Cutting gear teeth: Trimming straight teeth is certainly comparatively easier than cutting 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 wanted to cut tooth of spur gears; however, three simultaneous motions are necessary for cutting teeth of helical gear.
Impact load, vibration and noise: Since teeth of two mating spur gears will come in sudden contact, so they encounter a shock or impact load. This also generates significant vibration and noise, which occasionally impose limit on maximum permissible speed of procedure. On the contrary, gradual contact between mating teeth results a gradual load on the teeth and lower vibration and sound. Thus helical gears may be employed at higher quickness without much problem.
Contact scenario between mating teeth: Spur gears have directly teeth parallel to gear axis. Two mating gears are also mounted in parallel shafts. Thus teeth of two mating spur gears can be found in sudden contact and the contact is always a line of length equals to teeth encounter width. On the other hand, helical gears possess 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 starts with a spot and becomes a range and then steadily disengages as a spot. So contact length will not remain constant.
Orientation of driver and driven shafts: One fundamental advantage of equipment 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 various orientations of driver and driven shafts. Both spur equipment and helical gears are overwhelmingly utilized for parallel shafts; whereas, bevel gears can be used for intersecting shafts and worm equipment can be used 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 very similar to worm gear; however, crossed helical gear cannot provide high velocity decrease. Typically, it really is suitable 
for 1:1 to at least one 1:2 acceleration ratio (as compared to 1:15 to 1 1:100 in worm gear). Its application is also limited due to many limitations.