Design for washdown and sterilization

Feb. 1, 2006
Where food and pharmaceuticals are manufactured, it is often necessary to wash and sterilize equipment. But machinery typically doesn't appreciate high-pressure

Where food and pharmaceuticals are manufactured, it is often necessary to wash and sterilize equipment. But machinery typically doesn't appreciate high-pressure spray or heat. Designers, as a result, must take added measures to make sure every component on the machine can withstand whatever the cleaning process dishes out.

Food and drugs

Food and pharmaceutical environments, at first glance, seem worlds apart. Food plants are naturally messy and at high risk for bacteria and fungus growth. Not much could be more different than a sterile-bandage plant. Drug-making facilities likewise tend to sparkle and shine — so ingredients can be mixed with the highest precision. Even with their differences, however, food and drug plants have a few things in common, like cleaning solutions, hot water, and high-pressure sprays. Sanitation, product purity, and security, in general, play big in both industries.

Mark Saunders, who works with industrial enclosures at Hoffman, a division of Pentair Inc., Anoka, Minn., explains the cleaning process and the challenges it presents. “Pharmaceutical makers rely on wipedown cleaning and are concerned with contamination at a microscopic level. Food and beverage makers, on the other hand, focus on the particle level. Both, however, use chemicals to achieve the required level of sanitation.”

Large-scale systems, in particular, rely on chemical treatments because autoclaving (often used on small medical devices) is not an option. Even if it was, the heat would be too much for the lubricants, plastics, and seals encountered in machines. Common chemical treatments include chlorine washes and ethylene gas. Still, there are several challenges to a good washdown, says Saunders. “The first is simply the skill level and training of the individual doing the washing. Second is the chemical mixture. If not mixed in appropriate proportions, cleaning agents can be either too rich or too weak and can degrade the level of sanitation or even damage the system.”

Designers, of course, also play a role. In fact, the design of a piece of equipment greatly affects its cleanability. For this reason, designers are required by government agencies to have cleanability data not just on the materials used on a machine (like steels, plastics, and so forth) but also on whole pieces of assembled equipment.

“In most cases the FDA, USDA, 3A, and AMI do not approve specific pieces of equipment; they approve a process,” says Gary Wemmert of Dorner Mfg. Corp., Hartland, Wis. “This includes the equipment and the sanitation regimen. In accepting processes (coupled with onsite inspections) standards organizations can ensure the sanitation of products being produced.” In short, machine builders may have to prove that the product meets the design guidelines.

Design decisions impact more than just cleanability, however. They also determine how long it takes to clean a piece of equipment. The goal is to minimize sanitation cycle time without compromising effectiveness. When machines are down for cleaning, they're not producing product or return on capital.

“Besides USDA and FDA standards, there are UL, CSA, IP, and NEMA guidelines to consider,” adds Saunders. One critical consideration is washdown rating, such as UL type 4X, IP66, IP69K, and NEMA 4X. “These standards cover a broader industrial platform but are important as they pertain to the safety and operability of the system as a whole.”

But as Michael P. Flaherty, actuator product manager at Festo Corp., Hauppauge, N.Y. underscores, “The term washdown products is very broad and does not necessary designate food grade. There are different levels of protection for equipment to be approved for particular applications.”

And then there are guidelines for specific industry. For example, Baking Industry Sanitation Standards Committee (BISSC) certification is required for all baking applications; generally these call for increased ease of cleaning and built-in tolerance for higher ambient temperatures. The committee publishes these guidelines in the American National Standard for Baking Equipment — Sanitation Standard.


Another area of concern for designers has to do with materials, and choosing the right ones for each application. Machine parts not likely to contact food, for example, can be made of surface-treated aluminum or cast iron. However, both can oxidize. Chlorine is especially hard on aluminum, causing a whitish-colored rust.

Jason J. Kreidler, mechanical designer of ac development engineering at Regal-Beloit/Leeson Electric, Beloit, Wis., explains, “Treated aluminum and cast iron work well in most washdown situations where washing and sterilization is infrequent. However, stainless steel is best if components are put into service where they see frequent high-pressure washing and sterilization.” In such cases, cast iron and aluminum coatings eventually separate, creating areas where water and chemicals become trapped and contact unprotected material.

