Thread: Hot Spindle Bearings

Bob, sorry I'm away at the moment and struggling through with an iPhone so it's a bit tricky to jump forward and backward through threads, so this may already be pointed out. ALL bearings need some level of clearance in order to operate and some machinery uses the humble plain bearing since it's easier to get a bearing with small clearance in that configuration versus a roller bearing. Even with the pre-load adjusted it will still have some clearance if not it would simply seize. The higher the quality of the bearing the less this clearance need be. I believe there are figures available for the Hercus with regards the amount of movement you're measuring. If you haven't already done so, you can compare what you're measuring with the factory spec to get an idea of where you stand. Personally I would doubt your bearings are faulty if they are running quietly, feel smooth to turn, give a good finish, yet you're measuring excessive lateral play. That should be purely an adjustment issue.

Pete

Bob, I just got this darn phone to look at you images. I'd suggest you're measuring incorrectly for bearing adjustment. Take everything off the spindle and place your indicator on the spindle. You then lever the spindle and check the play meets factory spec. Personally I use a length of broom handle as it will pass right through the bore and won't damage it while providing plenty of leverage. The setup you show would be used to indicate runout at a specific distance from the nose with the lathe running. While it's true excessive bearing clearance may give excessive runout, the two really are 2 completely different animals.

I check mine with a dial indicator on the side of the end of the spindle, but with something longer in it to get some leverage to pull and push.

You can use the setup you show in the last picture, just move the indicators down to the end of the spindle and use that arbour to pull and push on.

Dave

FWIW there is an interesting discussion here on bearings for machine tools.. And what you get with cheap versus very expensive... All boils down to your needs..

Timken tapered roller bearings – truck bearings in my spindle? - Practical Machinist - Largest Manufacturing Technology Forum on the Web

Originally Posted by Pete F

ALL bearings need some level of clearance in order to operate and some machinery uses the humble plain bearing since it's easier to get a bearing with small clearance in that configuration versus a roller bearing. Even with the pre-load adjusted it will still have some clearance if not it would simply seize.

Pete

Sorry Pete F, I can't understand your reasoning. Preload on a tapered roller bearing means that all clearence between rollers and the races is negative, in other words the bearing is adjusted in a controlled manner to a loaded condition when it is installed.

To counter preloaded roller bearings from seizing and depending on the amount of preload they have when installed they are only run up to certain speeds for certain amounts of time, the amount of load is known in their intended operation, the cooling effect of the oil (usually an oil bath) and the addition of EP (extreme pressure) additives in the oil.

I decided to check the play again. With the indicator contact point touching the face of the spindle behind the threaded nose and pushing the spindle from both ends, there was no discernible play. I then mounted the indicator to read the radial play at the same location. Applying a fairly heavy load on the arbor about 6-7 inches from the spindle end resulted in a reading of about 0.0004".

I ran the spindle at 2530 rpm for 5 minutes and whilst the bearings were quite warm, I could comfortably leave my hand on the headstock casting. After 8 minutes of continuous running, the bearings were considerably hotter. The occasions that I might need to run the machine at that speed and for that length of time will most probably be few and far between. I might just leave it as it is.

RC. That was a very interesting and informative discussion on PM. Thank you.

Bob.

Originally Posted by Metmachmad

Sorry Pete F, I can't understand your reasoning. Preload on a tapered roller bearing means that all clearence between rollers and the races is negative, in other words the bearing is adjusted in a controlled manner to a loaded condition when it is installed.

To counter preloaded roller bearings from seizing and depending on the amount of preload they have when installed they are only run up to certain speeds for certain amounts of time, the amount of load is known in their intended operation, the cooling effect of the oil (usually an oil bath) and the addition of EP (extreme pressure) additives in the oil.

G'day, from what you've said, and I highlighted above, it would mean that the races and/or the rollers would actually distort under the pre-load. I'm afraid I'm a little sceptical that occurs in hardened races at the sort of pre-load levels being discussed here, so would be very interested if you could provide more information if you know that to indeed be the case. One of my precision spindles is pre-loaded with a simple spring, and again I'd be very interested to find that spring had enough power to distort 4 precision bearings and achieve negative clearance.

I'm keen to be corrected if wrong, but my understanding is that the pre-load removes unnecessary clearance, to effectively "zero", along the axis that they are pre-loaded. However I believe there is still an extremely small amount of clearance still there due to the physical imperfections of the bearing and the requirement to function without overheating.

Again, I'd be keen to hear more if you believe that I'm mistaken.

Pete

Hi Pete, Metmachmad,

I got interested in this topic, when doing the preload on the AC bearings for the mill ballscrews. Always happy to learn more.

I came to the conclusion that preload is just the amount of force applied, not a dimensional measurement as such. Although you can calculate the preload as movement if you know the elasticity of the parts involved. For very non-elastic parts like ball bearings, a tiny movement makes a big difference. Further, I suspect that at least some of the heat generated by an AC bearing with too much preload is caused by the deformation of the balls and races.

