Safe Spindle Temperature?

[RJS100]

Two other folks that converted this mill, used SKF bearings rated for 9000 rpm. One has no issues, the other continues to heat up after a long time. I removed the oils seal today. Here is what I found:

I managed to remove the OEM seal (45 x 72 x 10). It had a grove for a spring, but there was no spring. I was immediately apparent that it was rubbing on the spindle. You can see where it was rubbing. Even after cleaning it up, you can see where it burnished the spindle. The bearing seems to have quite a bit of grease in it, probably too much. Lastly, I do not think this seal is rubbing on the face of the inner race. There is a few mm of space above the top and bottom wear marks made by the seal(see below).

I am going to do some testing at different speeds and track the temperature with no seal installed. I should be receiving a 47 x 72 x 10 mm ring shorty. Keep you posted.

[RJS100]

With no oil seal installed and the original grease. Here is what I recorded for temps shown below. Sine I don't know what grease is being used or the brand of bearings, I stopped at 4500.

I inspected the bearing carefully during this temp test. The grease always stayed thick and nothing ran out at all. When I removed the oil seal, I did see a small amount of grease that that had turned to liquid and was sitting in the oil seal. This would have happened while I was running the mill previously.

I reached out to SKF, Timken and NTN. They all said these bearings run hot due to their internal resistance. Timken and NTN have a max temp off 250F with SKF at 300F. The folks at Timken said most of the time, they run at 160F to 180F. SKF is the winner with a top operating speed of 8000 rpm. NTN and Timken were rated at 6300 and 4000 with grease. For some reason, SKF claimed the 8000 is good with grease or oil. Quite a big difference from brand to brand. So I guess I would really have to know the manufacture to figure out the safe max rpm.

For now, I think I will not be running over 4000 to 4500 until I replace the grease with Isoflex NBU 15. Would you agree that the OEM grease is probably not suitable for higher speeds?

Thans as always... Richard

[RJS100]

I removed the spindle according to the manufacture's direction. They said it is standard practice to set the spindle in a vice with jaw protectors and use a dead blow hammer to gently drive the spindle out of the quill. They directed me to remove the bottom bearing in a similar fashion. This all worked easier than I thought. Prior to cleaning the bearings, I inspected the races carefully. I noticed some very small marks on the races (see below in red). These are very small and co-inside with the bottom edge of the roller bearings. I can actually feel them slightly with my fingers. Does this mean these bearings are toast?

I am guessing that these marks are a result of a rookie mistake I made some time ago when I flipped a part over, forgot to re-establish my z-height and plunged a 2 inch face mill directly into a piece of aluminum. Would you agree that I should not use these bearings and replace with a new set of SKF bearing that I ordered? My goal of course is to run at 6000 rpm.

Thanks... Richard

[suntravel]

If new SKF are on the way, use them. I am not sure if they have used high quality bearings in Taiwan.

Uwe

[RJS100]

Surprisingly, the OEM bearings are NTN, which according to the NTN are rated for 5500 rpm with grease. NTN indicated that I could probably do better with the Isoflex grease. If it wasn't for the marks on the races, I would probably just use them. My guess is that if I can feel the marks with my finger nail, I should probably not use them. Just my guess.

[Muzzer]

Those marks are suspiciously evenly placed, which perhaps suggests they are due to a spec of swarf or metallic particle. If so, they may be built up rather than worn away (ie convex rather than concave). Take a closer look and also use a sharp blade to determine which it is. You may well find they can be removed.

Over packing a bearing with grease seems to be a common problem. The requirement is simply to ensure the surfaces are always coated when in use and any excess is only really there as a reservoir to replace any that is thrown off and lost. The bearing mfrs recommend packing volumes which are fairly modest.

If you compare the drag torque of a clean bearing with light oil against that of a bearing packed with viscous grease, you can literally get a feel for the effect of the grease. Multiplying that drag torque by the spindle speed gives you the windage losses (power) due to the viscosity of the grease - and this will be dissipated as heat in the grease / bearing. What else could result in temperature rise? The seal would rapidly burn up if it caused any noticeable drag that was sufficient to heat up the bearing, while the rollers themselves have little opportunity to create drag.

[RJS100]

Thanks. I was hoping you were right so I could avoid removing the OEM races from the quill, but no luck. These marks are indeed indents and cannot be scraped off

In this case I do not think the overheating was due to too much grease. The mfg. recommends about 20% of the space between the rollers be filled, which is what it looked like. I think the issue here was a preload problem.

I noticed two things while taking apart this spindle. 1. It did not spin freely at all, suggesting too much preload. When I loosened the preload, it began to spin more freely. 2. The fit between the top bearing and spindle was too tight. It should be a loose press fit but was fairly tight. This makes adjusting the preload difficult as the castle nut that is used to adjust the preload has to work against this fit to be able to adjust the preload. In other words, it hinders the ability for the bearing to slide along the spindle. I spoke with a someone that has 11 of these benchtop machines and he said he has seen this before. He very lightly wet sanded the spindle with 2000 grit wet dry paper to SLIGHTLY loosen the fit, but not too much.

Now... I just have to get those bearing races out.

Thanks... Richard

[RJS100]

One other question, are you supposed to grease the splined portion of the spindle to stop if from rusting? The quill is locked in position since the machine is no longer used manually so it no longer slides up and down, but it is completely dry. Is there any issue applying grease?

[suntravel]

I use dry Teflon Spray for stuff like this, slight rust prevention and no dust or chips sticking on it.

If the bearings are greased correctly, spindle temp is a good indicator of ok preload.

Uwe

[RJS100]

Thanks Uwe. Do you lubricate the outside of the outer race before pressing it in place? I read that some folks use extreme pressure lube like this:

www.amazon.com/Allstar-Performance-ALL7 ... 135&sr=8-1


Some say leave it dry.

Richard