The FDA also has a say in the matter. “The FDA has regulations regarding the materials used in all machine surfaces that contact food,” says Jim Swiezynski, technical director at Stafford Mfg. Corp., North Reading, Mass. The specs call for either high-grade stainless or polymer. However, not all stainless steel is created equal. “There are many variations offering tradeoffs in corrosion resistance, magnetism, and machinability,” says Kreidler. “There's a trend towards stainless steel in many applications that would be serviced satisfactorily by aluminum or cast iron. This is most likely caused by the availability of low-cost, foreign-supplied components.”

300-Series stainless steel shafts and other components are able to withstand harsh and caustic washdown environments without corroding. 316 stainless surfaces make them impervious to corrosive acids and salts, present in fruits and vegetables, for example. “Generally 303-grade stainless steel is good for harsh washdown environments,” says Swiezynski. “But we usually recommend that 316-grade collars are used in applications where food or pharmaceuticals are present.” This is because 316-grade steel includes more molybdenum to better resist pitting.

The future is plastics

Cross Automation of The Cross Co., Belmont, N.C., designs linear positioners for beverage handling units — regularly subjected to highly caustic chemical washdowns. Besides a demanding motion profile (3 gs acceleration to 120 in./sec followed by a 3 g deceleration) the units operate year-round, under continuous 48-in. cycles/sec. Included is a belt-driven table from Parker Hannifin Corp.'s Daedal Division, Irwin, Pa. Originally, this piece utilized two 316 stainless bearing shafts combined with bushings to form the support bearing assembly. However, on subsequent units, Daedal designed an FR4 glass-filled phenolic substitute for 316 stainless and aluminum in the bushings.

As Todd Kanipe, who works on linear motion systems at Daedal explains: “FR4 glass-filled phenolic substituted for 316 stainless and aluminum in bushings reduces material costs about 25% and survives caustic environments. This material demonstrates machining and strength properties comparable to aluminum and has chemical resistance properties similar to 316 stainless.”

There are a couple caveats. “Plastics that contain formaldehyde or softeners are generally banned from food and pharmaceutical environments,” Flaherty explains. Also, moisture absorption, chemicals, and elevated temperature can reduce the tensile strength of other plastics.

“Take nylon six, for example,” says Georg Bartosch, Intech Corp., Closter, N.J. “Its mostly amorphous molecular structure — largely non-crystalline — allows moisture to penetrate into the material and the material swells. In a saturated state, there can be over 50% loss of tensile strength and up to 3% dimensional growth. In gear applications, such weakened plastic material (and inappropriate gear size) often lead to premature gear failure.”

That said, when engineers review load parameters and select non-hygroscopic plastic material, both tooth root stresses and tooth-flank wear are reduced. Adds Wemmert, “On conveyors, plastic components are more suitable than metal when operators need to see the materials being moved, eliminate jams, or monitor product flow to another process.”

Devil in the details

Seemingly small design details can make or break a system's washdown viability. Traditionally, food and beverage industries used stainless-steel NEMA enclosures with flat surfaces and piano hinges. These enclosures are inherently hard to clean and provide areas that can foster bacteria and growth of other undesirables. What's the solution? Ron Rotondo, principal engineer at Hoffman, explains, “Bullet-style liftoff hinges are easier to clean than traditional piano hinges. Sloping surfaces on tops, flanges, and covers help to minimize standing water, thereby minimizing opportunities for contamination.”

Also consider the legs on a system. It's highly desirable to have a small footprint (where legs and floor meet) as this is an area where contaminants collect. Says Rotondo, “Adjustable legs accommodate the slope in the floor for drainage without shimming, which minimizes areas for entrapment.”

The overall finish can also be critical. Smooth welds (with no spatter or crevices) make for a hygienic surface that minimizes opportunities for entrapment. Dairy food processing is a good example of an application that needs smooth, easy-to-clean components, without voids or pockets that support the growth of microorganisms.

