How is pulley different from wheel and axle

The construction of a "pulley" and a "wheel and axle" can be similar or even same. But in the simple machines classification their purposes differ: Pulley is used to change the direction of a rope or a chain. The actual rotation of the pulley wheel is a side effect. Wheel and axle are used to amplify rotational force. The force can be provided through a rope wrapped around the wheel, but the change of direction is a side effect. For example, a chain sprocket is an example of wheel and axle, while a chain idler would be an example of a pulley.