Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to powered shaft. The driver shaft, in most the cases, is part of prime mover (such as for example electric motor, hydraulic turbine, steam turbine, etc.); while, the driven shaft is 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 one engagement type rigid drive where motion and power are transmitted by way 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 toys, watches, etc.) as well as for heavy duty applications (such as gear package of machinery, marine drive, etc.).
Driver and driven shafts might have three mutual orientations, namely (we) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There exist four fundamental types of gears and the right gear should be selected predicated on the mutual orientation of the driver and driven shafts. Spur gear and helical gear are applicable for parallel shafts. Bevel equipment can be requested two intersecting shafts, which might not always be perpendicular. Worm gear arrangement is used for the third category (nonparallel nonintersecting shafts). Unlike spur gears which have straight the teeth parallel to the apparatus axis, helical gears possess tooth in helical form that are cut on the pitch cylinder. Although helical gears are generally used for parallel shafts like spur gears, it can also be used for perpendicular but nonintersecting shafts.
Accordingly generally there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, is used to for power tranny 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) should have same module, same pressure position and either same or reverse hands of helix. This type of gear has software similar to worm equipment; however, worm equipment is 
favored for steep speed reduction (1:15 to 1 1:100), whereas crossed helical gears cannot provide speed reduction beyond 1:2. Various differences between parallel helical equipment and crossed helical gear are given below in table format.