A Staff Report from the Straight Dope Science Advisory Board

What's the purpose of washers (the round metal things)?

June 28, 2005

Dear Straight Dope:

While putting a barbecue together tonight, I dropped a washer on the ground and couldn't find it, so I did without. That got me thinking: What do washers do, anyway? And lock washers? How to they "lock"? I can maybe see how the split kind, when squeezed by the nut, might cause some resistance that makes the nut stay tighter (like back at the dorms when you could jam some pennies between someone's door and door frame to make it almost impossible for them to open the door because the deadbolt was pinched against the strike plate). But what about those star-shaped lock washers? No way do those things "lock." Have we been bamboozled all these years by Big Hardware, or do these things actually work?

Dear Straight Dope:

How do screws come loose? I mean without provocation or interference. For example, I have one of those common cat carriers made of two plastic shells (top and bottom) which are held together by a dozen or so screws with plastic nuts. Invariably, after a year or so between vet appointments, I find that most if not all of the nuts have unthreaded themselves from the nuts. This happens in apparent isolation despite non-use of the carrier. I've found this phenomena in other places as well, such as threaded PVC connectors in my plumbing that, despite being strongly hand-tightened, will loosen significantly after a time. There seems to be some unnamed force at work that loosens screws. Any ideas?

Lock washers definitely do something. Little Ed tells the story of the time he was bolting the starter motor back on his old Rambler (remember Ramblers?), lost a lock washer, and, like you, figured he could do without. Silly Ed! The starter soon shook loose and hung off the engine at an odd angle, chewing up the pinion and costing Ed big bucks. Mark my words and avoid his sad fate.

A humble flat washer is used for many purposes, some proper and some not. The two most common proper purposes are:

  • To distribute the pressure of the nut or bolt evenly over the part being secured, reducing the chance of damage thereto, and
  • To provide a smooth surface for the nut or bolt to bear on, making it less likely to loosen as a result of an uneven fastening surface.

(Plumbing washers, the kind found in a faucet, serve a third purpose, namely to provide a water seal, but that's a subject for another day.)

The two most common improper uses of washers are:

  • To allow use of a bolt or nut too small for the hole in the part being secured, or
  • To make it possible to tighten a bolt or nut that's too long.

Using washers improperly (or in your and Ed's cases, not at all) can lead to damage or injury, so it's worth understanding how they're supposed to be used.

The following image shows several flat washers (from left to right steel, nylon, aluminum, and rubber):

Washers 1.

Nuts and bolts work work by placing a threaded shaft under tension. This tension, or preloading, is essential to strengthen the joint and reduce bolt fatigue due to fluctuating (variable) loads, as in the case of a machine part subject to vibration. If any of the parts involved deform under load--for example, if the contact area in the threads has any creep (or "plastic" flow), or any high spots on the contact surfaces between the threads flatten out, or the bolt or shaft lengthens microscopically, or the fastened parts compress--the joint will lose some of its tension. That's why metal car parts separated by a gasket need periodic re-tightening--as the gasket shrinks, the tension on the bolt decreases, and the parts aren't held together as tightly.

Lock washers can help with this situation by applying a spring force against the nut or bolt head.  Any stretching or flattening of parts is likely to be much less than the compression of the spring washer, so the fastener won't loosen much (see note 1).

Wavy washers are another type of washer meant to apply spring force, but they're not widely used as the spring force tends to be relatively small. Belleville washers, often called "Belleville springs," are washers shaped like a shallow cone or dish. The shape of these washers allows them to exert a constant spring force on a joint, even if the bolt or joint has significant slip or stretch after initial tightening. Belleville washers are typically used in high-stress applications such as structural steel.

I didn't have any Belleville washers handy to take a photo of, so I used my mad PowerPoint skilz to draw a cross-section of one:

Belleville.

Because the lock washers I've been describing generally have smooth surfaces, some people don't consider them true "locking" washers. They think the only real lock washers are the ones that, ironically, you turned up your nose at. Star-shaped lock washers help lock parts together because the points of the star both compress and dig into the mating surfaces when tightened. Thus they help combat the other enemy of the joint, loosening of the nut or bolt due to rotation. The downside, of course, is that these washers can damage the parts they're helping hold together. Sometimes people put smooth washers between sharp star washers and the parts to try to protect the surface, but that defeats the purpose, eliminating the washer's ability to grip the mating surfaces and allowing it to exert only a small spring force on the bolt. As a compromise, sometimes people use a washer with a rough-patterned surface to provide extra friction without damaging the working surfaces.

The following two images show some locking washers I had handy. From left to right, we have a wavy washer, a nut with integral rough-surfaced washer, a bronze split-ring lock washer used for blasting applications, and an ordinary steel split-ring lock washer:

Washers 2.

Top view:

Washers 2b.

The next image shows us three of the star-type lock washers an outside star, an inside star, and a nut with integral outside star washer:

Washers 3.

Why are washers called that? Using the Oxford English Dictionary (OED), my gal Fierra found that "washer," in the sense of "a perforated annular disc or flattened ring of metal, leather, or other material placed between two surfaces subject to rotative friction, to relieve friction and prevent lateral motion and unsteadiness," is "of doubtful origin." The earliest cite in the OED is from 1346 (spelled "whashers"), but the first quote that describes their use is from 1611:

. . . an iron hoope, amongst gunners called a washer, which serues to keepe the iron pin at the end of the axeltree from wearing the naue.

Now to address the second question: Why do nuts and bolts seem to magically loosen themselves? The most common reasons include corrosion in the threads (for metal fasteners), vibration or cyclic movement of the parts causing the nut or bolt to "walk" loose, and compression of the parts. Temperature changes can cause fasteners to loosen if the coefficients of expansion for fasteners and fastened are sufficiently different. Why might "Screwed" be seeing the nuts loosen on a cat carrier? Plastic fasteners are notorious for creep and "plastic" deformation (i.e., stretching) under load, loosening the nuts. If the parts being held together are also plastic, they too can deform, further reducing what little pre-loading exists. Some plastic fasteners are sensitive to temperature change, which could lead to loosening, although I doubt that's the case here.

Aside from that, I'd check behind the litterbox for a cache of cat-sized tools--maybe Frisky is planning a jailbreak.

Notes

1. Some engineering manuals warn not to tighten split-ring lockwashers till they're flat. For example, the NASA Fastener Design Manual says:

The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time, it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.

I've thought about this and discussed the matter with engineering colleagues, and confess I don't understand why a split-ring lockwasher is useless when flat. As far as I know, a spring's resistance is greatest when fully compressed. It's possible the manuals mean that, once a split-ring lockwasher is flat, its cut ends no longer dig into the surfaces being fastened together and thus don't prevent the nut or bolt from turning, as the star washes discussed later in this report do. But the manuals don't explicitly say that.

References

Parmley, Robert O., Standard Handbook of Fastening and Joining, 1989

Spotts, M.F., Design of Machine Elements, 1985

"washer" (definition), Oxford English Dictionary Online, accessed April 28, 2005

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