Can you damage a plain bronze bearing spindle by running it with too much clearance?

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Flynth

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As in the subject, suppose I have a high speed plain bearing spindle (2700rpm - that is high speed for oil filled plain bearing). It has runout over double of what it should be, but I learned to compensate for it and I can get good finishes on the machine (by varying speeds and feeds). Can my spindle get damaged by running like this for an extended amount of time?

For those interested below are more details.

I made an unpleasant discovery of a pretty substantial runout (at least when cold) on my plain bearing surface grinder spindle (0.04mm/1.5 thou, the limit for this machine is 0.01mm/4 tenths). Based on a finish I get on a single pass with a well balanced and dressed wheel I suspect somewhere around 0.02mm/8 tenths of runout when warm. I'll measure it properly when I have 2 hours to warm it up properly (that's the warmup time according to the manual). I can get good finishes by running multiple passes at varying feed stroke, table speeds and making light cuts. I would like to improve it if possible, but I think the plain bearing may already be on its full adjustment. So without some work I may not be able to remove that runout.

The spindle runs at 2700rpm.it uses a tapered plain bronze bearing on the front and two roller bearings in the back. It has a nut to adjust bearing clearance. The clearance now is similar to runout. I can push the spindle up and down 1.5 thou when cold. The adjusting nut feels like it's bottoming out. I can force it further by slamming my hand on a wrench (no cheater bars, just a 9~10 in long adjustable pin wrench), but I reverted to the previous setting in worry I'll distort the bearing by forcing it.

When I did my lathe's plain bearing I could make the spindle immovable before the nut got hard to turn.

So, if anyone knows, first, am I damaging my spindle further by running it with too much clearance. And second, am I correct in assuming the adjustment nut should be able to seize the spindle well before it requires force to turn?
 
I'm a bit surprised that your machine has a plain bearing on the spindle. Of all the brands I've used over the years I don't believe any has had such an arrangement. I'm guessing it might be an eastern European brand since the standard rpm for most grinders is 3,600 rpm and the recommended warm up time is usually 10 to 15 minutes.
 
Too much clearance is not a good thing, but not enough clearance is a bit worse.
Since you have a tapered bearing, the nut controls the taper clearance and end play of the spindle.
You may be lucky enough to split the difference between poor operation and heat generation by slightly over tightening the retainer to come to a better clearance.
While not ideal, it may add life to a bearing that’s already worn anyway
 
projectnut said:
I'm a bit surprised that your machine has a plain bearing on the spindle. Of all the brands I've used over the years I don't believe any has had such an arrangement. I'm guessing it might be an eastern European brand since the standard rpm for most grinders is 3,600 rpm and the recommended warm up time is usually 10 to 15 minutes.
Click to expand...

Indeed it is. It is Polish make Jotes Spc20a. It's a hydraulic machine with automatic feeds. I believe it was made in 1960s.

Back then in Poland it was common to use plain bearings on the front of spindles and a set of tapered roller bearings in the back. Some machines have a ball bearing on the end instead. I'm not sure why they chose to use plain bearings despite having access to good quality roller and ball bearings. I suspect it was a case of "if it works don't change it mentality" :)

Ulma Doctor said:
Too much clearance is not a good thing, but not enough clearance is a bit worse.
Since you have a tapered bearing, the nut controls the taper clearance and end play of the spindle.
You may be lucky enough to split the difference between poor operation and heat generation by slightly over tightening the retainer to come to a better clearance.
While not ideal, it may add life to a bearing that’s already worn anyway
Click to expand...

Well, I did warm it up today and indeed it was 0.002mm (8 tenths) when worm. That's double the limit for this machine. The spindle is a modular unit that should be able to be removed as one whole unit so I decided to remove it for inspection.

Unfortunately the spindle's tube is seized in its socket... I could detach the spindle cast iron enclosure and try pressing the spindle out, but I didn't want to loose the spindle to table axes alignment. So I had to tear that spindle apart on the grinder. That is not ideal as I was hoping to take it to a clean area. Instead I had to clean the grinder.

After opening it and measuring everything it seems (unless I made some mistake) there is still 6mm (a quarter inch) of travel left for the clearance adjustment.

However, it is an interesting type of plain bearing I never saw before. The outer taper is made of cast iron and it is a part of the spindle casing tube.

The inner bronze part is not slotted at all. It seems the design is that it gets deformed. The outside is relieved so it contacts the outer taper at only 3 areas. I'll take a picture tomorrow.

Hopefully I can get rid of that runout, but first I need to clean it. I found lots of crud inside. No shavings, more like old congealed oil etc.
 
