Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to powered shaft. The driver shaft, in most the cases, is a part of primary mover (such as for example electric electric motor, hydraulic turbine, steam turbine, etc.); while, the driven shaft is a part of the machine device. There exist four fundamental mechanical drives, namely equipment drive, belt drive, chain drive and rope drive. A equipment drive is one engagement type rigid drive where movement and power are transmitted through successive engagement and disengagement of tooth of two mating gears. It really is inherently free from slide and this it provides constant velocity ratio (positive drive). It can be utilized for light duty applications (such as toys, watches, etc.) aswell as for durable applications (such as 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 can be found four simple types of gears and the right gear ought to be selected based on the mutual orientation of the driver and powered shafts. Spur equipment and helical gear are applicable for parallel shafts. Bevel gear can be applied for two intersecting shafts, which might not always be perpendicular. Worm equipment arrangement can be used for the third category (nonparallel nonintersecting shafts). Unlike spur gears that have straight teeth parallel to the apparatus axis, helical gears possess teeth in helical type that are cut on the pitch cylinder. Although helical gears are generally used for parallel shafts like spur gears, it may also be utilized for perpendicular but nonintersecting shafts.

Accordingly right now there are two types of helical gears-parallel and crossed. Parallel helical gears, the normal one, is used to for power transmission between parallel shafts. Two mating parallel helical gears must have same module, same pressure position but opposite hands of helix. They offer vibration-free and quiet operation and can 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 hand of helix. This type of gear has application similar to worm gear; however, worm gear is preferred for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot offer velocity reduction beyond 1:2. Various differences between parallel helical gear and crossed helical gear are given below in table format.