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
A split pin can go through a drilled nut, through a gap in a castle nut, or outboard of a normal nut. In any case, it’s used because it is not subject to the same tendency to loosen due to vibration. If you’ve every dealt with a nut that has a burr in it, you know how a tiny piece of steel can prevent a large nut from moving. A split pin is the same idea, but introduced on purpose.
If you look at [this Wikipedia image](https://en.m.wikipedia.org/wiki/File:Clevis.svg) you’ll see that the split pin is not load bearing. All it has to do is stop the main pin from slipping out (to the left in this case).
Most all split pins never have to bear a load, and rather, stop something from moving in a direction it shouldn’t. And when it doesn’t have to hold a load, it can be fairly small while still being extremely effective.
It physically prevents things sliding around in a shaft, even if the nut ever was to come loose, the cotter pin will prevent it from unscrewing completely and ultimately prevent the two parts from separating completely
they are also used without a nut in situations where sideways sliding is not supposed to happen but needs to be prevented
They are used in situations where large loads aren’t expected so it’s not at risk of breaking, in those situations you would use something stringere
In your clevis example, there is a screw or pin penetrating both sides of the c- bracket / and wood on the left side of the picture.
The cotter pin would then go transversely through that screw or pin, then get spread/ split preventing the primary screw / pin from being able to slip out of the c- bracket.
Without a cotter pin, the nut or washer would be free to slide off the end.
In the case of a nut, the cotter pin stops it from being able to spin past the point of the hole the pin is secured through. You could achieve a similar effect by smashing the end, making a defacto rivet, bit the use of a spilt pin, allows for replacement of wearing parts, especially in cases of materials of differing hardness and friction.
By their nature, split pins are somewhat malleable and can be bent – as such they are made of metal. After many cycles of movement against plastic, wood, or a softer metal, wear will necessitate replacement of the wearing part. With a removable pin – this replacement can be done, if a riveting or mushrooming technique is used – then not so much.
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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