As i understand it, a helical gear should be approximately the same efficiency, if not better, than a spur gear for the same power rating (not size) and should in fact be smaller for the same power rating. I have not yet seen a FIRST gearbox that utilizes helical gears and i was wondering why this was? Is the cost actually the limiting factor in this case?
Parallel axis helical gears are stronger and quieter than spur gears of the same size but they are significantly less efficient.
There are 2 main reasons: sliding contact and trusts loads.
There is more sliding contact between the teeth of a helical gear than of a spur gear. This makes the gears quieter but it also makes them less efficient.
The second major factor is that that thrust loads have to be managed. If you put in lousy thrust bearings, you can quickly loose a LOT of efficiency.
I have designed many robots over the years. I have yet to use a helical gear on any of them.
Other folks have other ideas, but for me and mine, I have never seen a case where the pros outweighed the cons.
Joe J.
I agree with Joe on this one. The losses from the thrust angle and sliding friction use some of the precious power (battery) resources of our robots. Helical gears have their place in industry where excess power is available to drive them and strength in a smaller package and a quieter gear train is indicated.
I recommend miter gears if you really want to have a right angle, or any angle (I built a 45 degree gearbox for a machine at work), gearbox. Thrust loads still need to be accounted for but are significantly less.
I agree with Joe on this one also…we used them in 2003 and if you use them with the gear box first supplies you better be sure to put some aluminum plates on the side. The basic nature of the shaft the gear is on is to slide thru the bearing. And if your bearing is tapered it will blow thru the gear box 
. Joe and Andy were also right in saying you lose alot of efficiecy due to the sliding and thrust friction…we had to have 13.5V every time we went on the field or we would run out of juice.
I would also like to recommend worm drive reducers. We used a reducer from Boston Gear this season and they are suppurb with the atwoods.
-Pat
Actually I have always wondered if herringbone gears would be better than helical. From what I have read they should eliminate a lot of the problems from helical.
Could you explain how they eliminate those problems? I see how they eliminate axial thrust, but other than that, they seem to be equivalent to helical gears. They look pretty sweet though 
Could you explain how they eliminate those problems? I see how they eliminate axial thrust, but other than that, they seem to be equivalent to helical gears. They look pretty sweet though
Never mind from what I have read they only eliminate axial thrust. Though that is better than going through 50 plastic gearboxes. I should never reply to something when I am sick. Anyway what are hypoid gears? Just a question since we are talking about helical gears.
Hypoid gears are the helical version of bevel gears. If you’ve ever taken apart a differential, the pinion hypoid comes off of the drive shaft and the ring hypoid gear meshes with it inside of the differential.
-Rc
So vehicles only use helical gears for their compactness and lightness? Do any sort of high performance transmissions have spur gears for higher efficiency? Maybe the good lubrication car transmissions use make helical gears pretty close to spur gears in efficiency.
Vehicles also use helical gears for reduced noise level. As for high performance, Jerico and similiar transmissions used in racing have spur gears. Power transmitted is what counts!
One thing we found to be a problem with spur gears is that there is some “flex” between them. What I mean by this is that if you push another robot the gears seems to have a flexing/twisting/stretching action. We had this occur while trying a new drivetrain using just atwoods at river rage. To solve this effect we knocked the ratio down using a worm gear. With the worm gear we had immediate power and it actually simplified our drivetrain. We would have liked to use a planetary set but we didnt have the resourses this season to get/make some.
-Pat