Mechanical drives are used to transmit motion, torque and power from a driver shaft to driven shaft. The driver shaft, in most the cases, is a part of primary mover (such as electric electric motor, hydraulic turbine, steam turbine, etc.); while, the powered shaft is a part of the machine device. There exist four basic mechanical drives, namely equipment drive, belt drive, chain drive and rope drive. A equipment drive is one engagement type rigid drive where motion and power are transmitted by way of successive engagement and disengagement of teeth of two mating gears. It really is inherently free from slip and this it provides constant velocity ratio (positive drive). It can be utilized for light duty applications (such as for example toys, watches, etc.) aswell as for durable applications (such as gear box of machinery, marine drive, etc.).

Driver and driven shafts may have three mutual orientations, namely (we) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There exist four fundamental 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 always be perpendicular. Worm equipment arrangement is utilized for the 3rd category (nonparallel nonintersecting shafts). Unlike spur gears which have straight teeth parallel to the gear axis, helical gears possess the teeth in helical type that are cut on the pitch cylinder. Although helical gears are commonly utilized for parallel shafts like spur gears, it can also be used for perpendicular but non-intersecting shafts.

Accordingly there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, is used to for power transmission between parallel shafts. Two mating parallel helical gears must have same module, same pressure position but opposite hand of helix. They provide vibration-free and quiet procedure and can transmit heavy load. However, 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 angle and either same or opposite hands of helix. This type of gear has application 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 speed reduction beyond 1:2. Various differences between parallel helical equipment and crossed helical equipment receive below in table format.