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 prime mover (such as for example electric engine, hydraulic turbine, steam turbine, etc.); while, the driven shaft is a part of the machine device. There exist four fundamental mechanical drives, namely gear drive, belt drive, chain drive and rope drive. A gear 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 offers constant velocity ratio (positive drive). It can be used for light duty applications (such as toys, watches, etc.) as well as for durable applications (such as gear package 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 exist four simple types of gears and a suitable 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 equipment can be applied for two intersecting shafts, which might not always be perpendicular. Worm equipment arrangement is used for the 3rd category (nonparallel non-intersecting shafts). Unlike spur gears which have straight teeth parallel to the gear axis, helical gears have teeth in helical type that are cut on the pitch cylinder. Although helical gears are generally used 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 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 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 velocity reduction beyond 1:2. Various differences between parallel helical gear and crossed helical equipment receive below in table format.