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RussZHC
07-31-2011, 02:32 AM
Those that own or have had experience with lathes using plain bearings, particularly bronze main bearings, a question.

Restarted the lathe today after a year plus :D and since this is the first time I have owned it since "new", is it possible a slight tightening of the bolts clamping the bearings in place (bearings are one piece, split so one side is open, like a compressed letter "C") will help slow the flow of oil (expected) but more importantly could it also lessen the increase in bearing temperature since less oil would be running out? Seems a bit counter intuitive...:confused:

Bit more info if needed.
The main spindle riding on these bearings does not spin as freely as I had expected, though I have no point of reference, but there is only a very slight raise in temperature on the larger of the two, the smaller of the two has no noticeable change (very unscientific, simple touch). When switched on the motor certainly has no issue in starting movement of the main spindle and when switched off, the spindle does rotate 2/3/4 revolutions before full stop (sorry, did not count exactly and was also experimenting with pulley drive belt tightness).
There is very little "up/down" movement of spindle when I lean on it, though there is a fair bit of "lateral" movement (need to adjust pressure via screw collar on the end of main spindle...ran out of time today) so there maybe a "sweet spot" that is a little more worn and once always there, could ease rotation a bit.
Bolts in question were finger tight. It is also possible the shim on the front bearing needs to be slightly thicker.

Thanks, Russ

Evan
07-31-2011, 02:50 AM
The bolts that clamp the bearing on the shims are not an adjustment. They should be tightened on the shims which are the clearance adjustment. A slight temperature rise at max rpm is normal. So is a slight amount of clearance at rest. There must be clearance for the bearing to function. When it is running the clearance is filled with oil and is nearly incompressible. It also serves a damping function since the oil takes time to be squeezed out. This same principle is used in jet turbine bearings. It's called a "squeeze film damper".

Closing up the gap will make the bearing run hotter and risks the bearing surfaces making contact. It will also reduce the damping effect. This is assuming that the clearances are not too large due to wear. If the lathe has been correctly lubricated since new it should not display any wear and the clearance should be correct. When I overhauled my 1937 South Bend headstock recently there was virtually no visible wear on the spindle or the spindle housing.

philbur
07-31-2011, 04:18 AM
Hi Russ, you don't say what lathe you have. Not all plain bearings are the same design, although the principle of operation is similar.

1. There should be spindle/bearing clearance when the spindle is stationary. No clearance means no oil.

2. The spindle/bearing will run hotter with:

- smaller spindle/bearing clearance.

- higher oil viscosity.

- higheer spindle speed.

The higher temperature is not due to bearing friction (there should be no contact) it is due to the frictional flow within the oil film itself.

My Lathe has a specified static clearance between the spindle OD and the bearing ID of 0.02mm. The specified oil is ISO32. The stabilised temperature at 3,550 rpm is around a measured 35 degrees C.

If your bearings are not getting hot and you have no measurable clearance leave well alone.

If you do any adjustments, then before restarting the spindle, double check the spindle has oil and runs freely. After starting the spindle, monitor the bearing/housing temperature continuously until it stabilises. An IR temperature gun is great for this. If the temperature runs away there is a high probability the bearing will seize. Although this is highly undesirable it is not necessarily fatal. Let things cool down, back off the adjustment and try again.

Rate of oil consumption is not a criteria for adjustment!

Phil:)

SGW
07-31-2011, 06:08 AM
Are you using the correct weight oil? It should be about #6 or #7 machine oil.

RussZHC
07-31-2011, 08:03 AM
Thanks for responses so far.

Evan: unfortunately I doubt this lathe (very late 30s Sheldon) has been properly lubed all its life. Having said that, bearings themselves and the bearing surface area on the spindle were pretty good IMO.

I guess in my mind part of the question revolves around the difference in temperature between the two bearings after the same amount of "work".

I think I am right, given the numbers mentioned as clearance and other information of similar lathes I have gleaned, we are talking some small amounts.

So I will mention something I notices with the shims.
IIRC Southbend used laminations of shims where you could peel off layers, what Sheldon appears to have done is used a core and "dipped" or coated with brass/bronze.
That outer "skin" exists on both sides of the rear bearing (the cooler one) and is AWOL on one side of the front shim and partially missing on the other large flat side (as opposed to an edge; it flakes off and metal underneath is definitely different). It may only be a thousandth or so, each side, but suspect it could make a difference in this application.

Measure more complete shim and compare? Then adjust if needed?
Assume temperature change a valid marker?

I know that seems hit and miss as the shims to get clearances at the factory could be different thicknesses but as a starting point?
Will try and measure movement more accurately so that should give a bit of a clue as well.

quadrod
07-31-2011, 08:17 AM
For south bend lathes, on the cast iron plain bearing for the 9" lathe i think they wanted .001" to .0015" clearance measured by putting a length of stock in the spindle and lifting with 75lb of force, with dial indicator point touching the spindle at the head stock. On the lathes with bronze split bearing and expander the spec was .0007" to .001". If some one has a more correct procedure please feel free to correct me.

RussZHC
08-03-2011, 08:54 PM
Tightened the screws holding the caps in place, as opposed to just sort of finger tight and used an oil a bit lighter than what the factory suggested (the early Sheldon info I can find suggest a 20W but doesn't get much more specific; thanks SGW for the reminder) and that appears to have done the trick.
There is still a slight increase in temperature after running for awhile but now with an actual cutting load and the increase is less than before.
Before there was no oil visible down the fill cup when the lathe was stopped, now there is a slight "bubble" that disappears when the lathe is stopped and turned over by hand.

I still have not got around to actually measuring clearances (tomorrow?).

RussZHC
08-06-2011, 09:35 PM
Finally got to checking the spindle/bearings.
Best I can figure is .0014, so going by the SB specs earlier a bit much (double the "tight" bronze spec ?)...only disclaimer would be is I have no real way of accurately knowing the amount of force.