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The first thing to understand is this:
A bolt is a spring. It applies force by being stretched.
That means it needs to stay within the linear elastic regime of the stress-strain diagram.
"wow" you think. "I never knew that." Of course you didn't. Its not even 1% of
The effective spring constant of a bolt comes from the part of the bolt that is not engaged with any threads.
The clamping force of a bolt is generated by stretching this un-engaged part.
This is why some bolts have an unthreaded shank portion that bypasses threads.
There is an 'ideal' effective spring constant:
Too low and its like a sponge, soft, and doesn't provide much additional tension against a separating force.
Too high and its 'brittle', small rotations (e.g. from vibrations) will massively reduce the clamping force
But there is also the material being clamped which acts like a spring.
This is why you use washers - to increase the effective volume of material under load.
Stop killing your loved ones by skipping washers.
Washers save lives. Stiff material, elastic bolt. But why?
The clamped material and bolt act like springs in parallel.
Stiff material means less of the load cycling acts on the bolt.
Meaning the bolt pre-tension can be higher without it failing.
Not knowing about bolt pretension is like not understanding a stop light, you child.
Bolt pre-load or pre-tension is how tight the bolt is initially, and generates the constant clamping force.
It's this clamp force between the surfaces of a material, and the friction it produces, that resists shear.
High pretension means less shear taken up by the bolt itself
But most of the torque you apply to the bolt doesn't even go to the clamping force, it goes into thread and head friction.
This is good - it prevents the threads from 'unwinding' on their own, maintaining bolt pre-tension, off-loading axial and shear stresses to material
"So I'll just use tons of engaged threads so it never unlocks" you say - also retarded.
Almost all the forces on are taken up by the first few threads.
Finer threads are almost always better for generating tension, preventing failure, etc.
10 engaged threads is plenty.
This is just barely the beginning of introductory bolt physics.
Once you get into vacuum conditions you need to worry about trapped air in threads, off-gassing, start machining stuff out of tantalum.
Or nuclear reactors where neutrons make your bolts swell up
Sum up:
- Bolts are springs, they generate clamping forces by being stretched
- Washers help take axial loads off bolts and place them on materials
- Pre-tension means shear forces act on the clamped materials, not the bolt shafts
- Fine threads better almost always
- Neutrons
I was fortunate enough to work and design some sophisticated tools for the aerospace industry, measuring the clamp up force on a bolt.
If you know the coefficient of friction, and you know the torque on the nut, you can approximate this. However, what I chose to do was to send a
One of the mantras in our mechanical engineering course I helped teach was “A thread is not a seal”. Junior engineers always think you can plug a hole with a bolt. You just cannot.
A separate seal is needed that is put under pressure by the bolting force.
They're such an underappreciated technology. Looking back at the process of installing structural steel rivets makes you realise how lucky we are to have mass produced bolts at near zero cost.
I spent a good portion of my life in the threaded fastener business. It’s very interesting stuff. Hydrogen embrittlement anyone, anyone…….
Plus a bolt is really a spring…..What! Who knew?
Great thread!
casually getting an engineering class in an X thread
the internet is a wild place (and I'm here for it)
Sadly, GM engineers who designed the 2000 Northstar engine failed this class. Too few threads engaged to keep the heads on tight.
Besides the shank being loaded in tension, the threads are loaded in shear. You can strip the threads if you over tighten.
This is also an important time to point out that bolts should not be reused and that, from an engineering perspective, the joint is considered to have failed if the natural force created by the bolt is overcome.
Andrew
This is great …but
The only really important take away is this :
1/4-20 to 1/2-13 = 1/2 UGGA DUGGA
1/2-13 to 3/4-10 = 1 UGGA DUGGA
3/4-10 to 1-1/2” = 4 UGGA DUGGAS
1-1/2” + = All THE UGGA DUGGAS
thank you
maybe it's because I just got off a runner's high but this post felt deeply caring instead of hostile despite the insults
I have a Machinery's Handbook from 1931, there's no Unified Threads in it because they hadn't been invented yet.
Rule of thumb for bolt length I’ve always heard is 1.5 the diameter of the bolt needs to be threaded in. More threads doesn’t add a lot of strength.
A 10mm bolt should thread in 15mm.
You forgot to mention embedding - in my experience it’s the root cause of most bolt failure. The enemy of the people.
Read Bickford original smaller edition and new magnum opus.Fisher and Struik too.
"This makes you look retarded and incompetent, an abysmal failure, and kills people who trust you."
Hahaha, wow.
Ah yes, the classic 'screws are just fancy nails' approach. What could go wrong?
interesting thread,
can you recommend any sources if I want to dig deeper into this rabbit hole?
maybe i should've been in mechanical engineering i thought this was fairly obvious.
I was expecting to see antique screw machines running with the turrets, gears, and levers. This is boring.
