Locking nuts without loctite

[Barry_White]

Another solution is a castellated nut and a split pin.

[Master Splinter]

...a quick pass with the arc welder is good at making sure nuts don't come off

Or peen the exposed thread down a bit with a hammer and drift.

[BobL]

Here's an old school one I saw just a couple of days ago. Drill a shallow hole in the end of the bolt and use a centre punch to marginally splay the end of the bolt. If you do it right you can still get the nut off and doing so restores the bolt to it's former diameter for reuse.

[Woodwould]

... and doing so restores the bolt to it's former diameter for reuse.

... or stretches the nut beyond its safe use! That method would be for one use only. I wouldn't like to use either component again after doing that.