Plain bearings without lubrication? This is how dry operation works

igu-blog-adm | 20. January 2021

“Self-lubricating”, ” lubrication-free” or even “lubricant-free”. You often read these descriptions in bearing technology. The common denominator here are bearing systems that no longer need additional lubrication. In other words, no one has to stop the machine and relubricate it with a grease gun, there is no need for an oil change or, in the case of larger machines, no need for fully automatic relubrication systems. But what is behind it and how do plain bearings work without lubrication?

Whereas the designations “maintenance-free” and “self-lubricating” merely exclude the need for lubrication during operation, “lubrication-free” usually means the so-called dry operation. This means that more or less no liquid lubricants are used. The advantage: the bearing point remains clean and no dirt adheres to the bearing point. But how does this dry operation work? How do plain bearings manage without lubrication?

The devil in the detail: what does “lubrication-free” actually mean?

Lubrication is usually associated with grease or oil, but its presence does not necessarily mean dirty fingers when handling bearings. However, lubrication actually only means reducing friction and wear between (machine) elements or friction partners that move relative to each other. This can be a classic grease or a so-called solid lubricant.

Solid lubricants consist of fine particles or microscopic plates that reduce friction between friction partners. Common materials include graphite, molybdenum sulphite, ceramic particles or PTFE (Teflon). These are often also components of “liquid” lubricants. However, they can also be processed in solid form in plain bearings.

Solid lubrication: incorporated or layered?

In the plain bearing sector, there are two different types of bearings that are lubricated with solid lubricant. On the one hand, plain bearings with an exceptionally good sliding layer, the so-called composite plain bearings, have become established here. These consist of a support layer made of metal or fabric fibres and an inner sliding layer. While the support layer absorbs most of the mechanical forces, the inner and significantly softer sliding layer made of PTFE or other plastics ensures low friction.

Solid lubricants under the microscope, magnified 200 times (igus GmbH)

An alternative is offered by plain bearings that consist entirely of a material to which these lubricating particles have been added. These plain bearings are often made of plastic compounds. The advantages: the specifications of the many different plastic polymers can be combined with those of solid lubricants. At the same time, friction is low not only on the surface of the component, but across the entire wall thickness. The often thinly applied sliding layers of composite bearings also have the disadvantage that they are quite sensitive to mechanical stress. This effect can be observed, for example, on non-stick pans, which quickly becomes scuffed when metal kitchen utensils are used.

Plain bearings without lubrication: summary

Not all lubrication is the same. Not all lubrication means dirty fingers or oil in the groundwater. Solid lubricants can be integrated into plain bearings in such a way that they are released into the environment only to a negligible extent. For metal and fabric bushings, the solid lubrication consists of a thin layer of PTFE or other plastics with good sliding properties. The disadvantage is that the layer is sensitive to mechanical stress. In plastic bearings, the solid lubricants can be integrated or mixed directly into the material. In this way, a homogeneous structure is created. The entire wall thickness acts as a wear or sliding material.

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