Electronics > Mechanical & Automation Engineering

Threaded fasteners on cars

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Hi there,

How about the following topic. I have two cars and I like to work on them in the DIY car repair shop. When pulling parts I notice various threaded fastener components. You know: Nuts, bolts and washers. Some car assemblies use various types of locking mechanisms. For example for my FWD hatchback I do believe it is very common to mount the front axle to the wheel hub with a castle nut and a cotter pin. This is obviously a secure way to keep the nut in place during a car drive even with strong vibrations. But it is quite inconvenient to use castle nuts everywhere on the car. Another thing is lug/wheel nuts or lug bolts. In the Haynes manual for one of my cars they recommend to apply thread locking compound to prevent the nuts for loosening. But still I believe few people do this and few wheels are flying off. Okay, I’m definitely aware of proper torque and regarding this I do follow manufacturer recommendations carefully. Another example is some shock absorber where the mounting on the rear axle is just done with a flanged bolt and nut, without any wave/split lock washer.

So, my objective is to learn to judge for myself what assembly needs which kind of locking mechanism if any at all. I’ve got some books on mechanical design and a NASA fastener guide (available online) but I wasn’t able to find information on this very topic.

How could I learn more about this topic?

Thanks for your time!

Stray Electron:
   The first thing that you should probably understand is that there are different "classes" of threads. Automobiles generally use class I threads, the loosest fit, and those are very prone to vibrating loose.  I don't know if it's available in your country but 'Machinery's Handbook' has a huge section on threads. They print a new edition every year so used copies are cheap and widely available and things like threads don't change much.  I probably use my 1938 edition more than my newer editions.

  Unfortunately a lot of the fasteners used on automobiles are odd shapes and it's not easy to find replacements for them so one trick I've started using is to put a small diameter plastic fishing line through the nut or in the threaded hole and the then inserting the male thread. The plastic line makes the nut act like a Nylock lock nut.

  I've never used one but on Ebay and other sites, there are drill rigs that will let you drill diagonally through the corners of regular nuts and then you can use safety wire to secure them.

 FYI  I use anti-seize compound on just about all external nuts and bolts to prevent them from rusting up.  It doesn't seem to make them any more prone to loosening up than untreated threads but does make taking them off much easier.

The most important consideration is vibration resistance:
Shaking the joint in shear, the bolt should not come loose, or faster than some rate.

Some joints are, not only more important than others, but more prone to vibration.  You don't want your wheels falling off, or pistons or engine head -- these get locking mechanisms, like the castle nut, or lockwire, or at least being torqued properly.

Clamping force and torque are important as well, particularly when optimum amounts are needed.  Usually threads are designed for dry fitting, meaning the torque is calibrated for typical friction plus tension divided by the pitch slope (a screw is a ramp in revolution, after all).  Using lubrication drastically reduces the friction and therefore torque required, and may reduce the vibration resistance.

It could well be preferable to go with a dry joint, over lubrication or anti-seize -- indeed rust might be a feature, not a bug.  That last point kinda depends on how often they need to be serviced, how fast corrosion might happen, and if the risk of breaking fasteners is acceptable.  So, lug nuts, kinda eh: there's 4+ of them (usually), so some redundancy there, but subject to vibration and salt spray so they're kind of important; but they might also be serviced every few months/years (depending if you're following a rotation cycle or not :P ) so might not get too caked up in the mean time.  Other frame components, suspension, control arms, are rarely serviced, but taking more time to do them on the rare occasion they need to, probably isn't a big deal.

Suspension items BTW, are usually mounted in shear, or pinned, so they can be attached surprisingly simply, with just one bolt say, even for something as critical as that.  Well, shear is still the problem, the point, right, but evidently designed in such a way not to be a problem.  For example, they might use relatively large bolts, so the shear slippage is greater than design peak force maybe.  Control arms often have rubber bushings, which helps reduce peak forces, as does the tire itself.

Anyway, I'm just an EE, absolutely there are plenty of resources out there, from handbooks to textbooks.  Lots of hard won experience out there in the ME-sphere, a couple books should do 'ya.  Not... that I know which ones, but, there's a collection of linkages and slides and other mechanisms, that might also be of interest.  Probably one for bolted, riveted and other kinds of fixed joints.  Or, I forget how much of these is covered by Machinery's.


Generally the manufacturers fastening choice for a specific application is the correct one but like with electronics systems are constantly developing.
The motorsport industry provides the best feedback to manufacturers to develop better systems however as things improve so must our techniques to get the best from new technologies.

Fasteners in themselves wouldn't seem a complex topic however not only the types but the specific application and the metal alloys in use determine what's reliable.

As a kiddy when I first turned spanners thread retaining compounds were still a dream as were most of the high strength alloys each of which uses different fastening techniques for locking, thread size and pitch and fastening torque.
As some example I've worked a lot with magnesium high strength alloys that with the correct fasteners can give strength and fastener torque near to mild steels.  :o
Yet in the first half of last century locking was often done with wire with bolt heads drilled or castellated nuts and split pins for anything under high stress that in some applications is still used.

However without a workshop manual and significant experience we all at times need guidance for which any of the many thread torque guides can be invaluable and many torque wrenches come with one but there are plenty online discovered with a Gurgle 'thread torque guide pdf' search.

Then of course we need know exactly the tread size, pitch and tensile rating of the fastener we're dealing with so calipers, thread pitch gauges (Metric and Imperial) and sometimes even treading/tapping charts are all required to positively identify some fasteners so to determine their maximum assembly torque.

Then some bolts are single use, that is with correct use they are stretched when tightened to a specific torque but sometimes to a torque then an additional 90o turn.

No it's not a complex topic at all !  :P

I've never in my life seen a wheel fall off a car due to unsecured lugs. Wrong torque, yes.
The fasteners in a car will not work loose if they are torqued correctly (well, perhaps on British cars).
Wheel lugs are no big issue, they generally need 100...120 Nm (unless it's a truck).
For all other fasteners you need the Workshop Manual for the car in question, which will list the torques. In rare cases, threads will need securing, but generally plastering Loctite all over the place is not the solution.


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