Mechanical drives are used to transmit motion, torque and power from a driver shaft to driven shaft. The driver shaft, in most the cases, is part of primary mover (such as electric engine, hydraulic turbine, steam turbine, etc.); while, the powered shaft is part of the machine device. There exist four fundamental mechanical drives, namely equipment drive, belt drive, chain drive and rope drive. A gear drive is one engagement type rigid drive where movement and power are transmitted through successive engagement and disengagement of teeth of two mating gears. It really is inherently free from slip and this it offers constant velocity ratio (positive drive). It can be utilized for light duty applications (such as toys, watches, etc.) aswell as for durable applications (such as for example gear container of machinery, marine drive, etc.).
Driver and driven shafts might have three mutual orientations, namely (i) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There can be found four fundamental types of gears and a suitable gear ought to be selected based on the mutual orientation of the driver and driven shafts. Spur gear and helical gear can be applied for parallel shafts. Bevel gear can be requested two intersecting shafts, which might not necessarily be perpendicular. Worm equipment arrangement can be used for the 3rd category (nonparallel nonintersecting shafts). Unlike spur gears that have straight tooth parallel to the apparatus axis, helical gears have teeth in helical type that are cut on the pitch cylinder. Although helical gears are commonly utilized for parallel shafts like spur gears, it can also be utilized for perpendicular but nonintersecting shafts.
Accordingly presently 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 hands of helix. They offer vibration-free and quiet operation and can transmit heavy load. On the other hand, crossed helical gears are utilized for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also known as screw gears) should have same module, same pressure angle and either same or opposite hands of helix. This type of gear has application similar to worm gear; however, worm equipment is preferred for steep speed reduction (1:15 to 1 1:100), whereas crossed helical gears cannot offer velocity reduction beyond 1:2. Various distinctions between parallel helical equipment and crossed helical gear receive below in table format.