Cutting gear teeth: Trimming straight teeth is definitely comparatively easier than trimming helical teeth. Equipment milling or gear hobbing can be used to cut teeth of spur and helical gears. In milling, just two simultaneous motions are desired to cut teeth of spur gears; however, three simultaneous motions are required for cutting tooth of helical gear.
Influence load, vibration and sound: Since teeth of two mating spur gears will come in sudden contact, so they experience a shock or impact load. This also creates significant vibration and sound, which occasionally impose limit on optimum permissible speed of procedure. On the other hand, gradual get in touch with between mating teeth results a gradual load on the teeth and lower vibration and sound. Thus helical gears may be employed at higher velocity without much problem.
Contact situation between mating teeth: Spur gears have straight teeth parallel to gear axis. Two mating gears are also mounted in parallel shafts. Thus tooth of two mating spur gears come in sudden contact and the contact is always a type of duration equals to teeth face width. On the contrary, helical gears possess helical teeth and they are installed on parallel 
shafts. So the teeth of two mating helical gears come in gradual get in touch with. Their engagement starts with a spot and becomes a series and then gradually disengages as a point. 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 its likelihood to use for nonparallel shafts. However, several types of gear are suitable for various orientations of driver and driven shafts. Both spur gear and helical gears are overwhelmingly utilized for parallel shafts; whereas, bevel gears can be utilized for intersecting shafts and worm gear can be used for perpendicular non-intersecting shafts. There is a particular kind of helical equipment, called crossed helical equipment, which can be employed for transmitting power between perpendicular shafts. This is quite similar to worm gear; however, crossed helical gear cannot provide high velocity decrease. Typically, it is suitable for 1:1 to 1 1:2 rate ratio (in comparison with 1:15 to at least one 1:100 in worm gear). Its application is also limited because of many limitations.