A lawn mower pulley is a part that consists of two discs connected at an axle, where the space between the discs receives a belt. The component's axle fits onto the crankshaft, transmitting engine output from the axle to other parts of the unit via a drive belt. Pushed lawnmowers use a single drive belt to power the mower blade, while walk-behind and riding lawnmowers also use a second belt to power the mower's propulsion.
Lawn mower pulleys come in many different shapes, sizes and styles, depending on the particular make and model of the lawn mower and engine. They typically feature a notch or other attachment point around the center axle to connect it to the crankshaft. Pulleys are often made from metal or plastic.
A lawn mower idler is a similar but unrelated part to a pulley. An idler has a similar two-disc shape and space for a belt but is designed without an axle notch. This allows the part to freely rotate around the shaft it's connected to, allowing a mower engine to run without necessarily pushing the blade or unit. This part is commonly used on self-propelled mowers that feature an option to stop the mower blade but keep the engine running.