It's morning around here, I'll take the photos when I'm back home in the evening.

In the meantime. How does one stone a bore? Is it even possible?

Bores are definitely scraped with a special spoon scraper. So how are they stoned after scraping?


It seems the spindle must have been seized at some point. The damage is primarily in form of some scoring on the bronze part. It is likely the reason why it was set so loose in the first place. So I would like to remove the scoring if possible. With the lathe I rebuilt before I had to bore it out then I lapped it for the finish but it took lots of work. For example I made cast lead expandable(cut) laps etc. If possible I would prefer a simpler process this time. Also the lathe bearing had plenty of adjustment left. This one not so much.

Also to add. The thread that holds the wheel hub on the spindle is bent :-( I don't want to know how big of a hammer it was slammed with to end up bent like this (I haven't measured yet, but looks like 20 thou on the end of 3/4in thread).

I'm yet to indicate the whole shaft. I hope I don't discover it too is bent.

If the shaft is straight I don't think I'll risk bending it by straightening the threaded portion in a press. I'll probably make nuts (one for each wheel hub) that match the angle when tightened instead.
 
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Ok, I've managed to pop back home to take few photos to ponder on.

Here is the headstock of the grinder:
View attachment 445003
As you can see it can be removed, but then the alignment is lost. I'm not going to do it. Also I believe there is no taking out the spindle housing tube in one piece. I tried to tap it out through a brass rod. I managed to put few dimples in it regardless of using brass and it didn't even moved a thou (I measured). There is only one retaining bolt. I have the manual and the drawings so I'm certain nothing but corrosion (or some idiot glued it in - I wouldn't be surprised considering what I found in machines over the years).

That's the business end
View attachment 445002
You can see the face of the tube, where those 4 tapped holes go and the outer taper of the cast iron bushing (inside). The outer tube face is not a dimensionally critical part. It only holds a cap. So I'm not worried about those dimples. I would much prefer not to have them, but at the same time this would be do much easier if I managed to remove the spindle whole... But no.

So carrying on. Here is the spindle and the brass nut.
View attachment 445001
You can see a matching pair of tapered roller bearings on the drive end. The outer races are removable. The bearings are supposed to have spacers thaf are exactly the same in width due to preload. Unfortunately, not only the outer bearing spacer seems too short, the retaining nut was loose... So there was no preload whatsoever.

On the other end you can see the interesting features of this shaft. The bronze part has no oil grooves at all. The shaft has a spiral groove that pumps oil forward during use. Then towards the taper end there is a gap and a short opposing thread that I can only imagine is there to pump oil back to prevent pressure building up there. Also there is an oil hole on the bottom of the brass piece there as well as a notch. Bear that notch in mind...

Here is more pics of the brass piece. Has anyone seen anything like it?
View attachment 445000View attachment 445005View attachment 445006
B101 is a bronze alloy designation.

Then I have the drawing of the whole thing and few questions.
View attachment 445004
There is an oring groove on the shaft right behind the taper, but no matching groove on the drive end. This is a reason spindle looses oil and it is probably why it has been run dry before. I don't want to weaken the shaft by adding a groove as it is quite thin on the driven end (3/4in). But I might add a groove on the inside of the cap that goes over it.

I mentioned to remember that circiular groove/notch to discharge oil in the brass part. It turns out there still remains quite a big of travel to adjust, but the shaft worn itself everywhere but on that groove. As a result there is a quarter inch wide band of raised material on the shaft that prevents the adjustment.

I wish I could just grind it off on a lathe with a tool post grinder (mounted in cemters), but the front nut is bent... The centre is in it. Therefore mounting the shaft by a bent section produces huge runout. So I'll have to carefully tape the rest of the shaft and use sandpaper to sand that raised portion down (it is slightly over a thou).

So here we are.

Edit: Well, the shaft is bent too :( that's probably why the back bearings preload was set to "completely loose".

Its impossible to indicate due to the bent front thread. So I've put it all together (after cleaning up that narrow strip of raised material and adding an o-ring) and the spindle gets hot way too quickly. I started checking what is going on and with the front bearing set very loose. Then I tried to move it by hand rotating it a quarter turn between measurements. It should flex the same amount in every direction. It doesn't :-( This means the drive side bearings essentially press the front of the shaft to the bronze bushing which is not good. One way to solve it is to set them loose... But I'll try to straighten that shaft. I have a press so we'll see what happens.
 
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I realised this thread should probably be limited just to the question, being in the questions forum. I will start a new thread in machine restoration forum once I've done some more work on it.

I asked the moderator to delete two of my above posts. Thank you to those that answered :)
 
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