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 part 
of prime 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 gear drive, belt drive, chain drive and rope drive. A equipment drive is one engagement type rigid drive where movement 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 offers continuous velocity ratio (positive drive). It can be utilized for light duty applications (such as for example toys, watches, etc.) as well as for durable applications (such as gear box of machinery, marine drive, etc.).
Driver and driven shafts might have 3 mutual orientations, namely (i) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There exist four simple types of gears and the right gear ought to be selected predicated on the mutual orientation of the driver and powered shafts. Spur equipment and helical gear can be applied for parallel shafts. Bevel gear can be requested two intersecting shafts, which might not always be perpendicular. Worm gear arrangement can be used for the third category (nonparallel non-intersecting shafts). Unlike spur gears which have straight tooth parallel to the apparatus axis, helical gears have tooth in helical form 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 nonintersecting shafts.
Accordingly presently there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, is utilized to for power transmitting between parallel shafts. Two mating parallel helical gears must have same module, same pressure position but opposite hand of helix. They offer vibration-free and quiet operation and will 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 position and either same or opposite hands of helix. This kind of gear has program similar to worm gear; however, worm equipment is preferred for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot provide swiftness reduction beyond 1:2. Various variations between parallel helical equipment and crossed helical gear receive below in desk format.