Alton Vilhauer of Hub City Inc., Aberdeen, S.D. explains, “In messy applications like meat and poultry processing, stainless-steel gear drives with machined exterior surfaces — and no protrusions, undercuts, or ribs — are key. For example, stainless-steel motors that feature integral non-protruding rear conduit boxes increase cleanability and corrosion resistance.” The immediate benefit to processing applications like dairy, meat, and poultry is that pathogens and contaminants have no hiding place in which to multiply and become a sanitary concern.

Worm and bevel gear drives with a lot of bells and whistles can make initial cost seem prohibitive. However, as Vilhauer continues, extra features can make for maintenance-free washdown-duty systems that eliminate the need for scrubbing, sanding, and repainting (or removal and periodic replacement) of components. “When designers factor this into the cost equation, gear drives specifically designed for sanitation are sometimes the most economical solution to the persistent problems of repainting or replacing.” Kreidler concurs: “High-end stainless motors and gearboxes — more than just standard units built of stainless — incorporate features like hydrophobic breathers and O rings specifically for washdown.”

Housings isolated with high-pressure seals withstand the internal pressures of thermal expansion but keep out high-pressure washdown streams that can pulsate against the exterior of the unit. “Where sealing is more critical, locknuts designed for single or double seals are useful,” adds George Abbott, product manager at MITRPAK Power Transmission Products, Uxbridge, Mass. “Gear drives assembled with Viton seals (instead of Buna-N types) further reduce the possibility of contamination from the outside environment.” Another example: O rings on input-flange faces provide a positive seal against moisture intrusion in this critical area.

As Kreidler explains, seals are often asked to do the impossible task of standing up to various chemicals, enduring high-pressure jets, cycling through large temperature and speed ranges, all while operating statically and dynamically. “It's no surprise then that seals are quite often the first link in the chain leading to failure in washdown environments. But seals are relatively inexpensive compared to the cost of components they protect. That's why a seal made of the right material, even if the material is relatively expensive, is cheap insurance.”

Ideally, no lubricant contamination would ever occur in washdown or sterile situations. In reality, lubricants do sometimes make their way out of gearboxes and bearings. That's why any lubricants used need to be safe. “For applications where there is the possibility of incidental contact with food products, gear drives must be supplied with premium food-grade lubricant to meets FDA requirements (regulation section 121.2553) and carry the USDA AA rating,” Abbott notes. One example would be USDA Class 2 grease.

Another option is to keep lubricated components away from food and pharmaceuticals altogether. Wemmer explains how this is done on conveyor systems: “Bearings on conveyors can be located outside of frames, and motors and gearheads placed outside of the frame or under the conveyor, where lubricate contamination is virtually impossible.” In fact, this is a key design element that makes components acceptable to government organizations and food companies like Kraft and SaraLee.

Not an open-and-shut case

“The goal should be to either reduce places in which contaminants might remain or make those areas easier to access and clean,” says Rotondo. So say two shafts in a system need to be connected. What if there isn't enough room between them for a closed gearbox? Belts can transmit power here, but open geartrains transmit power with more precision. “When this open gearing is required in washdown environments, plastic gears are most suitable,” says Bartosch. “Plastics can be designed for 100% resistance to washdown chemicals.” Because the gears do not require lubrication, machines can start immediately after washdown with no time lost. Plastic gears also mate and wear well against stainless steel, common in washdown situations. But as Wemmer explains, “Simple and open designs improve sanitation but at a decrease in safety. This is an unacceptable tradeoff in most cases. So if a design must be open, manufacturers must also make sure the equipment poses no risks to operators.”


While washdowns and sterilizations effectively clean numerous types of equipment, they can also damage system components. Hans-Georg Jansing of Maxon Motor AG (U.S. headquarters in Burlingame, Calif.) explains, “Failed drives are expressed as a blocked shaft — increased current consumption, short circuits, and malfunctioning or failed electronics.” Corroded ball bearings, magnets, and loss of winding shape cause blocked shafts. Urs Kafader, also of Maxon adds, “Decomposing insulation layers and electrically conductive deposits cause short circuits. Electronic malfunctions and failures result from high sterilization temperatures.” Keith Kowalski of Haydon Switch & Instrument, Waterbury, Conn. notes that high temperatures may induce thermal shock, and thermal stress from rapid temperature changes can crack brittle items. One example is applying a cold wash to an overheated machine.

