Bolts, Threads & Fasteners

TT

Joined
Mar 7, 2005
Messages
53
What is the general theory behind coarse threaded bolts and fine threaded bolts/nuts?

Are coarse threads used in high load environments and fine threaded fasteners used in high vibration environments?

When re-installing my headsteady bracket on my 850 MkII I noticed that the 3 bolts that secure it to the head use only flat washers. Would stainless steel spring washers be useful or is that introducing an unnecessary component? Or is Loctite a better option?

Getting loctite on the middle bolt and not make a mess would be challenge.

Anything in conact with the engine is in a vibrating environment!

TT
 
Joined
Aug 11, 2003
Messages
32
Because a bolt with a fine thread has a larger cross section than one with a coarse thread, the fine threaded bolt will withstand a higher tensile load before shearing. Bolts used in aircraft engines are all fine threads.
If the thread is in a cast material such as cast aluminium or cast iron it is better to use a coarse thread as there is less chance of the thread stripping in the component.
Unfortunately if a thread has a fine pitch it is easier to overtorque it and snap it off. (the lower helix gives you a greater advantage when tightening the nut.)
 
Joined
Oct 7, 2005
Messages
1,170
Basic engineering rules say to use coarse threads in softer materials and fine threads in harder materials. Fine threads can take a higher load. If threaded into aluminum or cast iron, coarse threads are less likely to strip or come loose.

Of course, convenience and availability over-rule engineering, which is why the 26 TPI "Cycle" threads are so common on Nortons. There were a lot of screw-cutting machines that only had one setting!
 
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