Turning down the OD of a gear

[Chris is me]

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.

I do see the problems with the actually trimming the OD, depending on the team's machining expertise and capabilities it could be quite difficult to do. correctly

Adding three thou is a psuedo-standard in FRC. It's a good rule of thumb for custom gearboxes. It's also one way to artificially shift your tolerance band.

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.

Yeah... We had a 6(!) stage arm transmission this year and we had over seven degrees of slop in our original arm. We added the three thou but all of the loose hexes added way up. All of the shock loading of the arm flopping around in the straight up position led to multiple gear failures.

So yeah, I think we're going to .001 next year. I've heard people run zero backlash with Vex gears with no problems, but I'd like a little bit more space for grease in there, and this avoids having to break in gearboxes for any extended period of time. (The kids made me promise I wouldn't push for a geared arm again too)

[Tristan Lall]

My experience is similar to Eric's: the addition of up to 0.003 in was necessitated by gearboxes being produced by hand by inexperienced machinists on milling machines of uncertain provenance. If you're CNCing the gearbox plates, you can in all likelihood get away with less. But remember that the added centre distance isn't just for position of the holes, it's also for runout of the gear and perpendicularity of the axle. (For example, in 2006, 188 had a bad EDM job that screwed up some expensive hardened gears, and necessitated a lot of rubbing compound to "solve" the runout.)

I would be interested in hearing from some of the VexPro people about the design and tolerances of their gears. How far from standard tooth profiles can they be expected to deviate at perfect form (e.g. due to profile shifting, radiusing, etc.), and how much variance is introduced in manufacturing?

[chadr03]

Turning the OD of a gear can have some negative issues as people have stated you are removing 10% of the active tooth depth. Depending on the tooth counts used this can have a much more dramatic effect than a 10% reduction in torque capacity. One of the factors that drive the rating of a spur gear is known as the contact ratio. It is essentially a measure of what percentage of time one tooth is in contact verses two teeth in contact. When you truncate the OD of one gear you shift that contact ratio closer towards one tooth in contact more often than two teeth. You could imagine when the load is shared between multiple teeth more more load can be transmitted.

Additionally many times gears have what is known as tip relief. Where the involute is modified slightly on the tips of gears to allow them to enter and exit mesh more smoothly especially when a load is applied. I do not know if FRC gears have tip relief machined in, but if they do you will remove it and possibly cause the gears wear more quickly and be noisy.

How many teeth are on the CIM pinion? Would it be possible to use a larger pinion? If you are trying to keep a specific ratio vexpro might make just what you need. They appear to have some CIM pinions with modified addendums or shifted centers. (The modified addendums will allow you to add .025" of center distance without the increase in backlash or shaving the OD of your gears basically lets you set the center distance up as if the gear arrangement had an extra tooth).

See page 9.

http://content.vexrobotics.com/vexpr...ence_Guide.pdf

[hrench]

This is very case-dependent. Just remember that the teeth are designed to be in contact at the pitch-circle diameter, not at the outside diameter. So cutting the outside will have no effect on many systems.

So as long as you have 'regular' gears, not some kind of ten-tooth pinion or something, the outside two percent of the tooth should never be needed as the next tooth is engaged long before it disengages.

Also, technically, involute teeth aren't sliding, they're 100% rolling contact, but if you change the center-to-center distance, even by adding a measley 3 thousandths, you'll have some sliding contact and less gear efficiency, more wear.

But as someone above mentioned, what is the gear pitch?--if the teeth are very small, 0.020 could be alot. if they're big teeth, 0.020 is less concern.

[DonRotolo]

Originally Posted by hrench

Also, technically, involute teeth aren't sliding, they're 100% rolling contact

Quoted for truth: It may not be obvious unless you have studied gears, but the teeth if an involute gear don't just push on each other, they actually ROLL across each other. The sliding action (which leads to wear and wants a lubricant) is almost zero. That's one of the reasons center-to-center distances are so very important for gearboxes.

[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.

[hrench]

Originally Posted by chadr03

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. ...

...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.

Some really great discussion here and the graphic is amazing.

I think I'll add that chadr03 pointing out that the backlash will increase with added center-to-center spacing is correct because the teeth are no longer engaging along the theoretical pitch circle (or near it as the graphic shows) but instead, further out on the tooth--where the two teeth don't fit as well together. Also note, the teeth are narrower out there, hence weaker.

Gears are built with a bit of undercut already to take up a tolerance if the centers are too close. I'm still not an advocate of moving the center distance apart.