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I have a plate of aluminum 5 mm thick to which I would like to attach a hook. The plate runs roughly vertically, but only spans around 18 cm from anchor point to anchor point. The hook should be able to withstand upwards of 1000 kg.

I've drilled a 12 mm hole through the plate and my strategy is to insert the hook bolt in it and tighten it using a nut on each side.

However, in order to keep the aluminum from deforming, I'm going to need one or two steel washers.

My question is, whether it is necessary to use a washer on both sides or whether one side is sufficient?

If one is enough, does it make any difference, which side it rests on?

My reasoning goes like this: The job of the washer is to keep the hook from bending the aluminum by spreading the force over a large area. As long as a single washer is taut against the aluminum – held there by the nut – the hook won't be able to bend either way.

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While your setup will probably work either way, it's advisable to use a washer on both sides.

A washer does help prevent local bending as you note, but that isn't really its main purpose. The biggest reason to use a washer is to prevent the nut from cutting into or shearing through the base material. In your case where the hook bolt and nut are steel, and the plate is softer aluminum, this is especially a concern. As you tighten the connection, without a washer the nuts are likely to dig into the aluminum and start to act like a drill. It also wouldn't be too hard for the nut without a washer to be pulled through the plate (causing a shear failure in the shape of the nut) if there isn't a washer.

Note that in a similar case, using a shouldered eye bolt where the bolt, the substrate, and the nut are the same material, it would be customary to only use the washer on the nut side, because the bolt is not designed to ever be loaded in compression, only in tension against the nut. Because of this the bolts are designed with a shoulder that isn't as big as a washer, but meets the needs of the bolt for the angles it is rated at. Washers may be added when required for thread length or orientation.

Also note that based on your description it sounds like you may be side-loading the hooks which is generally not how they are designed to be used. If you want to make a separate question with the plate, the hook selection, the geometry, and the load, you could make sure you are using the hardware correctly. 1000 kg is a very serious load and if it's going to be anywhere near people, the setup should be evaluated according to standard rigging practices in your country.

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