Optimal change gear ordering?

[Bill Kahn]

I just realized that the same set of 5 change gears on my lathe (PM1030) can be mounted up to 12 different ways and still get the same TPI. Not all the 12 ways are always geometrically possible, but normally there are, indeed, multiple options.

What is the mechanical engineering or practical machining reason for selecting one of the options from another?

I'm thinking some principle like "Minimize the maximum interlocking-gear ratio" or maybe "Minimize the product of the three interlocking-gear ratios". I don't know. But engineers do choose one particular combination from the options.

What is the criteria?

Thanks.

-Bill

(Edited PM25 out, replaced with PM1030. (Dumb mistake))

 

[benmychree]

Engineering would first of all want specific ratios for threading, and also having the selected gears be able to mesh with each other at the center distances between the spindle and change box, using idlers to fill the gap.

 

[higgite]

The gear combinations must fit in the space provided. Not all combinations will.

Tom

Edit: I just saw in your other thread on the subject that you have already discovered this.

 

[mikey]

Wait, isn't the PM25v a variable speed mill?

 

[higgite]

Wait, isn't the PM25v a variable speed mill?

It sure is. Astute observation, Mike.

Tom

 

[mikey]

It sure is. Astute observation, Mike.

Tom

On some occasions, I do wake up not being totally confused. Today might be one of them.

 

[Bill Kahn]

OP here. Sorry, I meant PM1030 lathe. (Yes, I have a PM25 too. Sorry for the confusion. Original post corrected.). -Bill

 

[MrWhoopee]

Just a guess, but I would suspect that gear combinations are chosen to minimize the total number of gears required for the machine. Or did I misunderstand the question?

 

[MikeInOr]

The bigger the gear diameter the less load each tooth will have on it (Simple lever, the farther away you are from the fulcrum the less the load). So bigger gears closer to the driven shaft should technically be stronger and maybe even wear less. (But I really don't think it matters.)

 

[RJSakowski]

If possible, gears are usually selected so the number of teeth are relative primes. This is so each tooth will come into contact with every tooth on the meshing gear to make wear more uniform. This is done on automotive drive trains and other high power systems. This is seldom possible on a lathe's change gear setups though.