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, inhibitscorrosion, protects bearing surfaces from contamination, and helpsseals运行顺利运行并有效运行。
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边界和混合润滑, both of which dominate at lower speeds. Conversely, oil is often the better choice for high-speed applications due to its cooling effect.
Image credit: Yama Seiki
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.
选择合适的润滑剂的因素
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 theNational 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 asvacuum要么cleanroom兼容性，或在组件之间产生极压（EP）的高负载条件。还提供了延长温度范围的润滑脂润滑剂 - 以来的重要考虑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.
如何减少（或消除）对重新润滑的需求
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).
在这些设计中，轴承块内的塑料链将每个球（或滚子）置于链中的“链接”中。这在捕获润滑的每个球周围产生口袋，确保每个单独的球接收恒定的油脂或油供应，这可以显着地延长所需的润滑间隔。球链还防止球之间穿过轴承之间的碰撞，这减少了摩擦和热量并增加了润滑的寿命。
Image credit: THK
Another way that bearing manufacturers provide extended lubrication intervals and, in some cases,消除需要重新润滑, 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.
Image credit: NSK
Filed Under:Linear Motion Tips
