Grind my bearings?

[Oldmanandhistoy]

I would like your thoughts and opinions about grinding skate bearings with a radius to suit a round rail.

Would the benefit of full contact of the bearing with the rail make this a job worth doing?

I believe also that I could increase the pressure on the rail which would help increase over all rigidity.

There maybe other benefits that I don’t know of that some one may be able to enlighten me to also, or possibly reasons why this would be a bad idea?

If this subject has been covered else were could some one please point me to it? I have been unable to find anything.

Thanks in advance, John

[sendkeys]

In my opinion


Im sure it would help but i think it would be very hard to get any type of precision or a waste of time/money. I would just add more bearings to give it more support.

But time and money aren't everything, if you go for it maybe you should think about using a sleeve over the bearing. If you go this way you're not going to have to deal with any type of over heating or cutting away to much metal. You could also use a softer metal that would save the rails.

[Oldmanandhistoy]

Thank for your reply

I have already considered the idea of adding a sleeve to the bearing but decided this would be too much extra effort.

I have grinded a bearing just to see how thing went and had no problem with overheating.

As for precision I thought that the rail would quickly wear in so to speak and match the bearing?

What I really need to know is would there be any benefits to using this idea?

[NC Cams]

Hard to tell. Why not try it and see???

Approach seemed to work with the skate board bearings (as a former bearing engineer, I"d have advised agains it BUT you can't argue with success - at least at this point).

[joecnc2006]

I doubt the wall of the bearing would be thick enough to get the radius you are looking for.

[sendkeys]

Well if this is a router presision isn't going to matter i was speaking more for tir type error(out of round). If you have this type of error it will be hard to preload them without using some type of spring.

as for benfits. The main one would be a longer lasting surface. you wouldn't have to adjust them as often and over time would have more presision. Should also be a little quieter.

you would be able to add more preload if you used a sleeve but without i would guess you have to use about the same seeing you're taking away metal from the outside of the bearing.


Might just buy v bearings that should give 2times the surface contact. as long as the rails aren't to big or the bearings to small.


p.s this just my opinion only way to really tell is to test

[ViperTX]

Generally the bearing outer surface is hardened steel......

[Oldmanandhistoy]

Thanks all for your input.

I have calculated that grinding the bearings would remove 0.36mm if I don’t cover the full width. Do any of you guys know how much metal I have to work with?

I was thinking of running them on drill rod (silver steel over here in UK) but thus far I have been unable to find a supplier. Any one know of a supplier in the UK for 25mm diameter rod?

As I said earlier I had no problem grinding a bearing.

So would the improvement I would gain from grinding the bearing justify the time it would take to grind them? Would the time it takes to grind them be less than time I would spend adjusting them if not ground?

I think that with out a certain answer to this question and as I have made the bearing holder for my lathe already I should grind a couple more and do some simple tests. I was just hoping that some one might have tried this idea and could save me some time either way.

With the bearing I did grind, I ran it along a pipe in my hand and it seemed to almost grip the pipe. Having said this do you think that there may be a problem if I don’t get them set up dead straight with shear action?

Really wish I was a qualified engineer lol.

[WhiteTiger]

If the axis of the bearing varies from true 90 degrees to the axis of the rail, there won't be any sheer, it'll just mean that the ends of the radius make contact with the rail and the center doesn't (assuming you have true matching radii). The depth of the curve in the bearing remains the same, but when running out of alignment the outer edges are effectively closer together.

It can be an advantage as you would double your point contact on each bearing, but getting the skewed recess of every bearing exactly centered over your cylindrical rail would be a booger.

If you have full contact, "dead on" alignment of matched radii, then the lowest point in your bearing groove is smaller than the rest of the groove, and you end up with travel differences across the whole band of contact.

Basically it becomes a problem of a big wheel and a little wheel trying to travel the same distance in one rotation, and what was a rolling contact becomes a wiping/sliding contact across most of the involved surface.

This is why manufacturers of v rollers and ways tend to warn that they are best used in light load situations and will wear faster than plain surface equivalents.



Tiger

[Oldmanandhistoy]

Thanks WhiteTiger

A lot of sense in what you say and I understand your points.

Having read what you have said, it gets me thinking about the bearings I’m grinding.
Other than the point about the axis being 90 degrees the other points would come into affect in the way these bearings are constructed. A round ball running in a curved groove would have the same travel distances effect. Is something done to counter act this or is this an excepted effect.
Also the roller thrust bearings I have come across would also have a wiping/sliding action. Obviously the end of the roller closest to the centre would want to rotate slower than the opposite end. Seems to me they need to be conically in shape to avoid this effect.
But in both these cases it is obviously an excepted effect?

