A lawn mower clutch is a safety device that stops the blades from spinning while the lawn mower's engine is still running. It also can be a matter of convenience for the operator. On older lawnmowers, a clutch stops the wheels from turning so the operator can empty the catcher without shutting off the engine.
Mechanically, a clutch is needed anytime there are two rotating shafts that need to spin at different speeds. Clutches can be quite complicated and can be found in all kinds if electromechanical devices. In the case of a lawnmower, the clutch resembles a fairly simple on/off switch.
Both the shaft connected to the engine and the shaft connected to the blades have a plate attached. The engine’s shaft plate is called a flywheel. When the lawn mower clutch is engaged, the flywheel and the clutch plate are pressed together so that the engine spins the blades. This is done by springs, levers, hydraulics or some combination thereof.
There also are electric clutches. When the lawn mower clutch is disengaged, the clutch plate is separated from the flywheel. This causes the blades to stop.
In older lawn mowers, the blade shafts were connected to the engine’s drive shaft by gears or belts. Engaging the lawn mower clutch only connected the engine to the axle. These mowers did not require pushing, merely directing. The problem was that the blades kept turning even when the mower was stopped.
Some rotary lawn mowers have only one clutch, which is for the blades. Even though they have an engine, they have to be pushed. The best lawn mowers have a clutch connecting the engine to the rear axle and another connecting the engine to the blades.
There are also electric clutches. These are common in both small electric mowers as well as larger gas powered mowers with two sets of blades. They are referred to as power take-off (PTO) clutches.
When a PTO lawn mower clutch is engaged, a magnet is moved into position inside a coil. This creates an electrical current that completes a circuit between the engine and the blades. When disengaged, the magnet returns to its position outside the coil, and the circuit is said to be broken. Without a complete circuit, there is no power to the blades.