Mechanical drives are accustomed 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 part of the machine unit. There exist four simple mechanical drives, namely gear drive, belt drive, chain drive and rope drive. A gear drive is one engagement type rigid drive where motion and power are transmitted by means of successive engagement and disengagement of tooth of two mating gears. It really is inherently free from slip and this it provides constant velocity ratio (positive drive). It can be used for light duty applications (such as for example toys, watches, etc.) aswell as for heavy duty applications (such as gear box of machinery, marine drive, etc.).

Driver and driven shafts might have three mutual orientations, namely (i) parallel shafts, (ii) intersecting shafts and (iii) non-parallel nonintersecting shafts. There exist four simple types of gears and the right gear should be selected predicated on the mutual orientation of the driver and driven shafts. Spur equipment and helical gear are applicable for parallel shafts. Bevel gear can be requested two intersecting shafts, which may not necessarily be perpendicular. Worm gear arrangement can be used for the 3rd category (nonparallel non-intersecting shafts). Unlike spur gears which have straight the teeth parallel to the gear axis, helical gears possess teeth 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, can be used 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 will 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) must have same module, same pressure angle and either same or reverse hand of helix. This kind of gear has app similar to worm equipment; however, worm gear is favored for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot provide rate reduction beyond 1:2. Various differences between parallel helical equipment and crossed helical equipment receive below in table format.