164 THE DISTRIBUTOR’S LINK HOW TO BUILD A SAFE AND RELIABLE BOLTED/SCREWED JOINT continued from page 162 Fasteners Vertically walled drives like Hex Heads, 12-points, Torx, Sockets etc. should be used whenever possible. With these we don’t have to put pressure on the assembly tool and they are ergonomically suitable for assembly work (no or little operator fatigue). Use standard fasteners as much as possible. We have great, proven standards from ISO, ASME, ASTM, SAE and other standards organizations that can be used in most designs. External drives should be our preference for fasteners used outdoors where we may have dirty conditions. Socket Screws were originally intended only for use in machine tools (indoor equipment). They can be difficult to drive if the socket is filled with dirt. They are also, for the 12.9/alloy strength level very sensitive to stress corrosion cracking when exposed to the nasty out/door environment. Hex Screws in the higher property classes (grades) will show very high surface pressures against the joint material and we should either use hard washers or consider Flange Screws. Just keep in mind that Flange Screws require higher tightening torque than the standard hex heads due to the much larger friction area. Accessories A well designed and maintained bolted/screwed joint usually don’t need any extra help to perform as intended. For some added security, we sometimes use products like Prevailing Torque Nuts and Screws, chemicals, locking wires, specialty washers and other devices. Almost all joints, when using products that carry the subtitle “locking” will, when subjected to severe vibrations, loose some or all of it preload. When preload is lost, more and more of the external load will hit the fastener with a higher likelihood of fatigue failures. But, even if the preload is going down, a prevailing torque nut or similar will not freely spin off the bolt end. That way we may still be able to shut things down before parts separate entirely. There are today, however, several products on the market that still deserve the title “locking”, but I strongly suggest that you do your homework carefully before accepting some fancy sales pitches.
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