Ask The Search Engineer 12672

Dear Search Engineer: I am trying to find a way to have characters glowing through a tinted plexiglass (2064 Bronze) window using the power of an LED. The font size is about 6 or 8. When a function is selected, the LED lights up. I was wondering if I could use the LED to make the characters, which could be fluorescent, glow? Simple silk screening with back lighting would not give an even light dispersion. Could I use fluorescent tape for this purpose? Other suggestions?-J.R. in Canada

Howdy J.R.: A simple light sanding or sandblasting on the plexiglass can even out the light dispersion. There are also plastic filters either specifically made or used for this purpose. Look at an instrument cluster in a low-end auto and you will see how it is done. Another solution would be to check with Poly-Optical Products at www.poly-optical.com. They have a woven fiber-optic black light that provides light from a single LED.

Dearest Search Engineer: What is the exact threaded engagement required for different materials? What we need is the exact theory (not rule of thumb like 1.5x for ductile materials, 2x for brittle materials). We also need suggestions for a website or books that offer material properties, friction, factor of safety, pitch, and working conditions like temperature, duty cycles, fatigue, etc. -G.N.R. from India

Greetings: In the 25th edition Machinery's Handbook (Industrial Press, http://www.industrialpress.com/mh.htm), starting on page 1415, there are equations for calculating the thread engagement based on the material of internal and external threads. Another source of information is the literature that bolt and fastener manufacturers offer. Most give the minimum values of yield and tensile strength, elongation, and the typical chemistry and hardness of a fastener. Finally, this magazine published two articles in the '70s that still apply today. Check them out at www.manufacturing.net/contents/pdf/threadlength.pdf and www.manufacturing.net/contents/pdf/threadengagement.pdf

P.S. Looking for an article from Design News published before 1995? Drop us a note at [email protected] with the exact article name and issue date and we'll see if we can get them online.

Dear Search Engineer: I have several questions about surface adhesion of unreinforced, thin-layer, epoxy resin under static compressive load:

1. How long does an unreinforced, clear epoxy resin, 4cm in diameter and 2mm thick under 2-kg static load at ambient temperature maintain its surface adhesion (tac) despite compressive creep?

2. Which general types of epoxy resins and mixing proportions and techniques would render strong, long-lasting surface adhesion (tac)?

3. Which simple workshop test could be carried out to compare the above in order to find out by extrapolation long term results? -C.V. from TX

Dear C.V.: When an application involves a truly static compressive load, with no peel or shear-induced stresses, epoxies have stood up well for the timeframes described, particularly in aerospace applications. However, the life is contingent on several things, which are applicable to many adhesives, not just epoxies:

a) Assumes no UV or similar exposure, which degrades the mechanical properties over time. Use temperature is also very important and can degrade the epoxy over time, depending on the temperature history seen. Depending on the intended use, soaking in a hot, wet environment is often used to accelerate screening tests. If there is exposure to any other chemicals (solvents, oils, cleaning fluids, etc.) these can also degrade adhesives. Different adhesives work better for different environments. Supplier technical experts can usually make recommendations on what works best.

b) No mention was made of what materials were to be bonded. This can make a huge difference, as different materials require different surface preparation for bonding, and some (e.g. Teflon^(R), polyethylene plastics) do not bond well at all with epoxies. Any oils, fingerprints, or other surface contaminants can cause failure, usually shortly after bonding. Metals usually require an acid etch or special primers for long-term bonding applications. Composites and many other materials can be sanded, grit blasted, and solvent wiped. Glass and most other ceramics can get by with good solvent wipe to remove contaminants. Bonding should be done immediately after cleaning or preparing the surface if possible. Some materials have to be prepared if the time between the surface prep and bonding is more than a day or so.

c) Mechanical properties are generally degraded as thickness of the adhesive increases. As a general rule of thumb, keep bond thickness between 0.005 and 0.020 inches thick. For thicker bond lines, adhesives with fillers are usually better. Loctite EA9394 with aluminum filler is one of the best all-around adhesives, particularly if it is also used like a shim. Unfilled EA 9396 is clear and EA9392 is pretty good if being clear isn't required; but for the bond line thickness you are talking about, unfilled epoxies will usually run out and not provide a good bond.

d) Mixing proportions can be different for each adhesive system and should follow manufacturer recommendations for optimum properties. Good mixing is critical and one problem many inexperienced users have is to stir the two components in a cup, and then apply the adhesive. The problem with this is that one component invariably is not mixed well on the wall or bottom of the cup and if special care is not taken during mixing, this unmixed resin will be scrapped out of the cup and used and end up failing, and the user will not have a clue why some of the bonds worked well while a few failed.

