Bearings play an essential role in food processing from bakeries to seafood canneries to bottling plants. Like any component in a high-speed, high-volume production line, their unexpected failure or unscheduled maintenance can be devastating. To reduce the risks and avoid painful consequences, engineers must observe housing and insert materials as well as the type of bearing itself. What follows is a review of these important factors and practical advice for times when bearings are in contact with food.
In the house
When specifying bearings, designers must often choose housing and insert materials. Common housings include thermoplastic composites, nickel-plated cast iron, stainless steel, and polymers. In most cases, the application environment — daily exposure to high-pressure washdowns, humidity, caustics, and antibacterial solutions — dictates which housing material to use.
Sometimes, for example, acids, cooking fats, and other chemicals used in food and beverage processing may come into contact with the bearings. This can be avoided by employing composite or thermoplastic-housed units. Another option is cast stainless steel (usually made of AISI 300), which withstands strong chlorine and hydrogen peroxide concentrations.
Other materials such as high-strength, cast iron work best for red meat, pork, and poultry processing. Nickel-plated, cast-iron housings, on the other hand, are easier to clean and rid of debris and bacteria because of their smooth surface finish and solid-base mountings. The most common plating — CNC (copper, nickel, chrome) — durably resists corrosion and appears bright and shiny. Proper CNC sanitation prevents E. coli, salmonella, and staphylococcus aureus growth. Many of today's bearing housings also contain antimicrobial coatings to further thwart bacterial and fungal growth.
In addition, polymer housing offers excellent corrosion resistance, a chip-free exterior (as opposed to plated housings), and economical prices compared to stainless steel. But unlike stainless steel, polymer materials do not provide equal load strength.
Bearing inserts are made variously from stainless steel, thin dense chrome (TDC), and zinc. Stainless-steel inserts — although expensive — are regularly found in the food industry as they offer high-corrosion resistance. Generally, their outer and inner rings and balls consist of AISI 440C stainless steel. TDC coatings offer an alternative to this as they resist corrosion up to 15 times more than normal 52100 bearing-quality steel and provide greater surface hardness.
Zinc-coated inserts also guard against corrosion, come in most sizes, and cost less than stainless steel and TDC. However, plane bearings are needed during severe operating conditions where rolling-element inserts are not allowed. They usually install into any mounted housing that accepts rolling-element bearings. In addition, polymer and stainless-polymer combinations comply with requirements for a bushing-type design.
Normally, an FDA/USDA-approved grease (classified as H1 for incidental food contact) lubricates food-grade bearings. H1 lubricants are odor-free, tasteless, washout resistant, and harm-free to food. Whenever possible, bearings should be relubricated to replenish the old grease supply and purge contamination.
When relubrication is not possible, however, polymer-solid lubricants can be employed. During bearing operation, they release a small amount of oil to coat rolling elements and raceways, resist washout, and reach hard-to-maintain areas. On the downside, they handle speed and environmental limitations poorly.
Over the last 10 years, many special bearings (both mounted and unmounted) were created to improve food processing. Concerns influencing these designs include:
- Wet environments — due to water and equipment washdown
- Harsh chemicals — sanitation and cleaning requirements
- Excessive heat — baking and frying
- Excessive cold — freezer applications
In any of these environments, bearings must be shielded from corrosion. This is achieved through corrosion-resistant platings, coatings on metal surfaces, or materials that inherently offer corrosion resistance, such as stainless steel or polymers. Stainless steel offers the best corrosion resistance in the aforementioned environments and is therefore preferred in meat-processing plants for equipment and bearings.
Plated bearings, on the other hand, best suit wet areas with limited chemical washdown. Their lubricants must be USDA-approved food grade (H1 or H2) only. Many high-end, food-grade greases lubricate as effectively as conventional petroleum greases and contain additional qualities such as low-temperature operation and washout resistance.
Mounted bearings typically come in three basic housing-material combinations: plated or coated cast iron; polymers or composites; and stamped, cast, or machined stainless steel. Insert bearings are available in several corrosion-resistant platings or coatings: Zinc chromates, TDC, and black-oxide coatings are most prevalent.
Unmounted bearings are made of stainless steel and other highly corrosion-resistant materials such as high nitrogen corrosion-resistant steel (HNCR). HNCR resists corrosion better than any stainless steel while maintaining a material hardness similar to average bearing steel. HNCR materials therefore eliminate the capacity reduction inherent in stainless-steel bearings.
High-temperature bearings generally incorporate a graphite separator that lubricates bearings at temperatures above 600° F — something conventional lubricants such as grease and oil cannot do effectively. These mounted bearings increase life and lower maintenance costs in difficult baking, frying, and drying applications.
Plane bearings consist of a bushing-type design where two surfaces mate to each other without a ball or roller. They are available in several housing styles and bearing-material combinations for lubricant-free operation (sometimes required for sanitation) and easy cleaning. Unlike conventional ball bearings, they can be completely disassembled, cleaned, and reinstalled for safe use in and above the product zone — where conventional rolling-element bearings are unapproved. However, this style limits both speed and load compared to conventional ball bearings. Plane bearings are made of stainless steel or polymers for various operating requirements and temperature ranges.
Seals are one of the more common options used with bearings. They are intended to keep lubricants in and contaminants out and are an important consideration in food-processing applications. They vary from conventional single-lip rubber types to double-lip designs and are often coupled with metal fingers fitted outboard for added protection. For high-temperature applications, various silicone materials are available and advised.
End closures added to housed units further protect the inserts, prolong bearing life, and act as safety measures against rotating parts. There are open-type units, which allow shafts to extend through and closed types for stub shafts. Most often, covers are made from either stainless steel or composites.
Other options for a complete food-grade bearing assembly include stainless steel or nylon cages, stainless-steel setscrews, stainless steel or corrosion-resistant coated fingers, corrosion-resistant collars, and stainless-steel grease fittings. Survivability is key for the bearing, as well as the load.