Mechanical drives are used to transmit motion, torque and power from a driver shaft to driven shaft. The driver shaft, in majority of the cases, is a part of primary mover (such as electric engine, hydraulic turbine, steam turbine, etc.); while, the powered shaft is a part of the machine unit. There exist four basic mechanical drives, namely equipment drive, belt drive, chain drive and rope drive. A equipment drive is a single engagement type rigid drive where movement and power are transmitted through successive engagement and disengagement of tooth of two mating gears. It is inherently free from slide 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 durable applications (such as for example gear box of machinery, marine drive, etc.).
Driver and driven shafts may have 3 mutual orientations, namely (we) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There can be found four basic types of gears and a suitable gear ought to be selected predicated on the mutual orientation of the driver and powered shafts. Spur gear and helical gear are applicable 
for parallel shafts. Bevel equipment can be requested two intersecting shafts, which may not always be perpendicular. Worm equipment arrangement is used for the 3rd category (nonparallel non-intersecting shafts). Unlike spur gears which have straight tooth parallel to the apparatus axis, helical gears possess teeth in helical form that are cut on the pitch cylinder. Although helical gears are commonly 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, can be used to for power transmission between parallel shafts. Two mating parallel helical gears must have same module, same pressure angle but opposite hand of helix. They offer vibration-free and quiet operation and will transmit heavy load. On the other hand, crossed helical gears are used for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also called screw gears) must have same module, same pressure position and either same or opposite hand of helix. This kind of gear has application similar to worm gear; however, worm equipment is preferred for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot offer quickness reduction beyond 1:2. Various distinctions between parallel helical equipment and crossed helical equipment receive below in table format.