mortise wheel

A mortise wheel is a large cast-iron gear wheel whose teeth are not integral to the metal but are instead replaceable wooden blocks (called clogs or pegs) driven into mortises, or rectangular slots, cut into the wheel's face or perimeter. The wooden teeth engage with the teeth of a smaller metal pinion, transferring rotational force through the machinery. This hybrid construction, combining metal and wood, was the standard for heavy timber-working machinery from the 18th century through the mid-20th century.

The mortises themselves are rectangular cavities arranged in a circle around the wheel's diameter, typically spaced to create even tooth spacing. The wooden clogs, usually made from hardwood such as apple wood or hornbeam, are shaped to fit snugly into these mortises and are secured either by friction alone or by metal pins driven through the wheel and into the wood. The face of each wooden tooth is angled to match the required mesh angle with the pinion.

Why wood instead of all-metal teeth

The wooden teeth serve a crucial practical purpose: they act as a sacrificial element that can be damaged or worn without damage to the expensive cast-iron wheel body itself. When a jam or shock load occurs, the wooden tooth breaks or shears away, protecting the gear from fracture. A broken wooden clog is also easier and cheaper to replace than re-casting or re-machining an entire iron wheel. Additionally, wood runs quieter than metal-on-metal gear contact and is somewhat self-lubricating, which mattered in machines that ran continuously with primitive lubrication systems.

Mortise wheels were particularly common in sawmills, flour mills, and woodworking factories, where they formed part of the main power transmission from a water wheel or steam engine down to the saw or planer. Sizes ranged from 3 to 8 feet in diameter or larger, depending on the speed reduction and torque required. The pinion that meshed with a mortise wheel was often called a lantern pinion when it too had wooden teeth, or an iron pinion when it was cast solid.

The term 'mortise' refers directly to the construction method: the rectangular holes (mortises) cut into the wheel's body. This design has become obsolete in modern machinery, replaced entirely by all-metal cast or machined gears, which are more compact, require less maintenance, and are more precise. However, mortise wheels remain in use in some heritage mills and in restoration work on historical machinery.