Calculating preload of spindle-bearings. How to?

Gentlemen!

As some know, I'm building a Quorn-ish toolpost grinder. It works OK with

125mm disks and about 4000 RPM.

Now I made a bigger pulley to get 17000 UPM. The noise from the bearing is um ... I can't compare it with singing.

The bearing is a 7203 (angular contact ball bearing).

Admittedly, I used a plain 7203 and not the spindle version of it. Now I guess the problem is a wrong preload. I couldn't find data for that in the mfg's books.

My theory for calculating the propper preload is, that the balls wander to the outside (on the inner cone of the outer race) and lift off of the inner race. Knowing the weight of a ball, RPM, number of balls and the angle of the cone I'm tempted to be able to calculate the minimum preload to keep the balls where they should be. Am I right?

Any usefull link or tables or whatever?

Nick

[snip]

Looking at the quorn book, the quorn spindle bearings are spring preloaded. Are you using a similar method?

Wes

Yes. It is a metric () and a tad scaled up version of the Quorn's spindle-design. I know that he is giving some numbers for the preload (5 pounds, IIRC), but that bearing has -I guess- a different angle of contact.

So there should be a way to calculate the minimum preload.

What I forgot to mention: It's not just the RPM, but also the radial load that has an influence on the preload. Bigger radial load, bigger preload.

Funny that I couldn't find anything in my books or by googling. It seems that the spring here is an outdated concept. They all use paired bearings sitting side by side. Maybe I should quit the Quorn path and also use paired bearings (X arrangement) on the grinding disk end and a needle bearing on the pulley end of my spindle V2. The Quorn is using a tandem arrangement that can't take axial load in one direction. Throwing away what I have now would be OK, if only I could use it to make a more precise version with hardened and ground parts (outside and inside; so the high RPM) ...

Nick

Ive found a technical PDF on Nachi bearings site, but not read it yet. try

entos/NACHI/pdf/Tech.pdf another site recommended 2% of the dynamic carrying number (RPMxaxial/ radial load? ), upto 300N max. Im using a duplex pair on the nose end, and a single deep groove bearing for pulley support on the tail end on my spindle. at =A325 for a pair of super precision FAFNIR 2MM200 bearings it seemed easier than springs ;) of course the tolerences in the spindle housing and shaft means Ive had to buy some new measuring tools... so not so cheap afterall...

Dave

Nick, Pg 223 onwards in the Nachi PDF goes into preloading HTH

Dave

Take a look at:

Also at that sort of speed you would probably do much better using the version with a polyamide or phenolic cage and lubricate with oil rather than grease.

Bob

The 30K rpm grind spindles in our okuma grinders use oil mist oiling.

Wes

Ooops! Here they are, the numbers.

Thanks, Nick

OK, that's a ballpark. I'll try that.

The tools needed do not count! :-)

Nick

Yes, I probably needed the tools anyway, adn will certainly need them again.

7203 Precision bearing preloads: Extra light 30 N Light 70 N Medium 145 N Heavy 195 N

hth Dave

Thanks again for that one. Had a closer look at it and it seems that it will help me a lot.

Nick

My problem was having at least a usefull range to try within.

It takes 20000 with oil. With oil mist, it would go up even higher. OK, no nitpicking, sorry. This currently ain't the problem. If it fails after 100 hours, that would be OK.

Stupid me! I didn't have a look at it from this side! Sure such a preload would result in a bearing that can only withstand its own preload and nothing else, but ...

OK, thanks for the input.

Nick

OK. Increased the preload to 180..190 N and now it kinda works. Bearing is getting hot. About 50° guestimated, takes about 5 minutes and then temperature is quite stable. Measured the actual RPM to be 15350 (and then the reflective strips flew away..).

"Seems" (in fact it is a fact) that the bearings are ruined. The balls actually lifted with the little preload and bounced around in the races. When I rotate the spindle, it snaps in at several points. :-) Thanks god, I didn't buy the precision version of the 7203.

I'll invest in a new set of cheapish bearings and see how well they work without being abused by me.

It seems to me, that the Quorn spindle is really OK for the 4000 ... 5000 RPM, but with the big pulley suggested, it is beyond its limit. I think I'll build a new version with paired angular contact bearings (medium or high preload) in an X-arrangement and a needle bearing on the pulley end. Then, I also could use it for milling (quick start threads and the like) in the lathe.

"I even oiled them with WD 40"[tm] No, just kidding! I used HV 48.

Nick