Wednesday 17 April 2013

Lubrication Regimes

 Knowledge of the lubrication regimes in which our machines run is essential to choose the best viscosity and type of lubricant with the target to avoid wear and improve energy saving, that is why to know the Stribeck curve is needed.

 To choose the right viscosity for an application is of crucial importance to avoid wear in machines, for this we usually follow the manufacturer recommendations but rarely do we consider parameters as the surface relative speed or the real running temperature. If we are not sure usually we increase the ISO viscosity grade but this does not ensure the wear protection.

 In addition these criteria don’t consider the equipment energy saving which can get worse without to improve the reliability.

 The best way to solve this problem is to know the lubrication regime our machines run, that is why we need to know the Stribeck curve.

 Describe by Richard Stribeck during the first years of the XX century, this Curve provide us an idea of the friction coefficient variation between two surfaces in the function of the lubrication regime. This regime depends on a parameter related to the lubricant viscosity, the surfaces relative speed and the load.

 If we follow the abscissa axis, first we find the boundary lubrication regime in which the friction coefficient is too high due the film is too thin, lower than the surface roughness, so we cannot avoid the wear. If we cannot avoid running in this regime, due to the running temperature, very low relative speed and/or very high load, we must use solid lubricants and pastes. Another option is to increase the lubricant viscosity to move to the next lubrication regime.

 In the mixed lubrication regime, the film thickness is higher, around the surface roughness, so only there are isolated contacts. This regime provides a drastic friction coefficient decrease and we can find a curve minimum, it means is suitable for energy saving. To avoid wear to use anti-wear additives are needed. 

 Both regimes are considered unstable because the increase in the temperature reduces the viscosity and increases the friction, as a result of that the lubrication regime moves to the left of the curve, the area that generates more wear.

 If we increase the viscosity or the relative speed we move to the elastohydrodynamic and hydrodynamic regimes, where we avoid wear because the film thickness is higher than the roughness.

 In the elastohydrodynamic regime, described by Ertel and Grubin and developed by Cheng, Hertzian contacts are found due to very small contact surface and very high load, up to 3.0 GPa, that increase the viscosity of the lubricant, deform both surfaces and reduce the roughness. This lubrication regime is near the minimum of the Stribeck curve, in fact, some authors think the minimum is in this regime, so to keep in this regime increases the energy-saving and reduces wear. Gears, bearings and cams run in this regime.

 In the hydrodynamic regime, the film thickness is much higher than the roughness, due to the relative speed and the viscosity of the lubricant, this is why we avoid the contacts between the surfaces and eliminate wear. This regime is defined by the Reynolds equation. But the Stribeck curve indicates us that this regime increases the friction coefficient due the high viscosity so the energy-saving gets worse, mainly if the relative speed between the surfaces is too high, in this case, we must reduce the viscosity of the lubricant to move closer the minimum of the Stribeck curve. Journal bearings run in this regime.

 We can consider these regimes as stables because any variation of temperature produces a variation of the viscosity and the friction coefficient in the same direction so they stabilize themselves.

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