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Sorry to hear that - bummer.
Out of curiosity, how much grease did you add to each bearing? How did you measure it out and pack it?
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Discussion on Small CNC Mill Spindle and Axis Motor Performance
- Thread starter macardoso
- Start date
Out of curiosity, how much grease did you add to each bearing? How did you measure it out and pack it?
My apologies, I may have posted the last update a little prematurely. I spent the last few hours continuing to bed the bearings. I actually ended up getting a stable temperature at 8000rpm - it seems the bearings just needed more cycles getting hot, cooling down and then hot again. I did 3 additional cycles bringing the bearings up to 100C (about as hot as I felt comfortable) and then letting them cool to ~40C. On my last run, it took about 15 minutes running at 8000rpm to get them to 90C and they were stable at 91-93C for the following 45 minutes. The upper bearing runs a little cooler at 75C. Overall, these bearings run HOT... too hot to comfortably grab the spindle nose for any extended period of time. I'll have to monitor them over the next couple of weeks and report back. SKF states the maximum operating temperature is 120C, I'm not sure what the spec is for Kluber NBU-15.
SKF has an online tool to calculate grease quantity and relubrication intervals.
www.skfbearingselect.com
For the lower bearing (the one I'm more concerned about) the tool told me 6g of grease and a relubrication interval of 110hr based on 8000rpm. I used a medical syringe and a food scale to measure out the grease quantity. The lower bearing was packed like I would with a wheel bearing. The upper bearing was a lot more difficult to pack "properly". My spindle requires the upper bearing to be heated in order to slip over the shaft. Because of this, there wasn't a lot of time between pulling it out of the oven and installing it on the shaft to pack it like a wheel bearing (not to mention the bearing is hot and difficult to handle). I ended up measuring out the grease with my syringe, squirting some on the race that was installed in the quill and some on the rolling elements of the bearing before installing it on the shaft. After the bearing was installed I came back with a 22 gauge needle on the syringe and was able to stick it in between the rolling elements and squirt the rest of the grease in the bearing.
SKF has an online tool to calculate grease quantity and relubrication intervals.
SKF Product select
Please, use our online bearing selection tool, SKF Product select, which has a fully updated, clean and user-friendly interface. Calculation results can be obtained fast and are presented directly near the required input data. SKF Product select has been designed in alignment with the Bearing...
For the lower bearing (the one I'm more concerned about) the tool told me 6g of grease and a relubrication interval of 110hr based on 8000rpm. I used a medical syringe and a food scale to measure out the grease quantity. The lower bearing was packed like I would with a wheel bearing. The upper bearing was a lot more difficult to pack "properly". My spindle requires the upper bearing to be heated in order to slip over the shaft. Because of this, there wasn't a lot of time between pulling it out of the oven and installing it on the shaft to pack it like a wheel bearing (not to mention the bearing is hot and difficult to handle). I ended up measuring out the grease with my syringe, squirting some on the race that was installed in the quill and some on the rolling elements of the bearing before installing it on the shaft. After the bearing was installed I came back with a 22 gauge needle on the syringe and was able to stick it in between the rolling elements and squirt the rest of the grease in the bearing.
You might want to double-check the SKF bearing select. It looks like the reference speed for the 32007x is generated using oil - but I could be wrong as I just had a quick look.
AC bearings don't increase runout. Every high-speed spindle I've seen uses AC bearings (not talking about air bearings). Lathes have a big cylindrical roller at the front for radial loads, but otherwise they're all AC and not tapered rollers. If you haven't seen it yet, SKF's site has a great series of spindle/bearing configuration diagrams - and none of the higher-speed spindles they show use tapered rollers.
AC bearings don't increase runout. Every high-speed spindle I've seen uses AC bearings (not talking about air bearings). Lathes have a big cylindrical roller at the front for radial loads, but otherwise they're all AC and not tapered rollers. If you haven't seen it yet, SKF's site has a great series of spindle/bearing configuration diagrams - and none of the higher-speed spindles they show use tapered rollers.
Although not explicitly stated, it's my understanding that the reference speed for SKF bearings are generated using grease. It is stated throughout multiple documentation that if exceeding the reference speed and approaching the limiting speed, "oil lubrication may be required".
You're right. Almost every CNC I've seen uses AC bearings. In fact, there are a lot more options available for low runout AC bearings compared to tapered rollers. I guess what I was trying to allude to is the fact that these spindles were not designed to be used with AC bearings and thus, from what I've seen, there are very few AC bearing options that fit our spindles. All of the AC bearing options I have come across appear to be a lower tolerance grade than the X/Q tapered bearings I purchased and so I just assumed runout may increase if going with those options.
It's probably worth doing a deeper dive into AC bearings. I couldn't find any SKF's that would fit our spindles. I think the most common choice are the Nachi bearings (I believe @macardoso has been running them for a while). Maybe they're good enough for a hobby application?
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I don't have enough experience to comment if I am right or wrong, but I have had decent luck with my AC bearings purchased from VXB.
They required shims to be added to take up for the difference in size from the stock TR bearings, and they are not paired truly back to back like a spindle designed for AC bearings would be. In the case of the G0704/PM25, the two AC bearings are different sizes and preloaded by the adjuster nut which gives poor control of preload, and likely thermal variability and likely bow in the spindle from the poor quality of that nut.
