Design News for Mechanical and Design Engineers

No ground-braking innovation

Let me tell you why I know this. While I was vacationing in Europe in 1984, I was in a car driving from southern Germany to Innsbruck, Austria. Coming down a steep mountainside, there were signs that warned the drivers, all drivers, of the following:

• Put your transmission in the lowest gear, to control the speed of the vehicle.
• There will be 4 "brakeways" (Bremsenweg in German) coming up. If your brakes fail or you feel like you are losing control, steer into these brakeways to stop your vehicle.
• These brakeways are for everyone, trucks and cars as well.

I discussed this with my uncle, who lives in Germany and was the driver of the car I was in, and he said that these brakeways were first implemented in the 1960's. Considering how much trucking traffic must go through the Alps, because they cannot just drive around them, this should not be surprising.

Your article fails to show this type of detail and because of that, it is misleading. As we continue to become truly more global, you will find this detail to be more prevalent and it does need to be researched and discussed as well.

Thomas Seubert

Editor's note: In our bi-weekly newsletters, we publish questions from engineers looking for help and invite readers to respond. Here are a few recent exchanges:

I'm looking for a fluid level sensor that is primarily electrical as opposed to a mechanical float/switch. There are many high-priced non-invasive electrical sensors on the market, but I need something that is extremely low cost (less than $5). This sensing device will be subjected to temperatures up to 100C unfiltered water (could have corrosive elements in the water). I was thinking of a float with a magnet attached to a shaft that activates a hall sensor once the float goes above some predefined level. I'm concerned the float/shaft may get corroded and bind up. Any ideas would be appreciated.

R. Hower

Look at most late model automotive master cylinders. The float and the plastic tube it rides in do not corrode and are suitable for underhood temperatures.

J. Cole

A common practice in the semiconductor industry is to use a pressure sensor, flow air through a tube into your liquid, and at a "T" place the pressure sensor. When the liquid closes the air flow, the sensor will pick up the pressure change. Or you can replace the pressure sensor with a vertical tube with a plastic pellet. When the tube is closed by the liquid, the pellet will rise and block the light from an opto-iso device. For the opto device, use an LED and optical transistor.

B. Amick

I am trying to compile some industry in-formation on types of failure modes, thermally of LCD panels. I'm looking for this information for both TFT and DSTN LCD panels.

H. Piper

Failure modes of normal LCD panels normally occur when subjected to temperature extremes on the low end of the spectrum. They do not like cold—beyond -20C (the spec for extended range LCDs). This info may be applicable to TFTs and DSTNs. One will notice the refresh become sluggish, up to a point where nothing is displayed. Once the temperature creeps back up, then you will notice activity return from sluggish to normal. They behave better in the upper extreme.

D. Munoz

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". 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 from time to time.

C. Wise

This solution is described with a variation of the procedure in Machinery's Handbook (in the 24^th edition, it's on page 1324-25), Formulas for Stress Areas and Length of Engagement of Screw Threads, especially 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.

D. Cressman