Dear Search Engineer: Our company manufactures aftermarket add-on products for the wheelchair industry. We currently use a mixture of fasteners, 10-32 and 1/4-20 thread sizes, with button, flat, and socket style heads that are all hex socket screw styles. These are standard off-the-shelf alloy steel heat- treated fasteners. Our problem is rust. The end users of our products spend the majority of their time in their wheelchairs exposed to a variety of climates. We need to keep the cost down, which is why we use the steel fasteners. We've tried having them black zinc coated but the fasteners come back not coated evenly, or the coating has a tendency to flake with minimal handling.—M.H. in New Hampshire
Dear M.H.: Have you considered powder coating? It provides a uniform and durable outer layer. There are lots of powder coaters out there that may provide just what you are looking for.
Oh wise Search Engineer: I have been scouring CAD/CAM/CAE sites for any software for Mac OS X. Alas, I seem to be wandering alone in the wilderness. The graphics arts/design and architecture industries have used Mac OS software for years, and now is the time for some serious 'Windows-only' CAD systems to make it into the Mac OS world. I won't give up my Mac! —B. Arflack, someplace in California
Ah, Grasshopper: What you seek shall be found. There are two CAD systems that I am aware of for the Mac OS: VectorWorks from Nemetschek and a number of packages from Ashlar. VectorWorks is geared toward architecture B.A. where I understand it has a significant number of advantages over other packages. However, as a mechanical design and development package, Ashlar Cobalt has much more utility. The underlying technology is the Spacial Technology 3D engine so the data files are very portable across platforms and between packages. More details on the suite of CAD packages for Macs are available at www.ashlar.com.
Search Engineer: Help! I need to know how to calculate the force it takes to pull a 3/8-16 hex head cap screw out of one wall of a piece of #304 stainless steel rectangular tubing, wall thickness 0.125 inches. I need to know step-by-step with examples how to calculate this so I will be able to do this with different size bolts and material.—C. Wise from Kalamazoo
Dear Mr. Wise: You came to the right place! This solution is described with a variation of the procedure in Machinery's Handbook (24th edition, page 1324-25), Formulas for Stress Areas and Length of Engagement of Screw Threads. I recommend the section on Stripping of Internal Thread, which takes into account shear area of both external and internal materials and how they interact, and accounting for class of thread and fit. Substitute actual engagement for Le and back-calculate the failure force using the shear area calculated and the failure stress. This may be a little higher than yield strength because table values of yield and dimensions of the thread are minimum values, and some work hardening likely occurs. If you're trying to calculate the "safe" force to prevent pull-out, use the minimum values with an appropriate safety factor.
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