When selecting a linear system – ball screw, linear guide, or actuator – most of the focus is on application data such as load, speed, travel, and duty cycle. But there’s another factor that should be considered, especially for technically demanding applications or harsh environments. That factor is sealing.
How are the bearings protected against contamination? How is the integrity of the lubrication maintained? For linear systems with metal-on-metal rolling or sliding elements, protecting the load-bearing surfaces and keeping them lubricated are the two most important things machine builders and users can do to ensure long life from the system.
Keeping Contamination Out
Contamination can be in the form of coarse particles, such as wood or metal chips; fine dust from ceramics, fiberglass, or glass; or liquids such as cooling fluids, solvents, or adhesives. Each of these contaminants places different demands on the sealing system. When determining what type of seals are needed for the application, the first thing to check is the material compatibility between the seals and the type of contaminant. Standard seals are commonly made of a fluoroelastomer (FKM) material, due to its compatibility with many chemicals and lubricants. But some solvents will degrade FKM, damaging the seals and putting the system at risk for contamination.
The next thing to consider is the type of contact between the seals and the guide surface. Full-contact rubber seals are the best choice for keeping fine dust and particles out of the system, while metal scrapers are the best option for protection against larger chips and debris. Depending on the type of linear system, the application, and the mounting orientation, contamination can make its way into the unit via the front faces of a linear bearing or ball nut, through the sides of a linear bearing near the raceways, or through the top surface of an actuator. For example, grinding applications produce airborne contamination, which can enter a linear guide from both the front and the sides of the bearing block. In this case, adding side seals as well as front seals helps ensure the bearing is fully protected.
Keep in mind that the more contact the seal makes with the running surface, the more force or torque is needed to overcome the seal friction. This additional friction can affect the sizing and selection of the system’s drive components, so be sure to check the manufacturer’s specifications to determine how much friction is added by the seals.
Keeping Lubrication In
Preventing contamination is the more commonly recognized job of seals, but they serve another critical purpose – to keep lubrication inside the bearing. Whether the linear system is based on rolling or sliding elements, lubrication is critical for preventing metal-to-metal contact and heat buildup inside the bearing. Because it’s deposited along the running surfaces during operation, lubrication not only protects the internal elements, but also extends the life of the seals and reduces seal friction.
Many linear system manufacturers now offer extended lubrication units, which mount to the front of the linear bearing or ball nut. These are proprietary to each manufacturer, but the design typically consists of a porous material impregnated with grease or oil. The lubrication unit makes contact with the raceway, constantly supplying a light film of lubricant. This significantly increases relubrication intervals and reduces maintenance time. It also reduces the amount of lubrication used, which saves cost and has a positive environmental impact.
Applications that take place in uncommon environments such as vacuum, cleanroom, or radioactive chambers may require seals that have low outgassing or other special properties to withstand these conditions. Advancements in materials and manufacturing methods have allowed bearing manufactures to address even the most unique environments and contaminants and provide sealing options that are highly effective at keeping contamination out and keeping lubrication in.