What you say makes sense, it doesn’t jive with the way I’ve always visualized split lock washers working. I had this mental picture that, in the absence of the washer, should the nut rotate due to vibration, thus decreasing the normal force between the threads of the nut and the threads of the bolt, friction would decrease further, allowing easier rotation until the threads of the nut were in the backlash space between the threads of the bolt and would rotate freely. The washer would act as spring to take up the backlash.
In other words, the washer wasn’t preventing the rotation, it was just acting as a ‘last chance’ to take up the slack should the original joint fail.
I have nothing to base this theory on, it was just the way I imagined it working.
And your intuition is the general notion behind Theory #1; that the washer somehow adds to the preload or takes up it in its absence. But the actual force generated by the spiral washer when fully compressed is a small fraction (typically around 2-5%) of the correct preload. The bolt (and to an extent, the materal that forms the joint) should form a spring as well–when you torque the bolt, you’re actually stretching it, and compressing the material–that takes up this “backlash” and maintains sufficient normal contact between thread surfaces to generate the friction keeping it in place. In fact, in any properly designed joint with dry threads and torqued to the proper value, the axial component of the friction force is so much larger than the tensile load that there’s really no way it can ever back out, even if the load cycles from zero to the yield strength.
The reason bolts come loose is one of three causes:
[ul]
[li]The bolt is undertorqued (loose, insufficient friction);[/li][li]The bolt is overtorqued, or overstressed by the load, causing yield failure in the bolt; or most likely,[/li][li]The joint is overly compliant (spongy) and in operation/vibration lets the preload go to zero, and the bolt to progressively walk out.[/li][/ul]Potentially, a failure could also occur from a bolt that is excessively lubricated (with anti-seize, for instance) but in practice I have never seen this. (Anti-seize is sometimes inappropriately used to reduce friction in order to get an accurate torque value, which is okay for a test load, but not for a fastener that is supposed to make a permanent joint.) As long as you have that sucker torqued down to the correct value for the size, strength, coating, and joint compliance, and the bolt is correctly sized for the joint and load, it shouldn’t ever back out. If it’s a critical, single-point-of-failure joint or something that you can’t access to check and tighten, you use lockwire or a jam nut to positively secure it. Sprial lock washers to absolutely nothing for you.
And with that, I’m going to stop being a total engineering nerd, at least for the evening.
Stranger, I do believe that spiral lock washers are still used in the field to create a better bond for grounding purposes. They tend to scratch off paint and dig into the metals and the bolts thus creating a clear path for grounding purposes. There are better ways to accomplish the same result, but this is a simple and dirty method to ensure ground continuity through fastened pieces.
You should be getting good conduction between the threads, though (presuming, as one never should, that some injut didn’t paint the threads). Besides that, if you’re going to use them at all, they should be put on with a flat washer underneath, so it shouldn’t be digging into paint, anyway. (The flat washer maximizes bearing contact between the bolt head and the joint, which can be important when you have an oversized clearance hole.) But most of the time that I see them, they’re not in any application where a grounding path would be necessary. People just throw them on because “it’s what we’ve always done,” and getting them to stop mindlessly doing it is a Sisyphusian task.
I recall that british cars (when they were sold over here0 had a weird mixture of whitworth pattern (and english) standard bolts. that made for fun repair sessions-you were always wondering why NONE of your sockets would fit the bolt heads!
Ah, Triumph! How I hate thee! Let me counted the ways.
Interestingly enough, Whitworth threads are still used on camera tripod screws (though those are handscrews, so at least you don’t have to find a socket that kind-of fits.
In hydraulic fittings, there are a wide array off different thread types used, and people tend to stick with the one or two that they know, so if you get a machine with some legacy design on it, you end up with masses of adaptor fittings. A coworker once went through an aerial manlift deisgn and removed about $2500 worth of obscure fittings by commonizing everything to JIC 37 degree fittings. (The argument ensued as whether or not to go to ORFS, which is undoubtedly the superior fitting, but that would have largely negated the cost savings.)
