Toughing it out

Aug. 1, 2006
It's hot and gritty. Or it's cold and wet. Extreme environments are soaked, submersed, caustic, and chemically saturated areas, in baking-hot temperatures

It's hot and gritty. Or it's cold and wet. Extreme environments are soaked, submersed, caustic, and chemically saturated areas, in baking-hot temperatures and under vacuum. When the situation can't be improved, how can you improve your design to survive?

Moisture is perhaps the most challenging environmental concern. “High humidity or the presence of water and other liquids are very detrimental,” explains Scott Hewitt, president of Sick Stegmann, Inc., Dayton, Ohio. If liquid gets into an encoder, for example, it can blind the optics or short the electronics. But higher ingress-protection or IP ratings allow encoders to be subjected to liquid spray without damage. “Connectors with O-rings are more resistant to liquids, and gaskets between connector and encoder housings prevent leakage there,” says Hewitt. So even standard, economical connectors can boost encoder environmental ratings if thoughtfully incorporated.

Sealing also protects actuators in washdown environments that more rapidly strip away and degrade grease. But does that leave the ball or jackscrew inside vulnerable to internally generated particles? “Not really. Though not sealed to IP65, normal ballscrews are also contained systems — closed with AQ seals. IP65-rated units just include gaskets and more secure sealing,” says Chris Prior, senior technical support engineer at IAI America, Inc., Itasca, Ill.

That said, you can get pumping action due to the sealing that reduces actuator effectiveness or load capacity if not vented properly, explains Prior. “Quick connects on the rod body, to which small tubes are attached, address the problem; they can be run to where they won't pick up fluids so the body is exhausted while maintaining the IP65 sealing.” Longer cable from the actuator to the controller moves plug-in connectors away from the actuator, with extensions connected to the controllers — safe inside an enclosed cabinet.

Some large slider types rated IP65 employ double wiper seals in the slider, as well as an air purge. Strokes are substantially longer and they carry much larger loads than the rod type. One caveat: These sliders are limited to horizontal applications due to the sealing structure. “Other IP67 sliders withstand harsher water spray with a tube that fully encases the ballscrew, and magnetically couples the ball-nut to the slider,” says Prior. “This design's limitation is that magnetic particles should not come near this system; otherwise, increased wear will be a factor.”

Hot enough?

Many power generation and glass or plastic manufacturing processes run hot due to the elevated temperatures required to work with these products. This can deteriorate many components — even those deep inside a system. For example, penetrating heat quickly destroys controller chips. In this case, resistors can be made of tantalum nitride for high-temperature resistance. But how to secure these components to PC boards for temperature extremes? “100% tin or gold-plated terminations do not reflow at 200°C, so engineers can use heat-resistant epoxies and solders,” says Jerry Seams, applications engineering manager for IRC of TT Electronics, Corpus Christi, Tex. Similarly, materials-engineered greases resist melting under heat with additives that raise their dropping point. (This is the temperature at which a grease becomes runny enough to drip.) Then moving parts are properly lubricated, even when subject to high heat. Lithium soap and clay are two options.

Other times, heat doesn't cause full-on melting, but equally harmful thermal expansion and proportion changes. This is particularly detrimental on mechanical parts, where part dimensions affect quality of motion. “For example, during the baking process for semiconductor wafer fabrication, motion components can seize,” explains Stephen Fournier, engineering manager at Bishop-Wisecarver Corp., Pittsburg, Calif. Regular wheels in these temperatures commonly fail. Why? “Guide wheels and other linear guide technology rely on slight clearances between moving parts to rotate and support load,” says Fournier. When subjected to high temperatures, these components increase in size. That's when preexisting, necessary clearances are lost, causing moving parts to bind. “But stainless-steel wheels with heat-stabilized parts do not expand, even to 500° F — preventing seizing during operation,” Fournier explains.

Sometimes when it's too costly to engineer component materials for heat, air conditioning is a better option. “Thermal management solutions are typically less expensive than the components and controls they protect,” explains Greg Quick, product manager at Hoffman, part of the Pentair technical products group, Anoka, Minn. For wall-mount and stand alone units, conditioners can protect small, densely populated enclosures or large enclosures housing drives and motors with greater heat dissipation needs.

