# Hot hot hot! Mini-mill spindle



## patmat2350 (Jun 6, 2020)

My X2 spindle gets hot, almost too hot to hold, after a brief running time. The machine has a few years on it, and I've never opened the spindle box, nor adjusted anything. Am running an LMS belt drive though.
I can't sense any roughness or looseness in the bearings. Belt side load doesn't seem excessive. 
Any magic in the spindle nut tightness? Back it off a hair?
Or time for new bearings?


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## RJSakowski (Jun 6, 2020)

Too much preload on the bearings will cause heating.  A spindle will run warm to the touch with running though,  If the bearings are designed to load by compressing, as the spindle heats up, it will grow in length which increases the preload, causing even more heating.  At some point, heat lost will equal heat gain and you will have reached equilibrium.

Backing off the preload slightly should shift the equilibrium down and lowering the temperature.  As I recall, the proper preload on my Tormach spindle will cause a run temperature of 155ºF or lower.  This temperature will vary from machine to machine depending upon the distance between bearings.


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## mikey (Jun 6, 2020)

I agree with RJ that preload is probably excessive. However, if the bearings are shot then that will also cause them to run hot. I would start by checking your run out. If that is excessive then you need to replace the bearings; backing off preload will only make run out even worse. If run out is acceptable (typically less than 0.0002" TIR) then backing off preload is probably all you need.


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## GunsOfNavarone (Jun 6, 2020)

Since you haven't opened up the spindle, its got to be wear on bearings. It would be odd for the pre-load to have increased over time if you haven't adjusted. Related/unrelated, when I changed my bearings out recently, I used too much grease and it over heated until I pulled it back out and removed a good portion of it. I'm more used to wheel bearings...definitely don't over pack I learned...
Curious what you figure out...


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## patmat2350 (Jun 6, 2020)

Well, I lied. Forgot that I had to R&R the spindle nut when I installed the belt drive pulley.
Anyways, I loosened the nut and took it to light finger tight, just bottoming, and locked its grub screw. Ran for a while... seems a tad cooler, I can just hold on to it... my IR gun says 109°F inside the taper hole, just seems hotter than that to my touch. Still no discernible play or roughness.  

Well, if the bearings don't growl at me, I'll just keep running!


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## addertooth (Aug 20, 2020)

If it has been a years since you put grease on it, consider some lubrication.  It does not last forever, especially if it has run hot (which can bake off lubricant).  As others have said, while you are in there lubing BOTH bearings, check your pre-load.


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