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Know Thy Fasteners

Know Thy Fasteners

What are the biggest problems engineers have when specifying fasteners into a design? With the exception of a few engineers, most don't have a clear understanding of how bolted joints behave.

How should they consider them? They should know that a bolted joint is an integrated system of nuts and bolts. They should take into consideration clamped parts, mechanical properties of clamped parts, locking elements, tightening tools, applied tightening torque, and last but not least the person tightening the bolt.

Why is the apparent lack of knowledge a problem? If fasteners are simply used to hold a component in place such as a hose, a cable, a wire, or a lightweight sheet metal part, then for the most part there are no problems. Issues arise when fasteners fulfill a load-critical function, such as to keep the wheel from falling off the axle, to make sure the connecting rod is permanently secured to the crankshaft, or make sure the frame of tractor is fastened securely. In such cases all the parts used in the joint must be evaluated and properly laid out so that the joint and the piece of machinery it is holding together will survive both loads and the working environment's conditions. A bolt failure is often contributed to a bad fastener. There certainly are bad fasteners, but on average only one out of ten failures can be attributed to a fastener that is not mechanically sound.

What are the best strategies for overcoming those problems? Gather information. Spend as much time understanding a fastener as you do a jet engine-maybe not quite as much. An engineer should evaluate what the bolted joint has to deal with. Time can be allocated somewhere else if the fastener's function is to support something lightweight. However, if it turns out that the bolt has to deal with a great deal of pain (stress), then further research is necessary.

Do stainless steel fasteners last longer than fasteners with a protective coating or plating? That depends on the environment. Let's assume we use a plain regular unprotected steel fastener and an austenitic (300 series) stainless steel fastener on the same piece of machinery, somewhere in the Mojave desert. Both will last the same amount of time. On the other hand, if a fastener is used on a piece of machinery along the ocean shore the unprotected fastener will corrode within days whereas the stainless steel fastener will look brand new.

When should engineers use stainless steel? If a fastener is likely to be used in an air-conditioned room where the air-conditioner is on all day and night, zinc-plated fasteners will do just fine. However, seriously consider stainless steel if the same fastener is used in a room along the oceanfront with no air conditioning or the air conditioning is likely to be shut down overnight and or over the weekends. Stainless steel fasteners are certainly more costly. The most common corrosion-resistant (300 series) stainless steel fasteners have lower strength, typically about 90% of that of a grade 5 (or metric 8.8). Also, they actually may corrode internally without anybody noticing it, to the point where they fracture. Austenitic stainless fasteners are susceptible to corrosion under certain circumstances. A typical instance is the high concentration of chlorine fumes inside indoor swimming pools.

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