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 engine, hydraulic turbine, steam turbine, etc.); while, the driven shaft is part of the machine device. There exist four simple mechanical drives, namely equipment 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 slide 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) non-parallel nonintersecting shafts. There exist four basic types of gears and the right gear ought to be selected based on the mutual orientation of the driver and driven shafts. Spur equipment 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 used for the third category (nonparallel non-intersecting shafts). Unlike spur gears which have straight teeth parallel to the apparatus axis, helical gears have tooth in helical type that are cut on the pitch cylinder. Although helical gears are generally utilized for parallel shafts like spur gears, it can also be used for perpendicular but nonintersecting shafts.

Accordingly there are two types of helical gears-parallel and crossed. Parallel helical gears, the normal one, is utilized to for power transmitting between parallel shafts. Two mating parallel helical gears must have same module, same pressure angle but opposite hands of helix. They provide vibration-free and quiet procedure 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 program similar to worm equipment; however, worm equipment is favored for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot offer velocity reduction beyond 1:2. Various distinctions between parallel helical gear and crossed helical gear are given below in table format.