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garyhlucas
11-17-2013, 10:10 PM
I bought a mini-mill spindle from Little Machine Shop and disassembled it to remove all the gears and to cut a keyway into the spindle for a timing belt pulley. After putting it all back together and mounting a 1-3/4 hp treadmill motor the bearings run hot. I've run it for a couple of hours continuous starting at about 500 rpm and working my way up to the full 2800 rpm. The adjusting nut is tough to adjust as the bearings fit tight to the shaft, but I have it lightly snugged down. It ran for about three hours and at the end the cast iron housing was to hot to hold your hand on, however it seemed to have reached a stable temperature.

Looking for opinions. Damaged the bearings pushing it apart and pressing it back together? (No hammer used) Cheap Chinese bearings run hot? The spindle has some noticeable runout on the top end where the adjusting nut is. Building this was a lot of work and I invested in Tormach tooling because this is a CNC, so the heating is worrysome.

Thanks,

Mike Amick
11-17-2013, 11:26 PM
I think you may be looking for a problem where non exists. Too hot to touch isnt that
unusual. My understanding of adj bearings is .. after a warm up .. you speed the thing
up and the temp should rise 10 degrees or so .. and remain stable. It if continues to
rise, the bearings are too tight.

I'm sure you will get the exact poop in a min ... but I don't think things look that bad.

Mike

Jaakko Fagerlund
11-18-2013, 02:24 AM
Could you tell more about how you got the bearings off and reinstalled them, as they can be easily damaged even without a hammer. Also, if the bearings are the original ones, I would suggest replacing them with something of guaranteed quality or even with tapered roller bearings.

vincemulhollon
11-18-2013, 07:50 AM
Did you use a high temp grease? Not a grease that causes high temps (well, a high viscosity would be a little warmer) but a grease that doesn't mind running warm? If its permanently greased from the factory bearings I wouldn't change the grease unless its contaminated, just note perma-greased bearing grease mostly seems OK up to 140F or so depending who you ask.

For like $5 at walmart you can get one of those digital indoor/outdoor digital thermometers and stick the "outdoor" on the housing and kinda keep an eye on it. If it goes off the scale you've got a problem...

The short version of the link below is keep the bearing below 120 C (which is really hot, like 250F) and you'll be OK with most SKF bearings as long as the bearing is happy. At reduced loads, you can run plain old COTS steel bearings up to 500 or 600 degrees or so, assuming you keep the load down and the grease is OK at both room temp and that temp (good luck with that and you're not going to like the price of high temp grease it'll be weird silicone based stuff you can't get out of your hands for like a week). Note that the bearing is probably quite a bit warmer than the housing. Anyway if you get burned touching it, that's probably OK (other than the whole burn thing), but if it boils when you spit on it, that might (or might not) be a problem.

http://www.skf.com/group/products/bearings-units-housings/ball-bearings/principles/selection-of-bearing-size/selecting-bearing-size-using-the-life-equations/influence-of-operating-temperature/index.html

Heat in a bearing is no problem and they're designed to take quite a thermal beating. What heat is, is a symptom. If the cause of the heat is the bearing is packed with sand, grit, chips, rust, dust, dirt, either no lube or the wrong lube, problems like that, the bearing will probably die soon and it'll also be hot. The hot doesn't matter, the hot because its full of sand is what matters, get rid of the sand and the hot will take care of itself. On the other hand if the symptom of hot bearing is caused because its kinda tight, well, as long as its got good clean lube that doesn't mind the heat, it'll last forever, or at least until it doesn't have good clean lube.

A.K. Boomer
11-18-2013, 08:40 AM
It ran for about three hours and at the end the cast iron housing was to hot to hold your hand on, however it seemed to have reached a stable temperature.



Thanks,



That's allot of log time for immediate use of something that was most likely not designed for it, also keep in mind your using belts now and they will side load the bearings more as unlike gears they have to have tension on them to make them work - maybe the upper and lower bearing mount distance is not designed for this?

there is allot of factors - but first off is the questions of what you changed and how it could effect things.

