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Ask The Search Engineer

Ask The Search Engineer

PVC bonding . . . Transparent conductors . . . Linear actuators . . .

Dear Search Engineer: I need to bond a PVC extruded ring of 5-mm diameter. I tried adhesives, but I need some other method of joining. Adhesives leave the residue and it is very difficult to remove it. I tried ultrasonics but only lap joint, no butt welding, was possible. Any suggestions? -B.C., Mumbai, India

Dear B.C.: To coaxially weld the extruded rings, you could insert a heating metalling cylinder at the joint, oscillate the rings against each other with some axial pressure to locally friction-heat the PVC to just above melt temperature; then allow it to cool, and extract the heating insert. Alternatively, leave a gap between the rings and, after the insert has brought the temperature up to near melting point, rod-weld the gap.

Dear Search Engineer: I am looking for a transparent conductor that can be applied manually with some sort of applicator such as a brush or pen. I need this for experimenting with various patterns without going through the expense of a production technique. -R.O. in NV

Dear R.O.: Check out AGFA's Orgacon (www.agfa.com/sfc/polymer/). They sell both silkscreen ink and pre-coated films. The films can be washed with cleaners to remove the conductive coating to leave a pattern. Even scratching with a knife edge can define detailed test patterns. You can roughly paint test patterns by hand with the silkscreen ink, but you need to dry it in an oven at about 80C. Then, get a proper silkscreening done when you know the desired pattern.

Dear Search Engineer: I need linear actuators to work in a freezer, electrically powered-stepper or servo-operating 24/7 at -25F, moving to a maximum 75-lb load at 20 to 24 inch/sec over several distances-2, 4, 12, and 18 inches. Front and rear clevis is mounted with threaded cable connectors. Any suggestions? -D.H. in PA

Dear D.H.: If the problem is heat, mount the actuator outside the freezer. If the problem is lubrication freezing, use low-temperature lubricants. If the problem is thermal contraction of components causing mechanical hang-ups, again, mount the actuator outside. If the problem is incorrect feedback at the low temperature, either pass the loop through a software program compensating for the thermal offsets or build a mechanical differential, or pass the signal through an electronic signal factor multiplier. Both latter assume constant temperature.

Dear Search Engineer: What is the design and role of a worm and worm wheel contact pattern in the working of a gearbox? -U.S. in India

Dear U.S.: Different manufacturers use different worm cross-sectional profiles, though most are based on the most commonplace gear tooth equations. All have in common the creation of the largest possible contact footprint if the relative motion is without slip. If the relative motion includes slip by design (as most do), surface toughness, coefficients of friction, and lubrication become the primary concerns to prevent rapid wear. For in-depth knowledge, there is no alternative but to study the textbooks. Slipless relative motion is possible if the design introduces a rolling third member between the worm and wheel. This has been done and the student is advised to seek out the manufacturers and the relevant patents.

Dear Search Engineer: I have found programs that design the snap fit of plastic, but not aluminum. Is there an aluminum design guide that has the formula that sizes the snap leg?-E.B. in TX

Dear E.B.: Snap latches and similar devices are no more than elastically deformed members, calculable using the conventional formulae found in any engineering handbook. Just use the correct parameters (characteristics) for the candidate materials at the desired heat treatment, and you will soon reach an acceptable design. Be sure not to exceed the yield stress when multiplied by a factor of &1. Also, when dealing with aluminum, be sure the surface finish will not be affected by the strains, and that includes any oxidation finish (often dyed) and paint.

Dear Search Engineer: I am looking for black static dissipative heat shrink tubing. The volume resistivity should be between 10^6 to 10^8 O for a 1-inch length. Any suggestions?-P.M., Medina, NY

Dear P.M.: Years ago, I used a "chemical shrink" shrink tube that came in little foil pouches; exposing the shrink tube to the air caused the tube to start to shrink. The problem with the tube shrinking was that this material had enough conductance to cause a failure on the Hipot tests, so I went back to heat-shrinkable tubing. It may, however work, for your static dissipative application if you can find it.

Dear Search Engineer: I need to make cylindrical torpedo tubes with a diameter of 3 inches at both ends and 5 inches in the middle. The diameters vary parabolically. Shaft length is approximately 72 inches and is made of aluminum Al-6061-T6. We need to make about 10 parts for prototypes. Any way of making this part for under $20k in one piece without degrading the properties of the Al?-M.K. in MI

Dear M.K.: The cylindrical torpedo tubes could easily be made using hydroforming. The tooling mold would probably cost less than $20, and I estimate somewhere between $500 and $800 per piece. Alternatives to hydroforming are spinning-requiring either an internal mold or an internal spinning tool-and explosive forming, which uses an external mold and an internal explosive charge.

Dear Search Engineer: I work for a semi trailer manufacturer, and one of our major problems is keeping moisture from entering the trailers. We currently use a single-side adhesive-lined PVC foam tape. I would like to know if there is a manufacturer of a moisture-activated expandable foam tape. I have found some that make a heat-activated expandable tape, but the temperatures to expand the tape are quite high. (When my daughter was small, we bought her a package of foam dinosaurs that expanded about 200 percent when she threw them in her bath water); if we could find a closed cell foam tape that would expand when it comes in contact with moisture, then I believe this would be the perfect material to seal out moisture in our trailers.-D.G. in IN

Dear D.G.: Try a dual open cell foam-closed cell foam tape. The closed cell layer prevents moisture from penetrating via the tape, and the open cell layer will expand by hygroscopy and create sealing pressure.

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