature). Applied generously to the axle arm before fitting the hub.

Modern grease equivalent: Any oil or grease containing fatty acids — lard, tallow, vegetable oil thickened with soap. Petroleum grease is better if available.

Relubrication interval:

• Greased axle: relubricate every 40–80 km under heavy load
• Signs the bearing needs grease: squealing or grinding noise; wheel feels stiff to turn; excessive heat in the hub

Heat Buildup

A dry bearing generates heat rapidly. If a hub is hot to the touch after 1–2 km of travel, stop immediately and relubricate. A hot bearing can seize, destroy the axle arm, and cause the wheel to come off while moving.

Axle Material and Strength

Wooden axles:

• Traditional and adequate for light loads
• Use dense hardwood: oak, ash, elm, hickory
• Grain must run straight along the axle length
• Vulnerable to splitting and rot

Iron axles:

• Much stronger; standard from late medieval period onward
• Wrought iron was used historically; mild steel is the modern equivalent
• Size the arm diameter to handle the bending stress

Steel axle arm bending: The axle arm is a cantilever beam loaded at the wheel center. Maximum bending stress:

σ = M / Z

Where M = load × distance from bearing center to wheel center, Z = section modulus (= πd³/32 for round section).

For safety, design to a maximum stress of 100 MPa (mild steel yields at 250 MPa — this gives a safety factor of 2.5).

The combination of correctly sized axle arms, well-fitted bronze bushes, and regular lubrication produces a bearing system that can run for years with simple maintenance.