Fine threaded bolts vs Coarse threaded bolts ?

Mrwurm · Feb 12, 2006

My last project here got me thinking about fine threads vs coarse threads. Most things that I work on have coarse threads. But, once in a while I work on something like my new New Holland front blade and find that every fastener on it is 1/2-20. This is the fine thread version of the typical 1/2-13 coarse fastener. To make things even more complicated, metric fastners come in three pitches. For example a 14 mm bolt may come in 1.5 threads per mm, 1.75 threads per mm, and 2.0 threads per mm.

So, what gives here ? What are the benefits to use either fine or coarse threads for the same given diameter bolt ?

DocHeb · Feb 12, 2006

Fine threaded fasteners have higher tensile strength than coarse thread fasteners. They also take more work (i.e. - revolutions of the fastening tool) than coarse thread.

More info:
BoltScience.com
Engineering Fundamentals

Lazy · Feb 12, 2006

Hi
I think 1/4-20 is the standard and 1/4-28 would be the fine thread.

Charlie.

bobodu · Feb 12, 2006

"

Back to Tapping Handbook Contents

Coarse vs. Fine Threads

Coarse Threads

For most applications, course threads offer these advantages:

* Easier and faster assembly, providing a better start with less chance of cross threading.
* Nicks and burrs from handling are less liable to affect assembly.
* They are less likely to seize in temperature applications and in joints where corrosion will form.
* Less prone to strip when threaded into lower strength metals.
* More easily tapped in brittle materials and or materials that crumble easily.

Fine Threads

Fine threads may make for a superior fastener for applications with specific strength or other requirements.

* They are about 10% stronger that coarse threads due to their greater cross-section area.
* In very hard materials, fine threads are easier to tap.
* They can be adjusted more precisely because of their smaller helix angle.
* Where length of engagement is limited, they provide greater strength.
* Thinner wall thickness can be used because of their smaller thread cross section."

Well ...that answers one of my oldest questions!!!

I'm glad you asked...I will sleep well tonight!! /forums/images/graemlins/grin.gif

ksmmoto · Feb 12, 2006

Fine thread bolts have greater resistance to loosening than coarse. This is due to more contact area for the threads and less angle. That's why fine treads are used most of the time on engines and vehicles.

ksmmoto