Also, whats the extension ratio of the belt system as compared to where the gears were? are they leveraging the spindle bearings more? I could go on and on...

saltmine
11-18-2013, 10:36 AM
I wore out the bearings of my mini-mill. Decided to get the best I could find. Disassembled the spindle, removed the plastic gears (never used), and installed the new bearings. My mini-mill has the belt drive conversion. The belt really doesn't impart much side thrust on the bearings, since it's a small belt. You'd probably destroy the belt long before you could get it tight enough to harm the bearings. I discovered the adjusting nut doesn't have much to do with deep groove, spherical (ball) bearings. Yes, the spindle of mine still runs warm, even after using it for two years. So, I'm assuming the bearing adjustment is correct, there is no run-out or play at all in the spindle. It used to run pretty warm, when I first replaced the bearings, but it seems to stabilize, now. Yes, it would be nice to have available a tapered roller bearing spindle. Tapered roller bearings can carry a tremendous load compared to ball bearings. But, then you'd run into two problems; one, not too many people know how to correctly adjust tapered roller bearings. And two, tapered roller bearings would require a redesigned spindle with provisions for bearing pre-load and seals to retain the oil needed to provide a decent bearing life and freedom from constant bearing adjustment. That means that the spindle housing would have to be sealed to hold oil. I've thought of designing a tapered roller bearing spindle myself, many times. But as long as replacement bearings are cheap and easily replaced....why bother. It is food for thought...maybe later on to design a tapered roller bearing spindle for the mini-mill and lathe. Some models of the mini-mill use the same spindle and housing as the lathe.

hoffman
11-18-2013, 10:40 AM
I wore out the bearings of my mini-mill. Decided to get the best I could find. Disassembled the spindle, removed the plastic gears (never used), and installed the new bearings. My mini-mill has the belt drive conversion, but I discovered the adjusting nut doesn't have much to do with deep groove, spherical (ball) bearings. Yes, the spindle of mine still runs warm, even after using it for two years. So, I'm assuming the bearing adjustment is correct, there is no run-out or play at all in the spindle. It used to run pretty warm, when I first replaced the bearings, but it seems to stabilize, now. Yes, it would be nice to have available a tapered roller bearing spindle. Tapered roller bearings can carry a tremendous load compared to ball bearings. But, then you'd run into two problems; one, not too many people know how to correctly adjust tapered roller bearings. And two, tapered roller bearings would require a redesigned spindle with provisions for bearing pre-load and seals to retain the oil needed to provide a decent bearing life and freedom from constant bearing adjustment. That means that the spindle housing would have to be sealed to hole oil. I've thought of a tapered roller bearing spindle myself, many times. But as long as replacement bearings are cheap and easily replaced....why bother. It is food for thought...maybe later on to design a tapered roller bearing spindle for the mini-mill and lathe.

Are the factory bearings that bad?

saltmine
11-18-2013, 10:50 AM
I wouldn't say they were bad....serviceable. I bought my mini-mill new, and used the living daylights out of it for about six years before one of the original bearings developed a rough spot. My bad....I probably abused it more than the average guy would even consider. I also wore out the bronze nuts on the table lead screws also. And, yes, I've become quite adept at adjusting the gibs of every axis. My most recent project was to replace the bearings in the motor itself. Fortunately, a local bearing house had both of the bearings, American made, on the shelf. While I had it apart I was surprised to discover that the motor brushes were not even half worn yet. Everything got a good cleaning and it's back in service, without the annoying vibration.

dian
11-18-2013, 11:08 AM
big surprise:

"http://www.skf.com/group/products/be...ure/index.html"

i never would have thought, bearings could operate at 200c. how come, conventional wisdom seems to be to adjust them for around 50c?

lakeside53
11-18-2013, 11:18 AM
So they aren't loose at 20C... Don't quote "200C" unless you know the exact bearing in use. The cage construction, lube system and many other factors come into play.

