Gear Face Width

[Jared]

Originally Posted by Orion.DeYoe

I'm curious about the face width of gears in FRC. Especially as it relates to aluminum vs steel and diametral pitch.
Basically every 20dp gear in FRC is .5 inch face width. With that thickness there is certainly no reason to use steel gears.
However how much load can an aluminum gear stand at .5 inch face width? And how thin could they be made before becoming too weak for a relatively high load FRC application? Also, does there become a point where a steel gear with a very thin face width (so that the weight is comparable to a thicker aluminum gear) becomes as strong as a .5 inch thick aluminum one. At that point it would make sense to use steel as you're not sacrificing weight or strength but you're saving space.
On the same note, what if one were to use 32dp gears (or other comparable small tooth size) with a thicker face width instead? You would save a lot more weight and space than you would by making 20dp gears thinner.

Most FRC 20 DP gears have .375" face width, not .500", which is the thickness of the actual gear teeth. Vex sells almost all of their gears with little spacers on them that ride on the inner race of the bearing so that the gear's overall thickness is .500".

The gears we use see a huge range of torque. For some applications with the width they give us, aluminum is not enough. Drivetrains can see some very serious loads, upward of 120 ft-lbs at the wheel.

That being said, the first stage of reduction could possible be much smaller 32 DP gears.

If you're interested in getting more strength out of a gear, you can look at gears with a higher pressure angle. Most gears we use are 14.5 degree pressure angle, but 20 degree pressure angle gears are really common too. The higher pressure angle is a noisier and less efficient gear, but for FRC, the difference is not noticeable.

32 DP gears can't handle as much torque as 20 DP gears, but they work very well for the first stage of a gear reduction. I believe some teams have run 32 DP gears in the first stage of their drive gearbox, but I'm not positive on this.

If your main goal is to save weight, getting all non standard gears is a expensive and complicated way to do it. You'd likely get the gears in steel and have to do lots of machining to lighten them, and you'd additionally have to worry about putting the right keyway in them.

There are likely cheaper and far easier solutions to reducing weight. An entire 2 speed shifting gearbox, complete with side plates, mounting hardware, shifting parts, bearings, and shafts weighs under 2 lbs, and can be made lighter with more aggressive pocketing on gears.

[Orion.DeYoe]

Totally forgot about the hubs on the side. Not sure how that slipped my mind actually...

I'm thinking more along the lines of a team making it's own gears. I'm not asking if this is a good idea or not.
I just want to know if the gear sizes out there (20dp, aluminum, .375 face width) are the most efficient combination of DP, material, and face width or if there is noticeable room for improvement.

[hrench]

When I've designed gears for machines, I've followed the criteria from 'Design of Machine Elements' by Spotts; there are two criteria that need to be worked out before you decide on material, diameters and facewidth.

First you need to work out the bending capacity of the teeth. without copying the whole text and the equations, I'll say that the bending is rarely the controlling failure.
Second, you work out the 'tooth capacity in contact stress' This involves not only geometry but also the moduli of elasticity of the sets of gears in the train. If you've ever taken apart commercial gear trains, you'll often see that the output gears may be steel but the earlier gears in the train may be brass or even plastic. The equations show you that different moduli make for better wear /stress capacity.
But you can get more stress capacity by increasing the face-width, so sometimes this is a substitute for using varied materials.

So the point is, if you want to 'design' gears, I recommend actually running through the equations and see where you are. A 'standard' face width may be fine, but just assuming probably isn't. if you need these equations, I can get them for you.