Ordinary hex nuts don’t hold fast to anything; that’s why we have lock nuts. When something vibrates, expands and contracts (for months on end), it shake loose. It’s one thing if that happens in the wrong place, it can be a major headache. You’ll chase a bolt around your floor three times and then reach for the locking variety.
To make things simple, there are different locking styles because they’re making friction in slightly different ways. You have metal deformation, an integrated washer with teeth biting into mating surface, or a nylon collar gripping threads and forming to itself as it’s tightened the first time (but losing its grip forevermore). The side-by-side comparison infographic above show all of this.
How to Choose the Right Lock Nut
Compare the style to the situation instead of pulling stuff out of bucket hoping it’ll work. Use a nylon insert nut for generic machine applications under mild temperatures. Use all-metal prevailing torque for high heat and repeated removal. Use jam nut for when you need really precise adjustment but don’t want much profile. Use a castle nut for a sure-fire mechanical lock where a cotter pin isn’t a problem.
Most folks do not think temperature has this level of importance. Heat above the softening point begin to soften nylon. This is why you can tighten a bracket with a nylon nut just fine but it falls off the exhaust manifold. All-metal types maintains torque values over a much broader range different than others. This is why they are listed separately in the reference material.
Adding to the confusion is grade markings. For everyday purposes, a Grade C nut works well. However, outdoor applications using weathering steel call for higher proof loads, like Grade DH or DH3, for things like structural connectors. These torque numbers are not absolute but a place to start from a practical perspective. Over torquing runs the risk of stripping thread or cracking castings. Under torquing reduce the life of the locking feature by making it do all the work.
In most cases, the choice becomes reuse. An all metal nut can often be used multiple times with no obvious harm or damage to the threads, provided they’re still there. Use a nylon insert once and toss when removed if this is a safety critical joint. It’s cheap and works great the first time and after that, out she goes.
The same holds true with finishes. Use zinc plating in-doors, and go for stainless steel or hot dip galvanized when there’s any chance of moisture, salt, chemicals, etc. These are cross-referenced on the chart, which lets you run your eyes across the situation then select a nut that’ll survive it without constant tightening.
Match the locking method, temperature rating, and material to the actual service conditions. But ultimately it’s not complicated at all. Match your lock to the real-world application by choosing the right materials, temperature ratings, and locking methods. If you do this, you won’t have to settle for whatever happens to be sitting on the bench. And this one habit would of accounted for the majority of the fails that lead folks to look up the correct sizing chart in the first place.