Having said all this would you say my idea is a bad one? Or would the sliding/ wiping effect be acceptable as is in the roller thrust bearings I mentioned.

Like I said in an earlier post I have now done three bearing. Measuring the roundness it is with in 0.01mm (can’t measure more accurately than this with the instruments I have).
Heating was not a problem.
I will set them up tomorrow evening and see how they run. Nothing to loose now.

John

[NC Cams]

As a former bearing engineer, without a drawing, it is hard for me to see exactly what's going on and/or desired with regard to this bearing application.

Thus, if one starts to prescribe a rocket science solution, the inevitable "this is only a what-its that doesn't need to be that accurate" will probably be a reply sooner or later.

Basics to consider are this: For smoothness and running accuracy, one can and prefers to apply a 1%-2% of the radial capacity in the form of an axial preload. "Snug" is an irrelevant and dimensionless term.

You do want the load axis to run perpendicular to the bearing rotation axis. Some side thrust can be tolerated but only the internal geometry of the particular bearing used will limit the amount - NOBODY in the bearing industry gives out their internal geometery info.. This is why they say to "contact the factory" for cases where there is/will be severe axial thrust.

As stated previous, I never authorized any of my clients to modify bearings - they are too easy to damage internally. Folks who ignored the advice/recommendation usually did and usually did (modify and damage, respectively).

There are specialists out there who the bearing companies work with all the time. But, I assure you, someone who calls in and asks for info on how to grind skate board bearings at home on a lathe won't get a lot if any support....

The unequal speed issue is true - although a tire/wheel combo is running at a given rpm, the ft/sec (meters/sec) of the instantaneous surface speed is one thing at the axle and a whole lot more at the OD.

"Skidding" (unequal relative surface speeds) is a real problem when you deal with bearings that are being asked to make sliding contact with an adjacent moving component - this is especially thru with non-equal radii of contact situations. The V groove thing is correct and proof that you can't always "wing it".

Now, having said that, the "but i've already done this or that" with respect to the device in question has clearly occurred. This thread is surly NOT transpiring in real time as I doubt the member is sitting waiting for a reply before he acts....

In light of the success outlined so far, the design may simply be uniquely intolerant of engineered/misengineered excesses.

Or the user is lucky

Or he has created a very unique and tolerant design that is exploiting the daylights out of the bearings he's chosen.

I"m in no position to judge...

When it comes to the use of ball bearings, there are MANY situations where they are used incorrectly - sometimes well and sometimes with disasterous results. From the looks of things so far, the speeds and loads of this application are probably such that real bearing engineering calcs don't matter all that much if at all. The bearing may simply be overdesigned for the load conditions it is seeing.

After all, the bearings were modified (against what I"d recommend) and they worked. ANother example of the perverse nature of inanimate objects.

I'd be more inclined to ask the member to keep us posted with the results of his experimentation. He's clearly not needed an engineer's input and has done just fine without it. HOwever, when or if he does have problems, any improper useage of the bearings that is endemic to and a critical part of the design could make it difficult to utilize traditional fixes upon.

Then again, he's using skate board bearings which are techically the low cost 'loss leaders' of the industry. It will probably be easier and cheaper to feed it bearings than to re-engineer it, regardless.

I say to our member, "press on, enjoy, and keep us posted". Many times in my career, when a machine doesn't perform according to my theory, I"ve had to believe the results and invent a new theory.... This could be another example of that....

[WhiteTiger]

Originally Posted by Oldmanandhistoy But in both these cases it is obviously an excepted effect? Having said all this would you say my idea is a bad one? Or would the sliding/ wiping effect be acceptable as is in the roller thrust bearings I mentioned.

Yup, bearings seem to be no different than any other mechanical product. Production economies play as big a part in design as function, most times. Roller thrust bearings in most common sizes have a negligible surface inch of travel discrepancy, and where it becomes significant in relatively larger sizes, you find operating speed and load spec is cut proportionately way down.

I have to agree with NC Cams; you already have a process that is working to your satisfaction, and parts in hand. Might as well finish up and put it through it's paces. Worst case I can see is you end up replacing with unaltered bearings... best case, it turns out to be a "gee whiz this is cool".

"All the reasoned opinions in the world aren't worth one case of been-there-done-that"



Tiger