Mechanical drives are used to transmit motion, torque and power from a driver shaft to powered 
shaft. The driver shaft, in majority of 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 unit. There exist four simple 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 by means of successive engagement and disengagement of the teeth of two mating gears. It really is inherently free from slide and this it offers continuous velocity ratio (positive drive). It can be utilized for light duty applications (such as toys, watches, etc.) as well as for durable applications (such as gear box 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 basic types of gears and a suitable gear ought to be selected predicated on the mutual orientation of the driver and powered shafts. Spur equipment and helical gear are applicable for parallel shafts. Bevel equipment can be requested two intersecting shafts, which might not always be perpendicular. Worm gear arrangement is used for the 3rd category (nonparallel nonintersecting shafts). Unlike spur gears which have straight the teeth parallel to the gear axis, helical gears have the teeth in helical type that are cut on the pitch cylinder. Although helical gears are generally utilized for parallel shafts like spur gears, it may also be utilized for perpendicular but non-intersecting 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 position but opposite hands of helix. They offer vibration-free and quiet procedure and may 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 type of gear has software similar to worm equipment; however, worm equipment is preferred for steep speed reduction (1:15 to 1 1:100), whereas crossed helical gears cannot provide acceleration reduction beyond 1:2. Various distinctions between parallel helical gear and crossed helical gear are given below in table format.