Attack of the chemicals

When an application involves reactive chemicals, as in food processing and marine washdown, moving parts can deteriorate fast. Again, materials — in this case, stainless steel or plastic — prevent corrosion.

For example, traditional steel collars — even those finished with zinc or nickel — are quickly corroded by chemicals. On the other hand, acetal or passivated stainless steels resist water and many commercial chemicals, including chlorine and acids. “Passivation is the process of making a material ‘passive’ or nonreactive to other materials,” explains adds William Hewitson, director of manufacturing and engineering at Ruland Mfg. Co., Inc., Marlborough, Mass. This can be accomplished in two ways. “Materials can become passivated spontaneously by forming an oxidized surface film. In stainless steel, it's chromium oxide — but steels must be at least 11% chromium to do this.” Self-healing, this film quickly reforms a new protective layer if scratched. This is what makes stainless steels corrosion resistant.

“Passivation can also be chemically accelerated with nitric or citric acid,” says Hewitson. This helps remove surface contamination or impurities that can impede the natural process of the stainless steel, carbon steel chips or dust. If carbon steel is left in contact with stainless steel — scratched by it, for example — it can still cause corrosion in the stainless if it becomes wet or exposed to certain substances. But chemical passivation prevents this.

“Passivation is typically associated with stainless steel, but other materials can also be passivated,” says Hewitson. These processes usually have other names; for example anodizing of aluminum is also a passivating process. “Black oxide on carbon steel, as on shaft collars and rigid couplings, is another form,” explains Hewitson.

Magnetic drive pumps are another place where materials save the day. Most nonmetallic pumps include barium ferrite or neodymium iron boron inner magnet assemblies, which offer little corrosion resistance. “But nonmetallic drive pumps with samarium-cobalt magnet capsules better resist highly corrosive and toxic chemicals such as sodium hypochlorite,” says Peter Brule of Iwaki America Inc., Holliston, Mass. Sodium hypochlorite — increasingly used as a bleaching agent, oxidizer, sterilizer, deodorant, and water treatment — can release tiny gas bubbles that permeate capsules and attack the magnets in average pumps.

Damage from debris

Machine tooling is another place where chemicals can wreak havoc; aggressive coolants break down lubricants in a hurry. However, the larger challenge in tooling, woodworking, grinding, and polishing applications is the harsh abrasives and debris. Here, seal designs must be robust enough to deflect particles tight enough to exclude fluids, so active lubrication is protected and contained. “Dust, metal, and wood chips degrade linear bearings. Often, the sealing function of the ballscrew and nut assembly is not enough to fully protect against contaminants,” explains Clint Hayes of Bosch Rexroth, Hoffman Estates, Ill. Worst-case scenario, particulate clogs the nut's ball recirculation and prevents it from operating as designed.

Commonly, bellows are used to address this problem. “But while bellows are very good at protecting against contaminants, they're often cumbersome to handle and design around. Further, they consume up to 30% of possible screw travel for their compression,” Hayes points out. In contrast, clear anodized extrusion housings can be outfitted with stainless steel or plastic sealing strips: “This keeps even the finest debris from entering ballscrews.”

Washout extremes

In food and beverage manufacturing, operators spray caustic cleansing solutions onto conveyor lines and production equipment at each shift's end. “This washdown can create a film build-up on clear viewports of camera lenses and optical sensor windows,” explains Blake DeFrance, Cognex Corp., Natick, Mass. “Then, vision systems and sensors reject good parts, resulting in waste and downtime clearing fogged lenses.” If liquid enters cables, optical devices may malfunction or operate intermittently. Additionally, corroded or unsealed electrical connections can be safety hazards.

Some vision cameras resist caustic washdown chemicals and even submersion with stainless-steel housings. The design reduces the trapping food, bacteria, and particles; sealed industrial connectors protect electrical cables. In one robust package, units verify part presence on production lines, performs quality control checks, and reads bar codes.

