The basic concept of a drive belt involves transferring power from one part of the lawnmower to another; this is true of drive belts on any type of mechanical equipment. In the lawnmower application, the drive belt transfers the radial torque of the engine shaft to the input shaft of the transmission, which then is powered to turn and perform the function of the lawnmower: propelling the mower forward and spinning the blade.
The ability to transfer the torque, or power, from the engine to the transmission comes from the way the drive belt runs on the lawnmower. The drive belt is installed on pulleys which are attached to both shafts: the engine shaft and the input shaft of the transmission. When the engine fires and turns the engine shaft, it in turn causes the pulleys that hold the drive belt to turn. The first pulley causes the drive belt to move and the movement of the drive belt causes the second pulley to turn. The second pulley is attached to the input shaft of the transmission, so the movement of the second pulley causes the transmission shaft to begin turning.
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Wheels and Blade
In a lawnmower, the transmission shaft powers the drive wheels of the lawnmower, so getting power to the transmission shaft means that the mower will begin to move forward (or backward, if in reverse). Sometimes the lawnmower blade is directly attached to the drive belt or transmission shaft as well, and thus it is also powered by the drive belt. Other lawnmower might have a separate belt that powers the movement of the blade, but it works in the exact same way that the main drive belt works.