While I agree with most everything you have posted here about lockwasher and fastners, there are a couple of items above that I take some small issues with.
Anti-seize use. Anytime you are using steel to fasten to aluminum, you should use anti-seize. In practice I don’t think I have ever seen a bolt come loose that was properly torqued, and the failure was do to anti-seize. I have seen plently of threads torn out of aluminum due to seizing and galling. I guess it is possible to use too much and get a hydraulic lock in the bottom of a blind hole preventing proper torque, but that is not the anti-seize’s fault, that is an idiot installer.
Bolts that are subjected to vibration can walk out if not retained even when properly torqued. In this type of application chemical lockers, locknuts, or safety wire is used. Safety wire is probably the least useful of the mechanical retainers for several reasons. First, very few people own spools of safety wire and twisting pliers (If I had a dollar for everytime another mechanic asked me what those funny pliers were, we could go to a nice dinner) secondly there is a skill that is necessary to applying safety wire, and finally is the safety wire is applied /twisted incorrectly it can allow the bolt to back off anyway.
Much better to use a lock nut (Nylon or a deformed steel nut) or a glob of plastic locker pre applied to the bolt threads.
Personally, I think Locktite when properly applied is the best in all applications when the assembled joint is never exposed to more than 400F, and you can allow for proper cure time.
ralph124c British cars haven’t used Whitworth or British Standard for many many years. There were (and may still be) some legacy fastners on these cars.
If you want a real wrenching adventure try an old Volvo. The car was SAE (inch measurements) the Fuel injection was Bosch so it was metric, and if you had an overdrive, it was British Standard. Walk onto the Snap-On truck and ask for one of everything
Getting back to the OP, my question is why in the name of OG did they use 6 fastners to hold a seat in? I don’t hink I have ever seen more than 4 on any seat.
I agree with this; however, I’ve always taken to having Helicoil inserts in any aluminum manifold or bracket that was connected with steel fasteners specifically to prevent this. It’s pennies for c-notes to prevent somebody from galling it up (or it gets corrosion-bonded) and having to be drilled out and tapped later.
I would contend that if a steel/cast iron joint is properly designed (at least in the ideal world) that bolt walkout shouldn’t happen. If the installer torques the bolts to the specified range and the joint still comes apart, that’s a design problem (most likely due to bending or compressive stresses that totally, if just momentarily, relieve the preload). In reality, of course, it is sometimes not possible to prevent this vibration, especially with pipe fittings and other systems that can see high resonant vibrations, and so some kind of chemical/mechanical threadlocking is used. This can also be a problem with very compliant joints, such as those in brass or fiberglass, in which required preload can’t be maintained. (Any mechanical through-deck fitting on a fiberglass or wood boat hull, for instance, should be fixed with a bearing plate and a jam nut.)
I agree that lockwire is more complex and time consuming, and thus totally inappropriate on a consumer product like a car, where an untrained mechanic is likely to just leave the lockwire off. However, in a critical application with a lot of vibration or impact load where you don’t expect the bolt to be removed–say, a pivot pin for a piece of construction equipment, or hydraulic boss fitting on the fuel feed system of a rocket–lockwire (if properly installed by a trained technician) gives almost invoilate confidence in the joint.
The main objection to using adhesive threadlocker like LocTite, or a screw with a threadlocking patch, is that many people (even ones who should know better) will just stick the same bolt back into the hole sans threadlocker if they don’t have a bottle of LocTite on hand. Perhaps this isn’t a problem with auto mechanics, who should have this useful stuff on hand in gignormous quantities (not only does it assure locking, but it also helps prevent the threads from corrosion), but it was a major issue with field service techs working on construction equipment who often seem to bring less than minimal tools and never keep this with them. (Or maybe that was just the companies I worked for, all of whom are now out of business or absorbed by more successful corporations.) To use it properly, you should clean and degrease the threads, apply the recommended amount (a drop or two), torque it down and wait for it to cure before cranking up the machine on full, a process many techs just don’t seem to have patience to follow. I’ve used the stuff on cars, boats, firearms, motorized equipment, and other applications and it works great…provided you use it consistently and correctly. In the flying bomb industry, however, we just don’t trust people that much.
