View Full Version : Gear Train Question on Lathe?

japcas

02-26-2008, 02:45 PM

I've had a Jet 13x40 lathe for about 3 years now. I just completed a VFD install and mounted a tachometer so I would know what speed the spindle was operating at. Anyway, while I was mounting the pickup for the tach, I noticed that the tumbler reverse gears were two different sizes. One is a 26 tooth gear and the other is a 21 tooth gear. I may be overlooking something simple but I can't understand how it gets the same feed with two different size gears when the tumbler is reversed. I checked it with an indicator and really couldn't tell any difference in feed rate between the settings for the tumblers. I would have thought the two gears would have to have the same tooth count to give the same feed rate in both directions. Can anyone explain this?

The tumbler is in the neutral position in the picture. When the tumbler is set for right to left feed, the 26 tooth gear engages the spindle, when the tumbler lever is set for left to right feed the 21 tooth gear engages the spindle and feeds the 26 tooth gear and then on down the gear train.

http://i32.photobucket.com/albums/d39/japcas/DSCN20621.jpg

In one of the positions, the 2nd gear acts as an idler, which reverses direction but it will have no effect on the ultimate gear ratio.

japcas

02-26-2008, 03:02 PM

It still looks as if it would change the ratio between the spindle and the lower 50 tooth gear if they didn't have the same number of teeth. The 21 tooth gear is the reversing idler because it's only driven when the lever is set for left to right feeding. The 26 tooth gear is always engaged with the lower 50 tooth gear regardless of tumbler position. I'm not disagreeing with you SGW, I'm just having a hard time wrapping my head around it.

Try this:

In one position you have gear ratios of 50:26 then 26:50.

In the other position you have gear ratios of 50:26, 26:21, and 21:50. If you work that out, you'll discover that the intermediate gears don't matter, the ratio between the input and output is 1:1.

To make the math easier, try 64:32 and 32:64. The 32T gear will turn at twice the speed of the driving 64T gear....but then the 32:64 ratio will cause the output 64T gear to turn at half the speed of the 32T gear...or at the same speed as the input 64T gear. Or try 64:16 and 16:64, or any other intermediate gear, it doesn't matter.

Now, it also doesn't matter if you stick in another idler gear. Say 64:32:16:64.

The 32T gear will turn 2X the speed of the 64T gear, the 16T gear will turn at 2X the speed of the 32T gear (or 4X the speed of the 64T gear) but then the final 16:64 ratio will cause the output 64T gear to turn at 1/4 the speed of the 16T gear...or at the same speed as the input 64T gear.

John Stevenson

02-26-2008, 03:34 PM

In a straight train of gears no matter how many the only ones that count are the driver and driven, In other words first one and last one.

If you have a compound train, that where you have two side by side on the same spindle and locked to one another then you have to bring these into the equation.

.

DaveR

02-26-2008, 03:38 PM

You can look at it this way: when the 50t gear on the spindle advances one tooth, all 4 gears advance one tooth (including the lower 50t gear) regardless of the gear (or gears) between the two 50t gears. Idler gears don't affect gear ratios - but they can reverse direction.

My old (75+ years) Seneca Falls lathe is the same way - the two tumbler gears are different size.

Dave

japcas

02-26-2008, 05:12 PM

Thanks for the replies. I knew some one here would be able to shed some light. Gears calculations are really one of my strong points.

topct

02-26-2008, 06:18 PM

In one position you are driving through what would be an idler gear. Or one to one. In the other position you are driving through two gears of different counts, not only reversing, but changing the ratio.

Why?

japcas

02-26-2008, 06:30 PM

In one position you are driving through what would be an idler gear. Or one to one. In the other position you are driving through two gears of different counts, not only reversing, but changing the ratio.

Why?

That is the question I had also. DaveR says his old lathe has two different sized gears also. I checked my feeds in both directions with an indicator mounted to the lathe bed touching the carriage. I couldn't measure any difference in feedrates no matter which direction the tumbler lever was in. It must work, I just couldn't figure out how it was doing it.

BadDog

02-26-2008, 06:54 PM

As already stated, intermediate gears are ALWAYS 1:1. One tooth moves on input, that matches one on the output side. You could take out an intermediate 20 tooth gear, and replace it with a 2000 tooth gear (if there were room) and it would make no difference.

The only time you have a "ratio" is when you have a coupled differential. In a straight gear train, that is only the first gear and the last gear. Tthey are coupled to the I/O shafts so their relative turning rates matter. Or, if you have coupled gears of differing sizes within the gear train. For examples of coupled gears, see the the typical metric transposing gears as well as the low range transposer used to get really fast feeds in many lathes (like my Rockwell).

camdigger

02-26-2008, 07:01 PM

1.) there is no speed reduction induced by the idlers gears

2.) because there is no speed change, their size is irrelevant in terms of input speed vs output speed.

3.) the only determining factors for gear size is center distance of the driven and driver gears.

4.) the function of the idlers is to allow for reversal of the direction of rotation and to allow for #3.

5.) the gears are likely sized by other considerations like stacking for speed reduction mentioned in a previous post and /or space limitations of the design.