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Tool Talk Discussion Forum

Re: fine vs course thread bolts

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Author  [Modern View]

09-22-2013 14:16:09

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actually the limiting factor is the yield strength of the bolt. If you need more torque to produce the the same clamping force desired then a fine thread would not be as efficient as a course thread

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Brian G. NY

09-22-2013 14:52:21

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 Re: fine vs course thread bolts in reply to dpendzic, 09-22-2013 14:16:09  
You gotta run that by me once more dependzic!
I'm no engineer, but I think what Dean says sounds reasonable.
After all, walking up a less steep ramp gets you there the same as a steeper ramp but it's just a little easier walkin'.

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09-22-2013 16:24:34

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 Re: fine vs course thread bolts in reply to Brian G. NY, 09-22-2013 14:52:21  
what i am trying to say is that fine thread has more threads per inch so the friction working against the torque applied is greater. However the strength of the bolt is determined by the yield strength of the steel--not the tensile strength--the limiting area is not the root diameter of the threads but a greater value A- sub t--determined by tests--and of course fine threads have a greater area than course threads so the ultimate allowed clamping force will be greater-but a slightly more torque will be required to obtain that.--In real life situations it is probably insignificant unless you have a required clamping force required. In all of NY state bridge design connection requirements it is standard to use course thread A325 7/8 diameter bolts tensioned to a load of 30000 lbs.-
The torque wrenches are calibrated each day with a load cell---sometimes load indicating flat washers are used---lock washers are never used.

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09-22-2013 17:04:41

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 Re: fine vs course thread bolts in reply to dpendzic, 09-22-2013 16:24:34  
It's time for and engineering lesson.
Tensile strength and yield strength are material properties. The tensile load capacity of the the bolt is determined by the tensile strength an the cross sectional area of the bolt.
A fine thread bolt has a larger cross sectional area. There for it will carry a largr tension load.
All bolts in a bridge are loaded in a shear configuration. That means it is loaded across the diameter and not lengthwise. The thread type has no effect in this type of loading. It is also important that the thread area is not at a shear location as this will decrease the area.
The torque load fo the bolt is designed such that the working stress never exceeded the torque stress. This insures that the bolt will not fatigue.

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09-22-2013 18:08:56

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 Re: fine vs course thread bolts in reply to Joe(TX), 09-22-2013 17:04:41  
have to disagree Joe on the connections being in shear on the bolts. The connections are designed as friction connections with no shear load on the bolts--thats the reason for the very high clamping load to produce this friction.

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09-23-2013 18:42:38

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 Re: fine vs course thread bolts in reply to dpendzic, 09-22-2013 18:08:56  
You are mistaken. A bolt joint does not rely on clamping friction. I have been an engineer for over 40 years, and only an idiot would rely on friction for a connection.
If you have two plates bolted together, they have a shear connection.

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Owen Aaland

09-24-2013 09:00:00

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 Re: fine vs course thread bolts in reply to Joe(TX), 09-23-2013 18:42:38  
Quoting Removed, click Modern View to see

If you don't think friction plays a part just install a flywheel with the bolts only snugged up a bit and see how long the bolts last by themselves

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09-24-2013 06:16:03

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 Re: fine vs course thread bolts in reply to Joe(TX), 09-23-2013 18:42:38  
well never thought I was an idiot as being a licensed structural engineer in two states , having designed close to two dozen bridges, and retired as the Director of Bridges and Structures for a county in NY. I certainly hope those bridges don't fall down!

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