Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to powered shaft. The driver shaft, in majority of the cases, is part of primary mover (such as for example electric electric motor, hydraulic turbine, steam turbine, etc.); while, the powered shaft is part of the machine unit. There exist four fundamental 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 way of successive engagement and disengagement of the teeth of two mating gears. It really 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 package 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 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 requested two intersecting shafts, which might 
not always be perpendicular. Worm equipment arrangement is utilized for the 3rd category (nonparallel nonintersecting shafts). Unlike spur gears that have straight the teeth parallel to the apparatus axis, helical gears have the teeth in helical type that are cut on the pitch cylinder. Although helical gears are generally used for parallel shafts like spur gears, it may also be used for perpendicular but non-intersecting shafts.
Accordingly presently there are two types of helical gears-parallel and crossed. Parallel helical gears, the normal one, is used to for power transmission between parallel shafts. Two mating parallel helical gears should have same module, same pressure angle but opposite hands of helix. They offer vibration-free and quiet procedure and can transmit heavy load. However, crossed helical gears are used for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also known as screw gears) must have same module, same pressure position and either same or opposite hand of helix. This type of gear has application similar to worm gear; however, worm equipment is favored for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot provide swiftness reduction beyond 1:2. Various differences between parallel helical gear and crossed helical gear are given below in desk format.