In an earlier blog post, I discussed two examples of the need for oil over grease in a high-speed bearing. The reason for this rule has to do with the difference in how a lubricant functions in high vs. low speed applications. Simply put, hydrodynamic lubrication occurs when the fluid film in front of a bearing builds enough force to lift the bearing off the shaft, overcoming the naturally thin fluid film created by oil.
The principle at work is the same as what takes place when at a certain speed your car begins to hyd
roplane:
As you can see in the diagram, as the rotational speed builds there is a build-up of fluid in front of the rotating wheel. When a sufficient rotational speed is achieved the fluid will form a “hydrodynamic wedge” in front of the wheel that actually lifts the wheel off the surface of the road.
A bearing will obviously function differently, but the same principle applies. The diagram below illustrates how the forces within the bearing are distributed when a bearing is lubricated using hydrodynamic lubrication. As you can tell from the diagram, a higher vertical load on the shaft will increase the necessary speed of the bearing and/or viscosity of the oil.
When the hydrodynamic wedge from the diagram is formed, the bearing will have phenomenally low levels of friction between the two surfaces. However, the key challenge when to hydrodynamic bearing lubrication is that friction builds up within the hydrodynamic wedge and begins to heat the oil. As the oil heats up, the viscosity of the oil will change, causing the wedge and film to weaken/break down over time. The industry has developed additives and cooling mechanisms to overcome the effects of oil temperature on performance, but it is always a key concern. The reason for concern is that there is a danger that the surfaces can be damaged if the fluid film created by the oil is too thin for the materials in question or the speed drops low enough for the wedge to fail. In some cases, using oil with higher viscosity (thicker film) will help to generate the wedge at a lower speed, but this also raises the amount of friction. In addition, as I mentioned above, vertical loading of the shaft will only exacerbate these challenges.
As a result of these technical issues, lubricants have been created that operate under principles of boundary lubrication. Essentially, boundary lubrication uses low-friction solids to create a layer of lubricating solid particles between surfaces that act like mini ball bearings. The key to an effective boundary lubricant is that its solid additives have the lowest trade-off between pressure resistance and low friction. This is because as you include more solids in a lubricant you are reducing that lubricants ability to reduce friction.
Examples of solid additives:
▪ Dark inactive sulphurized fat
▪ Dark active sulphurized fat
▪ Dark active sulphur hydrocarbon
▪ Short and medium chain chlorinated alkanes
(see chlorinated hydrocarbons and chlorinated paraffins)
▪ Esters of chlorendic acid
▪ Polymer esters
▪ Molybdenum compounds
▪ PTFE
Boundary lubricants that utilize these additives can still have an extremely low coefficient of friction, and they are essential when surfaces are loaded. In addition, boundary-lubricating greases have the added benefit of staying on a surface that is at rest, unlike oils, to avoid “dry-starts”. For more on the tradeoffs you can read our post on Grease vs. Oil.
