Peer Advice

Q: We are manufacturing a retractable step that is made mainly out of aluminum for corrosion issues. We are using stainless bearing shafts. In isolated cases, we are experiencing oxidation build-up from the aluminum, although it is anodized. This build-up interferes with our DU bushing and sometimes creates binding. Any suggestions you have would be appreciated.

A.: It could be that you are seeing a little bit of galvanic corrosion between the aluminum and stainless steel. One possibility is to add sacrificial zinc anywhere on the aluminum piece; otherwise isolate the aluminum and stainless steel pieces.

Q: We design and build a product that must last 20 years in the field. The product is subjected to high humidity, temperature ranges from -40 to +70C, salt fog, freezing rain, etc. Below-grade installations can see pH levels ranging from 1 to 7. It must also have good impact resistance at the temperature extremes. It is easy to find a material, based on data sheets, that meets most of these requirements. However, most of the data is based on standard tests and not at the extremes of the actual environment. With a product life of 20 years, it is important to test the materials at an accelerated pace. Are there any guidelines on acceleration factors for plastic materials? I am especially interested in humidity/temperature combination and extreme temperature excursions.

A: Akron Rubber Development Lab (www.ardl. com/) is experienced in life prediction methods for elastomeric and plastic products. As long as the accelerated conditions do not create physico-chemical changes in the tested product different from those seen in the service environment, reasonable extrapolations can be made against a control benchmark material.

Q: What is the normal wall thickness for an insert for plastics that use bosses' post method? Is there a spec for this?

A: There are no hard and fast definite rules that apply for all plastics for all inserts and insertion techniques for sizing bosses for secondary insertion. You will find that some industries regularly oversize bosses for potential service rework to the next insert size. The most critical factors in sizing are: the allowable backside sink from molding the boss, the required strength of the assembly (pull-out and torque-out), and the characteristics (strength, stiffness, and elongation) of the plastic material. A simplistic starting point for boss sizing for general use in common plastic materials is the boss OD of 2× the insert OD. The interference fit for a specific insert should follow the insert manufacturer's guideline. Check with a big manufacturer like Dodge Helicoil.

Q: Is there a documented method that can be used to determine the deflection of a rectangular steel frame? Structural engineers must have a method that is not taught to mechanical engineers. Often diagonal members are required to reduce load deflection, but I have never seen a documented method to determine the loads transferred to and from the diagonal members. I know that if frame deflection exceeds 1/8 inch in 10 ft of length, broken weld joints will result.

A.: To answer your question, yes, there is a method used by mechanical engineers to determine deflection of any type of geometry. This would definitely fall into the realm of a structural engineer, but most mechanical engineers probably know how to solve for deflection in a beam. The main drivers in beam deflection are: type of material, geometry, wall thickness, how the beam is loaded, and how the beam is affixed. Once you have that data it is as simple as "plugging" the numbers in an equation to solve for deflection. You can check the Manual of Steel Construction (www.aisc.org/Template.cfm?Section=Bookstore&Template=/Ecommerce/ProductDisplay.cfm&Productid=203) by the American Institute of Steel Construction Inc. (AISC) for general guidelines for steel design/construction, including details of how to calculate allowable stresses for different shapes based on its unsupported length.

Q: Can you give me an idea of where to get a list of incompatible plastics, paints, and cleaning solvents? I'm interested in finding out what plastics should not be allowed to come in contact with others and which solvents should not be used to clean particular plastics. I would also like some understanding of the chemistry involved in these undesirable interactions, as well as a general set of rules. Is there a material science reference book?

A.: A good place to look may be the chemical compatibility section at the GE Plastics website (www.geplastics.com /resins/techsolution/designguide.html).

Q: I'm looking for an ESD safe cushioning material about 1/8-1/4 inch thick that will not age or wear and subsequently give off small conductive particles. Right now we use 1/4 inch thick sheets of black conductive ESD foam but the foam is giving off small black particles, which can then cause electrical shorting problems in the field. Any suggestions?

A.: Foamex makes non-shedding ESD foams (www.foamex.com). Or check out the Bradford CO. headquartered in Holland, MI (www. bradfordco.com/). They manufacture ESD dunnage for the automotive industry and may be able to help.

Q.: What national standard is used to define the metallurgical composition and the physical properties of 4140 steel?

A.: There are several standards, depending on the shape and treatment of the product, including: AISI 4140; UNS G41400, H41400, K11546; ASTM - A193, A194, A29, A322, A331, A506, A513, A519, A646, A752, A829, A914, A983; FED QQ-S-626C, Mil-S-5626C; SAE - 770, J404, J775.

Q: We manufacture industrial shock absorbers and would like to use a non-metallic, moldable material for our bushes. Typically, we use hard chrome-plated rods with 25 microns hard chrome layer and surface finish better than 0.5 Ra. The frequency of operation of our shock absorbers are between 20-35 strokes per minute. We need a material in which in inside diameters could be reamed or super finished. Any suggestions?

A: Try igus (http://igus.bdol.com/cbx.asp). They make a wide range of plastic bearings for many different situations. You may even be able to forgo the reaming as these plastic bushings can be made slightly undersized and wear themselves in.