Figure 1: Bacteria tends to build up in areas that are difficult to clean, such as grooves, crevices, and between the outer shell and plastic insert of two-piece bearings.
Designing linear motion systems for washdown applications requires careful component selection to solve problems associated with contamination as well as liquid and chemical interactions.
When food and pharmaceutical contact is a factor, it’s often necessary to wash or sterilize processing, packaging, handling, and automation equipment. When specifying components for such environments, design engineers must not only select components capable of withstanding high-pressure and high-temperature sprays with caustic materials. They must also consider the regulations, standards, and inspections required for maintaining and operating equipment in highly regulated food, beverage, and pharmaceutical applications.
Washdown environments require cleaning with water, chemicals, or a mixture of these either by hand or by automatic means. The washdown equipment may range from a simple cloth and bucket or hose to sophisticated high-pressure and controlled systems. Automatic cleaning of industrial equipment can involve CIP (clean in place) or SIP (steam in place).
Best Washdown Design Materials
Materials selection is critical for the bearing, shaft or rail, and seal components in linear motion design for washdown environments because it is key for meeting corrosion-resistance and machine performance requirements, standards, and regulations.
Stainless steel is typically the preferred material for general use in direct food contact areas because of its corrosion resistance and durability. Variations in stainless steel grades typically involve different levels of chromium and nickel.
300 Series Stainless Steel
In general, 300 series stainless steel is the most widely accepted material for food and medical applications. It is relatively soft, cannot be hardened, and is also nonmagnetic. Each grade discussed below might have different types with slightly different formulations and varying strengths and weaknesses based on additions to the mixtures.
303 — Called A1 under ISO standards, 303 is a free machining version of 304 due to added sulfur and phosphorus.
304 — Called A2 under ISO standards or 18/8 due to the 18% chromium and 8% nickel in its makeup, 304 is the most common grade of stainless steel.
316 — Called A4 under ISO standards or 18/10, 316 is the most commonly used alloy for food- and pharmaceutical-grade applications. The addition of up to 3% molybdenum aids in the prevention of corrosion from industrial chemicals and solvents, particularly pitting that can be caused by chlorides.
400 Series Stainless Steel
There are several types of 400 series materials. The most widely available and most used in industry is 440. It can be heat-treated and heat-hardened. It is often used for cutlery, linear shafting, and applications requiring good wear resistance. It can be hardened up to approximately RC58. However, due to added carbon in its makeup, 440 will oxidize under washdown conditions.
Stainless steels do not rust in the normal sense. If particles of red oxide appear on a stainless surface, it is most likely due to iron particulate coming from fillers within the bearing. To cleanse the surface, a solution of 10% nitric acid and 2% hydrofluoric acid at room temperature can be effective.
Aluminum and Coatings
Aluminum can be used in areas of a washdown environment where weight is a concern. However, bare aluminum has poor corrosion resistance and is susceptible to pitting and cracking. In washdown conditions, aluminum must be coated for protection. Anodizing ceramic coatings and other coatings like PTFE or other fillers are often used, but they may not provide the resistance or life that stainless steel offers. In more caustic chemical washdown environments, stainless steel is the preferred material.
Electroless Nickel Coatings
These coatings have become increasingly popular because of their corrosion- and wear-resistance combined with a smooth, polished appearance. Some forms include a PTFE infusion to aid in non-sticking properties. Most forms of this coating are FDA-compliant as well.
Plastics, Polymers, and Fillers
These nonmetal materials tend not to have the corrosion-resistance and durability of metal surfaces such as stainless steel over time and are thus not used as often as major components in food and pharmaceutical equipment. However, due to cost, weight, manufacturability, they are increasingly being used “under the hood”— in mechanical drive components, guides, bearings, fasteners, and more. Many solid plastics, such as injection-molded bearing inserts, present drawbacks in washdown applications in that most absorb liquid, causing components to swell and increasing the potential for binding and failure.
Also be aware that each of the standards organizations covered earlier has extensive information on various acceptable plastic materials. However, along with the base plastic, each polymeric material usually has fillers blended in by the manufacturer. These fillers are added to enhance performance, such as increased load capacity and lower coefficient of friction. Be sure that these fillers also are in compliance with standards.
Best washdown design practices for linear motion systems
Linear motion components offer their own unique challenges in washdown applications. Rotating components need to be mounted and sealed within a limited area, but because the moving component of a bearing, slide, or actuator system travels in a linear fashion, the space needing to be sealed or cleaned will be far larger — often up to several feet larger. Below are some tips on minimizing areas of potential bacteria buildup and maximize cleanability.
Linear Guide and Bearing Design
For linear recirculating ball bearings, use only stainless steel-sealed bearings with compliant seal materials and approved lubrication. There are two basic types of plain bearings. When using plastic inserts, be aware of moisture absorption that will lead to the bearing material swelling. This can result in binding issues. If the inside diameter is increased to deal with swelling, this often causes loose tolerances and inaccuracies in the system.
It is best to avoid open-ended bearings with grooves or inserts in areas that may be susceptible to bacteria buildup. These two-piece bearings will allow microscopic bacteria in crevices, grooves, and between the outer shell of the bearing and the plastic bearing insert. One-piece bonded bearings eliminate this potential for bacteria collection.
If they are to be used in a food-grade environment, ensure that materials and fillers are compliant with applicable standards.
The same design principles apply to recirculating ball bearing products, such as roundway linear ball bearings and profile rails. They provide advantages such as low friction and tight tolerances and are often available in stainless steel materials with FDA-compliant lubrication. However, they can present disadvantages in that they require grease lubrication to be used due to metal-to-metal contact. This lubrication picks up material from food items being processed, which can become trapped inside the multiple crevices and cavities around the balls and in the raceways of the bearing. This can potentially be a breeding ground for unwanted bacteria.
The best solution for most applications is a one-piece bonded bearing that uses PTFE-based, self-lubricating bearing materials that require no external lubrication. In addition, there are no grooves, crevices, or spaces between the liner and bearing shell where residue can become lodged, allowing bacteria to grow. The bearing material and outer shell are bonded together, creating a true one-piece bearing.
PBC Linear
www.pbclinear.com
Filed Under: LINEAR MOTION
