To read part I (introduction) of this series, click here.
To read part II (regulatory environment and safety) of this series, click here.
To read part III (lubrication) of this series, click here.
To read part IV ((materials) of this series, click here.
In the fifth (and final) portion of our examination of bearing considerations in the food processing industry, we will be taking a closer look at seals. In food processing plants, bearings are continuously exposed to liquids and various types of particulate debris. Sealed or shielded bearings are the best option for keeping foreign debris out of the interior of the bearing and keeping lubricant in.
The most common bearing seal material is a nitrile rubber. This type of seal is typically Buna-N bonded to a steel insert. The seal is fixed into a groove in the outer ring. The maximum operating temperature is 240° F. This type of seal makes contact with the inner ring providing better protection in contaminated environments than a metal shield. However, this results in an increase in torque and reduces the maximum speed capability of the bearing, but in almost all cases is a design tradeoff that improves the product life.
Certain lubricants and chemicals react with rubber. Aside from the Buna-n and nitrile rubber material, other materials are available for higher temperatures and chemical resistance. These include FDA approved silicone rubber seals that can handle a maximum temperature of 400° F and Viton that has good chemical resistance and a maximum operating temperature of 400° F.
The seal construction can greatly improve its ability to function in extreme environments. Seals can be shrouded in metal, such as stainless steel or galvanized steel. Often called slingers, this type of seal is often found on mounted bearings and the metal shroud helps protect the rubber from abrasion.
The portion of the seal that contacts the inner ring of the bearing is known as the seal lip. Seals can be designed as single, double or triple lips. The seal lips flare out where the contact the ring provides effective protection against lubricant loss and wet or particulate contaminants. Triple lip seals provide the most protection, but also increase rotational torque substantially compared to single lip designs.
When lower torque is a design requirement, a non-contact rubber seal based on the labyrinth effect can be utilized. These seals function by creating a path the fluid or debris must navigate in order to penetrate the seal. In bearings, this is typically just a groove in the inner ring. A contact lip seal can be used very effectively in combination with a labyrinth design to provide excellent protection and low torque.
Teflon and glass reinforced PTFE seals are an excellent material choice in food processing applications. They have outstanding chemical resistance, high and low-temperature capability and exhibit less torque than rubber seals. However, depending on the type of bearing and their construction, these seals are not as robust and can be dislodged if hit directly with a high-velocity stream of fluid.
Keep in mind, the seals found on most types of bearings are not designed for immersion and fluid penetration will eventually take place. They offer excellent protection from particulate contaminants or a fluid splash and wipe down situation.
In the food processing industry, bearing life is a major factor contributing to uptime and reliability. The environments bearings encounter in these applications are extremely harsh. Bearings are selected based in operational loads and desired life, but designers must also give careful consideration to protecting bearings from corrosion and contamination and maximizing lubricant life.
As evidenced, there are a wide range of bearing options to take into account for optimal application performance. Food processing engineers and designers are advised to consult a bearing applications specialist for assistance with calculating environmental factors, load capacity, bearing life and attributes.
Due to the wide array of products along with price and availability, both a lubrication specialist and the bearing manufacturer should be consulted before making a final lubricant selection.
There are a wide range of lubrication options to take into account for optimal application performance. Food processing engineers and designers are advised to consult a bearing applications specialist for assistance to best navigate these regulations. If you have a question regarding lubrication in the food industry (or any industry!) click here to pose it to our experts.