Dear Search Engineer: I need suggestions for a flat composite belt replacement for steel cable to be used in a hoist application. -R.S. from NJ

Dear R.S.: While I've never seen the actual wire rope on a hoist in a non-metal form, there are Kevlar slings readily available which can be attached to the hook block if product protection is an issue. If the concern is steel wire rope sparking, there are wire ropes in bronze and stainless steel.

Dear Search Engineer: I'm looking for a more thermally conductive and antistatic nylon composite or a suitable alternative that outperforms carbon black nylon. This product would be used with automotive fuel. -C. T. from CA

Dear C.T.: I suggest that you contact RTP Company. They are a specialty thermoplastic compounder that customizes more than 60 engineering resins. You can contact Sam Dahman, engineer, or Jeff Kronebusch, conductive product manager, at 800-433-4787.

Also LNP Engineered Plastics has some thermally conductive polymers that are nylon based. Check out their site at www.lnp.com.

Dear Search Engineer: I'm looking for a company that manufactures stainless steel filter media. The product will be used in diesel-emissions control. Any suggestions are appreciated. -M.S. from MI

Dear M.S.: Vacco Industries manufactures stainless-steel edge type filters. Check out their website at www.vacco.com.

Dear Mr. Search Engineer: I am working on a problem relating to heat transfer to fluid on an outside boundary. Can you suggest any software?-R.R., India

Dear R.R.: There is a product named PAM-FLOW that could help-check it out at www.esi-group.com/Products/Flow. There is an option in PAM-FLOW solver allowing you to take into account heat transfer equations. It is possible to assign some heat transfer parameters on a wall. This specific boundary condition allows you to specify time-varying parameters for conduction and radiative heat conduction at the wall.

Dear Search Engineer: I'm trying to locate a 20,000-or 30,000-rpm electric motor that will deliver at least 1 hp continuously. I also need high impact load bearings for the shaft. -M.W., CA.

Hey there M.W.: Try an SR motor. The motor speed could be controlled.

Dear Search Engineer: I have four 0.1% accuracy pressure transducers on a dial table. I need a solution to send these signals to a data acquisition system without using the slip rings. -F.M. , Ontario.

Dear F.M.: Signal conditioning and A/D conversion have to be done on the table. Communication with the DAQ system can then be accomplished digitally (best for error correction) with a wireless transmitter/receiver. Control power is best to be supplied with slip rings. Batteries are not recommended for an "always on" system. I've seen inductive systems (also called "air gap") where both control power and signals are transferred only when the unit on the dial table is in close proximity with the unit on the base.

A reader, J.T., responds to D.R.'s question in the 08.18.03 issue about photovoltaicsystems: If a standard solar panel of 12V with 18V at peak is brought down to 16V by a dc-dc inverter-converter module, will charging current be increased?

Many PV cell assemblies do indeed have the power peak at about 18V; my array certainly does. The significance is the mis-match with the 12V lead acid battery, which is no longer in a peak charge current range by 15 or 16V under charge. It should then generally be charged at a lower "finishing" rate. At the voltages where high current is very beneficial (10 -13V) the power from the PV system is far less optimal due to the inherent current limit of the PV array. Even if such a horrible downswitcher efficiency as 75% were correct, that could still be a win when the energy efficiency would otherwise be as low as 58% . Buck regulators can have much better efficiencies than 75% competently designed. In fact, PV charging systems, have for many years been available with down-switchers to increase current capability at low states of charge. Anything which will increase the current into the battery when in astate of deep discharge is extremely significant in improving system utility and battery longevity. The downswitching charger is capable of doing this.