Therefore, if the pinion drives the large gear, it reduces the speed of rotation by two-thirds but triples the torque. In such cases, the pinion is known as the reducing gear. When the large gear drives the pinion, however, it serves as a multiplying gear. It increases the speed of rotation but reduces the torque. Spur gears are simple gears with straight teeth and axles that are parallel to each other.
Helical gears have teeth that are set at an angle to the axle.
This design makes the gears quieter at high speeds, but it tends to make the gears generate a sideways force, making them less efficient. Helical gears called herringbone gears solve this problem. They have two rows of teeth, which generate opposite forces. The forces counteract each other, and the gears run smoothly. Bevel gears usually mesh at a right angle to transmit power between intersecting axles.