Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to driven 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 driven shaft is a part of the machine device. There exist four basic mechanical drives, namely equipment drive, belt drive, chain drive and rope drive. A gear drive is 1 engagement type rigid drive where movement 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 provides constant 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 package of machinery, marine drive, etc.).
Driver and driven shafts might have three mutual orientations, namely (we) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There can be found four basic types of gears and the right gear ought to be selected based on the mutual orientation of the driver and powered shafts. Spur equipment and helical gear are applicable for parallel shafts. Bevel gear can be applied for two intersecting shafts, which might not necessarily be perpendicular. Worm equipment arrangement is used for the third category (nonparallel non-intersecting shafts). Unlike spur gears that have straight teeth parallel to the gear axis, helical gears possess tooth in helical form that are cut on the pitch cylinder. Although helical gears are commonly utilized for parallel shafts like spur gears, it may also be used for perpendicular but non-intersecting shafts.
Accordingly generally 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 hand of helix. They offer vibration-free and quiet procedure and will transmit heavy load. On the other hand, crossed helical gears are utilized for nonintersecting but perpendicular shafts. Two mating crossed helical gears (also called screw gears) should have same module, same pressure position and either same or opposite hands of helix. This kind of gear has application similar to worm gear; however, worm gear is favored for steep speed decrease (1:15 to at least one 1:100), whereas crossed helical gears cannot offer swiftness reduction beyond 1:2. Various differences between parallel helical equipment and crossed helical equipment receive below in table format.