[Newbie] Gear Question

Keith Foor

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Aug 4, 2015
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Does an idler gear play into the ratio of a change gear setup on a lathe other than to reverse the rotation of the shaft? Obviously if the idler gear is attached to a second gear on a common shaft that is a different diameter it will have an effect, but if a 3 gear set, being the bull gear, the idler and the lead screw gear. If the idler is 80 tooth or 120 teeth is there a difference? I know this sounds silly, but I think it makes no difference other than direction of rotation
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The one thing I wanted to add is some of the simple math.
For gears it is a fairly simple to figure out the final RPM of a gear train.
You just multiply by each driving gear and divide by every driven gear.
In a three gear train, if your gears have teeth counts of A, B and C, then the final ratio is

A/B * B/C
and since B is on both the top and bottom it cancels to just A/C.

So
output RPM = (input RPM) * A/C

-brino
 
Thanks guys. That saves me from finding what the specific tooth count on the idler was. I will have to do some checking to see if the 80 tooth is big enough and if not find something. But if it will work at all I can use at least some of what i have once I put some sleeves in them to get them to fit the shaft, or pull the shaft and build it up with weld and turn it to the size of the gears I have.
 
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