Front hub nut tension

[AMB]

I can't believe that tensioning the hub nut to 225 lbs ft, has any effect on the bearing preload. I reckon the bearings in the sealed hub unit are loaded to normal specs (similar specs to the old removalable bearing setup), and all that the big 36 mm nut does, is hold the whole hub assembly on really bloody tight. However the 'how-to's' on here say that setting the tension to 225 lbs ft, is really important, and going too tight is going to ruin the bearings. Hogwash I say, or am I wrong?

[The Monty]

Id do it to what it says.

[lofty]

I'd say you're right AMB, but I'd still torque to spec if possible, though plenty of ppl will say 'till you fart' is tight enough.

[JC]

My torque wrench didn't go that far, so I used a long bar and tightened it as much as I could (a la lofty's method). Sold the car years ago, so no idea if this caused any issues.

[AMB]

Originally Posted by lofty

... 'till you fart' is tight enough.

Good one
I'm gonna use my torque wrench, goes up to 150 lb, then just do it heaps more with the breaker bar, and use loctite. Torque specs are usually just so you don't strip an alloy thread or stretch a bolt and weaken it, or squash a gasket. There's no chance of doing that on the hub.

[ronwest]

Originally Posted by AMB

However the 'how-to's' on here say that setting the tension to 225 lbs ft, is really important, and going too tight is going to ruin the bearings. Hogwash I say, or am I wrong?

You may very well be right but every single bearing company and vehicle manufacturer/service centre would disagree.

I mean you no disrespect at all but the fact that you don't have the correct tool does not mean that the correct torque is unimportant.

Don't you wonder why the specified torque isn't "150 lbs ft and then whatever you think is a fair thing" lol.

Seriously though, it's entirely up to you - your car, your money - but if I could, I'd do it to manufacturers stated torque.

[lofty]

Trouble is, 305 Nm is beyond most torque wrenches. So do you buy a monster wrench for a job you'll do once or twice in your ownership of the car? My solution was to buy a cheap nasty one from fleabay. I think it was $120. People have cast aspersions on the reliability of such a cheap thing. Apparently micrometer types don't hold their settings well. I haven't done the job yet, but when I do I figure it will at least give me a feel for how much grunt is enough.

I read a story years ago - could be an urban legend - that NASA once researched the most accurate type of torque wrench. The conclusion was a 'well calibrated elbow'. That is, experience and feel. Most of us probably don't do enough wrenching for that though.

[ronwest]

Originally Posted by lofty

The conclusion was a 'well calibrated elbow'. That is, experience and feel. Most of us probably don't do enough wrenching for that though.

Probably is an urban legend but you could get pretty close if you do a lot of it.

I really don't intend to get into a defensive position about correct tension for bearings. IMO it's a no-brainer.

I accept that not everyone has a tension wrench that's big enough and as is the case in most things we do the best we can with what we have to run with.

That doesn't mean it's unimportant and it may lead to premature bearing failure but if it doesn't, it's more good luck than good management

[AMB]

Hi there ronwest,
Sorry for any offense, should of realised that you might take it personally, was trying to be a little bit funny and just didn't think. I wasn't questioning the accuracy of your excellent how-to. And I'm sure Ford put quite a bit of research into the design of that special nut and the torque required to make sure it never loosens (with a drop of loctite to make sure). Yeh, if you've got the torque wrench then of course its good to do it up to specs, it is very important, don't want your wheel falling off.
What I'm questioning is whether that torque spec has much effect on the load on the bearings inside the hub, 150 lb or 225 lb, either way its a huge load that would crush the bearing and it just wouldn't turn. If I remember correctly, the preload on 'normal' replaceable wheel bearings is something like 2-5 lb (although I always do it by feel, the torque wrench setting always seemed to be a bit tight). And when you can't get the split pin in the hole, you back it off even further.
I wonder if anyone here has cracked open a hub to see how it works -I'll be doing that once I replace my ones. I think I'll find normal wheel bearings inside, with the races/retainers (or whatever) pressed in with a 100 tonne press to obtain a normal load on the bearings.
Now when I think about it more, you are probably right, if you did do the nuts up way too tight, you might be able press the bearing retainers (or whatever) in a micron or two, and overload the bearings. But I wonder how much extra torque would be required to do that - I don't think it would be that sensitive so that an extra 10 -20 % of torque would do it, but of course that's just a wild guess, and assumes that they were put in there with a huge press. End of rant. and open to all criticism.

