Bearing Design Considerations for the Food Processing Industry – Part 2
In the previous installment of this technical article, we explored different material options to consider for bearings used in food processing machinery. Here we will examine various types of seals commonly used in bearings for food and beverage equipment. The following is Part 2 of a three-part post. You can read Part 1 here.
Bearings are continuously exposed to liquids and various types of particulate debris in food processing plants. Sealed or shielded bearings should always be used. Seals are the best option for preventing the ingress of foreign debris and retaining lubricant in the bearing.
The most common bearing seal material is nitrile rubber, also known as NBR or Buna-N. This type of seal is comprised of a rubber profile bonded to a steel insert and has a maximum operating temperature of 240° F. Seals are typically fixed into a groove in the outer ring of the bearing and contact the inner ring, providing better protection than a metal shield in contaminated environments. However, this seal contact results in an increase in rotational torque and reduces the maximum speed capability of the bearing. Still, this limitation is usually seen as a design trade-off made to improve bearing life. Nitrile rubber reacts negatively with certain chemicals and lubricants and is unsuitable for high temperature applications. Alternative materials include fluoroelastomers, such as Viton, which has good chemical resistance and a maximum operating temperature of 400° F. FDA-approved food-grade silicone rubber, which has an operating temperature range from -80°F to 450°F, is another option for extreme temperature applications. Teflon and glass-reinforced PTFE seals are also excellent material choices for food processing applications. They have outstanding chemical resistance, can withstand high and low temperatures, and exhibit less torque than rubber seals. However, these seals are not as robust as those made from other materials - and depending on the bearing type and construction - can be easily dislodged if hit directly with a high velocity stream of fluid.
Construction of a seal can greatly improve its ability to function in extreme environments. The outer surface of the seal rubber can be shrouded in metal, such as stainless or galvanized steel. This metal shroud protects the rubber material from abrasion. On insert bearings found in mounted units, these shrouds are often referred to as slingers. The portion of the seal that contacts the inner ring of a bearing is known as the seal lip. Seal lips can be designed with a single, double, or triple lip configuration. The seal lip flares outward where it contacts the inner ring, providing very effective protection against the ingress of wet or particulate contaminants. Triple lip seals provide the most protection, but also increase rotational torque substantially when compared to single lip designs. When low torque is an application requirement, a non-contact rubber seal with a labyrinth design can be used. This type of seal configuration creates a labyrinth path which is difficult for fluid or debris to navigate. The seals found on most types of bearings are not designed for fluid immersion. If subjected to this environment, fluid penetration will eventually take place. In general, seals offer excellent protection from particulate contamination, fluid splash, and wipe down operations.
AST will discuss food-grade lubricants for bearings in the next installment of Bearing Design Considerations for the Food Processing Industry. Look for Part 3, coming February 22, 2023.
Cross sections of a mounted bearing unit, showing several common seal designs: a single lip (top), double lip (middle), and triple lip (bottom), all three protected by metal slingers.