Mechanical drives are used 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 for example electric engine, hydraulic turbine, steam turbine, etc.); while, the powered shaft is a 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 a single engagement type rigid drive where motion and power are transmitted through 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 toys, watches, etc.) as well 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) nonparallel nonintersecting shafts. There can be found four fundamental types of gears and the right gear should 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 always be perpendicular. Worm equipment arrangement can be used for the 3rd category (nonparallel non-intersecting shafts). Unlike spur gears which have straight the teeth parallel to the apparatus axis, helical gears possess the teeth in helical type that are cut on the pitch cylinder. Although helical gears are generally used for parallel shafts like spur gears, it may also be utilized for perpendicular but non-intersecting shafts.
Accordingly presently there are two types of helical gears-parallel and crossed. Parallel helical gears, the normal one, is used to for power tranny between parallel shafts. Two 
mating parallel helical gears should have same module, same pressure angle but opposite hands of helix. They provide vibration-free and quiet procedure and will transmit heavy load. On the other hand, crossed helical gears are utilized for nonintersecting 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 hands of helix. This kind of gear has application similar to worm equipment; however, worm gear is favored for steep speed reduction (1:15 to 1 1:100), whereas crossed helical gears cannot offer quickness reduction beyond 1:2. Various differences between parallel helical equipment and crossed helical equipment are given below in table format.