Cutting gear teeth: Trimming straight teeth can be comparatively easier than cutting helical teeth. Gear milling or gear hobbing can be used to cut tooth of spur and helical gears. In milling, just two simultaneous motions are desired to cut teeth of spur gears; however, three simultaneous motions are necessary for cutting tooth of helical gear.
Effect load, vibration and sound: Since teeth of two mating spur gears comes in sudden contact, so they encounter a shock or impact load. This also generates significant vibration and noise, which sometimes impose limit on optimum permissible speed of operation. On the contrary, gradual get in touch with between mating teeth results a gradual load on one’s teeth and lower vibration and sound. Thus helical gears may be employed at higher rate without much problem.
Contact situation between mating teeth: Spur gears have directly teeth parallel to equipment axis. Two mating gears are also installed in parallel shafts. Thus teeth of two mating spur gears come in sudden contact and the contact is always a line of length equals to teeth face width. On the contrary, helical gears possess helical teeth and they are mounted on parallel shafts. So teeth of two mating helical gears come in gradual contact. Their engagement starts with a spot and becomes a line and then steadily disengages as a spot. So contact length does not remain constant.
Orientation of driver and driven shafts: One basic advantage of equipment drive over other mechanical drives (like belt or chain drive) is the possibility to use for nonparallel shafts. However, various kinds gear are suitable for various orientations of driver and driven shafts. Both spur gear and helical gears are overwhelmingly used for parallel shafts; whereas, bevel gears can be used for intersecting shafts and worm equipment can be utilized for perpendicular nonintersecting shafts. There is a particular kind of helical equipment, called crossed helical gear, which can be employed for transmitting power between perpendicular shafts. This is quite similar to worm gear; however, crossed helical equipment cannot offer high velocity decrease. Typically, it is suitable for 1:1 to 1 1:2 speed ratio (when compared with 1:15 to 1 1:100 in worm gear). Its application is also limited because of many limitations.