Upgrading lathe motor for higher spindle RPM?

Hi all, I've got a '33 SB 11" underdrive lathe with a quick change gearbox that's working fine. I have been thinking about upgrading its 1150 RPM 1/2HP motor
with something capable of providing higher spindle speeds and perhaps more power. My lathe tops out at about 550 RPM and other SB lathes of the era (both smaller and larger) have spindle speeds as high as 1357 RPM. Since I'm using it for work smaller than what SB designers may have considered "usual" for an 11" machine, I'm wondering if the there's anything I can do to ramp up the speed a bit.
I realize there are likely bottle necks in the lathe's drive line (belt, bushings and gearbox?) and I'm wondering how severe they are.
Anybody have any experience doing something like this? Good idea? Bad idea?
Thanks,
Peter
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Peter, those lathes can be repowered with a 1 hp 1725 rpm motor with no problem. That will give you about 800 rpm at the spindle and that would be OK. I wouldn't go much over 1,000 rpm with out taking a good look at the spindle bearings. Many SB lathes had castiron bearings, excellent and highly accurate, just not made for high speeds. Even with bronze or babbit bearings, clearences and oiling become critical at higher rpm. Paul
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http://f1.pg.photos.yahoo.com/ph/ep213 /
Peter,
At above address you can check the speed chart [It was said to me, that this is for a 10" South Bend lathe]
Good luck. John
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Peter Grey wrote:

A couple of weeks ago I posted here: My 11" South Bend with 1.5" flat leather belt drive came with a .5 HP 1100 rpm 3 phase 230 VAC silent chain drive. Not having 3 phase I replaced the motor with a cap. start 1750 rpm single phase 230 VAC .75 HP motor. I also replaced the chain drive with a half inch V belt. Works fine. HTH.
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Why anyone would purchase any machine not having at least adjustable bearings like a timkin setup is beyond my imagination........
Is it any wonder the Japanese have kicked our asses, and the Chinese are soon to follow ???
Throw the whole kit an kabootle into the scrap bin is my opinion.......
Wait!!!
Then the Chinese might get hold of it !!!!
Nevermind..............
--
SVL



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Your question was: why use plain bearings? The answer is: accuracy. Most of your really precise older machines used plain bearings, including higher speed items like precision grinders. The best bearings weren't good enough at a practical level, and were enormously expensive. Bearing adjustment and oil viscocity were critical, but the bearings worked well. If you are going to speed up an older metal lathe in order to turn wood, you should loosen up your bearing caps and oil the bearings well with a slightly heavier oil than you have been running for metal (assuming your bearing clearance was proper for metal to start). I have been in machine shops, which having upgraded to a new ball or rollerbearing head lathe, kept the old plain bearing flatbelt lathe around just for that occasional job which required high precision. A true shaft running in good plain bearings will run true. Dead nut true. Paul
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6e70 wrote:

Let me add that once the shaft is rotating the only friction is in the oil film and that has lower friction than ball bearings. Only briefly during start up do ball bearings have lower friction. For low friction at higher speeds thin lube oil is used. To reduce the friction even more air bearings are often used with the higher speeds. Hint: Read up on turbines. HTH
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Thanks all.
I think I'll throw a new pulley on it to start and go from there.
Peter

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If you do run it faster, keep an eye on the bearing temperature. And you might look at the viscosity of the spindle oil closely.
I have a precision bench lathe and the bearings got hot if I ran it over 700 rpm. I used the right oil and it would do 1290 rpm, the rated top speed.
vince
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