Uh Oh BMC-20 spindle bearings?

[fasto]

I haven't used speeds much over 3000 RPM very much. Today I was running some parts that have a lot of contouring with a 3/16" EM. Since I didn't want to be there all day I was running the spindle at 5640 RPM (or something like that). The first part - about 15 minutes of contouring - was fine. By the third part, there was a really bad sound coming from the spindle/belt/motor somewhere. The sound didn't change with spindle load, even with the EM cutting air it was still the same. When it got to the drilling & tapping at 1000 and 500 RPM things sounded OK.

The fourth part was downright scary. It sounded like something was going to come apart. Slowing down to 4000 RPM helped the noise somewhat. The drilling and tapping (at lower speeds) were noisy, too.

After shutting down, then turning the spindle and backdriving the motor through the belt, the everything turns freely and very smoothly. Surface finishes are fine, too. The sound really does sound belt-like to me. Perhaps I've got hopeful ears. Running the spindle in manual mode (limited to around 1200 RPM it seems) everything sounds great. If I twist the spindle speed override knob quickly, I can occasionally get it to make a similar noise.

Reading back over the posts, I see that the belts are a known potential noise source. I also saw something about Wd40 to quiet them down. Anyone try this? The machine did sit for several years (in a warehouse) before I got it. Belt tension looks good, too, just slight deflection on moderate pressure.

Thinking about what else could be the problem: The spindle motor is a Fanuc red cap, and I suppose it could have bad bearings. I can take the belts off and run the spindle motor to check it out. The spindle could have bad bearings. I don't really know how to check the spindle bearings - except to say that the spindle does turn very smoothly without any notchiness at all, no discernable play, in fact it seems very good. The drawbar air cylinder has plenty of clearance from the spindle top. The spindle orientation sensor isn't dragging, nothing seems to be touching inside the head. Oh, temperature in the shop was 65-66 degrees and it doesn't get any colder inside.

I'd be happy to hear any suggestions!
Thanks,
Aaron

[fasto]

I emailed Hurco tech support and got a call from Steve Artz (spelling?) within an hour. He agrees that the belts are the problem. A new belt is about $200 and he's checking on availability. Can't beat that for support on an ancient machine!

[bloke]

If it was the bearings that failed, they are a piece of cake to change.
2MM9112WDUL (matched pair) and 2MM9110WDUL (matched pair) if I remember correctly.

[fasto]

I believe you remember correctly. The bottom most bearing isn't labeled in my parts breakdown; the top 2 are 2MM9110WD and the labeled bottom one is 2MM9112WD - presumably the unlabeled bottom one is the same!

It seems as though I have the oddball machine - again. Supposedly most of these have a two-speed spindle. At some point mine was changed to a high-speed only spindle as there is a note in the operators guide to this effect. This changes the drive belt from a 3-rib to a 6-rib version. Because the 6-rib versions are quite expensive, someone fitted two 3-rib versions. It seems as though one of these has stretched a bit, and the two are fighting each other.
A replacement 6-rib belt is $219 from Hurco, with a delivery first week of January. I can't wait that long; I am going to remove one belt and run with only one until then. I don't expect any power transmission troubles as I am less than 20% on the load meter.

[fasto]

Took one belt off, which was a lot easier than I expected. I ran a few parts, running 5000 RPM for a total of perhaps 90 minutes. Everything works fine! The spindle is a tiny bit noisy, which I can live with. Looks like a new belt is in my future.