The battle against corrosion is challenging, and some industries are more sensitive to it than others. Avoidance and prevention are particularly important in food, beverage and medical processing applications where damage from corrosive invasion is costly. Corrosion is damaging from a wide variety of perspectives. It weakens base materials, limits machine longevity, is cosmetically unappealing, can create resulting impurities and can ultimately impact production. Many manufacturers of power transmission products are continually researching methods and means to win this battle.
One of the main challenges for bearing manufacturers is providing products that maximize corrosion protection while maintaining material strength and load capacity. Typically the materials with superior corrosion-resistant properties lack the mechanical properties, such as strength and hardness, required for tolerable bearing operation. Conversely, steels with ideal characteristics for bearing operation haven’t had satisfactory performance when exposed to harsh environments and reactive chemicals.
Bearing housings are available in coatings, platings, and 304 and 316 series stainless steels, as well as nonmetallic polymers such as PBT, polypropylene and polyamide resins. Many of the nonmetallic polymers often are enriched with antimicrobial agents that help kill microorganisms. Common coatings used on cast-iron or steel-mounted bearing housings include powder epoxy paint, nickel and nylon. Some offerings are more effective against certain chemicals, loads and general environment than others. For example, stainless steel housings perform well when exposed to acetone, while a nonmetallic PBT housing doesn’t. The key is to select the right housing material option based on the application.
In addition to housing protection, the bearing itself should be considered. Stainless steels are offered for inner rings, rollers and outer rings in material grades such as 420, 410, 303, 304 and the most popular, 440C. However, many of the stainless steels offered don’t maintain the same load-carrying capacities of the ferrous steel counterparts. As a result, the capacity of the bearing may drop as much as 20% below standard steel offerings. Thus, they’re more practical for lower-stressed ball bearing products than the higher-stressed roller bearing products. Additionally, stainless steels are not inert with many chemicals and environments. Therefore, many common ball bearing product offerings include a plated option. These platings are included on the inner and outer ring raceways and vary from zinc chromate and low phosphorous nickel to nickel composites and variations of chrome.
Avoid choosing a bearing with pockets or voids in and around the housing where moisture and contamination can become entrapped. These areas allow for bacteria growth and can promote corrosion. Instead, choose a bearing with solid feet; solid underbodies; and a flat, smooth surface. This style is more effective at repelling contamination and hindering particle accumulation.
Beyond corrosion protection, product longevity is important to consider, as well. Many applications use washdown processes with a variety of chemicals to kill bacteria and microorganisms. This process can be lethal to a bearing that isn’t well-protected. Two critical components of successful anti-friction bearing operation are effective lubrication and contamination prevention. High-pressure wash threatens both of these components. Moisture ingress into the sealed bearing cavity will deteriorate lubrication, corrode raceways and rollers, degrade contact surfaces and lead to premature bearing failure. Therefore, the best means to prevent bearing failure is to protect lubricant stability and prevent contamination from entering the bearing cavity through a premium sealing system.
There are a variety of sealing systems available with washdown products. The most effective seals include 304 or 316 stainless-steel shields that include a rotating flinger and at least three elastomeric contact lips on each side of the bearing. Rotating flingers help propel contaminants away from the bearing entryway. The larger the flinger’s diameter, the more centrifugal acceleration is utilized, resulting in a more effective seal. These flingers may have elastomeric contact extensions, as well. The three contacting lips of the main, nonrotating shield are shaped to allow excess grease to flow past and yet prevent moisture and contaminants to flow into them. The contact pressure of each seal lip is adapted to maximize seal effectiveness, while minimizing drag to allow for a wide bearing operating speed range. When the bearing is lubricated, the area between each contact lip forms a grease dam that acts like a supplemental barrier to prevent foreign particle entry.
Additional sealing is often utilized with end closures that completely seal the insert on one side of the housing. The closures typically snap into place and are held rigid to the bearing housing. If the shaft ends at the bearing, the bearing can be completely closed and protected. If the shaft continues beyond the bearing, end closures are available that allow for the shaft to extend through them. These open-end closures will often utilize an additional rotating labyrinth for additional protection and reliability.
Some of the latest developments with ball bearings include revolutionary cage designs, which help channel and protect lubrication near the rolling elements. In the event that washdown practices do allow moisture to compromise the sealed bearing cavity, the grease could become contaminated and depleted. The new cages create compartments that safeguard the lubricant near the balls. This allows the balls to be in constant contact with effective lubrication. The result is less wear, minimized friction and less heat. In turn, the bearing doesn’t require the aggressive relubrication intervals that bearings with standard cages require.
Another effective tactic to protect mounted ball bearings in washdown environments is filling the sealed bearing cavity 100% with grease. This allows for a larger grease reservoir, builds a larger grease dam in and around the seal area, decreases the impact from moisture invasion into the bearing cavity, and adds greater reliability for extended relubrication intervals.
Reprinted with permission. This article originally appeared in the April 2014 issue of Plant Services (www.plantservices.com).