My experience is in airplane hydraulics, and we avoid threadlocker like the plague. The problem is that if you depend on it for anything vital, sooner or later the compnent manufacturer has a problem with properly cleaning the parts first, or they get a new assembly technician who hasn’t been trained in applying threadlocker, or some darn thing, and then you’ve got a bunch of defective parts out in service.
When a part really, really needs to stay together, we demand two separate mechanical means of retention, say for instance a locking insert and a tab washer.
We also don’t rely on lockwire to maintain torque. Vibration (which is the big problem in aircraft design) can cause a bolt to unscrew itself enough to lose the preload on a joint without breaking the lockwire. We typically use locking threaded inserts and lockwire.
If you install lockwire correctly (and it doesn’t break), it should prevent the bolt from rotating more than a tiny fraction of rotation. As Rick notes, though, correct installation is operator dependent; it needs to be both installed in the correct orientation and properly (but not over-) tightened. And it takes time–a couple of minutes per bolt–to do this. On a large joint, this could end up being half and hour or more.
The belt-and-suspenders approach is definitely called for, especially in some critical operational component in a machine that flies through the air, but we’ve gotten away from relying on locking inserts. I don’t know the whole story, whether it was due to defective/counterfit parts, or some kind of technician misinstallation, but we just use non-locking Helicoil-type NAS-spec inserts now. Lockwire is the last word as far as rocket propulsion people are concerned, and threadlocker is right out. It probably should be anyway, as in many applications we’ll see temperatures near or in excess of the rating for even the high temperature threadlocker (LocTite 272) anyway.
Stranger
Man, I love a message board where I can get in a continued discussion about threaded fasteners and bolted joints. That’s worth $15 alone. Now, if only I could find this in real life with a tall, shapely brunette…
Now that I think of it, the reason may be because locking interserts will change the torque value on your bolt for the same preload. Many of the fasteners we use–because everything needs to be as light as possible, and because it’s only going to see one use–are extremely high strength fasteners that are torqued up to >90% of yield, so even a moderate addition to torque can cause a failure. (These are “use once and throw away” bolts; unfortunately, some people think that this is merely an advisory statement on the ICD, and if the bolts look fine they decide to reuse them, resulting in small problems like having a thrust vector controller come loose in midflight. You’re not saving money if the thing falls out of the sky like a rock, guys.)
Actually, that should be 246 and 266. The 272 is permabond threadlocker.
Good ole 272. This is how you know if the guy assembling the joint knew his ass from 3rd base. If he used 272 and you can screw it apart without the use of heat, he did the job wrong.
If he did it right, you will have no threads left on the bolt unless you use heat.
A very destructive method of verifying a repair, but effective.
The wife’s car (SATURN ION) had a problem-the lamps beneath the climate control panel were out 9couldn’t see them at night). So i said, sure! Took the panel out-what did I find? SATURN had SOLDERED the lamp bulbs into the circuit board! No easy replacement-so i took the board out, desoldered the bulbs, and soldered two new bulbs (from radio Shack-thank og for RS)! Put the thing back together (carefully-don’t want to snap those plastic tabs!). GM-what the hell are you thinking? Why not design the board so that you can replace the lamp bulbs when they burn out?? :o
Because replacement bulbs are $1.29 and replacement circuit boards are $347.63, and only available through the GM parts department. It probably saves four tenths of a penny to solder the bulbs on manufacture as well as makes GM a couple of hundred bucks when replacement time comes around.
