Keep a wheel on its axle with a linchpin: a hardwood or iron peg through a hole in the axle tip just outside the hub, a cotter pin bent from heavy wire, or a carved key that wedges into a slot, and carry spares. The pin is rarely overloaded, since the sideways push of the uphill wheel on a 20% slope is only about 481 N, roughly 5 MPa on an 8 mm iron pin, so wheels are lost because pins work out, wear or are forgotten.
Part of Cart & Wagon Construction
A linchpin is the pin through the end of an axle that holds a wheel from sliding off, usually with a washer between pin and hub.
The Pin Is Loaded Only Sideways
On a level, straight arm the wheel’s weight goes into the journal and does not load the pin. The pin sees the sideways push of the wheel along the axle. On a side slope, the ground pushes each wheel toward the uphill side by friction. For the uphill wheel that push points outward, away from the cart, so that wheel bears on its linchpin. The downhill wheel is pushed inward against the shoulder of the axle, not the pin.
Traditional arms also slope slightly downward toward their tips (Engineering Principles), and then the weight itself has a component W × sin β along the arm, pushing the wheel outward. For an assumed slope of β = 5°, that is 2,453 × 0.0872 = 214 N on every wheel, on or off a slope.
How big is the push?
Take assumed figures: 250 kg on a wheel (2,453 N) and an 8 mm iron pin loaded in double shear (both ends of the pin that project from the axle), a shear area of 2 × π × 8² / 4 = 100.5 mm².
| Side slope | Slope angle | Push on the uphill wheel | Shear stress in the pin |
|---|---|---|---|
| 10% | 5.7° | 2,453 × sin 5.7° = 244 N | 244 / 100.5 = 2.4 MPa |
| 20% | 11.3° | 2,453 × sin 11.3° = 481 N | 481 / 100.5 = 4.8 MPa |
Mild steel fails in shear at something on the order of 150 MPa or more (a recalled handbook range), so even on a 20% slope the pin carries about a thirtieth of what would break it. The load on a pin is small, and the pin’s job is to be present and secure. A hardwood peg has a smaller margin, but the same logic holds.
Adding the 214 N from a cambered arm to the 481 N of a 20% slope gives 695 N, or 6.9 MPa on the same pin, still about a twentieth of the shear strength. What does fail is the axle tip around the hole. A hole drilled right at the tip leaves a short piece of wood beyond it, which can split off along the grain, so keep the hole back from the end.
What You Need
- Pins: a hardwood or iron peg, a cotter bent from heavy wire, or a carved key (Wooden Axle Construction)
- A washer, iron or hardwood, between the pin and the hub end
- A drill for the pin hole in the axle tip
- Cord to tether the pin to the axle
- Spares: at least two per wheel
Method: Make Pin Loss Unlikely
Step 1: Place the hole. Drill it just outside the hub’s outer end with the washer in place, with room to spare between the hole and the axle tip.
Step 2: Choose the type.
- A tapered peg driven through is simple, and its taper locks it.
- A wire cotter bent over at the ends cannot work out without being straightened.
- A carved key that wedges into a slot is a good field-made alternative.
Step 3: Fit the washer against the hub face and the pin against the washer, leaving just enough free play for the wheel to turn.
Step 4: Tie it. Tether the pin to the axle with cord, so that a pin that works loose hangs there instead of dropping on the road.
Step 5: Inspect. Check every pin at the start of each trip and at each stop; the parent article says to always carry spares.
Common Mistakes
- Relying on a snug fit alone — vibration works pins out over a few kilometres.
- No washer — the pin and hub end grain wear each other.
- Hole too near the tip — the end grain shears off.
- One pin and no spare — losing a wheel on a loaded cart in rough ground is a serious problem.
Safety
A wheel that comes off a loaded cart
When a wheel leaves its axle the cart drops on that corner, throwing the load sideways and often the shafts into the animal. Never work under a cart held only by a jack or the shafts, never stand downhill of a loaded cart, chock the other wheels first, and do not drive on with a missing or doubtful pin.
What’s Next
- Learn next: Maintenance & Field Repair — the pre-trip checks
- Combine with: Engineering Principles — why a wheel tends to work off its arm