Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to powered shaft. The driver shaft, in most the cases, is a part of prime mover (such as electric motor, hydraulic turbine, steam turbine, etc.); while, the powered shaft is 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 1 engagement type rigid drive where motion and power are transmitted by means of successive engagement and disengagement of tooth of two mating gears. It really is inherently free from slip and this it provides 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 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) non-parallel 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 gear and helical gear are applicable for parallel shafts. Bevel gear can be applied for two intersecting shafts, which may not always be perpendicular. Worm gear arrangement can be used 
for the 3rd category (nonparallel non-intersecting shafts). Unlike spur gears that have straight the teeth parallel to the gear axis, helical gears have tooth in helical type that are cut on the pitch cylinder. Although helical gears are commonly used for parallel shafts like spur gears, it may also be utilized for perpendicular but non-intersecting shafts.
Accordingly there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, is utilized to for power tranny between parallel shafts. Two mating parallel helical gears must have same module, same pressure position but opposite hands of helix. They provide vibration-free and quiet procedure and may transmit heavy load. However, crossed helical gears are utilized for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also known as screw gears) must have same module, same pressure angle and either same or opposing hands of helix. This kind of gear has application similar to worm equipment; however, worm gear is preferred for steep speed decrease (1:15 to at least one 1:100), whereas crossed helical gears cannot provide quickness reduction beyond 1:2. Various distinctions between parallel helical gear and crossed helical gear receive below in desk format.