Need Some Spur Gear Help

Dan_S

Dan_S

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I;m doing some prep work for an up coming project that will involve gears. Other than the basic stuff, i don't know a whole lot about gears. Something I'm struggling with, is if you know the center distance between 2 gears, how do you calculate the tolerance for that distance?
 
Bob, I've been searching for a few hours now, and i can't find anything about center to center distance tolerances. I've seen stuff about backlash tolerances built into the gears, but nothing about how accurate the center distance between 2 gears needs to be.
 
Hopefully a proper gear guru will show up and give a proper answer.
Until then, Why do you need to know the tolerance for center distance? Is there any reason you can not just set the tolerance to something arbitrary and adjust the fit while you are doing your project?
Any error in center distance directly affects backlash, and at the end of the day back lash is what is important.
Since spacing two shafts a precise distance apart is much simpler than machining a gear to a precise pitch circle, most stuff is designed to deal with gears that are not accurately made. Manufacturers have all sorts of tricks they use to get gears to mesh right, anything from lapping the gear to adjusting the center distance.
If this is a one off project, and getting your gears to mesh absolutely right, then do what clockmakers do - make a depthing tool. You then treat the gear space as having 0 error.
 
How accurate does what you are making need to be? We deal with gearing center to center issues whenever we swap out the change gears on our lathes. Mostly we just say that we need some backlash but not too much, and never even think of quantifying it. At most, we stick a random scrap of paper between the teeth and call it good. I am sure some gearing setups require strict accuracy tolerances, but that would be related to speed, torque, and in all cases, as Joshua said, ultimately backlash. You can control backlash while making gears and simply make them fit, if it is not a super critical application. You can also often make the center to center distance adjustable, like what we do with our lathe change gears or by adding a cam mount for one of the gear's center.
 
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what I'm working on is actually are a couple change gear banjos for my lathe.

Because of how my lathe is designed, it has something like 600,000 possible gear combinations. I'm working on a program to twiddle that down to whats mechanically possible, and then organizing the ratios so that at the very least I might not have to move the studs, just change the gears on them.

To work out all the math, I was hoping to get an idea of what kind of tolerance I needed to hold, because I have some gear sizes that are very close together, like a 75:72 & 66:80.
 
Dan_S said:
it has something like 600,000 possible gear combinations
Click to expand...

......at a dollar per combination you probably got a good deal on that lathe!
-brino
 
Dan_S said:
what I'm working on is actually are a couple change gear banjos for my lathe.

To work out all the math, I was hoping to get an idea of what kind of tolerance I needed to hold, because I have some gear sizes that are very close together, like a 75:72 & 66:80.
Click to expand...

It almost sounds as if you are confusing gear tolerances with gear ratios. In the post above you quote "gear sizes" of "75:72" which is actually the ratio between 2 gears. Because the tooth-to-tooth engagement does not slip with a gear set (as a V-belt might), you will have that ratio no matter how accurately or poorly the gears are made. Generally speaking, poorly made gears would simply make more noise.

Tight tolerance precision gears not only control the shape of the tooth and distance from the center of rotation, but the tooth-to-tooth spacing. A lathe really doesn't care about most of that. Suggest you look up "AGMA gear tolerances". Most off-the-shelf gears are AGMA Grade 3 to 5 and that's all gear sets on a lathe need.

You would only need super precise Grade 8 to 10 gear tolerances if you needed extremely precise position information taken at a slower speed from an adjacent shaft, such as a bi-directional turntable with synchro position indicator. In such a case, very, very low backlash could be tolerated due to the bi-directional rotation, and tooth-to-tooth spacing must be precise so that the synchro doesn't speed up and slow down when the turntable is in fact turning at a constant speed.

Does any of that help ?
 
Wobbles said:
It almost sounds as if you are confusing gear tolerances with gear ratios. In the post above you quote "gear sizes" of "75:72" which is actually the ratio between 2 gears.
Click to expand...
what I meant is the the gear ratios 75:72 & 66:80 will have very similar center distances (within say 15 thousands of each other).


Wobbles said:
Suggest you look up "AGMA gear tolerances". Most off-the-shelf gears are AGMA Grade 3 to 5 and that's all gear sets on a lathe need.

Does any of that help ?
Click to expand...

I think so, the AGMA search term, is turning up some stuff i think i can work with.
 
Dan_S said:
what I meant is the the gear ratios 75:72 & 66:80 will have very similar center distances (within say 15 thousands of each other).
Click to expand...

That would depend upon the gear tooth size which you did not quote.
 
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