OP : Did the spindle run hot before you disassembled it? How much grease did you put in the bearings? if it's much more then about 30% fill of the SPACE in the bearing, it will overheat. "Preload" is tricky - even a small adjustment can have a large effect on running temperature, but I'm not familiar with the mini-mill spindle - publish a diagram of the spindle. Deep grove bearings can have a preload applied to achieve some angular contact - a cheap method used by some.

chipmaker4130
11-18-2013, 01:47 PM
. . .OP : Did the spindle run hot before you disassembled it?. . .

My first thought too. Chasing a shadow?

Toolguy
11-18-2013, 02:20 PM
One thing I have found that helps a lot once you make sure everything is mechanically correct is to clean out the bearings good with solvent, blow out the solvent and put Kluber grease in the bearings. www.kluber.com. I was having my mill spindles get hot enough to burn my hand on the casting after running 3000 rpm for 15 minutes. I took the spindle cartridge out and changed to Kluber, now I can run 3200 all day and the spindle hardly gets over room temp. They have different lubes, find the right one for your application.

velocette
11-18-2013, 04:14 PM
Hi Gary
"The spindle has some noticeable runout on the top end where the adjusting nut is".
This "May be the key to the problem the spindle is bent or has not been machined properly.
The original bearings are only just adequate for the job.
Taper Roller Bearings or Angular Contact Bearings are available but are more expensive than the originals.
Consider fitting an Amp Meter to the motor leads. This is useful to monitor bearing adjustment by measuring the load on the motor also helps to achieve the best rate of material removal without overloading.
To get the best out of the extra power you will need to stiffen the column to base as this does not cope very well.
My X2 mill fitted with Taper Rollers runs at 3200 RPM and draws 1 Amp No Load and the bearings are only warm to touch.
You can expect a bearing to run hotter until it is "Run In".

Eric

garyhlucas
11-18-2013, 09:49 PM
Could you tell more about how you got the bearings off and reinstalled them, as they can be easily damaged even without a hammer. Also, if the bearings are the original ones, I would suggest replacing them with something of guaranteed quality or even with tapered roller bearings.
I don't remember exactly how I did it, but I suspect I pushed the shaft out and the bearings cam with them, I didn't use a hammer though.

garyhlucas
11-18-2013, 09:51 PM
I didn't grease them, the whole head was brand new with sealed bearings before I took it apart and I never ran it, so nothing to compare to.

garyhlucas
11-18-2013, 09:54 PM
I didn't change anything as I never ran it previously. The belt is not real tight as it is a 3mm HTD x 25mm timing belt. The pulley is installed on the spindle below the lower bearing, so it is very close to the bearing, nothing in the way of overhang.

garyhlucas
11-18-2013, 10:04 PM
Hi Gary
"The spindle has some noticeable runout on the top end where the adjusting nut is".
This "May be the key to the problem the spindle is bent or has not been machined properly.
The original bearings are only just adequate for the job.
Taper Roller Bearings or Angular Contact Bearings are available but are more expensive than the originals.
Consider fitting an Amp Meter to the motor leads. This is useful to monitor bearing adjustment by measuring the load on the motor also helps to achieve the best rate of material removal without overloading.
To get the best out of the extra power you will need to stiffen the column to base as this does not cope very well.
My X2 mill fitted with Taper Rollers runs at 3200 RPM and draws 1 Amp No Load and the bearings are only warm to touch.
You can expect a bearing to run hotter until it is "Run In".

Eric

Eric,
Yes I have considered that the spindle is bent a little. I am an electrical guy, and I actually have a DC clamp on meter. However I previously head the situation where the machine axis were far heavier duty than the spindle, which previously was off a Sherline mill. Now the spindle is more powerful than the machine is rigid, so I am unlikely to be able to use the full motor horsepower anyway. I only have one belt speed and the motor is constant torque so you really need a big motor to actually get any torque at low speeds. Today I cut an orifice plate out of 1/16" thick 316 stainless using a 4 flute 1/8" end mill and it did a very nice job. So I suspect the heat is going to be just fine.

On the other hand I need just one more tool, an infrared thermometer. Actually I really do need that as I have a 3D printer head on this machine with a heated bed and I'd like to be able to test temperatures in lots of places.