driving wheel

A driving wheel is one of the large wheels on a steam locomotive that receives power directly from the engine's cylinders through connecting rods and crossheads. Unlike the smaller leading and trailing wheels that merely guide the locomotive, driving wheels transmit the force needed to move the entire train. On most steam locomotives, there are between two and four pairs of driving wheels, all connected to the same axle or crankshaft so they rotate in sync.

The size and number of driving wheels depend on the locomotive's duty. Freight engines typically have larger driving wheels with lower numbers, sometimes just one pair, because they need maximum pulling force at low speeds. Express passenger locomotives often have smaller driving wheels, sometimes three or four pairs, to reach higher speeds while maintaining adequate tractive effort. A typical driving wheel diameter ranges from 4.5 feet on a freight engine to 7 feet or more on a fast passenger locomotive.

Driving wheels must be considerably stronger than other wheels on the locomotive because they endure the full tension of the driving rods. They are usually cast iron or steel, with the rim often made as a separate piece and shrunk onto the wheel center for better wear characteristics. The wheel is ribbed or has a pronounced flange on each side to grip the rails and prevent slipping, though the flange must be precisely ground to specification to avoid derailment.

Adhesion and slip

The critical challenge with driving wheels is adhesion: the friction between wheel and rail must be enough to transmit engine power without the wheels spinning uselessly on the rails. Poor adhesion causes wheel slip, which damages both wheel and rail and robs the engine of pulling power. Sand or other abrasives are applied to the rails in front of the driving wheels during poor conditions such as wet leaves or ice to restore grip. The total weight resting on the driving wheels directly limits how much tractive force the locomotive can develop.

The term "driving wheel" distinguishes these from the idle wheels forward and aft that serve only to support weight and guide the locomotive around curves. In modern diesel and electric locomotives, which use different transmission systems, the equivalent function is performed by traction motors driving the wheel axles, though workers may still use "driving wheel" informally for any powered wheel on a rail vehicle.