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.
