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I am new to fasteners in general, and was wondering if anyone knew if it was acceptable to use a regular washer underneath a split lock washer. There are two applications, one is on top of a 3D printed part (PLA or ABS), and the second which is on a 1/8" 5052 aluminum sheet.

In my mind, the only benefit to using a regular washer would be to better distribute any clamping forces and protect the material underneath. But I'm worried that adding extra parts would increase chances for things wiggling out and losing torque on the screw.

enter image description here

enter image description here

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    $\begingroup$ This is a very common arrangement - I often see it when disassembling things $\endgroup$ Jul 11 at 21:01
  • $\begingroup$ IF you really want to be sure nothing will ever come loose, use a castellated nut and a split pin or a locking wire., not a lock washer. $\endgroup$
    – alephzero
    Jul 11 at 22:08
  • $\begingroup$ {flat-washer, split-lock washer, socket-head-screw} was standard practice at the place where I had a mech design job. That said, I'm not super confident of split-lock washers in terms of effectiveness, especially in smaller sizes (M3, M4). For more important situations we used belleville, serrated belleville, or even wedge-lock (aka nord-lock) washers ($$$). Also, IME, flat-washer vs no-washer does noticably reduce random self-loosening. I'm not sure the mechanism of it. Maybe it gives a more consistent relationship between torque and axial load on the screw. $\endgroup$
    – Pete W
    Jul 11 at 23:38
  • $\begingroup$ @TwoWaySpeedOfLight There's a lot of myths around how split washers are supposed to work. Enough that I don't know which is true and wouldn't trust anything I see anywhere unless it comes from someone who actively uses them all the time in critical applications. There's a chance they don't work at all or make things worse, and a higher chance there are only specific situations or methods to effectively use them. It seems to me your configuration needs a nut so use a jam nut or nylon lock nut. No threadlocker (unless it's superglue) because there's plastic in your assembly. $\endgroup$
    – DKNguyen
    Jul 12 at 0:43
  • $\begingroup$ Then again, there's also a NASA study that says they makes things worse. So here's a chance they don't work at all or make things worse so that might outweigh all the other hearsay. $\endgroup$
    – DKNguyen
    Jul 12 at 1:12
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There are subtle differences. Its definitely not wrong. IMHO, its a matter of priorities. The lock washer usually improves the behavior of the bolt from coming undone. However, there are a lot of ifs and details that need to be considered.

1. components

First of all a short discussion about the individual components (lock washer and plain washer).

1.1 plain washer

In general, the plain washer helps by distributing the loads in a greater surface (see my answer to this question).

1.2 lock washer

The lock washer has two distinctive functions:

  1. it digs into the surfaces that is in contact
  2. it behaves as spring and it can maintain a minimum clamping force over a greater distance.

Especially the second one is very important (you should look into joint stiffness, because its hard to explain here), however the idea is that the lock washer behaves as a soft spring (relatively to the bolt). When you tighten a lock washer you can displace the lock washer for at least mm without significant increase in the force. If on the other hand you didn't use a washer, then the force for tightening would increase very fast upon contact of the bolt with the clamped material. The benefit is that if there is displacement induced vibration, there is almost always a residual force, that doesn't allow the bolt to rotate.

2. Configurations

Additionally there is a difference whether its a bolt-nut or just a fastener.

2.1 fastener

enter image description here

Figure: blind fastener configuration

My preferred way when there is a blind fastener (sometimes called cap screw) clamping two pieces, is just using a lock washer. Only using a lock washer and tightening it sufficiently will probably create some sort of marking on the surface, which will inhibit further the untightening of the screw.

Using the plain washer, is helpful because it spreads the loads, however, if you are worried about dynamic loads, and untightening due to vibrations, the plain washer will probably won't help

2.2 bolt nut

When using a bolt nut, - in general- the preferred way (at least the one that I was told by experts), is that its best to have the plain washer at the bolt head, and the lock washer on the side of the nut.

enter image description here

By digging in the surface, the lock washer reduces the probability of the bolt rotating.

Bottom line

If the main worry is to

  • secure the bolt, then just the lock washer is better (although there are better ways - castellated nuts and lock pins).
  • to protect the clamped material (especially if the clamped material is soft see PLA), then I would prefer using the lock washer with the plain washer (clamped material - plain washer - lock washer - bolt/screw)

I guess the bottom line is that you need to consider the priorities on each application.

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Per this article, place a regular washer beneath the split lock washer seems not a good idea. Which says:

Flat washers and lock washers are often used together, but many people still cannot get the lock washer flat washer placement right. Flat washer then lock washer or vice versa, which one is correct?

