Nature isn’t concerned with your design’s purpose or performance. Untreated metal rusts, sticks, and warps because that’s what it does naturally; rubbing surfaces have an inherent coefficient of friction, and motor power transmission always generates heat. To ignore these environmental effects is to design in a vacuum. (Designing for a vacuum is another problem altogether.) But widely varied lubrication, sealing, and coating methods are eradicating the unwanted effects of nature.
Because the strength of a coating depends on the bond strength of its atoms, ceramics can rule in this area. However, making hard and tough ceramic crystals by minimizing the formation of poorer crystalline forms is challenging. Ceramics tend to break catastrophically, with little or no warning. Research has helped inprove the material, but it still has its drawbacks.
Knowing this, when Spühl AG, St. Gallen, Switzerland, developed its new generation of wire processing machines for mattress springs and spring interiors, it decided to replace the equipment’s ceramic- coated aluminum grippers with ones made of VICTREX peek polymer. Says Kevin Jennings, General Manager for Victrex North America, “During manufacture, there is constant friction generated by the wire sliding up and down inside the gripper. This requires a high level of wear resistance.” First a wire is coiled into a spring; then the grippers move the spring to a knotting station. After knotting one end, the gripper moves across the spring interior to knot the other end. The PEEK polymer lowers resulting friction and with a melt temperature of 340°C, it withstands the remaining friction heat.
Take a coat
In vacuum or food-grade applications, lubricants can cause unacceptable contamination. In extreme temperatures the tables are turned, and lubricants get the abuse.
To rectify these problems, self-lubricated components with engineered surfaces are popping up everywhere. Says Joe Constance, an analyst for Technical Insights of Frost and Sullivan, New York: “Tough coatings, and the continuing development of this technology, are allowing novel and diverse approaches to extend component life, and are lowering machinery replacement costs industry wide.” Constance explains that lubrication remains where it is needed, between adjacent surfaces on mechanically stressed shafts, bearings, and gears. Though engineered surfaces alone are not appropriate for applications that rely on oil baths for heat dissipation, they can be designed for special applications. Increased wear resistance, decreased friction coefficients, or specialized reflectivity, conductivity, and surface tension are several benefits.
If during a plant breakdown cans were filled with 16 ounces of solid grease instead of green beans, someone would probably notice. But what about more miniscule contamination? In case of incidental contact with food, food processing machinery in bakeries, breweries, packing plants, and dairies require inert lubricants. Many companies are switching entire plants to food-grade lubricants for consistency and easy maintenance. To make this possible, Keystone Lubricants, Linden, N.J., has expanded their line of food-grade lubricants to include synthetic, semi-synthetic, mineral base, and spray formulations. Used to lubricate and deter rust on equipment, Wet Lube 100, for example, is an emulsifiable lubricant that protects machinery in the presence of water. For flushing old oil and residue off of equipment, another NSF and USDA H-1 rated oil features a lightviscosity formula.
Sometimes though, H-1 compliant lubes don’t hold up. At food processing plants where rotary screw compressors are used in hand equipment to add power, H-1 mineral oils were only serviceable for a couple months. After that, the heat, high circulation rates, and oil sump capacity limits of the equipment took their toll. As the oil degraded, it formed a sticky goop that glazed internal metal surfaces. Every couple years, the compressors had to be disassembled and painstakingly cleaned.
To solve this problem, one plant switched to using Molykote, a lubricant from Dow Corning of Midland, Mich. The oil was used and observed in equipment for 6,000 hours — with no problem. Even after 5 million hours of continuous observed operation, the oil did not goop up or varnish onto internal surfaces.
Scoffing at scoring
For some components, high loads, shock, and acceleration are all in a day’s work. But these conditions, as well as poor maintenance, extreme operating environments, and vibration can be detrimental to bearings, gears, and other precision parts.
So just change geometry to handle these difficult loads, right? In designs where this tradeoff causes more harm than good, one option is to use tribological coatings and finishes to change surface material, surface design, or subsurface properties. Many options are available, and are based on a thorough analysis of the material, lubrication conditions and application requirements. Integrating surface engineering techniques into new designs (or modifying existing ones) may help to eradicate premature component failure.
Special thanks to Gary L. Doll at Timken Co. in Canton, Ohio.