fine vs course thread bolts

Charlie M

Well-known Member
Any advantage to a fine thread bolt over a course thread. Fine threads seem to be more difficult to find.
 
All else equal, fine threaded fasteners produce more clamping force than course threaded fasteners at the same tightening torque and are less prome to self loosening.

Dean
 
I find fine threaded,(SAE) bolts/nuts at Menards,
(China), Home Depot, "Blains Farm-Fleet", and,
of course "Fastenal"
 
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
 
Henry Ford loved them. I have found that proper choice of fasteners are usually used in the right places. I could never understand why Ford used fine thread on plow bolts.
 
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'.
 
For a given grade, a fine thread bolt has higher tensile strength than a coarse thread bolt. That's because the threads on a fine thread bold aren't as deep, leaving more of the bolt diameter.

Check out the table at the site below. A grade 5 1/2-13 bolt as a tensile strength of 17,000 lbs. For a grade 5 1/2-20 bolt, it's 19,200. That's why for applications requiring high tensile strength like head and main bearing bolts, fine thread fasteners are used almost exclusively.
Fastener tensile strength
 
Bingo.

Fine thread fasteners have less thread engagement. This is the reason that fine thread fasteners are not recommended for use in materials such as cast iron, aluminum, etc.

Dean
 
That's why for applications requiring high tensile strength like head and main bearing bolts, fine thread fasteners are used almost exclusively.

Really? Most head bolts and main bearing bolts I've seen are coarse thread. I don't think fine thread would last long in cast iron or aluminum.
 
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.
 
As an example, Cummins V8-470 and V-8 504 engines have fine- thread head bolts going into cast iron. Probably some pretty good stuff, maybe high nickel content???

Been so long since I worked on a 855 or other big Cummins I don"t remember if those bolts are course or fine.
 
I made a puller once, to pull a brake drum off the axel on a tractor. I used a fine threaded bolt thinking that it would have more pulling power. I stripper the threads off the bolts. Would a coarse threaded bolt have worked better? I don't know, I gave up and used another approach.

Dusty
 
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.
 
Generally fine threads are for clamping force on parts to a fine thread nut and coarse threads have less chance of pulling out of a casting. An example would be a stud on a Harley base nut. coarse screws into the aluminum crankcase and fine sticks up for the nut to hold the iron cylinder down tightly. But as I said generally.You can google lots of info on this.
 
For making pullers it is best to order a rod of square or acme thread for the most force .One three foot piece can make alot . Or old jacks are a source too. MSC sells rod.
 
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.
 
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.
 
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!
 
(quoted from post at 21:42:38 09/23/13) 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.

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
 
60 acre hillside:

"I could never understand why Ford used fine thread on plow bolts."

To get you to BUY MORE plow bolts when the threads stripped out!
 
(quoted from post at 15:39:00 09/22/13) That's why for applications requiring high tensile strength like head and main bearing bolts, fine thread fasteners are used almost exclusively.

Really? Most head bolts and main bearing bolts I've seen are coarse thread. I don't think fine thread would last long in cast iron or aluminum.

Head STUDS into the block-coarse thread. Head studs THROUGH the head-fine thread.
 
(quoted from post at 15:39:00 09/22/13) That's why for applications requiring high tensile strength like head and main bearing bolts, fine thread fasteners are used almost exclusively.

Really? Most head bolts and main bearing bolts I've seen are coarse thread. I don't think fine thread would last long in cast iron or aluminum.

Head STUDS into the block-coarse thread. Head studs THROUGH- the head-fine thread.
 

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