Micropitting is a type of fatigue wear commonly found on gear teeth. The gradual loss of material in the form of small surface pits leads to a change in the geometry of the component; if this is not controlled, it can result in failure of the part.
The standardised method of evaluating a lubricants ability to reduce micropitting is with a back-to-back gear machine (the FZG Gear Test). This test for micropitting is very long – in the order of 3-4 weeks.
We have developed an accurate and repeatable accelerated micropitting test. Which is being used to help lubricant formulators develop new products, in a fast and cost-effective manner.
This test method uses the PCS Instruments MPR, in a 3 ring on roller configuration.
The loss of material of our test roller “MPR Profile Deviation” correlates with the gear profile deviation as measured in the FZG FVA 54/7 or DIN 3990 part 16 method. This gives oil formulators a useful and accurate tool to develop new lubricants.
If you would like to hear more, please get in touch. [email protected]
We are currently working on an exciting project with the Railway Standards Safety Board (RSSB) and the European Lubricating Grease Institute (ELGI), to develop a new performance test for Top of rail (TOR) materials. These TOR materials are used to ensure traction and reduce noise at the wheel/rail interface. This allows trains to start without the wheels spinning and stop without skidding. The use of TOR materials is primarily a safety concern, but they can also reduce the squeal emitted by the train wheels in curves.
Our new test uses creep curves to evaluate the performance of TOR materials. The test consists of ball and disc loaded against each other and driven independently, while measuring the friction force acting between them. The linear speeds at the point of contact of both the ball and disc can be varied from being equal to a difference of 10%. This difference in speed introduces sliding into the contact and produces the creep curves. The height of the creep curve gives an indication of the traction available for the train, while the shape of the creep curve can give an indication of noise. Eleven top of rail materials were tested using the method to show the difference in their expected performance in the field. These materials were supplied blind-coded.
An example of the creep curves is shown in the figure below:
After testing it was revealed that samples A, G, H and J were the TOR materials approved for use in the field, and would be expected to perform well in this test. Samples F and L are “dual products” – those which can be used on both flange and TOR. Samples B, C, D, E and K are flange products, expected to have low traction. The four TOR materials can be identified easily from the creep curves, as they all produce higher traction in this system.
This method is now being considered for addition to the DIN EN 16028 standard “Railway applications –Wheel/rail friction management –Lubricants for trainborne and trackside applications”
A Stribeck Curve (sometimes also called a Stribeck Friction Curve) is a common name used to described friction vs entrainment speed results, measured on a tribometer. These are very useful as they give a good overview of the frictional performance of a lubricant.
The Stribeck Curve is named after the German engineer Richard Stribeck, who conducted pioneering research on lubrication and friction in the early 20th century.
The variation in speed allows friction to be measured across the main lubrication regimes, as shown in the diagram below:
In the boundary regime the load between the two surfaces is supported at the surface asperities. Friction is dominated by the properties of the surface and any surface active additives. In the elastohydrodynamic (EHL) regime the load is supported by the fluid film. Friction is controlled by the physical properties of the lubricant. In the mixed regime the load is supported by a mixture of the surface asperities and the fluid film.
Stribeck curves can be used to quickly assess the lubricants ability to reduce friction in the boundary, mixed and EHD lubrication regime.
An example Stribeck curve taken for 3 very different fluids is shown below:
In this example the boundary and mixed regime can be clearly seen for the classic motor oil (blue) and the heady duty engine oil (orange). The heavy duty engine oil showing higher overall friction in the boundary and mixed regime. The axle fluid shows very low friction in the boundary and mixed regime. At higher speeds ~ 3000 mm/s, the friction in the EHD regime for the three oils is similar.
Common effects in MTM type Stribeck curves:
Stribeck curves can be used to measure and investigate a large range of different effects. These include surface active additives, which will control friction in the boundary regime and the position of the mixed regime. Base oil viscosity and viscosity index improvers can vary the position of the curve (at a constant speed). Some of the more common effects studied using Stribeck curves are shown here…
Friction Modifiers
Additives such as organic friction modifiers reduce friction in the boundary and mixed regime:
Antiwear additives / ZnDDP
Thick tribofilm forming additives, such as ZnDDP can increase the roughness of the surface and extent the speed at which the contact remains in the boundary and mixed regime. Pushing the curve to the right.
Viscosity
Increasing the viscosity of the oil has the effect of keeping the contact in full film lubrication to lower speeds. Pushing the curve to the left:
Using Stribeck curves to compare lubricant formulations
Stribeck curves are commonly used to study fully formulated products. This can give an indication of how the combination of different additives effects the friction in the boundary and mixed regime, along with the position of the mixed regime.
The Stribeck curves formed by 6 different 5W30 engine oils are shown below:
This type of result can be used by lubricant formulators and chemists to directly compare oils. For this particular result – it can be seen that lubricants MTMD003, 16,17 and 19 have a very similar frictional performance in the boundary regime. Lubricants MTMD003 and 16 have a similar response in the mixed regime (200 – 3000 mm/s) suggesting a similar thickness and morphology tribofilm. The same can be said for MTMD003 and 18.
Lubricant MTMD018 has a slightly lower boundary friction coefficient – and interestingly the friction increases with speed (an effect sometime seen with friction modifier type additives).
Lubricants MTMD017 and 20 have a lower overall friction coefficient in the mixed regime – an effect which is likely due to the formation of a thinner tribofilm – or very little tribofilm. This is commonly observed with ashless type antiwear additives.
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)
