Reusability of nuts and tapped holes + Choosing the right material

  • #1
Juanda
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TL;DR Summary
Shigley's recommends considering nuts disposable items. Would this apply to threaded holes too?
Also, trying to understand what's the best procedure to choose the right material for a given bolt.
In the section "8-3 Threaded fasteners" from Shigley's the following text can be seen:
The material of the nut must be selected carefully to match that of the bolt. During tightening, the first thread of the nut tends to take the entire load; but yielding occurs, with some strengthening due to the cold work that takes place, and the load is eventually divided over about three nut threads. For this reason you should never reuse nuts; in fact, it can be dangerous to do so.

Replacing a nut is simple enough. However, replacing a tapped part or having to update the tapped hole with something like a helicoil could be more complicated. Would that recommendation from Shigley's apply to any tapped/threaded hole assuming the bolt is torqued to its proof load?

Secondly, although not mentioned in that text, I found the reason the nuts are to be replaced but the bolts are OK. I didn't understand why it's the threads in the nut that get damaged and apparently, it is because nuts are typically made of softer materials. Nuts are designed to be able to fully develop the tensile strength of the bolt so they're technically stronger but they also are softer which implies their threads yield locally. Here is some text from Chapter 4 of "An Introduction to the Design and Behavior of Bolted Joints".
  1. In general, we want the nut or tapped hole to support more load than the bolt, because bolt failures are easier to detect than stripped nuts. One rule of thumb: the nut's proof load should approximately equal the ultimate tensile strength of the bolt [21].
  2. In an apparent contradiction of the above, nuts are usually (but not always) made of slightly softer (less strong) material than the mating bolts, so that the nut threads will yield locally and better conform to the bolt threads when loaded. This better distributes the stresses in both parts. Note that the strength of a part depends on dimension, shape, etc., as well as choice of material. This is why it is possible to make a "stronger" nut from a "weaker" material.
  3. If you need a nut for a bolt made of a special material such as titanium, you'd presumably make the nut of the same material. The nut might be heat-treated differently, however, to satisfy point 2 above. Again, this is not a universal custom.
  4. The strength of a nut depends, in part, on the length of thread engagement. To increase safety or reduce failure rates, you can consider increasing the length of engagement. Doubling the length of the nut, however, won't double its strength; the relationship is more complex than that, as discussed in Chap. 3.
  5. A nut's strength also depends on its width "across flats." Nuts with thinner walls "dilate" more-partially disengage themselves from the bolt under load-and so have less strength. This is one of the reasons why standard nuts are available in several configurations-hex nuts, thick hex nuts, and heavy hex nuts. These differ in height (thread length) too. Your choice will be based on economics and on the fact that most bolts (and nuts) aren't loaded anywhere near the limit of their strengths. As a result, a regular hex nut-or equivalent in a tapped hole-is sufficient for most applications. Thicked, heavier nuts would be preferred if the bolts are to be loaded to target preloads beyond 60-70% of yield perhaps, or if the consequences of failure suggest that an extra degree of safety would be prudent.

From all that said, if I had a specially strong bolt made of something like stainless steel 15-5 PH H1025, I could use it with a COTS nut made of a simpler and softer material like stainless steel A2 (AISI 304) as long as the nut is tall and thick enough. It'd also be OK to use a 15-5 PH nut but with a greater H (temperature at which is treated) so it is softer. That'd imply the nut wouldn't need to be as big but finding a COTS bigger nut made of A2 seems easier and cheaper.
And it'd be advisable to replace the nut after being used.
 
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  • #2
This subject is too broad to be completely summarised, or fairly answered.

It is NOT normal practice to replace nuts, but there will be special situations. Where weight minimisation is important, such as in aerospace, fasteners will be lighter and more critical, and the possibility of reuse must be clearly specified.

I see it said that Nylon insert locknuts must be replaced every time, but the manufacturers often state it is possible to reuse their product 15 times. It is up to the mechanic to intelligently assess the degree of wear in a nut, and to replace it at the appropriate time. In the same way, you can sometimes get away with reusing a split pin, but the low cost of the split pin does not justify the practice. Nuts are more specialised, expensive, and harder to find.


It is really not possible to replace tapped holes in machine parts without replacing the part. If worn, the threads may be repaired with an insert, but an insert would probably never be replaced, since inserts are usually a harder material, stainless steel, with a greater external diameter than the original threaded bolt or screw, so will be deformed less. Removing and replacing inserts is particularly difficult.

A screw, tightened by turning the head, will wear the material in a threaded hole, but a stud that is first screwed into the hole will not wear the hole when the external nut is tightened, as is done when torquing a nut on a bolt.


Follow the instructions, take care, and act responsibly. Ask yourself if your work has met the safety requirements. Emulate a torque wrench, by saying a ritual "click", or "Allahu Akbar", to confirm safe completion of each and every fastening procedure.
At the end of the day, you must know that you have done your job safely for others.
 
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  • #3
As an aircraft mechanic, I used the rule of thumb of “if in doubt, throw it out” when it came to hardware and fasteners. Replacing a screw is cheap. Replacing an engine because a worn out screw rattled loose and chewed up the fan blades is expensive.
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  • #4
same in racing , flyboy exactly correct !
driver safety is it!
 
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