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Fastener Tech 101

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I find myself reiterating often about fastener functions, torquing, yield strengths, etc, and I've finally decided to put it all on paper and in one thread. :) Enjoy.



What is a fastener?
A fastener is a threaded bolt or stud (and nut) used to clamp (or fasten) and hold two or more objects together. Generally, all automotive fasteners are made of some sort of steel or alloy of varying strengths.


How exactly does a fastener work?
A fastener works by using a measure clamping force between two or more objects. All fastener materials have an advertised yield strength and tensile strength. Simply put, yield strength is the point when the fastener will begin to permanently deform and become "failed" and unusable. Tensile strength refers to the point in which the fastener's diameter begins to "neck down" due to far exceeding it's yield strength. We typically only use a fasteners yield strength in the automotive world, as any fastener that is pushed beyond it's yield strength becomes questionable.

We use a fasteners advertised yield strength and diameter to calculate it's maximum torque value. A good rule of thumb is to torque a fastener to 75-85% of it's yield strength per given diameter. This is usually a safe and conservative amount that will allow the fastener to function to the best of it's ability and without resulting in failure. A properly torqued fastener will actually stretch a small amount when installed since no material is incompressible. This force occurs when a fastener is tighten beyond "finger tight" and is commonly called a preload or clamp load. In a sense, a preloaded fastener acts much like a spring. We torque it down and stretch the bolt/stud to a measured amount, and it's natural reaction to want to rebound is what creates the clamping force. When the fastener is removed, it should completely rebound back to it's initial free-standing length. If a fastener does not completely rebound, then it's considered to be compromised and is no longer fit for use. Even if the difference in length is only +.001", that's enough to deem the fastener as weakened and failed. This happens when a fastener is torqued to a value beyond it's yield strength. This is why a greater torque value does not always equate to more clamping force. That only stands true if you're still working within the limitations of the materials yield strength. If you find yourself in a situation where you've maxed out a particular fastener and you are in need of more clamping force, then it's time to step up to a stronger material. NEVER TORQUE BEYOND IT'S RATED YIELD STRENGTH!


Strengths of commonly used materials:

Grade 5 - YS: 90,000 psi, TS: 120,000 psi
Grade 8 - YS: 120,000 psi, TS: 150,000 psi
8740 chrome moly - YS: 180,000 psi, TS: 200,000 psi
L19/H11 tool steel - YS: 200-230,000 psi, TS: 260,000 psi


Proper Installation?
To achieve an accurate torque measurement, we need to overcome as much friction as possible using the least amount of "extra" force. The prime area for friction occurrence are the threads (more specifically, the top side of the threads) and the 'flat' underneath the nut or bolt head. To combat against friction, we use a lubricant on these areas. Without a lubricant, the threads will drag and create friction and that will "inflate" the torque value, which will result in less clamp load than you intended or measured. Not all lubricants were created equal either. For example, moly lube will relieve much more friction that when using motor oil. In a sense, 120 ft/lbs with motor oil is basically 80 ft/lbs of actually fastener torque and 40 ft/lbs of friction drag. Using moly lube will help overcome the greatest amount of friction, and will yield a much more accurate clamp force across the board. Different materials and surface coatings will also have an affect on friction, and most manufacturers will account for this when they recommend a torque vale for their fastener, so it's always important to take their recommendation into account rather than only use a rule of thumb every time. It's also important to periodically have your torque wrench check for accuracy. It's not uncommon for even the best wrenches on the market to lose a small amount of accuracy.


Measuring stretch?
This won't apply to most of us. I can personally attest that I've never bothered with measuring any of my fasteners. I've never felt that I was at the level where it becomes important enough to be deemed mandatory. However, if you're picky and have access to a stretch gauge, then it definitely doesn't hurt.

Basically, you just need to measure and record the free-standing lengths of each fastener. And when the fastener is removed at any point, re-measure, record, and compare your findings with previous measurements. As previously mentioned, the fastener should ALWAYS rebound. If it doesn't completely rebound, then it's junk and will need to be replaced.


For any questions and/or comments, feel free to post in this thread: http://www.dsmtuners.com/forums/bolt-tech/381426-fastener-tech-101-a.html
 
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