Cutting gear teeth: Reducing straight teeth is comparatively easier than slicing helical teeth. Gear milling or gear hobbing can be used to cut the teeth of spur and helical gears. In milling, only two simultaneous motions are desired to cut tooth of spur gears; however, three simultaneous motions are necessary for cutting the teeth of helical gear.
Effect load, vibration and sound: Since teeth of two mating spur gears will come in sudden contact, therefore they encounter a shock or impact load. This also produces significant vibration and noise, which sometimes impose limit on maximum permissible speed of operation. On the other hand, gradual get in touch with between mating teeth outcomes a gradual load on one’s teeth and lower vibration and noise. Thus helical gears can be employed at higher velocity without much problem.
Contact scenario between mating teeth: Spur gears have straight teeth parallel to gear axis. Two mating gears are also mounted in parallel shafts. Thus teeth of two mating spur gears come in sudden get in touch with and the get in touch with is always a type of size equals to teeth face width. On the contrary, helical gears have helical teeth plus they are installed on parallel shafts. So tooth of two mating helical gears can be found in gradual get in touch with. Their engagement begins with a spot and becomes a series and then gradually disengages as a spot. So contact length does not remain constant.
Orientation of 
driver and driven shafts: One fundamental advantage of gear drive over other mechanical drives (like belt or chain drive) is the likelihood to use for non-parallel shafts. However, several types of gear are ideal for varying 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 gear can be utilized for perpendicular non-intersecting shafts. There is a particular kind of helical gear, called crossed helical gear, which can be useful for transmitting power between perpendicular shafts. This is very similar to worm gear; however, crossed helical equipment cannot offer high velocity reduction. Typically, it really is suitable for 1:1 to at least one 1:2 speed ratio (when compared with 1:15 to at least one 1:100 in worm gear). Its application can be limited because of many limitations.