Ask The Search Engineer 13107

Dear Search Engineer: I need to find a quick electrical interconnect system that can be mounted to the patient side of a common blood pressure cuff and serve as an interface to a flat mylar sheet with printed circuits on it. A standard cuff is approximately 4 inches wide, but they can vary from the size of a standard Band-aid to thigh cuffs, which could be 6 inches wide.-C.D. in OR

Dear C.D.: I suggest a reusable molded flexible connector system with miniature integral edge connectors and integral strain relief for a wire connector. Try www.connecters-ez.com or www.cables-ez.com

Dear Search Engineer: I would like to get a circuit diagram for a low cost, one shot multivibrator/oscillator with the following requirements:

I also need a supplier for small-size permanent magnets.-M.A. from MI

Dear M.A.: The simplest solution for a circuit like this is PIC 12C508A. Check with Microchip at www.microchip.com. It will run on low voltages under 4.5V, is inexpensive, and can be programmed to produce any sequence of timing pulses desired. With its internal oscillator, no external components will be required. For small size permanent magnets, check out Jobmaster Magnets at www.jobmaster.com

Dear Search Engineer: We manufacture electronic energy meters and use brass screws with tin plating and a nickel undercoating. These screws are engaged with brass cage clamps, also tin-plated with a nickel undercoating. This assembly is used to fasten electrical cables. We have a problem with the brass screws breaking while fixing the cables when fastened at a torque of 1.2 to 15 Nm. We did not have this problem when the screws and cages were not plated. Can you suggest a solution? -E.M. from Udaipur, India

Dear E.M.: Check your screw threads before and after plating. Chances are your threads are closing up after the plating process. If you are manufacturing the cages, you may need to open up your threads before plating to get the required dimensions after plating. If you are not manufacturing the cages, you may need to run a tap through the holes after plating. You will also want to re-evaluate your screw torque. You may have reduced your coefficient of friction with the nickel plating and thus will require a lower screw torque. Another question to ask is, has the screw vendor changed. I have seen a great deal of variation between domestic and off-shore brass screws. Some are hard and very brittle while others are like peanut butter. I would do the dimensional checks first as you will have more control over those variables.

Dear Search Engineer: I work for a magnetic company developing a product that requires the manufacturing of air coils using 38 AWG "bondable" magnet wire. I selected heat bonding method using a heat gun, because I believe it will be easier since we only need to build a small amount. Can you recommend a material for the winding fixture (mandrel). I'm using stainless steel, but it takes awhile for the bonding agent to cure (about 30 seconds) and then it's somewhat difficult to eject it from the mandrel (shaft) where I wound it. -F.M. in NY

Dear F.M.: Instead of letting the SS madrel be a huge heat sink, how about heating it, so it would help in the curing process? Or have you tried ultra high molecular weight polypropylene (UHMW-PP)? It's inexpensive, wears well, and can take moderate heat-check the temperature rating against your curing temp. In either case, if the coil design can accommodate it, put a degree or so taper on the madrel-so that if it's moved slightly, it will come off easily.

Dear Search Engineer: I am looking for symbols for stereo jacks (head phone) DC jack, USB port to put in my plastic part. Where can I get those standards? -H. R. in CA

Dear H.R.: The IEEE, ANSI, and IEC organizations provide most of the standards for symbols used on electrical equipment. The symbols are often included in AutoCADor Visio drawing packages, or they may be acquired from the organizations themselves.

The IEEE maintains a library of graphical symbols and standards -http://standards.ieee.org/catalog/olis/gsdu.html. IEC 60617 standard covers graphical symbols used on diagrams, while IEC 60417 provides graphical symbols for use on equipment. Look at www.iec.ch or through the ANSI site-www.ansi.org. Finally, the USB organization maintains its own logo license requirements and you can find the info at www.usb.org.

Dear Search Engineer: I'm working in automotive design in India and am trying to measure the force exerted by the webbing of the seat belt on the chest/body of an occupant. Any suggestions on how to measure this without sophisticated instruments? -R.J., India

Dear R.J.: For an extremely unsophisticated instrument, how about using a rubber balloon between the belt and body with a pressure meter connected to the balloon's opening. Measurement errors occur when the balloon is not completely covered by the belt. Of course, this gives local information only. Offset is through the balloon's inner tension, which is measured as pressure without the balloon applied.

Dear Search Engineer: I currently design machinery to cold roll various forms onto various materials. One customer requires a taper to be rolled onto a 0.75-inch diameter shaft. I have all the material specs and dimensions to calculate the rolling force required for cold forming. I can then size the machine. Please help me locate the equations or method of calculation. -W.M.H. from MA

Dear W.M.H.: This is a question on how to calculate plastic deformation. There are texts that have dealt with plastic deformations from theory, using deformation energy as the approach. However, for practical purposes, try using the UTS point as if it were the yield point for a calculative point of departure, then stressing the material experimentally while measuring the rolling forces. The difference will be large but the ratios reliable for any given setup and material of same history (think dislocations and grain boundaries)! If the material cracks, I suggest deforming it when hot. In any event, in use the residual stresses might cause havoc, so I suggest heat treatment stress relief.