Traction curves can be used to develop lubricants for specific applications, for example high traction fluids for CVTs. They can also be used to help understand the physical changes of the lubricant under high contact pressures. This can then be used to help develop new lubricants with special properties, for example with low traction to help in machine efficiency.
The measured traction depends on the physical properties of the lubricant in the central film area. Under EHD conditions the lubricant film experiences high pressures and high shear rates momentarily. Traction curves gives an approximation of how the EHD film shear stress varies with strain rate.
An example of a idealised traction curve is shown below:
At lower SRRs the strain rate is small and the fluid is considered Newtonian. Hence as the proportion of sliding increases, so does the strain rate along with the shear stress of the fluid. This results in an increasing measured traction. This is the linear region.
At higher SRRs the strain rate becomes significant, resulting in shear thinning. The shear stress of the EHD film cannot exceed a critical value called the “limiting shear stress”. This results in the levelling out of the traction curve, indicated by the isothermal region. The limiting shear stress is dependent on the molecular structure of the lubricant.
The example of a traction curve below uses a high traction fluid – Santotrac 50. This fluid consists of a high traction cyclic hydrocarbon structure.
Traction curves can be used to compare the EHD traction of different base oil formulations. This type of measruement can be linked to machine efficiency. The plot below shows some traction curves formed by engine oils, with different base oil composition.
As the traction curves are lowered, a gain is expected in machine efficiency – especially those with a large number of non-conformal EHD contacts.
Traction curves are also used frequently in rolling element bearing research. The traction forces in the bearing influence the fatigue performance of the surface. Traction curves can be used to find suitable lubricants and greases to reduce the shear forces on the bearing surfaces, extending their lifetime.
We can conduct traction curves in the following test envelope:
Temperatures: ambient – 150 °C
Speeds: 0 – 4 m/s
Contact pressures: Up to 1.25 GPa with steel specimens (up to 3 GPa with WC)