Saturday, October 15, 2011

Macroscopic engineering: Playing with hardware

How to get it

If you've never bought hardware before, I'd recommend two places: Grainger and McMasterCarr. They sell basic parts. Basic like threaded rods and gears.

Of course, there's also Ebay. But then you have to deal with buying things on Ebay: Anything you buy will take forever to ship, you'll never be able to get another one just like it, you have to shop for it using the Ebay interface which is not at all designed to help you find the right part.


One of my options for how to raise and lower the fetching shelf is to have threaded rods that can be turned to push it up or down. However, if I put that rod off to one side of the fetching shelf it will torque the shelf as it goes up. I'll need a threaded rod on each side. But I want to make sure they rotate at exactly the same time or else I'll again be torquing the shelf. So my plan is to have a chain that connects them. A single motor will drive the chain.

Originally I was thinking bike chain. It's strong enough. It's universal. There's just one problem: it's only ever used on bikes. You want to buy a bike gear that's 1/2” instead of the really specific hub threading? Well no dice, because that's all anyone ever uses bike chain for.

The rest of industry uses something called 'roller chain'. In fact, #40 roller chain is just a wider, beefier, version of the standard #40 bike chain. And when you go to that, you can get all sorts of random gears and other quipment.


The grabber itself will have to have a way of holding onto a shelf it's grabbing. A claw which hooks around the shelf would get complicated. Perhaps it would be a solenoid with a spring to counteract the electromagnet and it would push a pin through a hole in the shelf to grab it. That's a lot of parts. You also need to find the hole or slot in the shelf to grab on to. And you'll need to put a lot of lateral force on the pin to pull the shelf out.

The alternative is an electromagnet that can latch on to the side of a table. I picked one up that's intended to be used on locking doors and can hold at least 100lbs when powered at 20V. Running it at 14V I saw a 14mA power draw and it was invincibly strong. The only downside I saw was that it would never let go. After turning off the power on it for 2 minutes I was still unable to unstick it from the metal plate it was on unless I used significant force. Running it at 5V was able to get objects off in about 30 seconds. If it's actually too weak to pull the shelf at 5V, I'll just bump it back to a higher voltage and have a counter-acting magnet on each shelf to hold it once the grabber is supposed to have let go, as opposed to relying on friction to separate them when the engine starts pulling the magnet back.

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