The first build of the spindle with AC bearings ran HOT. Too hot to comfortably touch and extended hours at 5000rpm could get a rag to gently sizzle on the lower bearing housing. The was set too high of preload and packed 100% fill with standard synthetic grease. These bearings were shot after 4 years. My second attempt was more careful during assembly, proper 30% fill with Kluber IsoFlex NBU15, and a much lighter preload. The lower bearing gets warm, but plenty comfortable to hold onto (~110*F). The AC servo running my spindle gets the hottest at about 160*F during extended operation - this is normal. Total TIR is 3.5 tenths at the spindle taper, but I cannot comment how much of that comes from the spindle taper being ground not true to the bearing seats vs. inaccuracy in the bearings. I want to say the bearings are rated <2 tenths TIR so assuming I did OK in the assembly, some of that has to be in the taper.
In practice, with the Tormach TTS collet in the mix, I get about 4-5 tenths of runout. Not ideal but doesn't mess up the work I do.
If I ever scrape my machine and get more accurate screws, I'd love to hard turn the spindle taper in-situ to give an absolute minimum TIR.
I did a fun measurement using the current feedback on my servo and found that the unloaded spindle and drive train consume about 11% of the spindle load which is about 200W of heat being generated within the head of the machine. This is actually pretty efficient for a belt drive + 6 bearings.
-Mike
They required shims to be added to take up for the difference in size from the stock TR bearings, and they are not paired truly back to back like a spindle designed for AC bearings would be. In the case of the G0704/PM25, the two AC bearings are different sizes and preloaded by the adjuster nut which gives poor control of preload, and likely thermal variability and likely bow in the spindle from the poor quality of that nut.
The first build of the spindle with AC bearings ran HOT. Too hot to comfortably touch and extended hours at 5000rpm could get a rag to gently sizzle on the lower bearing housing. The was set too high of preload and packed 100% fill with standard synthetic grease. These bearings were shot after 4 years. My second attempt was more careful during assembly, proper 30% fill with Kluber IsoFlex NBU15, and a much lighter preload. The lower bearing gets warm, but plenty comfortable to hold onto (~110*F). The AC servo running my spindle gets the hottest at about 160*F during extended operation - this is normal. Total TIR is 3.5 tenths at the spindle taper, but I cannot comment how much of that comes from the spindle taper being ground not true to the bearing seats vs. inaccuracy in the bearings. I want to say the bearings are rated <2 tenths TIR so assuming I did OK in the assembly, some of that has to be in the taper.
In practice, with the Tormach TTS collet in the mix, I get about 4-5 tenths of runout. Not ideal but doesn't mess up the work I do.
If I ever scrape my machine and get more accurate screws, I'd love to hard turn the spindle taper in-situ to give an absolute minimum TIR.
I did a fun measurement using the current feedback on my servo and found that the unloaded spindle and drive train consume about 11% of the spindle load which is about 200W of heat being generated within the head of the machine. This is actually pretty efficient for a belt drive + 6 bearings.
-Mike
Found this on the SKF site. They state that low-viscosity grease is 'also valid' but that greased bearings will run hot on run-in.
Your results may be to be expected if the preload is a bit high. I think investigating some AC bearings might be the ticket.
And Tormach gets 10kRPM with AC bearings, just like everybody else. I believe they have a 5kRPM motor, and the high range of the two-speed drive gets the spindle up to 10k.
Your results may be to be expected if the preload is a bit high. I think investigating some AC bearings might be the ticket.
And Tormach gets 10kRPM with AC bearings, just like everybody else. I believe they have a 5kRPM motor, and the high range of the two-speed drive gets the spindle up to 10k.
In spindle motor news... I've run in to a bummer with those Siemens servos.
I'm unable to get them to spin faster than about 4200rpm. Looks like I didn't do enough homework on the datasheet and my bus voltage (~340vac) just isn't enough to get them to rated speed. That, or I'm completely lost on the drive tuning and motor parameters.
Unless someone has some suggestions on drive tuning, looks like I may be back to the BLDC plan.
I'm unable to get them to spin faster than about 4200rpm. Looks like I didn't do enough homework on the datasheet and my bus voltage (~340vac) just isn't enough to get them to rated speed. That, or I'm completely lost on the drive tuning and motor parameters.
Unless someone has some suggestions on drive tuning, looks like I may be back to the BLDC plan.
If bearing temps vs. speed is at all a linear correlation, it seems like your AC bearings run a lot cooler (which is to be expected, and a little encouraging). I recorded my TR bearings leveling out at 65C (150F for you american folks
) at 5000rpm. 3.5 tenths doesn't seem too bad.I completely missed that in my research, thanks! I'm going to stick with the TR bearings for a little bit and see what happens. In the meantime, I'll dive into researching some AC bearing options. I'm definitely intrigued to try out some different bearings and report back my findings. As for preload, I'm afraid if I reduce the preload any more than I already have, I'll end up with axial play, but more than likely there could be some fine-tuning to be done (the preload nut is extremely touchy).
That's super unfortunate... I was going to comment earlier but forgot. It seems like those servos are rated for 480vac?
Not according to the datasheet. Ke is 50v/Krpm, so it should go faster. But it doesn't.
I'm not an expert by any stretch, thus I'm hoping its a drive setting I can fiddle with.
Curves from the datasheet. "Vmot" is defined in the obscure "general data" brochure as the bus voltage:
I just wanted to share this Timken resource that I came across. It specifically addresses machine tool spindle design and I have found pages 22-31 particularly insightful.