Adjustable Split Nuts on Spindle - How to use them

So I got into a situation I have never run into before. Unfortunately, I have to explain the whole scenario before it makes sense why I am asking.

So I am reassembling the vertical spindle for my horizontal mill. The direction change (horizontal to vertical) is done with a bevel gear that resides on the center of the spindle. See picture of the apparatus:



Notice the nut that holds the gear in place (it is the same nut as the bearing preload nut). The nut is split, (see picture), with a socket head cap screw to allow you the adjust the split. You can see (a little) in the picture where after the factory (?) assembly, the split adjustment screw was ground down to be flush with the nut, which allowed me to see where this screw was originally positioned. Additionally, there is a set screw with a copper plug against the threads.

When reinstalling, I realize it was critical that this nut was tight against the gear as it coming apart would certainly be bad news for the unit.

So I left the adjustment screw semi loose and set screw loose, ran down the nut with a my spanner wrench, then gave the wrench a couple good whacks with the rubber mallet to make sure it was tight. When I went to tighten the split adjustment screw and noticed it was not tight, and to get it tight I had to go past the ground down part of the screw. This made me think that perhaps I needed another turn on this screw.

So I loosened the nut, tried to get another turn on the screw, but this made it too tight to turn the nut down (lot of interference). So I adjusted the screw so it was just barely in tension, then again, screwed down the nut, gave the wrench a good whack or two with the rubber mallet, and then tightened the adjustment screw. Again, it tightened PAST the ground down section (why I do not know) but it got tight, and then I tightened the set screw securely.

So I went on and installed the top tapered roller bearing. The preload nut for the bearing is a similar nut (little smaller thread, so I did not get the two mixed up). I did a similar routine for that nut, although I used less torque given this was a bearing preload. Just some light taps to make sure it was taught, figuring I would have to tighten again after bearing break-in. Again, the screw, it tightened at all prior to installation, made the nut too hard to turn, so I again, left it just in tension, screwed down the nut, tightened the adjustment screw, then the set screw. All seemed to be fine.

Fast forward to the next day: I subsequently had to remove the top nut to check a marking on the bearing. When I did, I found a few little skinny metal shards under the nut threads. Nothing like actual threads stripped, just super skinny but long pieces, one of them was copper (remember, the set screw has a copper plug against the threads. I did notice, that the threads on the spindle showed quite a bit of chatter marks and were far from smooth. Not sure if this meant everything was never cleaned up very well, and these shards are just the nut and spindle seating together, or if something was wrong with the way I assembled everything.

Anyway, in short my question is this:

1. I'm concerned that if the top nut created some metal shards, did the bottom nut as well, and they are just waiting to come loose and get in my new spindle bearings? Should I disassemble the head again and check? What should I do differently?

2. Did I do something wrong in how I assembled these nuts? Is there a correct way/order of operations to tightening a nut like this? Was getting them tight with the rubber mallet too tight? (keep in mind, this was me with a big screwdriver holding the NMTB40 lugs with my leg and hitting a 6" handle spanner wrench, not clamped in a vise with a 24" Stilson.

I would look for printed instructions to be sure. So just guessing. If there is no torque spec then gutentight is about right. Was a lock washer omitted? The grind mark may be for clearance and not meant to be a mark. Is backlash on the gear important? Maybe the adjustment is made after the shaft and bearing preload is set?

The nut shown looks like it would lock the gear into place. That makes it just a nut, not a preloading nut. The gear needs a definite location and should be shimmed into position and then locked against the shim with the nut. If the gear locates against a shoulder on the spindle, you can perhaps assume it is in the correct position without shimming. But, never assume.

The top nut would be the preload. However, I don't like this system of preloading a bearing against 'air', because it could shift because of vibration or whatever. I would rather patiently make up a spacer to go between the top bearing and the other nut, or some shoulder. Fiddle with the thickness of this spacer so that you can tighten the top nut down firmly and get the proper preload spacing, independent of the tightness of the nut.

The lower nut, when tight, needs to have it's top face running nice and square to the spindle axis. I'd lightly face it using a lathe when it is tightened. It would not be unusual for that nut to be cocked slightly, so hence the custom facing operation after it has been tightened against the gear.

I would clean out the burrs raised by removing the nut (was the setscrew loosened off first?) and reassemble.

Make the proposed top spacer purposely a bit too long to begin with, and measure the up and down movement of the spindle in the outer bearing races. Sneak up on the proper length of the spacer by trial, deduction and error.

deltap - Yeah, that would be nice. This is an obscure Italian made machine. I have absolutely zero literature. The grind mark was indeed for clearance, the nut fits in a (relatively) tight orifice below the top bearing. Even with my new torque position PAST the old position, I did not see or feel it contact.

HuFlungDong - First, yes, the bottom nut simply secures the gear. The gear sets against shoulder on the spindle + a machine bushing. This nut lock this gear down against said shoulder/washer. I understand your method, but I will say this, to create this spacer via trial and error, I would be forced to press the top bearing on and off for each trial. I think the risk of damage from this repeated process far exceeds the benefits of the set preload spacer. I do, however, understand the theory. My Clausing lathe "air" preloads. When I put new bearings that machine, I got the same recommendation and information regarding newer machines that use this hard stop preload method.


I have discussed this issue with a couple people. My thoughts are now this: I don't see what I would do differently on this bottom gear nut. The fact it did not tighten to the same spot on the screw is a mystery. Perhaps it was not that tight from the factory?

The top preload nut likely has either a rust spot (this was the reason for bearing replacement), or a small ding that likely created the shards. I have cleaned all of this off, and reassembled. It is a chicken/egg scenario, where tightening the nut created the shards (yes, the set screw was backed out), but I have to remove the nut to clean the shards. I suppose I just have to hope my disassembly/cleaning/reassembly was the last of the shards from the preload nut. I just would hate to have a shaving get into any of my brand new bearings.

Certainly not an optimal scenario, but I can't pinpoint anything I did wrong, therefore, I have a hard time getting motivated to tear the spindle apart, only to do the same thing putting it back together a second time. If someone has a suggestion otherwise, I'd be interested to hear.

I was wondering- you stated that along with the split and the bolt through the lock but that there was a copper tipped set screw. If you loosened the bolt, could you turn in the set screw a little and then re tighten the bolt? I would think that the bolt might not goin as far if the set screw tip was mashed into the thread a little by the clamping torque