I saw on Wikipedia the following
> Split pins are frequently used to secure other fasteners, e.g. [clevis pins](https://en.wikipedia.org/wiki/Clevis_pin), as well as being used in combination with [hardboard](https://en.wikipedia.org/wiki/Hardboard) discs as a traditional joining technique for [teddy bears](https://en.wikipedia.org/wiki/Teddy_bear)
I don’t understand how such a small element would help secure other fasteners, say a nut for example. How does it secure it, I mean what value does it exactly add?
In: 3
Lets say you join two things together but want them to be able to rotate, like a digger bucket on the end of the arm. You make a hole on the end of the arm and a plate on each side of it welded to the bucket then stick a big pin through the holes and grease it so it rotates smoothly – this looks a bit like the clevis arrangement in your link.
Now, in theory, you are sorted – the parts rotate freely but the digger bucket cant slide off the end of the arm because the plates either side hold it in place. The problem is that vibration or any unbalanced friction on the pin can make it slide in and out. You can put a wide head on one side to stop it sliding one way but not the other so eventually, it’s going to fall out and the bucket will fall off.
To prevent this, you need to lock the other end of the shaft somehow. This isn’t taking load from the bucket catching on anything, all it’s doing is stopping the pin vibrating out so the lock doesn’t need to be very strong. You could thread the end of the shaft and use a nut, but that will eventually vibrate loose and fall off then you’re back to square 1. You can use a lock nut or thread glue to hold it which may work for the joints on the arms but the bucket is a part that gets swapped out quite often to fit different attachments so lock nuts will be a pain.
The easiest way to deal with this is to have something that hold itself that is easy to remove – small diggers use [lynchpins](https://en.wikipedia.org/wiki/Linchpin) with spring clips to do this but cotter pins and R clips are used for this in other applications. They’re quick and easy to remove and strong enough to stop the big pin sliding out and the equipment is designed so that the real load can’t be applied to them. That’s one of the reasons why the clevis pin image that you linked has to go on both sides of the part it connects rather than sitting on one side only – if it was able to sit just on one side then you could pull directly against the small retaining pin which isn’t strong enough for that
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