This copied from the Magazine thread - comments by Paul Maddox:
Super 'Technical Topic' in M176 on Rear Hub Bearings. I agree with Simon's view that a high torque on the retaining nut seems unnecessary, since no pre-load is expected on a ball bearing - compared to a Taper Roller bearing where the correct pre-load is critical - and the presence of a lock washer surely supports this view.
I was intrigued by the comment that some traders sometimes offer an equivalent to 3LDJ40, and am curious whether these might be the self aligning type, e.g. SKF 1208, as the offset ball arrangement means they maintain the narrower 18mm width, compared to the 23mm of 4208, but its load ratings are only ~60% of 6208.
My reference is a 1981 SKF General Catalogue which contains a lot of useful information on selection criteria, but nothing obvious on retaining nut tolerance. Interestingly, it does give an example of tolerance fit onto hollow shafts for 6208 - is the axle shaft hollow?
BR,
Paul
Rear hub bearings
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Simon Johnston
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Re: Rear hub bearings
Many thanks for your kind words, Paul, it’s good to see that there’s at least one reader! I had to dig deep to get information on how tight to tighten the nut but everything I found pointed to around 50-60% of the static load bearing capacity of the bearing. When you look at the standard tool for tightening the nut - the tube with a Tommy bar at one end and eight tangs at the other - there’s no way you’d ever get 120 ft. lbs.
As far as I could determine, the self aligning bearings are not suitable for a hub application. The one trader I contacted about his double row ball bearings - £120 each as I recall - couldn’t give me any technical specification whatsoever. It seemed to be a case that they were double row and therefore original and better than the 6208. My own conclusion was that even the original 3LDJ40 was inferior as it is a slot filled bearing and thus lower axial capacity than the 6208.
The bearing is fitted to the axle casing which is of course hollow. Hence my comment about the tightening torque for a carbon steel pipe rather than a solid steel bolt and nut. What does your catalogue say?
As far as I could determine, the self aligning bearings are not suitable for a hub application. The one trader I contacted about his double row ball bearings - £120 each as I recall - couldn’t give me any technical specification whatsoever. It seemed to be a case that they were double row and therefore original and better than the 6208. My own conclusion was that even the original 3LDJ40 was inferior as it is a slot filled bearing and thus lower axial capacity than the 6208.
The bearing is fitted to the axle casing which is of course hollow. Hence my comment about the tightening torque for a carbon steel pipe rather than a solid steel bolt and nut. What does your catalogue say?
Re: Rear hub bearings
Simon, I believe you are correct about self-aligning types not being suitable. I only mentioned them in passing. Equally, I don't recall being aware of slot-filled bearings.
There is a lot of information in the SKF General Catalogue, but I've selected 4 scans that might prove interesting. 'Selection of bearing type' reinforces what we already know; 'Fits for hollow shafts' covers the reference I mentioned previously and is obviously of relevance; then we get onto Axial & Radial location, where axial is what we're interested in wrt to the hub nut.
There is nothing to suggest any degree of tightness, rather under Accessories it simply describes hub nuts as 'a simple means of axially locating bearings and machine components on the shaft. In view of this I'd be inclined to take SKF's conservative rule-of-thumb, thus 1070lbs in your equation, to give a lower torque of 28 ft.lbs (38Nm), which 'feel's more than adequate to me. Or, if you prefer a range of 28-55 ft.lbs.
There is a lot of information in the SKF General Catalogue, but I've selected 4 scans that might prove interesting. 'Selection of bearing type' reinforces what we already know; 'Fits for hollow shafts' covers the reference I mentioned previously and is obviously of relevance; then we get onto Axial & Radial location, where axial is what we're interested in wrt to the hub nut.
There is nothing to suggest any degree of tightness, rather under Accessories it simply describes hub nuts as 'a simple means of axially locating bearings and machine components on the shaft. In view of this I'd be inclined to take SKF's conservative rule-of-thumb, thus 1070lbs in your equation, to give a lower torque of 28 ft.lbs (38Nm), which 'feel's more than adequate to me. Or, if you prefer a range of 28-55 ft.lbs.
- SKF Selection of Bearing Type pgs 22-23.JPG (198.63 KiB) Viewed 1634 times
- SKF Fits for Hollow Shafts pgs 80-81.JPG (177.92 KiB) Viewed 1634 times
- SKF Axial Location of Bearings pgs 82-83.JPG (203.05 KiB) Viewed 1634 times
- SKF Radial Location of Bearings pgs 68-69.JPG (233.68 KiB) Viewed 1634 times
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Simon Johnston
- Posts: 5
- Joined: Sun Feb 17, 2019 9:00 pm
Re: Rear hub bearings
Paul,
These make interesting reading - thank you for sharing them. I’m afraid that a lot of (most of?) the theory goes over my head but I think it is clear that the figure bandied about of 120 ft. lbs. is excessive. Indeed, as you’ve noted, 25% of the static loading is probably sufficient and would be consistent with both the tool used to tighten it and the reference in the Instruction Manuals to 'slackening' the nut with a brass drift. However, the bearing surfaces on both sides of my axle weren't in wonderful condition and I used Loctite to try and make up for this. In these circumstances I felt that going for around the 50% of the static loading was prudent to try and prevent the inner raceway from fretting the bearing surface any further. I think that the stresses that the hub is under, and thus the bearing, are probably greater than would be found with, for example, a stationary machine with a bearing on a immobile shaft.
These make interesting reading - thank you for sharing them. I’m afraid that a lot of (most of?) the theory goes over my head but I think it is clear that the figure bandied about of 120 ft. lbs. is excessive. Indeed, as you’ve noted, 25% of the static loading is probably sufficient and would be consistent with both the tool used to tighten it and the reference in the Instruction Manuals to 'slackening' the nut with a brass drift. However, the bearing surfaces on both sides of my axle weren't in wonderful condition and I used Loctite to try and make up for this. In these circumstances I felt that going for around the 50% of the static loading was prudent to try and prevent the inner raceway from fretting the bearing surface any further. I think that the stresses that the hub is under, and thus the bearing, are probably greater than would be found with, for example, a stationary machine with a bearing on a immobile shaft.