NSK Nakanishi Spindle Rebuild

I got a used NSK spindle from eBay, and on hooking it up, my E2530 motor controller gave an immediate overload warning when trying to run it. I have not been able to find any information online on how these spindles are constructed or rebuilt, so I thought it might be useful to share my process.

This is the spindle as I got it. After talking to NSK support, it turned out to be a non-standard spindle, a custom made modification of their standard line. The model number is NR-401E.

NSK uses two types of couplings on their motors, one looks like a + and the other like a -. This is the former type:

The rear coupling simply unscrews:

And right away we can see at least one of the problems. The rear bearings are completely gummed up.

Moving on the the front of the spindle...

The front protective ring screws off simply. No funny retaining mechanisms.

Underneath, we find the ring used to pretension the front bearing pair. This is a bit dirty and would not easily budge at first, but soaking with a little WD-40 helped things along.

In spite of the dirty retaining ring, these bearings look much better.

The shaft now simply slides out through the front with little force, thanks to some super-precise machining on NSK part. And the damage on the rear part of the shaft is now clearly visible. After the rear bearings seized, the shaft started spinning against the race.

The outer housing is ok.

The outside of the front bearings are marked ZL-0484A and ZL-0484B. I am not sure what that means.

However, the bearings are of type 7901, and there is very thin preload washer in between them. This has to be treated with great care or it will break.

The rear bearings also push out with little effort. These are of type 7900C. It is interesting that this spindle design is very constrained with dual angular contact bearings on each end. It seems that the thermal expansion of the shaft is handled by a tight slip fit between the rear bearings and and the shaft.

Then everything got a good cleaning in an ultrasonic cleaner. First in CRC 3-36, then in acetone. I have found that acetone is not good at dissolving every type of lubricant used. It seems that there are waxes in some lubricants that acetone cannot dissolve. So my approach is to first use the CRC to dilute and wash out the dirt and lube and then use acetone to remove the CRC. This has worked well.

Interestingly, after cleaning, all the bearings seemed ok. Since I did not have replacement bearings and since the wear on the shaft left me pessimistic anyway, I just re-lubricated everything with NSK high-performance bearing grease and put it back together.

One thing I could not figure out is how to remove the drive coupling on the end of the shaft. It is moveable and spring loaded, but it is not clear to me how this should detach. I wish I had an xray machine....

I should note that the front bearings have a marking for where they are matched. One should be careful to line these up:

In the end, I put the spindle on the controller and ran it at low RPM for a couple of hours, since I just moved everything and re-tightened all the preloading mechanisms. The spindle now turns freely and runs very smoothly. However, I don't think the rear bearings can be relied upon to take any sort of load. I think this spindle will work fine as a gang tool in my lathe for drilling some holes, but I would not use it for continuous duty milling.