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 electric motor, hydraulic turbine, steam turbine, etc.); while, the powered shaft is part of the machine device. There exist four simple mechanical drives, namely gear drive, belt drive, chain drive and rope drive. A gear drive is a single engagement type rigid drive where movement and power are transmitted by means of successive engagement and disengagement of teeth of two mating gears. It really is inherently free from slip and this it offers continuous velocity ratio (positive drive). It can be utilized for light duty applications (such as for example toys, watches, etc.) aswell as for heavy duty applications (such as for example gear package 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 exist four fundamental types of gears and the right gear should be selected based on the mutual orientation of the driver and driven shafts. Spur equipment and helical gear are applicable for parallel shafts. Bevel equipment can be requested two intersecting shafts, which may not always be perpendicular. Worm gear arrangement is used for the third category (nonparallel nonintersecting shafts). Unlike spur gears that have straight 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 right now there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, can be 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 offer vibration-free and quiet operation and may transmit heavy load. However, 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 reverse hands of helix. This type 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 provide velocity reduction beyond 1:2. Various variations between parallel helical gear and crossed helical equipment are given below in desk format.