Turning down the OD of a gear

[dtengineering]

And if anyone reading this thread hasn't had a chance to study a little bit about how gears work... take a look. We generally tend to look at gears and say, "Oh, it's just a gear." and take them for granted, but the math and science that goes into gear design is truly fascinating. The modern gear is the product of centuries of engineering evolution.

I know I've still got a lot to learn about how gears work, wear and are manufactured.

Jason

[Rauhul Varma]

Originally Posted by hrench

Just remember that the teeth are designed to be in contact at the pitch-circle diameter, not at the outside diameter.

Isn't that a simplification of what the contact actually looks like. Gears contact along the line of action, which is some angle (the pressure angle) off the line tangent to both pitch diameters.

Originally Posted by hrench

So as long as you have 'regular' gears, the outside two percent of the tooth should never be needed as the next tooth is engaged long before it disengages.

Wont removing that outside bit increase the stress per tooth?

[chadr03]

Originally Posted by Rauhul Varma

Wont removing that outside bit increase the stress per tooth?

Absolutely.

Involute gears do not simply contact at the pitch diameter the contact moves from the tip to the root along the line of action as can be seen on the previous graphic. Gears all have sliding contact and most have at least one place on the line of action that has pure rolling contact. A gear mesh has three distinct actions during mesh. When a tooth begins in contact it is called the approach action there is a combination of sliding an rolling on the mating surfaces. When the contact along the line of action passes through the pitch diameter there is an instant of pure rolling. Then the contact goes into recess action where there is a combination of sliding and rolling once again. A shift in center distance will not change the fact that the contact will move from the tip to the root along the line of action it will simply make the line of action shorter and change the amount of backlash.

[Rauhul Varma]

Originally Posted by T^2

I think this practice is also becoming slightly less common because of the prevalence of VEXPro gears. Their hex is oversize, while the shafts are undersize; this leads to slop. Adding .003 to c-c can lead to very noticeable backlash.

We did notice that too, but since we machine our own hex, we make it oversize to mate their bores nicely and haven't had those issues.

[DampRobot]

Originally Posted by Rauhul Varma

Is it really that common in FRC to add 2-3 thou? (I honestly don't know) For every gearbox I've designed I've only added half a thou and haven't had any problems with wear.

Can you really hit a half thou accuracy? Even running finish and float passes and really getting to know the machine, program, and material, I can usually only hit around 1-2 thou accuracy on our CNC for stuff like hole to hole distances.

Yeah, 3 thou is the standard number. I've heard that it mostly came into being in a time when FRC gears were pretty low quality, VP gears nowadays are high enough quality that you don't really need the adder. Another reason for the adder is that if you have a bit of runout on your shafts, or poor quality gear teeth, and you put the gears at the nominal pitch diameter, there'll be places in the rotation where the gears get sort of jammed together, and you lose efficiency. Adding a few thou helps make sure that the gears aren't getting jammed into each other in case of tolerance issues somewhere else.

[asid61]

Quick question to the OP:
Why does this matter so much? If it's for clearance to spacers, just move the spacers. And if it's for ground clearance, forget it. You will need more ground clearance than 0.02" due to the tread wear, compression, and random stuff on the ground.

CIMs are always 2.536" outer diameter unfortunately. You could look into RS-775, but if this is for drive CIMs would work better.

[Tristan Lall]

Originally Posted by magnets

I'm designing a gearbox that would work really nicely if a gear with an OD of 1.700 had an OD that was .020 less. How bad would it be to to turn down the OD of the gear .020? Also, does anybody know exactly the OD of a CIM motor? The website claims 2.54 max, but do they come in under this?

By the way, what are the dimensions and diametral pitch of this gear? It's kind of important to know how much of the tooth is being lost. But maybe that's part of the solution: you could go with the largest teeth that will work. In most cases, the limit will be set by the undercut of teeth on the pinion as you go down to small diameters. (To improve that, prefer a higher pressure angle given gears of a certain diameter and the same pitch.)