How to Determine Motor Speed?

ShagDog said:
I have the motor show in the attachment on a South Bend 9 with flat belt drive cone pulleys. For Rpm, the motor is labeled "1725/1140". I did a calculation on the current set-up based on 1725 Rpm which if my calculations are correct(?) I come up with the slowest spindle speed of about 415 rpm. The countershaft drive pulley has 2 steps, and it is on the smaller step. The motor drive pulley is 3" diameter.

If the motor is running at 1140 rpm, I think I would come up with a spindle rpm of about 274, again, if my calculations are correct (?).

How do I tell if the motor is set up to run at 1725 or 1140 rpm; and if at 1725, how would I reduce it to 1140 rpm? Also, would there be any issue in running it at 1140?
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That is cooler motor .
They will stall a lot
A 9A takes a 1725 rpm.
May look a DC motor for viable speed

Dave
 
Well I am a little leery of working on electrical, so I decided to forego the option of slowing down the motor if at the higher speed (still don't know which rpm it is set at), unless I was able to find the wiring diagram specific to this old motor.

I found a better solution (I already knew it would be better but was being penny wise and pound foolish). I found a smaller 2" pulley on another motor I had laying around that fit the Dayton motor shaft. I was able to pull off the 3" pulley of the Dayton and replace it with the 2" pulley. I was able to line up the 2" motor pulley to the 10" (low speed range) countershaft pulley (instead of the 9" high speed range) and was able to get the speed down to the lower speed range of the 12 speed drive (South Bend Model 644-R). I wanted this lower speed range because the South Bend book, How to Run a Lathe, states in a footnote that "When using high countershaft speed, back gears should not be engaged." Again, I don't know which rpm the motor is set at; so, I cant' determine spindle speed. However, at least the spindle speed is now in the lower range due to the pulley change. I need to invest in a tach.

Now I need to search whether the speed ranges listed by South Bend are based on a 2" motor pulley.
 
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ShagDog said:
Thanks. Ordered it. Had to add to cart to get free shipping, and I ordered these machine lights again. They work great.
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I have a couple of those lights, and I agree that they are really useful. For the tach: I didn't find that the adhesive on the included reflective tape was very useful. I had some SOLAS reflective tape (marine version of truck reflector tape) and a small piece of that worked well. If the rotating surface is reflective (like the mill quill) I just wrap black electrical tape around and then cut a 'slot' in the tape to leave a reflective area.
 
ShagDog said:
I wanted this lower speed range because the South Bend book, How to Run a Lathe, states in a footnote that "When using high countershaft speed, back gears should not be engaged."
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They do state that. That's based on having the original two step motor pulley. The countershaft is good for the high speed option obviously, but if you want to "slightly" overcomplecate something...

If you DO use backgears on the high speed range, but NEVER, EVER allow the flat belt to go into the fastest of the three positions, you'll accomplish the same goal, which is to slow the lathe and not send the backgear "stuff" into geosynchronous orbit.

That does take dilligence on your part though.

ShagDog said:
Again, I don't know which rpm the motor is set at; so, I cant' determine spindle speed. However, at least the spindle speed is now in the lower range due to the pulley change. I need to invest in a tach.

Now I need to search whether the speed ranges listed by South Bend are based on a 2" motor pulley.
Click to expand...

The original two step pulley on the motor shaft had (obviously) two sizes. The larger being something shy of three and a half (3.4 maybe but I use three and a half for math in my head), and the smaller one being two inches. Two point zero something insignificant, for all intents and purposes, it's two inch. As old as these are, it's probably worn to two inches or less anyhow.... If you can get the two inch motor pulley to drive the larger countershaft pulley, then at the high speed of your motor, that's what South Bend would call "low range".

I'd suggest that the odds are high of the motor being wired at 1740. Setting it slower will cut it's horsepower tremendously, with little if any gain. The lathe already (with a motor at 1740 rpm) will go down to something near enough to 50 RPM at the spindle. There's not really a lot of opportunity to turn work that's gonna require any less than that.

Once you redo my math yorself and get where you're comfortable engaging the back gears (And disengaging the bull gear pin at the same time... :cool: ) stick a bright red sticker on the chuck. and the flat belt in low, then turn on the lathe and watch the sticker pass by. The 1740 option will give you a speed that's very easy to mistake for one sticker pass in one second. (Technically just less, but go with that). The 1140 option will give you a spindle speed that's far easier to mistake as one sticker pass per two seconds. (That's not it either, but it's close, go with it). No those aren't dead accurate, but you're not finding a speed from scratch. You've got two to pick from. Go with the closest one, you'll confirm your motor speed, and you'll know where to start your real math from.

And no, this does NOT beat having a cheap tack to shoot at old analog spinny things when you wanna quickly find out where you're at, but that will get you a key data point to start off with.
 
Harbor Freight has a laser tach too, seems like it is in the $35 range.
 
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