Helical Inline Gear Motors Helical gears are not specifically much like worm or bevel gearbox systems. They are in fact an alternative solution to spur gears where in fact the tooth are parallel to the axis of the gear itself. For example, within an inline application, you could have either spur OR helical gears. An inline helical gearbox engine will most likely be quieter and will carry higher loads than motors utilizing a spur gearbox. They could be more costly in initial cost, and manufacturers have to take into account 
axial thrust. However, the fact that the helix position can vary from 15 to 30 degrees allows for flexibility in terms of design. They are found in in-line applications along with 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. In addition, it produces less friction and permits more teeth to be engaged simultaneously as the teeth roll across each other.
There are myriad types of gearing. Some manufactures use spur gears helical spiral bevel gear motor rather than helical gearing, for example. However, there is not as much surface contact, as mentioned above and consequently there is more noise and less torque transfer possible.
Another system where noise is definitely common are planetary equipment systems, where the one gear is completely within the other. They also require grease lubrication and they must be constructed with expensive materials to be able to maintain a long-life and bearing integrity. An easier variant, internal gearing, is suffering from the same problems and can only be used with parallel shaft.
Finally, hypoid gearing may be best understood as similar to bevel gearing, but with the gear axes not intersecting. The sliding actions can create huge amounts of heat and the alignment needs great care.