Given the huge body of information regarding the properties of materials commonly employed in valve bodies, seals, and O-rings, the material selection process is becoming a routine exercise. The challenging part of determining material choice is actually verifying that the chosen material best fits the application.
Most material selection problems at Beswick Engineering, Greenland, N.H., have arisen from specifying common types of materials such as elastomers and 300 series stainless steels. Because they are so widely specified, they are often used in applications where their less robust material properties can create problems. Various fillers and additives employed in O-rings may affect performance in specific applications.
Elastomers are a good example of materials that can require some extra legwork during the selection process. Off-the-shelf compounds for the common elastomers (such as Buna [nitrile rubber], Viton [fluoroelastomer], and EPDM rubber) will often serve applications without a problem. However, extreme temperatures, pressures, and fluids may require more due diligence.
If you check with the manufacturers of these materials, you will find that they offer a variety of compounds and durometers (measure of hardness of a material) within a given family of elastomers. They provide these formulations for a reason. If you think your elastomer is causing problems, a call to the manufacturer may reveal that it is not the type of elastomer that is the problem; it is the compound or the durometer.
Periodically, applications are encountered that require a modified version of an off-the-shelf compound. One application in particular involved sealing against a fluid compatible with EPDM seals. This fluid was a thermodynamic fluid used at temperatures near the lowest recommended for the elastomer up to beyond the boiling point of water. A standard, off-the-shelf EPDM compound, ideal for many applications, was specified and initially used in the application. It is important to note that the seal was manufactured in purple color for easy identification.
Numerous sealing problems were experienced and became a source of frustration — to the point where a quantity of the fluid was obtained and sent to the seal manufacturer for evaluation. It was discovered that this fluid had a propensity for leaching the plasticizer out of elastomeric seals and causing dimensional changes. The standard purple EPDM O-ring material was also proving to be weak and prone to damage.
In the end, the solution was an EPDM compound with a low extractable plasticizer and formulated using carbon black, not the purple talc filler. The low extractable plasticizer minimized shrinkage when exposed to the fluid, and the carbon black formulation increased the strength of the material. After changing over to the new seal material, the leakage problems were eliminated.
A material problem with a Buna component fabricated from sheet stock was also encountered. The finished parts began to have a “leathery” consistency, and the surface of the component was shiny with a sticky feel. Since the Buna sheet stock was purchased from a distributor and not the manufacturer, details about the elastomer’s formulation and processing parameters could not be provided. As a result, the sheet stock had to be sourced from a manufacturer that supplies a known compound. There are advantages to sourcing components from manufacturers that know their products and have staff on hand to assist when there is a problem.
Another time, a costly FFKM (perfluoroelastomer) O-ring was specified that required a great deal of stretching during installation. Since none of the other elastomers had given us a problem, it was assumed that it would install without a problem. This proved wrong; the installation process on the standard component required the FFKM material to stretch well beyond the manufacturer’s recommended limit. The O-rings then split and failed. Since use of this particular material design was specified and locked in, the only option was to redesign the component in two pieces so that the O-ring could be installed without damage.
Stainless Steel Challenges
Desirable for their corrosion resistance, 300 series stainless steels also have some undesirable traits and present some challenges. The 316 grade of stainless steel is very difficult to machine, and none of the 300 series stainless steels can be thermally hardened. But their biggest problems are their tendency to gall and their need to be passivated after fabrication.
Galling is an adhesion or welding-like phenomenon which occurs between two surfaces in sliding contact. Pressure and friction conspire to cause the two surfaces to stick to each other and cause pitting and tearing of the two surfaces. In the case of the 300 stainless steels, galling is witnessed most often on threaded components. Threaded components are fertile ground for galling, and in combination with 300 stainless steels the result is often a seizure of the two components. Stainless steel can be a tricky material to process, while close attention needs to be paid to the possibility of inclusions in brass.
Galling can be reduced by minimizing friction during installation. Anti-seize coatings and lubricants are used to alleviate this problem. Assembling threaded parts slowly without applying pressure or tension also helps to minimize friction.
Most of our galling issues arise when applying compression fittings. 300 series stainless compression fittings are often used where cleanliness is an issue. The need for purity in the fluid lines precludes the use of anti-seize coatings and lubricants and calls for clean-room-level cleaning processes.
Alas, galling can be aggravated by certain cleaning processes. Many applications involve compression fittings subjected to a sequence of acid baths during cleaning. These baths cause “micro-pitting” of the material surface, as they scour away all remaining lubrication. The results are extreme cases of galling. To prevent this from occurring, we use a solution involving water and alcohols as lubricants, specify custom threads, and design tools to pre-assemble the fittings.
300 series stainless steels are passivated after machining. The passivation process uses acids to etch out any embedded iron or other contaminants which may lead to corrosion of the part. However, even a passivated part can have a corrosion issue known as rouging. Rouging is a type of corrosion which is also caused by iron contamination on the surface, but it can also occur due to an iron-bearing compound carried in the fluid. Rouging can also be caused by high-purity or de-ionized water. It has a red appearance and a gel-like consistency.
This gel-like substance can occlude the waterways in some miniature components. When waterways are small, capillary forces can prevent the passivation fluids from treating all of the internal details of a component. When these components are used with high-purity water, rouging occurs and begins plugging the small orifices in the fluid line. To alleviate this problem, one needs to ensure that cleaning and passivating processes sweep entirely the internal volumes of the components.
Even brass can cause some difficulties. If brass bar stock is used, be aware of possible inclusions that may be drawn through the bar. Brass bars are often drawn from large slugs which are heated to a plastic state and then extruded. If there is a contaminant in the slug, it can be drawn through one or several bars.
Porosity issues have arisen from these inclusions on parts that looked fine in terms of outward appearance. To ensure the porous brass does not cause problems, parts are pressure tested and offending components are rejected.
Sometimes you are left in situations where you are not so much specifying a material as you are making the best of what you have. An appreciation for the complexities of applying even common materials can allow us to foresee some of their pitfalls and anticipate solutions before they become problems.
Gary Treadwell is chief engineer of Beswick Engineering, based in Greenland, N.H., which designs and manufactures brass and stainless steel fittings, valves, regulators, adapters, and cylinders. Its products support a wide variety of miniature fluid-power industries, from industrial grade to high-tech applications to educational research and development projects.
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