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Old September 12th, 2007, 03:19 PM   #74
I'm not old, honest...
whiterhino's Avatar
Join Date: 03-07-06
Location: Davisburg MI
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I was going to dig into some reference material and the machinery hand book but have decided to simplify it for now.

My first reply was based on looking at a simple screw chart that every old time designer carried. How many of you guys have used the Holo-krome slip chart? Well, a simple look at a few different bolt sizes show me that the average tension induced when tightened to yield is about 10% higher, along with Minimum tensile strenght of the same size bolt and finally that you can torque a fine thread higher.

So, then, I did a quick search (what you younger guys doA) and found a writeup on a website. Their descriptions are below.

Unified national coarse. UNC is the most commonly used thread on general-purpose fasteners. Coarse threads are deeper than fine threads and are easier to assemble without cross threading. The manufacturing tolerances can be larger than for finer threads, allowing for higher plating tolerances. UNC threads are normally easier to remove when corroded, owing to their sloppy fit. However, a UNC fastener can be procured with a class 3 (tighter) fit if needed (classes to be covered later).

Unified national fine. UNF thread has a larger minor diameter than UNC thread, which gives UNF fasteners slightly higher load-carrying and better torque-locking capabilities than UNC fasteners of the same identical material and outside diameter. The fine threads have tighter manufacturing tolerances than UNC threads, and the smaller lead angle allows for finer tension adjustment. UNF threads are the most widely used threads in the aerospace industry.

One thing to keep in mind: the threads on a bolt are never the weakest part. It is the bolt itself. I can do the math if you like. Also, keep in mind, as mentioned in the writeup, the root diameter of the coarse thread is smaller, thus creating a "smaller" cross section of bolt.

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