Mechanical drives are used 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 driven shaft is a 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 a single engagement type rigid drive where motion and power are transmitted through successive engagement and disengagement of tooth of two mating gears. It is inherently free from slide and this it provides continuous velocity ratio (positive drive). It can be used for light duty applications (such as toys, watches, etc.) as well as for heavy duty 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) nonparallel nonintersecting shafts. There exist four basic types of gears and a suitable gear ought to be selected based on the mutual orientation of the driver and powered shafts. Spur gear and helical gear can be applied for parallel shafts. Bevel equipment can be requested two intersecting shafts, which may not necessarily be perpendicular. Worm gear arrangement can be used for the 3rd category (nonparallel non-intersecting shafts). Unlike spur gears that have straight teeth parallel to the apparatus 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 used for perpendicular but nonintersecting shafts.
Accordingly right now there are two types of helical gears-parallel and crossed. Parallel helical gears, the normal one, is used to for power transmitting between parallel shafts. Two mating parallel helical gears should have same module, same pressure position but opposite hands of helix. They offer vibration-free and quiet procedure and can transmit heavy load. On the other hand, crossed helical gears are used for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also known as screw gears) should have same module, same 
pressure position and either same or reverse hand of helix. This kind of gear has app similar to worm gear; however, worm equipment is preferred for steep speed decrease (1:15 to 1 1:100), whereas crossed helical gears cannot offer acceleration reduction beyond 1:2. Various variations between parallel helical gear and crossed helical equipment receive below in desk format.