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Diamond Bearing Tribology Fundamentals
Tribology comes from the Greek word ‘tribos’ which means ‘rubbing’. It is a relatively new science that considers the friction, wear and lubrication at moving contacts. Diamond bearing tribology looks at the physics between two diamond surfaces moving relative to one another, most of the time in the presence of a lubricant.Diamond Bearing Tribology Fundamentals
Diamond Bearing Technology
Diamond bearings utilize Polycrystalline diamond (PCD) in their primary working surfaces.
In general, bearings allow relative movement or rotation of two machine elements with respect to one another while minimizing heat generation and maintaining alignment between the respective machine elements. There are several classes of bearings. Three of the most common would be rolling element, sliding and fluid film bearings.
1) Rolling element bearings accommodate relative motion by inserting a rolling element between the races that move relative
to one another.
2) Sliding bearings use special bearing materials that work well together as sliding parts to reduce friction and thus reducing heat
and wear. These bearings are operated with and without lubricants.
3) Fluid film bearings comprise two surfaces moving with respect to one another in the presence of a fluid lubricant that separates the surfaces. The speed of the relative motion, the viscosity of the fluid, and the geometry of the bearings work together to create the fluid film. The fluid film keeps the friction and heat generation to a minimum and makes wear non-existent as long as the surfaces remain separated. A crankshaft in an automobile is an example of this.
Polycrystalline diamond could, in principle, be used in all three bearing types. Currently, however, they are found in only sliding and fluid film bearings. Diamond bearings consist of precision machined polycrystalline diamond surfaces that run relative one to another. In operation diamond bearings may act as dry sliding bearings with little or no lubricant, semi-lubricated sliding bearings or fully lubricated fluid film bearings.
Because of the properties of polycrystalline diamond, diamond bearings are capable of operating under harsh conditions which include operating with process fluids and severe loading. They are extremely resistant to abrasion and provide bearing lives that are 2 to 8 times longer than tungsten carbide or other hard metal bearings when operating in abrasive fluids. Successful applications include the near-bit bearing set in drilling mud motors and turbines. In these applications, bearing life has been extended, overall tool length has been shortened and bearing reliability improved. Diamond bearings have also found application in power generation and rotary steerable tools where bearing life has been at least tripled when compared to conventional tungsten carbide bearings. The outstanding toughness, thermal conductivity and hardness are the main properties of polycrystalline diamond that make it such an outstanding bearing material.
Characteristics of common classes of bearings:
|Fluid Film||Sliding||Rolling Element|
|High Temperature Limit||Lubricant||Material/Lubricant||Lubricant|
reference with permission: Khonsari and Booser, Applied Tribology, Bearing Design and Lubrication, John Wiley and Sons, 2008