It's time for me to create an exception for one of the big three rules I set out at the beginning. Under certain circumstances, you can
use a mostly-tightened nut to make flat linkages.
This is one area which I have not personally built anything, so all the illustrated examples will be from others, but it's a tactic that is commonly attempted enough that I think addressing it is important.Use prevailing-torque nuts
Also just called "locknuts", these nuts have a bit of deformed thread or a big chunk of nylon plastic in them which tightly grips the thread. These cannot be spun on a screw with your finger - generally, at least a wrench or set of pliers is needed, and a driver bit for the matching screw. The idea is you can tighten the nut to a very well known degree and have them stay there. If the mechanism needs less slop, you can tighten the nut a controlled amount.Use back-to-back nuts
Called "jam nuts", or literally 2 nuts tightened against each other, this method is less reliable for setting an exact tension but is handy for when you do not have locknuts available. A dab of threadlocking compound before tightening helps give the threads some extra friction.Use washers with the nuts
, preferably plastic ones
The washer helps to add a small amount of spring compliance to the joint, and also acts as a bearing surface. It helps prevent the material from torquing on the nut or screw directly and causing it to unpredictably tighten or loosen. Plastic washers give bearing properties as well as more compliance than bronze washers.Make the mating faces wide
In a typical unsupported planar linkage, the two links are basically braced against eachother. Making them wider not only adds rigidity in flexing, but the wider faces have more leverage against the flexural forces. They are less likely to twist and bind. Lubricating the faces, or using a bearing washer between, also helps.
There's another cheap way of making a planar linkage which involves pop rivets (blind rivets)
that is popular with small mechanisms (like on 2.007 robots!):Paper shimmed rivet linkages
This method uses a pop rivet as the hinging mechanism, since they're easy to install. Insert a small piece of paper or other very thin and sturdy material into the space between the two links, and apply the rivet. Normally the rivet would tightly clamp the parts together, but with the shim in, there is a small gap on the order of thousandths of an inch.
After the rivet is installed, slip the paper shim back out. The artificially installed slop allows the linkage to move freely. This method does not allow for much post-installation slop tuning, but is fine for prototyped mechanisms or things that are only carrying light loads.
The example picture of the 2.007 robots (Images 3 and 4) show locations where this method was used.