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I recently had a project where I made an aluminum linkage (similar to a scissor lift) operated by linear actuators. Aluminum pieces were fabricated from 6061 "standard" aluminum stock, 6mm (0.25 inch) thick. The bars are at most 750x75mm (30x3"). The application was mostly cosmetic (for a special effect) rather than lifting or exerting much force.

I used shoulder bolts with a nylon washer between the aluminum pieces to reduce friction, and standard zinc-plated steel washers on the exterior sides.

The problem I encountered was tightening the bolt too much would result in too much friction, whereas too loose would result in one of the aluminum pieces coming slightly off the shoulder of the bolt, skewing, and causing similar problems.

What would be a better way to create a reliable pivot point that doesn't require constant adjustment?

I put together this illustration as an example:

Linkage Description

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  • $\begingroup$ As you say the loads are light, you might just get away with adding spacers under the nut - probably just a couple of washers. A spring washer might keep things a little smoother. $\endgroup$ – Chris H Feb 8 '15 at 17:39
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You can use a sort of "double diameter washer". I've made that name up, but I am pretty sure such a device exists:

enter image description here

That's a cross section of one of your linkages. The white things are the aluminum pieces, the grey one is the screw, the yellow is the nut while the red is the "double diameter washer". If you can found some washer that is slightly longer than your aluminum bars thikness you are good to go. Of course if it's too long the linkage may become lousy, but with a good cutter you can trim them down, or you can also add some regular washers between the red one and the nut.

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    $\begingroup$ What you have drawn in red is called a flange bearing. Very cheap ones are available, usually made out of lubricated bronze for situations like this. The system will work better if you use two flange bearings, so the bearing rotates on the pin rather than the aluminum arm rotating on the bearing. The most important thing is that the nut not apply a clamping force to the joint. Usually this is accomplished with either a lock nut in the case of a shoulder bolt or circlips in the case of a unthreaded pivot pin. $\endgroup$ – Ethan48 Jan 21 '15 at 16:45
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A shoulder bolt is a good approach in applications with light loading, but it needs to be specified such that it can be fully tightened while still allowing the necessary clearance for movement without binding. Use shims where needed.

You can reduce some of the play in the joint by having the bolt threaded into one of the aluminum pieces (but still backed up with a nut for strength). In other words, only one of the aluminum bars needs to turn freely on the bolt.

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