Often I take something apart and when I go to reassemble it, I don't remember which side the washer was on. When there's only one washer, does it generally go on the nut side (pressed up against the nut) or the bolt side (pressed up against the bolt)?
Washers are used for multiple purposes when mechanical parts are assembled using bolts and nuts. Here are some usages and as you can see it is not likely that a generalized answer can be devised to directly answer your question!
Some washers have a special design that attempts to help keep the nut and/or bolt from coming loose. Known as a lock washer these may be a split ring type, star type, wavy or any one of a number of other designs. A lock washer will be used with the part of the assembly that most likely could turn such as the nut. It could be used under the bolt head in instances where the bolt screws into threads in one part of the assembly.
A washer is sometimes used to protect the surface of the assembled parts. A nut or bolt head being turned during the tightening process can mar the part surface around the hole and a washer can be used to take the abuse as opposed to the part. This may be particularly applicable when the parts are a softer material such as plastic, brass or aluminum and a washer made of a harder material is used.
Sometimes a thin material is part of the assembly and a larger diameter washer is used to help distribute the pressure of the tight bolt across a larger part of the thin material. In these cases the washer would be used against the thin material. It can help to keep the material flatter or to prevent pullout of the fastener.
There are times when a large diameter washer used as in #3 above may be used with yet another washer smaller in diameter washer on top of the first. This would be done for cases where the larger diameter washer may tend to mushroom when the bolt is tightened.
Assemblies often have a slotted or oversize hole on one side. This is designed to allow for tolerance in the hole size and location in the parts being assembled. Another variation of this would be a long slotted hole that permits the adjustment of one part relative to the other before the bolt and nut are fully tightened. A washer would be used over the slotted hole to allow the pressure of the tightened bolt to be spread across the slot and up onto the part itself. In some instances the washer will also help to prevent the turning bolt head or nut from deforming the slotted part during the tightening process.
Some assemblies are designed to have the parts move with respect to one another when the bolt is actually tight. The travel of the nut may be restricted by a shouldered design of the bolt or a sleeve can be installed around the bolt in the hole. In these cases a washer is often used to keep the moving part from wearing the bolt head or nut. The washer may be made of a material designed to reduce the friction between itself and the moving part. This can also help to keep a turning part from trying to loosen the nut through friction.
Many times washers will play a role that is a combination of the above usages. One common example is to see a lock washer used along with a larger diameter flat washer.
If there is only one washer used with a nut/bolt, it usually goes on the nut side.
The nut in most circumstances is more movable, and is more commonly turned to tighten the assembly. The washer helps prevent damage to the surface of the object being fastened. In most cases where the bolt is easier to turn, the bolt has a round head that will cause less damage anyway.
For the same reason, lock washers always go on the nut side to stop the nut moving. Lock washers are almost always used with a flat washer as well.
The nut is also often smaller in profile than the bolt head. Not always, but often. The washer helps distribute the pressure that the nut places on the surface of the fastened objects.
Triple use (as I saw after years of mechanics) :
1) Washers are ofter used to avoid grinding of the support when you screw the nut.
2) Washers avoid the bolt head or the nut to penetrate/damage the support.
3) Washers help things moving in boltened assemblies if you oil them a bit (eg. articulations).
Following Niall C comment : observation and a bit of logic will make you put the washer on the right place, now you know what it can do.
There are so many types of washers, i will advice on the more common ones :
Flat washers (first row of the picture) :
- "Flat not wide" : avoids degradation of the support (paint, ...)
- "Flat wide" : for pivots, when the pieces you bolt have to move
- "Flat wide and thick" : for avoiding bolt/nut penetration in not-so-hard supports (pine wood, plastics, ...)
Lock washers (second and third row of the picture) :
- All types : penetrates the support material in order to lock the bolt/nut and the support
All others types are for specific use...
Generally when bolting up you insert the bolt through the various components that are being bolted together, place a washer over the thread and then secure the nut. Normally you do not want to rotate the bolt inside the components being held together. The washer provides a smooth relatively friction free surface on which the nut can rotate. If you are interested in a torque value to set the bolt, it is not possible to allow for the friction of the bolt shank rotating inside the components being bolted together as this is a vastly varying value whereas the nut on its thread up against a washer gives reasonably consistent torque values. If the member under the head of the bolt is badly corroded or the hole is worn large then you may consider a washer under the bolt head to provide a better distribution of the load.
When there's only one washer, that could be due to someone forgetting to put on a second washer during initial assembly, or when the item in question was last taken apart and reassembled - there's no absolute guarantee that where the washer was, before you took it off, is where it's supposed to go.
While the odds favor putting the washer under the nut, rather than under the head, there are so many variables to consider that a simple 'always under the nut' answer is more misleading than useful.
Best thing to do is look at the situation, and decide where it seems to make the most sense to put the washer. Some trial & error might be necessary, and/or the addition of a second washer, and/or a lock washer, etc.
Chances are that you wouldn't be taking the thing apart in the first place, if it had been working perfectly, so making changes might be an improvement.
I generally agree with the consensus that the lock washer should butt against the nut. I have had times where this was not practical (inadequate space on the nut end, for one), and I did not hesitate to use the lock washer on the bolt side. This is no sin, and functions almost as well as the other way.
Consider: What you have is a steel ring, cut in one place. Then, the two ends are bent so they are not in the same plane, then tempered to form a basic spring. Therefore, when the nut is threaded against the lock washer, it takes substantial force to flatten the lock washer back to a single plane. Because it is spring steel, it does not simply bend the washer back to flat; the force applied is retained as potential energy stored in the washer. Pushing back against the nut, hence discouraging the nut from coming loose. It will not allow the nut to vibrate loose, as this counter force prevents any space developing where physical vibration can occur.
The same affects apply, if one puts the lock washer against the bolt. I think it is safe to say that the control of vibration (loosening) would be slightly better on the nut side, because the same spring force need constrain a somewhat smaller mass of the nut. I posit that the difference in effectiveness would be hard to measure, whatever side one puts the lock washer on. Usually, if there is only one flat washer, it too, will go on the nut end. Besides applying a more even fastening pressure, this practice provides a nice flat surface for the lock washer to contract against.