Besides causing vision component and cable failures, washdowns may contaminate a motor's interior. “Improperly drained liquids enter windings and break down the insulation, creating a path to ground,” says Chris Medinger, Leeson Electric Corp., Grafton, Wis. “To guard against this, windings should be well insulated during the varnish dip and bake cycle.” Encapsulating windings in a resin-type material protects against dielectric breakdown from moisture and chemicals. On single-phase design motors, encapsulated solid-state switches can be used, as they don't corrode or stop operating when moisture is present.

When moisture or chemicals enter a motor's housing, the wiring harness becomes wet, causing short circuits and loss of a feedback signal. “Consequently, the drive registers a feedback fault and disables the axis,” explains Adam Shively, Rockwell Automation, Eden Prairie, Minn. However, encapsulated windings prevent direct attack of the copper.

One solution is cable entry to the motor, sealed at the factory. “Side exit of the cables keeps length short, while promoting a ‘drip-loop’ that keeps liquids away from the motor,” mentions Shively. In addition, plugged holes apply positive pressure to a motor's interior, eliminating the vacuum that can result when a motor cools.

On the same page

It is important to establish standards across industry so automation components can withstand extreme environments. The International Electrotechnical Commission (IEC) rates the degree to which electrical components' enclosures are sealed against foreign bodies, such as dust and moisture. This classification system is indicated by the letters “IP” followed by two digits. The first digit represents the degree of protection against solid objects, and the second, water ingress.

An example is IP69K for washdown environments. “IP69K protects electronic components against high temperature, high-pressure water spray from a few inches away,” explains Shively. By contrast, IP65 and IP66 protect against lower-pressure, room-temperature spray from about 10 ft.

The IEC 60721/DIN EN 60721 classification is a document describing typical situations for machine and component use. “It classifies environmental parameters and their severities and covers extreme short-term conditions that may result during transportation, installation, storage, and use,” relates Frager. The document also provides standards in weather-protected and non-weather-protected areas.

Motors operating in food processing often contain a paint finish, which the USDA must approve. “The USDA's Food Safety Inspection Service (FSIS) adopted the Hazard Analysis and Critical Control Points (HACCP) to prevent chemical and bacteriological contamination during all food-processing steps,” states Medinger. And motors operating in the baking industry must satisfy the Baking Industry Sanitation Standards Committee's (BISSC) specifications.

Motors and temperatures

Washdowns are just one type of extreme environment in which electrical systems must survive. Equally important are temperature extremes and vibration. To address these, most solutions include choosing alternate materials and components.

Take ferrite magnets for instance. Low ambient temperatures can demagnetize them, weakening the motor. “Dc motor operators know when this happens because the no-load speed increases. Magnets such as AlNiCo or Neodymium better survive low temperatures,” explains Urs Kafader, Maxon Precision Motors Inc., Fall River, Mass.

When plastic and resin in the insulation system are exposed to hard frost, windings may crack and become inflexible. “Furthermore, electronic component operating ranges, such as encoders, limit the temperature range that a motor reaches,” says Carsten Frager, Lenze/AC Technology Corp., Uxbridge, Mass. So, Lenze/AC Tech uses the temperature range of resolvers, which is greater, to increase the motor's operating temperature range.

At the other end of the spectrum, high ambient temperatures damage electrical components too. “Overheating can deform the winding and block the shaft,” says Kafader. Reinforcing windings with a glass-fiber bondage permits winding temperatures up to 150°C. High temperatures can also weaken magnets. “When thermally activated, the elementary magnets don't all point in the same direction. But, this is reversed if the magnets cool,” relates Kafader. At too high a temperature, demagnetization is irreversible because the magnet's inner structure changes. “To prevent motors from overheating, some servos have a temperature sensor that switches a drive off upon reaching the temperature limit,” comments Frager.

In addition, high temperatures can damage the magnet wire in stepmotors. “A standard class of magnet wire, which has an inherent insulation characteristic, is used in standard motors. Under extreme circumstances, the wire can melt, and thus, short out,” says Mindy Lin, Lin Engineering Inc., Santa Clara, Calif.

To accommodate extremely high temperatures, Lin suggests choosing a higher-class magnet wire with better insulation. Or, engineers can use a vacuum-rated stepmotor. To construct this, one should remove all paint and oils from the motor, and employ high-temperature windings and Teflon leads.

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