"Ideally the lock washer goes into the threaded fastener first followed by the flat washer. This way the lock washer adds tension to the fastener assembly. It creates pressure on the bolt or nut (depending on where it is used) to prevent the assembly from loosening when it is exposed to vibration.The flat washer, through its surface area, serves to distribute the clamping pressure caused by tightening the fastener in place. It could also serve as a spacer when the bolt is a bit bigger than the ideal size for the assembly. The flat washer can be used on the bolt side, nut side or both."

enter image description here

But since the load suitable for using the split washer is usually small, the difference in effectiveness may also be small.

Edit

Additional information regarding the split washer is provided for information. Per Wiki Article:

"Spring washers are a left hand helix and allow the thread to be tightened in a right hand direction only, i.e. a clockwise direction. When a left hand turning motion is applied, the raised edge bites into the underside of the bolt or nut and the part that it is bolted to, thus resisting turning. Therefore, spring washers are ineffective on left hand threads and hardened surfaces[citation needed]. Also, they are not to be used in conjunction with a flat washer under the spring washer, as this isolates the spring washer from biting into the component that will resist turning."

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    $\begingroup$ You say not a good idea in your first sentence, but the body of your answer contradicts this? $\endgroup$ Jul 12 at 0:10
  • $\begingroup$ I see, thanks for the reply. I guess I was wondering whether or not the split on the lock washer is designed to dig into the material in any way, and as a result its effectiveness would be hindered by the regular washer. But I guess it's primarily the springiness which keeps things tight. Do you know why people would choose something like a split lock washer then as opposed to a belleville washer? $\endgroup$ Jul 12 at 0:32
  • $\begingroup$ @Jonathan It was my first impression too, and that's why I decided to add the graph to clarify the ambiguity in the sentence. The plain washer can be added on one side on top of the lock washer, or both sides - bolt side, and the nut side (with the sequence on the nut side stay the same). $\endgroup$
    – r13
    Jul 12 at 0:34
  • $\begingroup$ I believe the choice depends on the need for the application and the cost. This article will give you an idea of the Belleville washer, its advantage, and disadvantages. en.wikipedia.org/wiki/Belleville_washer $\endgroup$
    – r13
    Jul 12 at 0:44
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    $\begingroup$ It says in bold the lock washer goes onto the fastener first, followed by the plain washer. This means that the plain washer touches the part, the lock washer touches the fastener head, exactly as you say is not a good idea! Your illustration doesn't show the washers... $\endgroup$ Jul 12 at 7:29
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Flat washers aren't just about spreading the load, there is a whole list of possible reasons to use them. But if spreading the load is why you want a flat washer, then the combo of split washer and flat washer isn't the way to go.

Split-type lock washers usually have a smaller diameter than the diameter of the bolt head, so they make that problem more difficult. They are probably best for easily accessible, noncritical and relatively low load applications that may need repeated removal. An examples would be bolting thinnish sheetmetal panels into rivenuts. The rivenuts have a bit of a boss to them, so that's what the panel actually lands on. The bolts are far bigger in diameter than the sheet metal thickness and rivenuts aren't all that stout, so a split washer does a good job where you can't put any energy into bolt elongation.

For what you are asking about, use a flat washer to spread the load, and use a flange-head bolt, and a serrated washer-head nut or a flange-head nylon nut. The flat washers could be a hard plastic.

You need to figure out what the pressure distribution on each face is and look at the coefficients of friction to figure out which bearing surface is going to slide. With low surface energy plastics, it can be a challenge to get the nut to turn on the washer. You can switch to a rubber bonded washer if it helps.

enter image description here

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Yes, the lock washer stops the nut from coming undone while

The washer spreads the load over a greater area than the nut. There is a similar question on here about that.

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  • $\begingroup$ The related question was engineering.stackexchange.com/q/44701/3505. $\endgroup$
    – Transistor
    Jul 11 at 21:35
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    $\begingroup$ Also the plain washer stops the lock washer damaging whatever is underneath. There's no point stopping the nut shaking loose if a crack develops around the bolt where the lock washer digs into the surface. $\endgroup$
    – alephzero
    Jul 11 at 22:04
  • $\begingroup$ With a regular washer, a lock-washer, and a nut: The high clamp force of the nut and lock-washer makes the friction between the regular washer and the surface of the part being bolted fairly high. Shock and vibration sould not be a problem. The regular washer makes replacement less damaging to the part that is bolted. A nut and lock-washer alone causes the lock-washer to bite into the part and nut being fastened. Removal makes the lock-washer scrape and dig material off the part and/or nut as you unscrew the nut. $\endgroup$
    – Jim Clark
    Jul 11 at 22:15
  • $\begingroup$ If it is critical that they not come undone, you might use bolts that have holes through them then wire them together after assembly, and use nuts that also have a hole in them and wire them together as done with the bolts (or nuts that are castelated and use a cotter-pin through a hole in the bolt as with alxes where tightness is not a requirement). $\endgroup$
    – Jim Clark
    Jul 11 at 22:28

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