All metal-on-metal recirculating bearings require lubrication in order to function properly and provide a reasonable bearing life. Lubrication reduces friction and heat by separating the rolling elements from the bearing raceway, inhibits corrosion, protects bearing surfaces from contamination, and helps seals run smoothly and operate effectively.
Although the importance of lubrication is clear, choosing the best lubricant for recirculating linear bearings can be difficult, due to the array of criteria that factor into the decision and the range of choices available.
The basics: Grease or oil?
Recirculating linear bearings can use either grease or oil lubricant, and the choice between the two often comes down to a few application criteria together with ease-of-maintenance requirements.
For example, low-speed applications typically require grease because it provides better protection when the bearing operates in the conditions of boundary and mixed lubrication, both of which dominate at lower speeds. Conversely, oil is often the better choice for high-speed applications due to its cooling effect.
Orientation also factors into the grease-versus-oil decision. For vertical or inclined applications, grease is typically preferred, since oil migrates (flows) more easily and can collect, or “pool,” in the lowest available space, preventing it from reaching critical surfaces. And some manufacturers recommend that oil lubrication only be used with a central lubrication system to ensure the proper amount of oil is delivered to the bearing at the proper intervals.
Factors for choosing the right lubricant
The choice of oil lubricant for recirculating linear bearings is based primarily on viscosity. Grease lubricants should be lithium-based with a consistency class between 00 and 3, with the most suitable consistency class being chosen based on the application’s speed and load.
Consistency class, as defined by the National Lubricating Grease Institute (NLGI), defines how the grease will flow and disperse within the bearing. Greases that are too “soft” (low consistency class) can migrate away from critical areas, while greases that are too “stiff” (high consistency class) may not reach some areas.
It’s also important to ensure that grease lubricants do not contain solid additives, such as PTFE (Teflon), MoS2 (molybdenum disulfide), or graphite. While solid lubricants and additives are suitable for sliding contact bearings, they can cause significant damage to the surfaces of rolling bearing elements.
Greases are also available to meet special environmental requirements, such as vacuum or cleanroom compatibility, or high load conditions that create extreme pressure (EP) between components. Grease lubricants with extended temperature ranges are also available — an important consideration since temperature has a significant influence on a lubricant’s performance.
When used outside its rated operating temperature range, grease can break down into its components (base oil, thickener, and additives) or undergo a change in viscosity significant enough to reduce its effectiveness. In general, lower temperature applications require a lower viscosity grease, while higher temperature applications require a higher viscosity grease.
Look to the manufacturer for guidance
In any system where a bearing is being re-lubricated — weather manually, via an automatic system, or through a lubricator — it’s important to use the correct lubrication. Oil and grease aren’t generally compatible, so once a bearing has been lubricated with one, it’s not possible to switch to the other without extensive purging and cleaning. (Preservative oil used for shipping is an exception. Grease can be applied to a bearing in addition to oil preservatives.) Even mixing lubricants of the same type with different viscosities (oils) or different consistency classes (greases) can degrade their performance.
Fortunately, linear bearing manufacturers are keenly aware of the importance of lubrication to their products, and they typically provide extensive lubrication guidelines — including lubrication types, quantities, intervals, and even guidelines for the design of lubrication systems — in product catalogs and maintenance manuals.
How to reduce (or eliminate) the need for re-lubrication
Two features of linear guides can help reduce the need for re-lubrication during a bearing’s life. First, virtually all recirculating bearing manufacturers now offer a significant portion of their product line with ball chains (also referred to as ball separators or ball cages).
In these designs, a plastic chain inside the bearing block encases each ball (or roller) in its own “link” in the chain. This creates a pocket around each ball where lubrication is captured, ensuring that each individual ball receives a constant supply of grease or oil, which can significantly extend the required lubrication intervals. The ball chain also prevents collisions between balls as they travel through the bearing, which reduces friction and heat and increases the life of the lubrication.
Another way that bearing manufacturers provide extended lubrication intervals and, in some cases, eliminate the need for re-lubrication, is to attach a lubricator to one or both ends of the bearing block. Although designs vary according to manufacturer and bearing style, linear bearing lubricators work by providing continuous delivery of lubricant (typically oil) directly to the raceways through oil-impregnated resin or a felt wick.
Filed Under: Linear Motion Tips