Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to driven shaft. The driver shaft, in most the cases, is a part of prime mover (such as for example electric motor, hydraulic turbine, steam turbine, etc.); while, the powered shaft is part of the machine unit. There exist four simple mechanical drives, namely gear drive, belt drive, chain drive and rope drive. A equipment drive is 1 engagement type rigid drive where motion and power are transmitted by means of successive engagement and disengagement of the 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 package of machinery, marine drive, etc.).
Driver and driven shafts may have 3 mutual orientations, namely (we) parallel shafts, (ii) intersecting shafts and (iii) non-parallel nonintersecting shafts. There exist four basic types of gears and a suitable gear should be selected based on the mutual orientation of the driver and powered shafts. Spur gear and helical gear are applicable for parallel shafts. Bevel equipment can be requested two intersecting shafts, which might not necessarily be perpendicular. Worm gear arrangement is utilized for the 3rd category (nonparallel nonintersecting shafts). Unlike spur gears that have straight teeth parallel to the apparatus axis, helical gears possess teeth 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 used for perpendicular but non-intersecting shafts.
Accordingly generally there are two types of helical gears-parallel and 
crossed. Parallel helical gears, the common one, can be used to for power transmission between parallel shafts. Two mating parallel helical gears must have same module, same pressure angle but opposite hands of helix. They provide vibration-free and quiet procedure and may transmit heavy load. On the other hand, crossed helical gears are utilized for nonintersecting but perpendicular shafts. Two mating crossed helical gears (also called screw gears) must have same module, same pressure angle and either same or opposite hand of helix. This type of gear has program similar to worm equipment; however, worm gear is preferred for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot provide quickness reduction beyond 1:2. Various differences between parallel helical equipment and crossed helical equipment are given below in table format.
