maxium safe speed for lathe spindle with bronze bearings?

how do you figure this out? I know that it would depend on the lube system and the exact materials in question, but is there a general rule of thumb for drip/wick oilers and regular old bronze bearings from the 1950's and before? how does diameter of the bearing affect it's ability to handle speed? am I right in assuming a bigger diameter spindle has to go slower because of the SFM of the spindle where it is in contact with the bearing surface?

thanks in advance,

jon

Yes the larger diameter the of spindle the less rpm that it can spin.

Oil is only so thin... High speed Turbocharger bearings in equipment are rarely .75 in OD.

And they have full pressure oiling...

http://en.wikipedia.org/wiki/Plain_bearing

Is a start, search for Plain bearing...

Get used to terms like PV V Max P Max and temperature range.

If you had stated bearing sizes we can help more...

Most old machines were built to same specs tho, and an RPM increase is not usually possible...

Is it possible to fit an oil pump system to your application?. I have a phosphor bronze bush Graziano lathe, the bush has a journal surface about 3" ID and about 4 " long from memory. It's an oil pressurised fluid bearing and goes to 1800 rpm top speed. I'd say with pressurised oil feed you're really only limited by input power within reason.

Gas turbines AFAIK mostly use fluid bearings and they have pretty impressive rpm ratings in the tens of thousands of revs.

From memory 500 ft / min surface speed is a dead safe round number rule of thumb maximum for a felt wiper or drip feed fed bearing.

Most of the small lathes with bronze or CI (usually head-stock material) seem to check out at 550 (ish) although some run up around the 600 range. Hard steel in hard steel bearings as on precision bench lathes and the like can go faster but I'm not sure how much.

Best guess is that with everything in top condition, modern synthetic oils and proper attention to oil supply something in the region of 1,000 sfm is safe for a reasonable life. Top condition means plenty of attention to surface finish and achieving the right bearing clearance. Synthetics typically run rather smaller clearances than dino oils.

Speeding up an old machine would be chancy.

Clive

I think it depends almost entirely on the lubrication system. I did a quick search and found this link: http://www.machinerylubrication.com/Read/779/journal-bearing-lubrication

This is a small excerpt which explains how the rotation itself creates a hydrodynamic oil film that separates the shaft from the bearing:

Journal bearings operate in the boundary regime (metal-to-metal contact) only during the startup and shutdown of the equipment when the rotational speed of the shaft (journal) is insufficient to create an oil film. It is during startup and shutdown when almost all of the damage to the bearing occurs.2 Information on plain bearing failures was discussed in an article in the July - August 2004 issue of ML magazine. Hydrostatic lift, created by an external pressurized oil feed, may be employed to float large, heavy journals prior to startup (shaft rotation) to prevent this type of damage. During normal operation, the shaft rotates at sufficient speed to force oil between the conforming curved surfaces of the shaft and shell, thus creating an oil wedge and a hydrodynamic oil film. This full hydrodynamic fluid film allows these bearings to support extremely heavy loads and operate at high rotational speeds. Surface speeds of 175 to 250 meters/second (30,000 to 50,000 feet/minute) are common. Temperatures are often limited by the lubricant used, as the lead and tin babbitt is capable of temperatures reaching 150°C (300°F).

thanks for the info and replies. I am not trying to speed up a lathe, per se, but wanting to make sure that I don't run one of them too fast, since it doesn't have it's original drive unit, and I don't have exact specs on how fast it was supposed to be run. and just curious in general to boot.

maxium safe speed for lathe spindle with bronze bearings?

Click to expand...

Here is Hendeyman's thread on the subject. Hendey taper journals were very high class ring oiled jobs.

http://www.practicalmachinist.com/vb/blog.php?b=232

John Oder

I had a nine inch South Bend lathe spindle start to seize up at 800 rpm.

The spindle slowed down and the the flat belt slipped.

This happened several times ,over the years, and never caused even the slightest damage to the bronze bearings or steel spindle.

It would probably be safe enough to do some tests while monitoring the bearing temperature and motor amp draw.