From NASA publication RP-1228, 1990, pp. 9-10:
View attachment 183366
I always thought these things were the way to go for washers. Apparently not.
Greg
Lock washers have their drawbacks to be sure... one problem is, over time, they tend to split or break. They can crack somewhere around the circumference and then the smaller of the two pieces has a tendency to fall out completely, especially if there is any play whatsoever between the parts... Then you have a PART of a washer, basically something slightly over a half-circle, holding stuff together, and exerting force on only part of the surface instead of all the way around. Sometimes BOTH halves fall out and then the whole thing is loose by the thickness of the lock washer... not good.
That said, I don't believe everything they say just because "it's NASA"... after all, any organization that would take nearly 20 years and $40 billion bucks to turn existing shuttle engines and stuff into a new rocket doesn't exactly inspire confidence anyway. Plus, HOW in the world could you POSSIBLY have an ALUMINUM lock washer?? Aluminum is extraordinarily soft-- a lock washer works on the principle of the springiness of the material forcing the cut open, which is usually cut on a bias to create a beveled edge, which then "digs in" to the steel face of the bolt head or nut tightened down against it, preventing it from backing out. Aluminum can't be hardened to the point of being "springy" enough to make a lock washer work, that I can see... A GOOD lock washer will be made of a hard enough metal and cut at the appropriate angle so that when the nut is loosened, it will ACTUALLY CUT INTO the face of the nut pressed against it, or into the metal underneath it that the other side of the cut of the lock washer is bearing against, whichever is softer... I've seen MANY a nut or flat washer I've had to take apart on farm equipment that will have a "fish hook" or metal burr sticking out by as much as 1/8 of an inch or so, where the lock washer cut into the surface as it was being unscrewed...
In short, they're full of beans...
That said, yes, for CERTIFIED jobs where stuff HAS to be locked securely, a lock washer is NOT the preferred method... usually when you talk about locked fasteners on aircraft or rockets, typically LOCK WIRE is used... on automotive and vehicle applications, and farm equipment where it is VITAL that the nut not come loose, (which in a locknut, it really only starts to get a bite and prevent further backing off of the nut only AFTER it has turned enough to loosen up pressure on the lock washer and allow it to spring open, so the cut edge can get a "bite" on the nut or surface below, preventing further loosening... by that point the "torque" or clamping force of the fastener was originally tightened to has already been compromised...), it is common to use a "castle nut" with a crenellated upper surface and a hole drilled through the threaded portion of the fastener, through which a cotter key or pin is installed, locking the nut to the threaded part. A variation on this method often used on farm equipment, especially on large nuts used to tighten axle shafts on disks and other ground-engaging equipment exposed to the elements and working in the dirt, is a "lock plate", which is typically a flat rectangular piece of steel about 1/16 inch thick or so, with a hole for the bolt in the middle. The standard nut (usually at least for a 1 inch or larger threaded axle, which is usually square itself with a rounded, threaded end machined onto it) is tightened normally and then once completely tightened, the lock plate is "hammered over" onto/against the flats of the nut, and the "ears" hammered over the corners, to lock the nut in place. Typically these lock plates fit into a "notch" in the end thrust washer (which is usually a fairly large cast piece that distributes the clamping force of the axle bolt to the disk blades and spacers between them, forming a long stack "sandwich" of parts comprising that particular disk gang). The inside of the thrust washer is typically a square bore that fits over the square end of the square axle shaft, effectively locking the nut to the axle shaft via the lock plate. To loosen the nut for repairs, the lock plate must be hammered away from the nut, usually using a screwdriver or drift punch or something to wedge between the nut and plate and bend the metal lock plate back.
Of course more commonly used for general applications is the so-called "lock nut". These take various forms, usually either as a 'deformed' nut or "crushed nut", where a press typically partially crushes the nut or deforms the metal into either a slightly oblong "oval" shape after threading that creates an interference fit between the nut and the threaded bolt or part it's screwed onto, or a "ny-lock" nut that has a raised top hollow portion of the nut, which after threading a small nylon "collar" is inserted, and then the nut is run through an additional press machine to curl this top hollow part over tightly against the nylon insert, locking it in place. When the nut is installed on the fastener, the threads of the fastener cut their own threads in the insert by crushing the nylon plastic out of the way between the threads, locking the nylon collar to the threads with an interference fit. Somewhat better than a "crushed nut" or deformed locknut, but still not 100% secure.
On some shafts like for shredders or mowers, it's also typical to use a nut with a fairly thin flange extending from the top. This flange is part of the nut itself, typically machined out of the parent metal when the nut is formed, and threaded along with the rest of the nut. The nut is installed on the threaded shaft, typically using fine threads for superior strength, and the shaft typically will have a "groove" of up to 1/4 inch wide machined in it the full length of the threaded portion... once fully tightened, the outer flange on the edge of the nut is then hammered down into the groove in the threaded part of the shaft, using a drift punch and hammer. This "dimple" on the edge of the nut then effectively locks the nut to the shaft, ensuring it cannot turn and "back off" because of the interference between the dimple and either side of the slot. Of course disassembly for repairs requires that the dimple either be cut away, or a slim chisel be hammered under the dimple in the shaft groove, to force the dimple back out of the groove sufficient that the nut can be unscrewed... Typically the nut is a sacrificial part and is replaced with a new one after repairs, since the bent metal rim can never be completely "unbent", nor is it reliable when bent a second time (metal fatigue).
Then there's various epoxy-based "thread locking compounds" like "Loc-Tite" and such... these are usually pretty good solutions, depending on 1) the heat the parts are subjected to, and 2) the need and frequency of disassembly for service, adjustment, or repairs. Epoxy-based threadlockers can be incredibly difficult to get apart in some formulations, and usually heat softens them to the point where they can fail to lock the parts together-- they can melt and actually form a LUBRICANT on the threads! Usually the more heat-resistant the formulation, the harder it is to get the parts apart when necessary (which is why there's different colors and "hardness" of Loc-tite. In a similar way, Ny-lock nuts can also fail to lock when exposed to heat sufficient to soften the plastic so that it loses its "grip" (interference fit) on the threads it's locked to...
That's the main ones off the top of my head, anyway. Basically, if you want a GUARANTEED lock, you need a cotter pin or locking wire... everything else is "not 100%" guaranteed not to come apart...
Later! OL JR
