fine threads VS coarse threads on set screws...........................

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goldenchips2

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Does anyone have a chart on the holding power of fine thread set screws VS coarse thread set screws???
I have some info on the coarse threads, but not the fine threads.
Needing info for choosing the best for this application
Years ago I came across this information and wrote it down for future reference....

A 1/4x20 with hand hand knob will clamp 300 PSI, using a wrench will yeild 2200 PSI
A 5/16x18 with hand hand knob will clamp 400 PSI, using a wrench will yeild 2500 PSI
A 3/8x16 with hand hand knob will clamp 700 PSI, using a wrench will yeild 3000 PSI
A 1/2x13 with hand hand knob will clamp 900 PSI, using a wrench will yeild 6800 PSI
A 5/8x11 with hand hand knob will clamp 1300 PSI, using a wrench will yeild 7800 PSI

Just putting that out there for confirmation and or more information.
Any information sheding light on the subject will be highly appreciated.
 
Generally, fine thread anything can be torqued to higher values than the coarse thread equivalents.
 
Billy,

Great document.

I think page 28 talks about the torque that should be used during installation.

I think the bottom of page 4 and the top of page 5 discuss the actual load carrying capacity of the threads ... with fine offering "slightly higher load-carrying (in shear) and better torquelocking capabilities".

Regretfully there does not appear to be a page that defines the exact load carrying capacity of either other than Appendix C which just covers mechanical properties ... and does not enumerate differences based on thread pitch.

Arvid
 
The potential benefits of fine threads are:
1. Size for size a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).
2. Fine threads have also less tendency to loosen since the thread incline is smaller and hence so is the off torque.
3. Because of the smaller pitch they allow finer adjustments in applications that need such a feature.
4. Fine threads can be more easily tapped into hard materials and thin walled tubes.
5. Fine threads require less torque to develop equivalent bolt preloads.


On the negative side:
1. Fine threads are more susceptible to galling than coarse threads.
2. They need longer thread engagements and are more prone to damage and thread fouling.
3. They are also less suitable for high speed assembly since they are more likely to seize when being tightened.


Normally a coarse thread is specified unless there is an over-riding reason to specify a fine thread, certainly for metric fasteners, fine threads are more difficult to obtain.
 
Coarse threads were designed for cast iron,and fine was for steel originally.
 
check on metric threads while your at it..
 
While I'm not trying to cast doubt on the figures you originally posted, there is insufficient information presented there to either confirm or deny those strengths. There are numerous factors to consider in fastener design and selection. I'll just give the few that come to mind readily. Length of engagement, class of fit, material of both screw/bolt and nut or receiving thread, torque applied, direction of initial stress, static or dynamic load, temperature of environment, ....and I could come up with others should I drag out a book or two.
 
Since a fastener should fail before the threads pull out, what you really need to know is what the minimum thread engagement is for a specific fastener in a specific material. There are calculators available at the following link for just about anything you need regarding the strength of fasteners:

Nut and Bolt Stress Calculators

Tom
 
Threads !!!!!

Now if that isn't a bundle of information I'll never find any. For Hobby Machiists that pretty much gives a really good explanation of thread applications. It would take an awful lot of time to go through all the books, pamplets,etc. to come up with anything close to that. My Hat is off and a deep bow to all who answered his question.
dickr
 
YES YES YES....great stuff,.. thanks guys, and please keep the info coming if there is any more out there worth noting....
I know there are volumes of the stuff on the subject at hand, but what I am after are just layman rules of thumb,
with HUGE safety factors allready built in...
 
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