Canadian Plastics

VIEW FROM THE FLOOR (March 01, 2000)

Mechanical fasteners simply don't get the respect or attention they deserve. I believe it's a lack of sex appeal. Who cares about nuts and bolts in a world full of engineering resins, advanced composi...

March 1, 2000   By Jim Anderton, technical editor



Mechanical fasteners simply don’t get the respect or attention they deserve. I believe it’s a lack of sex appeal. Who cares about nuts and bolts in a world full of engineering resins, advanced composites and Internet IPOs?

Last month I described how screws and bolts stretch like a spring when torqued, and how the “elastic limit” describes the failure threshold for over-torqued fasteners. Screwing assemblies together reliably and repeatably requires torque values that take the fastener into the elastic range without getting too close to the dreaded elastic limit. The key is getting that spring effect to clamp the parts together.

Split ring and bellville-type lock washers exploit this effect by adding spring. I prefer to avoid lock washers wherever possible, because if they break, the bolt is completely unloaded. Given the minimal clamping redundancy built into modern tools and molds, the resulting loose assemblies are the start of big troubles.

The best technique for the measurement of bolt stretch is by micrometer, but this is rarely feasible. Generally, we’re forced to measure torque, an indirect measure of the desired axial stretch. Unfortunately, many outside factors affect torque measurement, whether by torque wrench, instrumentation, or bare hands.

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The most common factor is friction, both between threads and under the bolt head. Accurate torque values require minimum friction, meaning clean, well-machined threads and mating surfaces. Pneumatic impact tools can contribute to the problem both by shocking the fastener and by concealing rotational friction. Ideally, bolts should be lubricated, spun up with an air ratchet, and then finish-torqued with a calibrated instrument. Alternatively, a torque-sensing device could isolate the impact gun from the socket or Allen key, such as the “torque sticks” used by truck wheel installers.

Debris in blind threaded holes is another major cause of loose assemblies. Conventional blowguns may drive material deeper into the hole, resulting in loose assemblies with fasteners that feel tight. The solution is to blow out the gunk from the bottom of the hole upwards. I’ve used a length of soft copper tubing brazed to the end of a blowgun (along with an approved face shield and gloves) with good results.

Misalignment is yet another cause of assembly problems. Screws and bolts are clamps, not dowel pins. Unless you’re using shoulder bolts (common on stripper assemblies) don’t laterally locate precision assemblies with fasteners. If you forget that dowel, tear down and put it in.

Cross-threading a fastener is an obvious red flag, but remember that impact gun? Pneumatic tools can deaden even the most sensitive of hands. If a tool or mold features a wide variety of screw sizes, there is a temptation to change sockets without selecting the appropriate impact setting. Ninety-nine times out of 100, it’s just a matter of easing off the trigger at the right moment, but if that little 1/4-20 strips or breaks, what then? At the current price of down time, a stripped or broken bolt will easily exceed the cost of the pneumatic tools. The best set-up I’ve seen uses a manifold attached to the “crash cart” with a 3/8-inch air ratchet dedicated to smaller fasteners.

Friction, debris, misalignment and over-torquing are just some of the ways to abuse fasteners. Clean threads and calibrated tools may not seem sexy, but the alternative is downright ugly.


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