This is an explanation of two methods of creating multi-use steel locknuts for threaded bolts, and a reminder of a few other methods that you may already use.
Nylon Locknuts work well, but sometimes they are not available, and after several times on and off a bolt they lose much of their locknut properties as the nylon shapes to the threads of the bolt. Twice in my work traveling, I have needed locknuts and been unable to find them.
1. “Cut and Crush”—distort the threads on a nut by cutting out a narrow section of threads and crushing the remaining threads into that gap.
2. Ovalized nut –distort threads into an oval
3. Reminders They nylock. and other commercially available locking nuts, 2 nuts tightened against each other, Loctite, etc.
**The “cut and crush” and “ovalize” methods were taught to me by Ralf Hotchkiss, my colleague at Whirlwind Wheelchair. Ralf travels and works extensively in places with limited access to modern manufacturing technology and supply chain, and has been learning and designing tricks for “low tech” manufacture for the past 40 years.
These methods can be replicated pretty much anywhere—but I made these locknuts at Techshop SF (amid the whir of CNC machine tools).
where i made it: http://techshop.ws/
where i learned how to make it: http://www.whirlwindwheelchair.org/
Step 1: Split and Crush a Nut (Thread Interference)
1. Cut and Crush: (For steel nuts)
This locknut works by distorting the threads of a nut, so that threading it onto a bolt will spring the nut to its original position. This method does weaken the nut somewhat so use with caution. First, cut into the nut with a hacksaw blade as shown, approximately 1/3 of the way through the inside hole. This will remove a width of approximately 0.8mm .
Then crush the nut in a vice to bend the two split sections of the nut to touch. The nut will deform into this shape. When threaded onto a bolt, the thread pattern forces the nut to spring back into shape, remaining under spring tension until it is unthreaded.
Don’t cut farther than halfway through the nut, in order to retain strength, but if you don’t cut far enough into the threaded are it won’t be possible to crush the two sides inward enough to get a good thread interference for locking. I recommend cutting 1/3 of the way though the hole in the center of the nut.
With a ½”-20 nut made with this method, I measured a torque of 8 foot-pounds at both the first and tenth time tightening the nut.
Step 2: Ovalized Nut
2. Ovalize nut
(These are commercially available as Elliptical offset nuts.)
Crush a nut in a vice to deform the round threads into an oval. Threading this nut onto a bolt will force the nut to become more circular, under spring tension. You could also crush the nut with a blow from a hammer, but the results would be less consistent without a jig.
There is a relatively narrow range of amount of ovalization that leads to a good locknut. To make more than a few of these nuts, you could make a jig that has a calibrated hard stop for the crushing force (vice or hammer).
This 1/2"-20 nut moved with a torque of 20 ft-lbs at the first and tenth time tightening it onto the bolt.
Step 3: Other Locknut Reminders
B) Castle nut (with cotter pin)
C) Flex-Top Expanding Hex Locknuts
D)2 nuts tightened against each other
F) Spring Washers
McMaster-Carr is a good source of locknuts, and information about commercial options.