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Rules of thumb for setting spindle bearing preload

Mr. J

Plastic
Joined
Nov 4, 2022
Every application is different. And this was a thread about "rule of thumb." So, no amount of friendly chat here can begin to be an education on bearings, gearboxes, oil reservoirs, gear grinds and how all affects heat... as alluded to in previous posts.

I just gave my $0.02 worth. I've been repairing and building machine tools since before there even was a CNC. I've spent my entire life at it, not just dabbling here and there. The important thing for me is that I know I'm still learning.

I suggest when setting used bearings: Go lighter on preload than your application would call for when new. If your application calls for clearance, keep the same clearance. Expect gearbox oil splash to take away bearing heat to a great degree. If it can't run cool, something is wrong somewhere and may or not be part of a poor design. Break in can cause a LOT of heat, or in some setups will cause very little heat. Different setups are different.

Your lathe is pretty basic. Just tighten until your deflection is OK. You'll never get rid of it all, If warm, loosen it again. If you still have problems, maybe new bearings?
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Every application is different. And this was a thread about "rule of thumb." So, no amount of friendly chat here can begin to be an education on bearings, gearboxes, oil reservoirs, gear grinds and how all affects heat... as alluded to in previous posts.

I just gave my $0.02 worth. I've been repairing and building machine tools since before there even was a CNC. I've spent my entire life at it, not just dabbling here and there. The important thing for me is that I know I'm still learning.

I suggest when setting used bearings: Go lighter on preload than your application would call for when new. If your application calls for clearance, keep the same clearance. Expect gearbox oil splash to take away bearing heat to a great degree. If it can't run cool, something is wrong somewhere and may or not be part of a poor design. Break in can cause a LOT of heat, or in some setups will cause very little heat. Different setups are different.

Your lathe is pretty basic. Just tighten until your deflection is OK. You'll never get rid of it all, If warm, loosen it again. If you still have problems, maybe new bearings?

Yes, most of your info I agree with. But not that the bearings will always stay cool. At low RPM sure. But they need to be tested at high RPM also unless the machine doesn't ever rotate that fast. Not all machines have an oil reservoir and oil lubricated bearings - many have grease, and only what's in the bearings. The possible range here is massive, so really, to my mind, there's no real rule of thumb possible except "don't get too hot."

In this particular lathe, being a small swing I expect it spins up pretty quick, and with roller bearings, there is extra friction. They won't run cool to the touch at high RPM. Switching to angular contact ball bearings will let it run cooler but it's still not going to be anywhere near room temperature. The freshly changed spindle bearings in my Wells Index mill have been through break-in just recently and at 4,200 they will settle in at about 110°-115°F after around 15-20 minutes.

And I've rebuilt a cubic ton of various spindles and gearboxes and slitting knife gearboxes also. At high speed, they get warm.
 

Mr. J

Plastic
Joined
Nov 4, 2022
You are, of course, exactly right. Just as I said in my earlier post: higher rpm causes more heat. Set to the desired "happy medium" rpm to run cool. It will be hotter at higher rpm and sloppier at lower rpm.

Glad to see we agree.
 

triumph406

Titanium
Joined
Sep 14, 2008
Location
ca
If it can't run cool, something is wrong somewhere and may or not be part of a poor design. Break in can cause a LOT of heat, or in some setups will cause very little heat. Different setups are different.

I have a Graziano SAG14, the headstock adjacent to the chuck can get quite hot, even at 550rpm. That's just the way they run.
 

CarbideBob

Diamond
Joined
Jan 14, 2007
Location
Flushing/Flint, Michigan
I do not know a single spindle rebuilder of any sort of reputation that does not use bearing temp measurements.
A temp probe here is like owning a micrometer when you get your first lathe or mill.
What is the right temp for speed and time and the temp curve become a bit of an art.
If it runs cold or room temp something is wrong. If you spit on it and it boils something is wrong.
Bob
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
I quote Teddy Roosevelt:

It is not the critic who counts, not the one who points out how the strong man stumbled or how the doer of deeds might have done them better. The credit belongs to the man who is actually in the arena, whose face is marred with sweat and dust and blood; who strives valiantly; who errs and comes short again and again; who knows the great enthusiasms, the great devotions, and spends himself in a worthy cause; who, if he wins, knows the triumph of high achievement; and who, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who know neither victory nor defeat.

I thank those of you who are "the man in the arena" and share some of that knowledge you have paid for with significant of both dollars and sweat.
 
Joined
Apr 19, 2006
Location
Manchester, England
I have a Graziano SAG14, the headstock adjacent to the chuck can get quite hot, even at 550rpm. That's just the way they run.
Another obvious factor regarding headstock heating is where in the World you are and how hot the climate gets. Lots of machine tools have different lubrication specs for say Sweden as opposed to Nigeria etc

Regards Tyrone.
 

4GSR

Diamond
Joined
Jan 25, 2005
Location
Victoria, Texas, USA
Beat this dead horse a little bit more...
Way back in time at a place I worked at, they had several trepanning lathes that were basically government surplus gun barrel boring lathes. We would take there machines and retrofit with up to 150 HP DC spindle motors and all of the necessary jewelry it took to make one of these into what we called a trepanner. My handle is from one of the models of one that LeBlond built for the government arsenals back in WWII. The head stocks on these lathes were equipped with large Timken tapered roller bearings. I don't recall the exact dimensions of the bearings, something like 21" ID x 26" OD x 3" wide. There were two of these single row bearings at each end of the spindle placed back to back. The mechanic would re-adjust the running clearance from almost zero to about .015"-.030" clearance under the rollers by using a feeler gage. These spindles would run at about 700 RPM! Day in-Day out, almost 24 hours a day six days a week during the oil boom of the late 1970's early 1980's. Them old head stocks would sometimes get so hot they would blister your hands or arms if you touched them. I complained to the mechanic one time that he was putting too much clearance into the bearings, Timken only recommended about .006-.007" under the rollers. He told me to buzz off, I've been doing this for the last twenty something years, I know what I'm doing. Whatever!
This particular bearing arrangement requires much more clearance to run at the RPM's than with bearings place face to face. But the reason this was done this way was for war time emergency use to get the machines built quickly in a timely manner. Ken
 








 
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