[ronwest]

Absolutely no offense taken, mate.

The point I made about manufacturers etc rigidly stating the torque spec for the hub nut is the important piece of this discussion.

If you look back through the how-to pics you'll see a pic of the inner race remaining on the shaft after the hub was removed. What that says is that the bearing itself (not the hub with the bearing somehow retained within it) is pre-loaded by the hub nut. That means that it has to be tight enough so that it will hold the inner in place without it spinning but not so tight that it will deform.

I don't know how much extra the bearing will stand before deforming (remember it's a slide fit on the shaft) but I fancy that it's not a great deal.

I used to have a really informative .pdf from either Timken or SKF with all sorts of info re pre-load, tension, etc but I can't find it. I just had a quick squizz on both those sites but can't see the one I have/had. Maybe it was NSK or Koyo, lol.

Anyhoo, as I said, your car, your money. I doubt that over tightening (within reason) will cause catastrophic failure but I don't doubt for a minute that it will cause premature bearing failure.
How premature? Not the faintest idea, although I read once that correctly installed wheel bearings were designed for one million revolutions. That may or may not be current info.

[AMB]

Yeh just realised I'm rambling on, should really find out more about how these unit bearings work. That's why I want to pull one apart.

I did find this info that may be a very useful compromise for people in my situation. It's a torque Wrench Extension Calculator, see here: http://www.norbar.com/calculators/to...alculator.aspx

So my torque wrench is 400 mm long from the centre of the drive to the centre of the handle. If I slip a snug fitting pipe over the handle and extend it by say 100% (total length 800 mm), then I can set my wrench to half of 225 lb ft. Or if I extend it by 50% (total length 600 mm), then I could set the wrench to 150 lb.

[AMB]

Scratch that last idea, the extension needs to be at the other end of the torque wrench, doh! And it doesn't apply to non-length dependent wrenches. Here's a better idea: http://www.specialpatrolgroup.co.uk/...ue/torque.html

[lofty]

AMB, I'm not a bearing expert so I'm going out on a bit of a limb here, but... the traditional front wheel bearing arrangement is a pair of opposed tapered rollers. The only way the AU setup makes sense is if they're not tapered but straight rollers, or more likely balls so they can take side loads. There would be a tubular spacer inside the hub, between the two bearing inner races. When the nut is tightened, the two inner races and the spacer form a solid sandwich against the stub axle boss. Preload is not really relevant to this type of bearing, or if it is, it's manufactured in and not adjustable. That's both the strength and the weakness of it - nothing to get wrong, but no servicability.

[AMB]

Hi there Lofty. Nah they are ball bearings. have a look at the ronwest's how-to here:
http://www.fordforums.com.au/vbporta...article&id=319
Half way down the page you will see a picture of the bearing and the next picture shows the race left on the shaft. Note the sleeve in front of the race, that looks like the part that transfers the 225 lb torque load to the back of the bearing race. I suppose if you over tightened it, you could distort it and ruin it. Here's a cut away view of the hub, might not be exactly the same, but similar principle:
http://www.skf.com/portal/skf/home/i....690410.867532

[lofty]

Reading back, my main point was a bit redundant, and I'm in over my head, so I'm bailing. I will just add that the factory manual says 260 - 350 Nm, which is a pretty wide range. Make of that what you like.

Originally Posted by AMB

Hi there Lofty. Nah they are ball bearings.

Like I said:

or more likely balls so they can take side loads.

Thanks for the links. And thanks to ronwest.

[AMB]

The menu on the left of that SKF site page, if u click on "wheel end" it shows all different variations of the hub bearing unit. Looking through them all, I can see how over-tightening could distort the assembly in some cases. I read that bearing unit 3 (3rd generation) is now the norm, and skf says that the bearing preload is 'independent from the final vehicle assembly conditions'.