Design News for Mechanical and Design Engineers

Hey K.S.: Semi-trailer axle manufacturers, such as Arvin Meritor, make axles that are drawn from a tube. The spindles are swaged down from the axle tube and are hollow. But I do have a caveat—even though the loads are distributed in a uniform and manageable way, forging and castings will not go away! Numerous crash test data point to the advantages of solid vs. tubular matter when forces are put upon those loads. In other words, "bigger (mass) is better," when it comes to automobile crashes. The current improvement trends are moving in the direction of composite materials.

Dear Mr. Search Engineer: I am trying to find a fastening solution that will allow me to fix a stainless steel machine screw into some sort of blind hole in an aluminum casting and ensure I can still remove it easily years later without corrosion problems. The product is waterproof so the solution needs to be some sort of blind threaded fastener. The client doesn't want to use helicoil type inserts.—S.C. in SC

Right back atcha, dude: What about the use of "Never Seize" or other brand of anti-seize, anti-gall compound at assembly time? A suggestion might be to inject a measured amount into a pre-tapped (blind) hole so that it gets forced out around the threads upon fastener insertion. Coat the fastener at the time of assembly. Another solution may be an insert, such as those offered by AVK or Atlas Engineering. Check out www.atlas-eng.com or www.avkfasteners.com for more information. There is also a speed fastening system developed by Advel Cherry Textron that may work. It uses a threaded rivet cold form that threads into the aluminum casting. The force to cold form the threads comes from a hydraulic rivet gun backing out a mandrel from the Rivscrew® leaving a hex hole that can be unscrewed later with standard allen wrenches.

Dear Search Engineer: A standard solar panel of 12V has about 18V at peak output power and is used to charge a battery of 12V nominal, but a high drop-out voltage is wasted. If this 18V is brought down to say, 16V, by dc-dc inverter-converter module, we can increase the charging current. I'm looking for design details in this type of conversion that can be compensated by the gain in charging current.—D.R. from IL

Dear D.R.: True, a solar panel can output 18V, BUT the current load is low at that setting. Most panels are designed to operate with that open circuit voltage (for 12V nominal systems), but the maximum derived power is at the 'knee' of the load curve, where the voltage drops, and the current output rises. The greatest current output is near short-circuit conditions. The voltage is for the cell electrochemical reactions, but the current is what does the actual charging. A voltage converter as requested, would be about 70-80% efficient, reducing the overall efficiencies, and will not increase the charging current.