The most common one is a worm gear with a low lead angle. In a self-locking worm gear drive, the torque applied to the worm gear is blocked by friction force preventing its rotation. There are parallel axis self-locking gears that, unlike worm gears, can utilize practically any gear ratio. Face gears typically have a straight or skewed tooth line and a varying normal section tooth profile along the tooth line. These face gears are engaged with spur or helical involute pinions at intersecting or crossed axes. Spiral face gears that utilize a convex-concave contact at the high pressure angle drive tooth flanks have potentially greater load capacity in comparison to conventional face gears. Application of asymmetric gear teeth with the high pressure angle concave flanks and the low pressure angle convex flanks of the spiral face gear allows a minimal contact ratio greater than 1.0 to be achieved in both tooth flank meshes.