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The Search Engineer finds solutions to all your questions, problems, and dilemmas. Occasionally, he could be wrong. But he doubts it.
By Design News Staff -- Design News, April 18, 2005
Mechanical diaphragm . . . Wet/dry contacts . . . Concave roller . . .
Dear Search Engineer: I'm looking for a metal, rubber, or combined mechanical diaphragm. The idea is to extrude meat (crème) through a round pipe, and at the end I want to cut and form the meat at the right length in order to obtain a final form similar to an egg. I've tried to build one only with rubber and it worked fine to make the form, but the cutting requires a sharp edge when the diaphragm is closed. How can we progressively close a circular hole and keep it circular at every moment? —M.D., Portugal
Dear M.D.: One way would be with a device similar to a camera shutter. If there are no space restrictions around the tube, try multiple guillotine blades every 30 degrees, for example, advancing linearly and closing the circular port. If space is a problem, try multiple cam-shaped blades, rotating in place until the circular hole is closed. You may also want to consider Iris Diaphragm Valves that are used with gravity feed dry materials.
Dear Search Engineer: I have seen applications where normally open and normally closed contacts have been classified as wet and dry. Do you have any information regarding the difference between wet and dry contacts in switching applications?—C.T., Orchard Park, NY
Dear C.T.: Dry contacts usually refer to metallic contacts in air, which when closed provide metal-to-metal contact. Wet contacts refer to spaced contacts (electrodes) that are closed when a conductive material such as mercury bridges between them. An example of a wet contact would be the lowly home thermostat housing a mercury switch. Wet contacts are sealed from atmosphere by a capsule or container that isolates or contains any arc or spark within; dry contacts are usually open to atmosphere and may or may not have a cover plate. Wet contact power relays worked well for high cycle rates or long cycle rate life times. For the most part, they have been replaced by electronic relays (SCR, IGBT, etc.) due to the hazardous nature of mercury.
Dear Search Engineer: I am having a difficult time finding information on optimizing the "negative crown" radius of a concave roller relative to the rail radius. I also would like to be able to calculate/estimate the contact patch from deformation of the roller to estimate roller life. The roller would be urethane (approximately 50 shore D durometer) with an aluminum hub. The initial "negative crown" radius of the roller is 0.862 inch and the rail has a 0.750-inch radius. Contact diameter of the convex roller is 2.375 inches. If the roller is pushed to the side, squealing may occur due to relative sliding from the velocity differential. Any suggestions? —J.R., Franklin Park, IL
Dear J.R.: To calculate the curved surface contact area and curved surface compressive stress, an excellent resource is provided in Fundamentals of Machine Component Design by Juvinall and Marshek (1991), page 322. The text refers to an original analysis of elastic contact stresses, published in 1881 by Heinrich Hertz (http://rbi.ims.ca/4390-511). Otherwise, you can make some samples with different geometries but the same surface finish, and try whichever is best; then with that geometry, vary the surface finish.
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