So, you are both correct (in my view) the negative clearance metmachmad refers to is the amount of compression that the components are under, which is directly related to the force applied and the elasticity of the parts.

I have heard of instances where bearings were dimpled (crenellated sp?) by excessive preload.

Regards
Ray.

Ray, I agree, excess heat will be created by excessive pre-loading of the bearings, indeed it could lead to a situation where the bearing heats, reducing the clearance still further, which heats it more, etc until the bearing is destroyed. In principle I understand what is being alleged with regards negative clearance but I'm afraid I'm still very sceptical that this is in fact the case in these situations. I raise the point as I'm curious to know whether that is in fact what happens. I have the manual for my surface grinder on my desk, and the configuration for its spindle is a very common configuration of a spring pre-loading 4 back-to-back bearings. Precisely the same configuration was used in a spindle I was looking at making for a toolpost grinder. I find it very difficult to believe that spring has enough power to actually distort those 4 bearings such that the elasticity of the bearing material itself was coming into play. I haven't had that spindle apart yet (but soon need to), so don't know precisely how strong it is, but the one in the toolpost grinder spindle wasn't especially strong, and as mentioned above I believe simple reduces the excessive clearance.

That's my story and so far I'm sticking to it. Ha ha

Hi PeteF

You ask a very good question, do the balls and races actually distort under preload?

I don't know the answer with any degree of certainty, (I think yes they do) but here is my logic..

1. Where does the heat come from?
I think it comes from the balls and races compressing under the preload.
More force (preload) == more heat.

One way of thinking about it is to imagine a truck, with no load on it, drive around for a while and check the tyre temperatures.. Then load it up to maximum and drive the same distance. The tyres will be a lot hotter when loaded.

The extra heat in the tyres is caused by the tyres compressing more each revolution.
I think that something similar happens in preloaded bearings.

2. Can you even apply enough force to deform the balls into the races?
The answer must be yes, if you can actually dimple bearings with too much preload.

Interesting topic, it would be good to find out for sure, if what I'm thinking is correct..

Regards
Ray

Originally Posted by Pete F

I have the manual for my surface grinder on my desk, and the configuration for its spindle is a very common configuration of a spring pre-loading 4 back-to-back bearings.

Pete
Are the "4 back-to-back bearings" tapered roller? I doubt it. A surface grinder wouldn't have much axial load compared to a lathe.
As I understand it, preload on tapered roller bearings is to ensure the bearing doesn't lose preload under load. The higher the load the higher the preload and the shorter the bearing life. Once preload is lost the load is carried by one or two rollers.
As always I could be wrong. I'll try and find a graph of bearing life V preload.

Stuart

http://www.skf.com/files/005031.pdf

Stu, I'd suggest it doesn't matter what the bearings are, the principles of the pre-load are the same, to remove excess clearance.

Ray, I love cycling! I say that because I've just come back from a ride and thought about this very question for much of the ride. Here's my conclusion, for what it's worth.

I'd suggest the heat is coming from 2 sources, one is the hydraulic action of the lubricant being squashed between the rollers/balls and the races. Forget that, it's another issue for the moment.

The other is indeed a distortion of the surface of the bearing. Perhaps it seems like I'm being pedantic, but I see a major difference between this situation and deforming the whole bearing itself, which is what "negative clearance" implied to myself. What I'm supposing is that the bearing surface is not perfectly smooth, but if magnified greatly can be thought of as a series of "mountains and valleys". Everything put in contact with another surface will deform somewhat and it is these "mountains" that are being squashed; the very peaks on the surface of the bearing. That process generates heat. However the overall shape of the bearing isn't really being changed much at all. As greater load (whether pre-load or actual load) is placed on the bearing more "peaks" on the surface are being "squashed", hence more heat is generated. Eventually the point would come where the overall shape of the bearing is being changed by the load, but I'd expect that to be way beyond a pre-load state.

What I expect is that under no load at all there is a physical "gap" between the bearing components. As a load is applied (eg pre-load) that gap (clearance is reduced) to the point where the surface of the bearing begins to be affected by the load. This is the point at which we pre-load a bearing.

Now I need somebody to tell me if I was close ... or do I need to stick to cycling

A clear, analogous decription of the events that create bearing heat. Thank you Pete.

Originally Posted by Anorak Bob

A clear, analogous decription of the events that create bearing heat. Thank you Pete.

Maybe, but is it correct? The link Stu posted implies the whole bearing yields, thus creating the heat, yet I've read other papers (also by SKF interestingly enough) that imply something different and, sadly without saying as much one way or the other, more along the lines I've what I suggested. Mind you, that SKF paper linked to above talks about tapered bearings, the diagram isn't displaying well on my monitor, but it looks like a plain roller bearing from what I can see. All very strange! It may come down to semantics as to precisely what is defined as "clearance" in this situation.

Pete