Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to driven shaft. The driver shaft, in majority of the cases, is part of prime mover (such as for example electric motor, hydraulic turbine, steam turbine, etc.); while, the powered shaft is part of the machine device. There exist four fundamental mechanical drives, namely gear drive, belt drive, chain drive and rope drive. A equipment drive is a single engagement type rigid drive where motion and power are transmitted through successive engagement and disengagement of teeth of two mating gears. It is inherently free from slip and this it provides continuous velocity ratio (positive drive). It can be utilized for light duty applications (such as toys, watches, etc.) as well as for heavy duty applications (such as for example gear package of machinery, marine drive, etc.).

Driver and driven shafts may have three mutual orientations, namely (i) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There exist four basic types of gears and a suitable gear ought to be selected based on the mutual orientation of the driver and powered shafts. Spur gear and helical gear are applicable for parallel shafts. Bevel equipment can be applied for two intersecting shafts, which might not necessarily be perpendicular. Worm equipment arrangement is utilized for the third category (nonparallel non-intersecting shafts). Unlike spur gears which have straight tooth parallel to the apparatus axis, helical gears have the teeth in helical type that are cut on the pitch cylinder. Although helical gears are generally used for parallel shafts like spur gears, it can also be used for perpendicular but nonintersecting shafts.

Accordingly generally there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, is utilized to for power transmitting between parallel shafts. Two mating parallel helical gears should have same module, same pressure position but opposite hands of helix. They provide vibration-free and quiet operation and may transmit heavy load. On the other hand, crossed helical gears are used for nonintersecting but perpendicular shafts. Two mating crossed helical gears (also called screw gears) should have same module, same pressure position and either same or opposite hands of helix. This type of gear has software similar to worm equipment; however, worm gear is favored for steep speed decrease (1:15 to at least one 1:100), whereas crossed helical gears cannot offer velocity reduction beyond 1:2. Various variations between parallel helical gear and crossed helical equipment receive below in table format.