If liquid contaminates the motor's interior without draining properly, it can seep into windings and break down the insulation system, creating a path to ground. “Interior surfaces corrode when improperly protected or coated. Liquids may even wash out or contaminate grease, causing the bearings to fail,” says Chris Medinger of Leeson Electric Corp., Grafton, Wis. Moreover, high humidity and moisture tend to ruin control systems that are improperly sealed. According to Ralph Whitley of Boston Gear, Charlotte, N.C. and Kal Vanlaningham of Warner Electric, South Beloit, Ill. (both of Altra Industrial Motion), these problems can range from intermittent cycle interruptions to complete ground faults.


Jansing and Kafader recommend encapsulating critical parts and components, passivating surfaces, and employing coatings that resist moisture to prolong component life. In addition, all connections should be sealed; otherwise, water between two different metals can act as an electrolyte. “To combat thermal maladies — especially in medical devices — it is often more effective to make the motor a one-time component, then discard it,” says Kowalski. Other options include adding a sealed enclosure to completely protect the motor and applying corrosion-resistant coatings. When addressing liquid contamination, encapsulating the windings protects against dielectric breakdown and on single-phase motors, encapsulated solid-state switches can replace mechanical-type starting switches. “Free of moving parts, they don't corrode or malfunction in moist environments,” says Medinger. “Applications employing parts outside their design intent must not let liquids pool at enclosure joints, especially when interior temperatures fluctuate,” says Adam Shively of Rockwell Automation, Milwaukee, Wis. He also suggests supplying low pressure, clean, dry air to enclosures.


Besides system failures, end users must also contend with component malfunctions. For instance, electronic parts unable to operate at high temperatures and pressures corrode. “The minimum steam temperature in an autoclave,” states Clyde Hancock of MicroMo Electronics Inc., Clearwater, Fla., “is 121° C at 15 psi. By comparison, high-pressure washdowns can be up to 100° C at 1,200 psi.” John Lauffer and Chris Kamboris of AC Technology Corp., Uxbridge, Mass. point out other negative effects from washdown. “Moisture and chemicals can contaminate printed circuit boards and create high impedance paths with electromigration, thus shortening life.”

Sensors are also at risk when moisture enters the housing. “Moisture ingress can induce electrical shorts that temporarily or permanently hinder sensor operation,” says Eric Henefield of Turck Inc., Plymouth, Minn. “When hot liquids contact the sensor's housing, its materials expand, cool, and contract, pulling in moisture.” Besides sensors, encoders are also susceptible to damage during. Brian Winters of Avtron Manufacturing Inc., Cleveland, Ohio relates, “Air pressure differences and flow pressure force water (or vapor) into encoders, preventing the sensor from seeing fine lines on the disk.” This can lead to seemingly random drive faults.

According to Joe Dolinsky of Banner Engineering Corp, Minneapolis, Minn., some cleaning agents fog a photoelectric or vision sensor's lens. Irradiation can turn glass black or make it brittle.


There are several ways to address component faults. “In high-temperature environments, electronic circuits and boards can be conformal coated or encapsulated,” notes Hancock. Lauffer and Kamboris elaborate, “Conformal coatings use a thin polymeric layer that adheres to printed circuit boards and components. They act like insulators by preventing moisture and shorts from forming.”

“The best defense against intrusive moisture is housing materials with desirable thermal properties,” says Henefield. To further protect against moisture, Winters suggests magnetic encoders with fully encapsulated electronics to see the rotor clearly under any condition.

A safeguard solution for sensors is using remote sensing heads or fiber optics. “Plastic fiber optics withstand temperatures up to 70°C. Glass fiber optic cables survive corrosive chemicals and temperatures from -140°C to 480°C,” says Dolinsky.

About the Author

Elisabeth Eitel

Elisabeth Eitel was a Senior Editor at Machine Design magazine until 2014. She has a B.S. in Mechanical Engineering from Fenn College at Cleveland State University.

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