Gears are toothed wheels that transmit constant angular velocity between two shafts, which can be parallel, intersecting, or non-intersecting. There are different types of gears classified by the orientation of their shafts, including spur gears which have radially arranged teeth parallel to the shaft allowing transmission between parallel axes, helical gears which have teeth arranged at an angle to the axis, and bevel gears which connect intersecting shafts. For gears to maintain a constant speed ratio, their tooth profiles must be designed so that the line of action passes through a fixed pitch point along the line connecting their centers.
