Bolts - Cut vs Rolled threads 1

[victorynox]

Hi,
I know that when comparing cut and rolled threads of a bolt, rolled are superior as is comes to fatigue and stress corrosion cracking.

If you're using the bolt in an aircraft suppose to fly - maybe 20 000 hours (or whatever), the obvious choice is a bolt with a rolled thread.

However I'm working in the space industry, and as soon as you're about to use a fastener in the design, everyone starts shouting "rolled threads, use ROLLED THREADS!!!". My question is if that is really necessary? The fatigue environment is nowhere near that of an aircraft. The loads in our products are present for at most 10 minutes into the flight. After this time it's all over for this specific product. Is it really that important for this application to have rolled threads? What do you think?

Thankful for replies.

 

[3DDave]

I'd put it this way - if the bolt fails do you want to be explaining how you saved a couple of dollars and blew tens of millions to hundreds of millions on the launch?

 

[phamENG]

I'm not a fatigue expert, but isn't the fatigue life of a part related to the stress range as well as the number of cycles over that range? How do the stresses compare to that of a bolt in an aircraft? The forces required to get a rocket into orbit are much higher than those needed to get an aircraft into the air, right? Even though your bolt may not see near the cycles of a bolt in an airplane, it may be exposed to significantly higher stresses that reduce the fatigue life and could present a concern for low-cycle fatigue failure. Again, not an expert on this - I could be mistaken.

3DDave has a pretty solid point either way, though.

 

[rb1957]

1) "penny-wise, pound foolish"
2) the cost of rolling the thread is probably less than the mark-up for space rated h/ware.
3) the fatigue environment at launch is "awful". the thermal cycling on orbit is another issue.
4) is this the hill you want to die on ? I reckon you'll be pretty lonely on that hill … ie I doubt there'll be many others supporting this proposal.
5) yes, we are as conservative as ...

another day in paradise, or is paradise one day closer ?

 

[Artisi]

Are there any likely cost savings?
No,stick with rolled threads.
Yes, is the saving worth the hassle?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[CWB1]

The answer is the same as always when it comes to fasteners - either use a COTS fastener that meets the appropriate standards or pay the cost to properly design and test a custom fastener. There should be no unknowns, guessing, or bad generalizations.

 

[dgallup]

I think tensile strength is superior as well for rolled threads. If you are using an oversized cut thread fastener then you are spending a lot of money to put that oversized fastener into space. It costs something like $10,000 to put a pound into orbit. Do the cost benefit analysis.

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[Stick.]

Rolled bolts also have a smaller diameter on the unthreaded portion, so a bolt with rolled threads will weigh less than one with cut threads.

 

[Sparweb]

The weight savings do not come from comparing 2 bolts with similar sizes.
The weight savings come from detailed design that shows that fatigue-critical joints must have UPSIZED cut-thread fasteners to have the same life as a rolled-thread fastener.
Fasteners add weight. Using the smallest fastener (or anything) while maintaining safety saves structural weight. In space everything is literally worth its weight in gold, even in the SpaceX era of lower launch costs.

http://www.sparweb.ca

 

[EdStainless]

It isn't true that rolled threads are always superior to cut threads, I have seen plenty of really bad rolled threads over the years. But in well made fasteners there are many reasons to prefer rolled threads.
There are some exception to this. In some alloys rolling threads can greatly complicate the heat treatment. In most cases the starting bar/rod will have some degree of cold work, you have to make sure that the additional cold work of thread rolling does not make areas too hard and brittle. It is also an issue with some age hardening alloys that the cold work changes their aging response. Depending on the alloy the heavily cold worked areas may come out higher or lower in strength than the body of the fastener.

I will say that over the years I have seen a larger number of issues with heading than with the threads. Simple and often overlooked the head to shank junction is critical.

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P.E. Metallurgy