12" Commercial Reduction Gearing

[miken]

Hi, I know this is a very old thread but I'm hoping someone is still around and can help me understand this. I have an Atlas 3990 undermount with QCGB. If the QCGB is driven by a 40 I can understand how adding the 20 will directly cut the feed rate in half. I also have read that any combination (64/32, 48/24, 40/20) that will physically fit together will also cut the feed rate in half. What I cant seem to understand is why the additional number of teeth doesn't matter? With all other gears remaining the same, input 32/16 and 48 at the QCGB, why does switching the 40 to a 48 not change the original rate. Those additional 8 teeth don't matter? Seems like only a 40/20 should work, but I know the other options work as numerous people have verified. Thanks for any help, have a great July 4th. Mike -

 

[Rob]

when it is a single gear on a shaft it is 1 to 1 ratio no mater how many teeth. when you have a compond gear ie 2 different tooth counts the change is the ratio of the 2 gears.

 

[wa5cab]

I sorta struggle too with understanding why changing the tooth count of any of the single gears causes no change to the overall ratio, but it doesn't.
Consider only three gears - suppose that in the gear train you had a 32 tooth gear on the spindle driving a 32 tooth gear driving a 32 tooth gear on the lead screw. It is easy to be able to say that the three gears will all turn at the same RPM as will the spindle and the lead screw. Now change the 2nd gear to 16 teeth. It is intuitively obvious that the second gear will turn faster than it had before the change but that the 1st and 3rd gears and the spindle and lead screw will still turn at the same speed.

 

[miken]

I sorta struggle too with understanding why changing the tooth count of any of the single gears causes no change to the overall ratio, but it doesn't.

Thanks Rob and wa5cab, I appreciate your replies. I am going to accept it and move on.