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Ask The Search Engineer

Ask The Search Engineer

Dear Search Engineer: I'm interested in learning how to weld Tefzel(R) and the kind of welder needed. I'd be working with a thickness of 20 mil.-B.T., CA

Hey there B.T.: Tefzel, fluoropolymer film sold by DuPont Films (www.dupont.com/teflon/films/E-80420-1.html) can be heat welded by pressing the seam edges to a hot plate at 520 to 530F and then pressing the edges together. It can also be ultrasonic welded. Typical welding conditions are 25-psi contact pressure and a one- or two- second cycle time.

Most Wonderful Search Engineer: What are the basic considerations in designing a shaft? -Omar from Bahrain

Greetings Omar: Where do I begin? Most folks think this would be a simple question, but hey, engineers know better! First of all, the considerations in designing a shaft depend on the application as well as the following:

1. Forces and torques. Shafts, by virtue of their name, are always subject to torsion. In addition they are usually subjected to bending. You would need to work out the combined stress resulting from these loads.

2. Distortion and vibration. Most applications would limit the extent to which a shaft may be allowed to deflect. Also, a shaft, particularly if it operates at high speeds, needs to be checked for natural frequency vibration. Torsional vibration is a factor that also needs careful analysis. The design would depend on the forces and torques involved and their locations and directions.

3. Wear and corrosion. Wear of contacting surfaces could ultimately be a factor, particularly if bushings are used. Contact pressure, when using narrow anti-friction bearings, may necessitate spot hardening of the shaft at these locations (induction hardening is a good method). A corrosive environment may also necessitate the use of specific stainless steel materials, hard chrome plating, metallizing, etc.

4. Fatigue factor. Fatigue is, by far, the most insidious of factors and tends to be ignored too often. If you look at the history of shaft failures, you will find that more shafts fail from fatigue than from any other reason. The fatigue factor is influenced by the material, variations in profile of the shaft, and small, seemingly insignificant aspects like circlip grooves, keyways, lubrication cross-holes, etc. If the shaft diameter is greater at some points than others (which is usually the case), use small incremental steps, preferably at a shallow taper because this is particularly effective against fatigue. Incorporate generous radii or chamfers, rather than sharp corners. Use splines, rather than keys. To avoid both, use friction-locking devices like Ringfeder. They may be a little more expensive but are extremely effective particularly with slow speed shafts and may even help you to reduce the diameter of the shaft significantly.

5. Lubrication and cooling. Though an engineer would naturally look into the lubrication of the bearings used, I have seen cases where shafts have failed because of insufficient lubrication or overheating of the contact surface.

With this information, you should be able to get a rough idea of the size of shaft you'll need. You can select standard bearings to support your shaft taking into accent speed, loads, and expected life.

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