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Ask The Search Engineer

Ask The Search Engineer

Dear Search Engineer:I am in search of the "ideal" screw fastener, if such a fastener exists. In my engineering work, after I've designed a fabrication the arduous task of assigning screw fasteners begins. Dictating factors are: clearance room (hex head, socket head, etc.) and strength (screw sizes). Isn't there a compact-small head-screw fastener of superior strength? The end result being that the designer merely tallies the number of screws required in the design and submits it to manufacturing, making everyone's job easier. -Respectfully, Raghav S.

Greetings Raghav: I've got bad news for you, partner-there's no such thing as an ideal minimum-size head screw. Why? Because fastener choice depends on so many variables. For instance, you may have a relatively hard joint or a soft joint. The shear strength varies in the material of the nut or tapped hole. In another case, you may have oversized holes for easy alignment and wish to use a relatively large diameter head. Or perhaps you may need different head types for best tool access, straight in versus side access only. The medical and sanitary processing industries prefer heads with no pockets to hold dirt; and the list goes on and on.

Search Engineer: Here's one for you .... What are good material choices for injection-molded light pipes? UL 94 V0 is required for flame retardancy in this application.-Marty from Illinois

Hello there Marty: Have you checked into using GE Plastic Lexan 141-111? A good choice for injection-molded light pipes.

Dear Search Engineer: I want to determine the effect of an impact load between a metal pin and a plastic bushing. We are using the plastic bushing to thermally isolate two different frames. The assembly sees a random vibration environment. I have tried to calculate impact loads on a kinetic-potential energy basis, but I don't like my results and am looking for an alternative. Any hints? -C. M. from Ohio

Howdy C.M. It would help to have an idea of the size of parts involved, severity of impact, etc. But without more to go on, let me ask you-does the thermal isolation have to be plastic? How about a rubber or elastomer bushing? Since such things are routinely used for engine mounts, body/frame isolation, inside the eyes of leaf springs, in shock/strut mounts, and so on in the automotive field, I believe that off-the-shelf isolaters are available in a wide range of sizes, stiffnesses, and other properties. Their design basis is typically the vibration-isolation feature and their performance should meet your requirements. In addition, their thermal isolating properties are a by-product of the materials, molded-in voids (if present), and multiple boundaries between one side and the other. On the other hand, if the bushing must be plastic, can it be designed as a plastic spring, with the ability to absorb some of the vibration and impact by deflecting? Stiffer suspension bushings, such as required on race cars, are commonly made of urethane or even acetal; the trade-off for better control and handling is faster wear.

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