Ask the Search Engineer

RF modems . . .Threaded modems . . .Braking mechanisms . . .

Dear Search Engineer: I'm required to transmit information using RF. The minimum and maximum frequency of the pulses are 3 Hz and 20 Hz. The distance between the transmitter and the receiver is a maximum of 1m. How should I go about it? —V.I. in U.K.

Dear V.I.: There are standard interfaces available that work as "RF modem" (i.e., can be connected to the RS232 port of the user's board). On the other side, there is a connection to the RF antenna, with the same arrangement at the other end. Such RF modems commonly use an SOC, such as SiRF (www.sirf.com). In your application, you have a signal that equals pulses of 3-20 Hz, so you might build a simple circuit using a SiRF chip and an RF antenna.

Dear Search Engineer: We are looking for a process to make 2 × 3/4 inch through-hole × 3/16-inch thick rubber or urethane washers with metal impregnation for weight characteristics. We would prefer corrosion resistance of impregnating the metal. Is there such a process? —M.G., DN reader

Dear M.G.: To produce clean holes through metal-impregnated rubber urethane, try a water-jet cutter with a ruby orifice to restrict the stream and clean up the overspray. This will allow you to lay a sheet of the material on the platen and cut multiple units.

Dear Search Engineer: Often when repairing a piece of equipment, you run across a threaded fastener of unknown specification. You can get the measurements of it, but they may not match anything familiar. Is there some resource (website, printed table, book, etc.) that consolidates various thread specifications from around the world? The idea is that you could enter a thread parameter (pitch, major diameter, etc.) and get a list of matching available fasteners. —R.L., DN reader

Dear R.L.: There is a book that describes the threads of the world, called Guide to World Screw Threads, published by Industrial Press Inc. Check it out at http://rbi.ims.ca/3857-511.

Dear Search Engineer: I am looking for a solution to this application: a machine runs fom 200 to 800 cycles per minute. When I press "stop," a braking mechanism comes in to stop the machine. The machine stops at a different position, depending on the speed; that means that at 200 cycles per minute, the machine stops at 1:00, and at 800 cycles per minute, it stops at 7:00. I can tabulate these various stopping positions at the various speeds. These stopping positions are consistent at the various speeds. I would like to stop the machine, regardless of the running speeds, at a fixed position for me to decide. Feedback is via a rotary encoder. —M.K., Singapore

Dear M.K.: Try inserting an adjustable time-delay relay or circuit between the "stop" button and the braking device. By adjustment of the time delay, you should be able to pre-set, or at least make adjustments on the fly to engage the defined stop point at various operating speeds.