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Rotate
10-28-2002, 12:43 AM
Is it true that lathe spindle actually rises as it spins up and settles down when it's not spinnning? Is this measurable amount? Is this just with babbit bearing or does it apply to ball and taper roller bearings also?

Albert

docsteve66
10-28-2002, 12:59 AM
Albert: This is pure speculation on my part. This is first time I have heard of this phenomena. but: they say any bearing rides on a wedge of oil as it spins, so even though the oil is spread evenly around the spindle, it must settle some when it is stopped and rise when spinning. Measureable? Sure but i have no idea how to measure or how much it would measure. Reminds me of Thrud's 6 millionths of an inch taper when the lathe tool bit is set low. Maybe a insulated wire, or even a mike (still insulated) a bulb and battery. spin it up (THE spindle) run the wire or mike down till it lights or flickers, turn the machine off, let it sit and measure the gap. I think I have too much time on my hands ! http://bbs.homeshopmachinist.net//smile.gif
Steve

metal mite
10-28-2002, 08:33 AM
Rotate,
Probably true for friction bearings.
The planer tables rose about .005 on the oil film when I was a dumb ----- apprentice.
Had to let the table stroke a couple of times prior to setting a cut.
In that application the lift is not restricted as in a circular bearing.
Mite

SGW
10-28-2002, 08:44 AM
I expect this is applicable mostly to journal bearings, not ball bearings, but yes, it happens.
The clearance in a lathe spindle bearing is pretty small though -- South Bend specifies about a thousandth of an inch, as I recall -- so you're not talking about much movement. SB also specifies very thin spindle oil, about #6 (the actual specification is 100 Saybolt, but that tranlates into roughly #6 oil. I'd guess you might get a movement of...what? 1/4 thou, maybe?

It is definitely important to use the correct oil though. A while ago I had the bright idea that if I used #40 synthetic oil as my way lube, I'd be virutally guaranteed of an oil film, no contact, and virtually no wear. Well, it sure didn't contact. The lathe carriage floated on that #40 oil so well, it would move around by as much as 0.002" and it was impossible to take a really accurate cut.

mbensema
10-28-2002, 09:17 AM
The spindle will rise, but it depends on the oil viscosity and the speed of the shaft and the bearing size as to how much. A higher speed or higher viscosity will cause the shaft to rise more. The FAG bearing catalog shows a chart of how the bearing size and shaft speed relate to the minimum oil viscosity needed to maintain the minimum oil thickness of 0.004 mm I believe it is, so you are not going to easily measure it on a small bearing. For example, a ball bearing with a shaft size of 25 mm with normal clearances has a radial clearance of 0.005 to 0.02 mm, not an easily measured value on a rotating shaft.

Fluid film bearings use proximity sensors to measure vibration by measuring the displacement of the shaft during operation. These probes also measure the oil film thickness during operation by calculating how high the shaft rose from rest to operating speed.

Mike

docsteve66
10-28-2002, 06:31 PM
SGW- lol! we once traced a "random " problem to a man who regularly used some super way lube. made a large machine float as you described.

Thrud
10-28-2002, 06:56 PM
Albert
This is true for any hydrodynamically lubed bearing (Mains, conrods) as the oil actually provides a barrier between the shaft and the bearing supported by the oil pressure in the system. Most of the engine wear occurs from dry start ups when metal to metal rubbing occurs. The shaft rides on a cushion of oil.

In a spindle bearing it is a little different in that the oil gathered at the bottom of the bearing acts as a solid (almost) allowing the spindle to climb up (like a ramp) the oil until the oil is evenly distributed as the forces acting on the spindle are equalized. More or less.