Author: David

Maintaining safe and quiet railways

Posted on by David

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”

Want to know more? Download the paper

Stribeck Curves

Posted on by David

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:

stribeck curve with lubrication regimes

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:

Stribeck curve example log scale

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:

friction modifier effects on stribeck curves

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.

ZnDDP tribofilm effect on stribeck curve

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:

high viscosity oil effect on Stribeck curves

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:

Example stribeck curves for 5W30 engine oils

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.

Covid Response 2020

Posted on by David

At the beginning of the first COVID lockdown in the UK – March 2020, we like many other people didn’t know what effect the lockdown would have on our business.  But we did know and feel like we needed to help our local community. 

This included the manufacture of protective face visors, using our 3D printers.  These were first manufactured and distributed via a collaborative print farm, organised by Swansea University. 

We then decided to manufacture and supply directly to our local community, following the Prusa face visor design.  These were supplied via friends and the volunteers at the Carmarthen Round Table to those who need them. 

The local council then got in touch and started ordering the face visors at 100 pcs a time, which were distributed around the town by the Council. 

To make them more comfortable, we also enlisted the help of Daisychain fabrics to improve the headband for the wearer.

At around the same time we manufactured hand sanitiser and decanted into handheld bottles. 

There were again supplied FOC to the local community via various channels. 

We estimate about 50L of hand sanitiser was supplied – equivalent to 500 bottles. 

Our efforts were covered by the Carmarthen Round Table and the local paper.

We were contacted by some staff members from Glangwili hospital requesting help to manufacture small adaptor parts for the CPAP machine. 

We were able to reverse engineer the parts and 3D printed some copied parts for trial. 

Two of these adaptor parts are required per patient.  At the time only two of these adaptor parts were available at the hospital, and one was on my desk!  Highlighting the desperation of the situation. 

We also liaised with a larger company to produce these parts on mass via injection moulding for the hospital.  Thankfully the original parts eventually arrived through the normal distribution channels at the hospital and our emergency adaptor parts were never needed.

We also realised most of our colleagues, friends and clients were now working at home.  So wanted to do something to brighten their days. 

We placed a big order with Alfies coffee co, for a custom coffee roast dubbed “Stribeck roast”.  We then offered the coffee on social media free to anyone who likes coffee and tribology.

These were then posted out all over the UK, Europe and the USA. 

The response we received from this gesture was huge, with many people posting about the coffee on social media or getting in touch to say thanks.