nuts, bolts and washers

Say you are securing one metal plate to another using a bolt, nut and spring washer (split washer). Does it matter which side the washer goes - the nut side or the bolt side? It’s much easier to put it on the bolt side but someone told me it’s wrong (without providing any logical reason).

Also, many nuts have rounded edges on one side. Does this side face towards or away from the metal plats I’m trying to bolt? I always thought the flat (non-rounded) side faces the plate, but someone else said he thought that was wrong.

I don’t think it matters where you put a split washer. The rounded corners def. face away from what you are bolting down.

The lock washer is useless under the head of the bolt. It is designed to prevent the the fastener from spinning. Most folks assume the spring action is why lock washers work. This is not true. When properly torqued, the lock washer digs into the nut and other mating surface to prevent CCW rotation of the assembly. If there is nothing to prevent the nut from spinning, it can easily fall off. Also, most everyone overtightens most bolts defeating the purpose of lock washers anyway. Something to consider, around 90% of the threaded fasteners used to hold an average 747 together are tightened to less than 50 inch/pounds of torgue. This is about only 4 foot/pounds. It is overtightening of fasteners that causes problems with airplane parts failing, not fasteners falling out from not being tight enough.

Also, you want the rounded part of the nut away from the surface being bolted together. The greater the surface making contact with the nut and material insures better torque retention and greater strength.

You get a bit more friction on the nut when tightening if you put the washer on that side. That’ll make it a touch easier to tighten. Putting the washer with the bolt also improves the resistance of the connection to vibration.
When you bolt two things together it’s usually because there will be some stress applied that will tend to move the two pieces apart. When this stress acts on the bolt in the bolt hole, it typically causes the bolt to jam up against on side of the hole or the other. This jamming essentially freezes the bolt in place, and prevents any springiness on one side of the bolt, such as that produced by a lock washer, from transferring to the other side of the bolt. That means that if a lock washer is put on the side of the bolt head, it’s springiness can do nothing to increase the friction that holds the nut in place.

Thanks guys. Now excuse me, I’ve got to take apart tjos frame I made and move all the split washer to the correct side…

By the way where’s a good place to learn things like this? Is this the sort of thing they teach in mechanical engineering courses, or just common sense for those who tinker with or fix machines?

I believe some of the above points are valid, but I had a hard time following them.

Here’s my take on it:

Let’s say you’re trying to fasten two metal pieces together. And let’s say you’re using a bolt and nut. The inside face of the bolt head would be touching one plate, and the inside surface of the nut would be touching the other plate. So the bolt and nut are pretty much equal in this respect. Therefore, it doesn’t matter where you put the lock washer.

Ah, but it turns out it does. I’ve left one thing out: in most situations, the thread/body part of the bolt is also touching the inside surfaces of each plate. This means that the bolt is less likely to move (under vibration) than the nut. (The nut is only making contact with one plate on its inside surface, while the bolt is making contact on its inside face and most of its entire length.)

The upshot is that, under vibration, the nut is more likely to loosen than the bolt. So the lock washer should go on the nut.

the other thing that happens is that with a simple cut spring washer (the ones like an ordinary washer with a cut and slight twist, not the star type), the spring action is applied off-centre to the nut, tending to ‘twist’ the nut slightly against the thread, making the thread surfaces make better contact with each other.