NASA-grade hardware?

Pull up a floor mat or pass a magnet around the floor of most shops and you’ll find a common part: lock washers. Next to nuts and sheet-metal screws, washers are the most commonly dropped part during repair operations, and unlike the others are just as commonly left off when dropped. Chasing a non-critical five-cent part when the job is already late isn’t going to happen, but what if you need to replace a lock washer and there are several alternatives available? The U. S. National Aeronautics and Space Administration actually has a technical manual, Publication 1228, “Fastener Design Manual” by Richard T. Barrett, that summarizes everything anybody needs to know about keeping nuts and bolts from falling off.

In a nutshell, Barratt’s thesis is that for fasteners that clamp something important, from a Space Shuttle Main Engine mount to a Chevy’s thermostat housing, bolt or stud stretch is doing both the clamping and the locking. What’s the NASA opinion of split ring lock washers?

“The lock washer serves as a spring while the bolt is being tightened. However, the spring is normally flat by the time the bolt is fully torqued. At this time, it is equivalent to a normal flat washer and its locking ability is nonexistent.”

The only exception to Barrett’s criticism is where the material being clamped is soft enough for the spit ring’s edges to “bite” and resist counter clockwise rotation…assuming the underside of the bolt head or nut is also soft. This is one reason why head bolts don’t use lock washers. The other is equally important: If the spit ring breaks, and they can, the bolt will be fully loose and will fall out. Split rings are for non-critical assemblies.

How about spring or “Belleville” washers? Barrett comments: “unless they have serrations on their surfaces, they have no significant locking capability.” This leaves toothed lock washers, as commonly found in auto body and interior applications. “Although this washer does provide some locking action, it damages the mating surfaces. These scratches can cause cracks in highly stressed fasteners, in mating parts or both, as well as increased corrosion susceptibility.”

So lock washers are limited in important applications, and can cause corrosion in others. What do you do? Alternatives such as anaerobic thread locking compounds are good, although Barrett warns about their use at temperatures over 450F. While Mom’s minivan isn’t going to re-enter the atmosphere at 17,000 miles per hour, exhaust manifold bolts do get hot enough to defeat the thread locking chemistry. Fortunately, when it comes to header bolts, it’s usually an issue getting them off, not keeping them on.

In automotive applications, bolts and screws are commonly threaded into tapped assemblies, so how do you apply the right amount of torque to secure the bolt, but not strip the thread? Barrett gives a mathematical formula which roughly states that the allowable pullout load on a tapped hole is directly related to the thread diameter, the strength of the material you’re bolting into, and the number of threads engaged. I’ve always used the old toolmaker’s assumption that anything threaded to a depth beyond half the bolt diameter won’t increase the resistance to pullout. Barrett shows that this is wrong; the deeper the better.