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To be safe, always replace the hub nuts and properly torque to spec. Almost all manufacturers and service manuals say the nuts for this kind of bearing design should be considered single use. Some hubs are even supplied with a new nut so mechanics won't cut corners if they don't have new ones handy. The Volvo Pocket Data Manual fails to indicate the front hub nut is single use (but does in other torque tables such as for the engine and brakes), but that should be considered an oversight. Every other technical source I have found says to replace them, including respected bearing manufacturer SKF (a known supplier to Volvo).
For the later 700/900 series (with no castle nut such as in 140s/240s), the front hub tightening spec is a two-step process: 100 Nm (=74 ft-lbs) with a torque wrench while turning the wheel plus a further 45 degrees with a sturdy extra long breaker bar or cheater pipe. This is with a clean, dry thread. As much as you might think it's a good idea, thread locker should not be used in torque to yield applications. Thread locker acts as a lubricant and will result in over-tightening (in the order of 20% or more), possibly taking the fastener beyond the yield point, even more likely if it's a used fastener. The later bearings used in the 700/900 series are a sealed, maintenance-free design so extra grease is of no added benefit and in fact could lead to over-torquing if it gets on the threads or outer face of the bearing.
-----------------------end of advice, beginning of discussion ----------
Unfortunately, many of us old Volvo types are used to reusing the front hub nuts on the 140/240 series (where replacement is not called for). They use a different bearing design where you do add grease and occasionally have to re-adjust using a very different tightening procedure, 50 ft-lbs -120 degrees, with a cotter pin to prevent the castle nut from working loose. That's nowhere near the yield point so that makes them safer to reuse. This perhaps gives some of us a greater sense of false security and a tendency to re-use the hub nuts on our "newer" 700/900 cars, in the process suggesting to others it's okay.
Now having said all that, in practice it's probably not all that huge a safety issue to re-use the nuts; it's just not a great idea. While investigating this topic I found some interesting technical points, so I'll carry on here for those wanting to know a bit more about re-using hub nuts in torque to yield applications:
o According to SKF, their high quality bearings are engineered for a certain amount of deformation under compression. Both over-torquing and under-torquing changes that compression which can lead to premature bearing wear according to them.
o The yield point is the point of maximum tension for the weakest part of the fastener before spring-like stretch becomes permanent deformation and weakens the metal. If you stay well under that point, there is no permanent damage and in theory the fastener can be reused. Not all two-step angle tightening procedures are actually torque to yield, but it's always safest to assume they are unless it says otherwise.
o This is a nut application, not a bolt. The nut threads are the weak point (which is why you replace the nut). While deformation can be measured by comparing the length of a bolt, you can't do that with a nut. The odds of the axle sheering are remote. What's more likely to first start happening if the nut ever starts to work its way loose is the bearing prematurely wearing out. As with any bearing starting to go, you would almost certainly notice noise, handling and braking problems long before it becomes a safety problem or the wheel falls off. It's mostly when you ignore signs of bearing wear or in the rare case of a faulty bearing causing a sudden catastrophe that you've got safety issues, which can happen whether or not you use a new nut. No manufacturing process is perfect. A faulty bearing can suddenly fail anytime and lead to the above scenario, just that it might be sooner with a used nut.
o Threads in the old nuts may already be deformed and torquing them close to yield a second time can lead to even more thread deformation and thus closer to the failure point. When the threads start to fail the nut can start working its way loose. Also, if nuts are re-used you can never be 100% sure how many times they were previously torqued or if they might have been slightly over-torqued the first time. If you were to re-use a hub nut, you would thus want to under-torque it to be safe, but wouldn't know by how much.
o If a hub nut ever did start to come loose it could lead to rapid bearing wear. If someone managed to ignore the warning signs long enough, the bearing could start to seize before it disintegrates. If you think of that left hub nut when moving forward, it had better be on good and tight when it starts to seize otherwise rotational forces will rapidly start to loosen it, hastening a more catastrophic bearing failure and possible loss of control. In a bearing seizure, you want the rubber to start skidding, not the bolt coming loose.
o It would certainly make sense to use thread locker if you were going to under-torque a used nut. Because it acts as a lubricant (as noted above), you would want to under-torque it even further to avoid over-tightening, but again wouldn't know by how much and it would depend on the amount of paste used. Thread locker fails under high heat (which is why a torch can be used to deal with a frozen bolt that uses thread locker). The excessive heat from friction as a bearing starts to fail may well reach the point where the locking compound isn't doing it's job properly.
These are all the kinds of thing engineers worry about and why they build in safety margins, including specifying new nuts.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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