Friday, May 8, 2009

Bolt Tensile Strength

Diane here, posing the question: What’s the difference between my world (the world of the marketing person) and the engineer’s world? Answer: I only deal with three fasteners: staples, clips and tape. Engineers deal with a staggering number of fasteners when creating a design, and even after they’ve selected the appropriate fastener—say, a bolt—there are still numerous things to consider. Today we’ll look at just one: tensile strength.

In my very simplified world of fasteners, my options might boil down to this: which kind of clip will have the strength to hold this number of pages together? The regular staple? The ginormous binder clip? Well, that’s not really an accurate analogy to tensile strength, but the point is, an engineer must be much more particular than I, math- and physics-wise, to confirm the bolt of choice can handle the stress of the job.

Stress of this kind is typically measured in psi (pounds per square inch) or MPa (meter-pascals). Those are measures of pressure, so what you’re looking to find out is the amount a pressure your bolt can handle. This is not a simple question, even if you know the strength of the material used for the bolt. Other factors involved are size of the bolt, whether it’s square or round, whether the threads are fine or coarse pitch (fine threads, logically, are stronger), and much more.

Because of these complexities, bolts are given ratings to simplify your choices. SAE or DIN/ISO grades will provide you with an accurate measurement of the stress a particular bolt can handle. Our Web site’s Technical Section includes a handy chart of bolt minimum tensile strength.

But what exactly is meant by tensile strength? A lot of things, and you can break it down more specifically. First, you have Proof Strength, which is used to determine tolerable torque, and is ~95% of Yield Strength. Yield Strength is the point at which your bolt will start to deform. And lastly there is Ultimate Tensile Strength, the point at which your bolt will break.

There are other factors that influence the stress a bolt can handle in a particular application. For one thing, tensile strength ratings assume the nut is fully threaded onto the bolt. Meanwhile, the nut itself needs to be adequate strength and support the shear load. The machine is assumed to be operating under typical room temperature, and the torque on the bolt needs to be greater than the load torque so that tension changes don’t result in fatigue. There may be other factors besides load that effect the bolt. In an application that is subject to vibration, that means additional stress as the load on the bolt repeatedly reverses.

When I think about all this, I have one of those rare moments when I’m glad I’m not an engineer. I can pretty much eyeball whether I need a paperclip or a ginormous binder clip. If you become as baffled as I am about selecting the right bolt, just call our Technical Sales Associates for assistance. Bolts are included in our Section 19, Fasteners and Seals.

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