Mechanical drives are used to transmit motion, torque and power from a driver shaft to driven shaft. The driver shaft, in majority of the cases, is part of prime mover (such as 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 equipment drive is 1 engagement type rigid drive where movement and power are transmitted by way of successive engagement and disengagement of teeth of two mating gears. It is inherently free from slip and this it provides constant velocity ratio (positive drive). It can be utilized for light duty applications (such as for example 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 (we) parallel shafts, (ii) intersecting shafts and (iii) non-parallel nonintersecting shafts. There can be found four simple types of gears and a suitable gear should 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 requested two intersecting shafts, which may not necessarily be perpendicular. Worm gear arrangement is used for the 3rd category (nonparallel nonintersecting shafts). Unlike spur gears that have straight the teeth parallel to the gear axis, helical gears possess tooth in helical type that are cut on the pitch cylinder. Although helical gears are commonly utilized for parallel shafts like spur gears, it may also be utilized for perpendicular but nonintersecting shafts.
Accordingly 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 angle 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 used 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 opposite hands of helix. This kind of gear has software 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 quickness reduction beyond 
1:2. Various variations between parallel helical equipment and crossed helical equipment are given below in desk format.