Helical Inline Gear Motors Helical gears are not specifically comparable to worm or bevel gearbox systems. They are actually an alternative solution to spur gears where the teeth are parallel to the axis of the gear itself. For example, within an inline application, you can have either spur OR helical gears. An inline helical gearbox motor will usually be quieter and will carry higher loads than motors utilizing a spur gearbox. They may be more expensive in initial cost, and manufacturers need to take into account axial thrust. However, the actual fact that the helix angle can vary from 15 to 30 degrees allows for flexibility with regards to design. They are used in in-line applications in helical spiral bevel gear motor addition to parallel shaft applications.
The benefit of helical gears is that it produces a rolling action, is quieter, and has less vibration in comparison to spur gears. It also produces much less friction and permits more the teeth to be engaged simultaneously as one’s teeth roll across each other.
There are myriad types of gearing. Some manufactures use spur gears instead of helical gearing, for instance. However, there is not as much surface contact, as observed above and as a result there is more noise and much less torque transfer possible.
Another program where noise is definitely common are planetary equipment systems, where the one gear is totally within the other. They also require grease lubrication and they must be built with expensive materials in order to maintain a long-lifestyle and bearing integrity. An easier variant, internal gearing, is suffering from the same 
problems and can only be utilized with parallel shaft.
Finally, hypoid gearing may be best understood since similar to bevel gearing, but with the gear axes not really intersecting. The sliding action can create huge amounts of warmth and the alignment requires great care.