January 19th, 2021
At Saint-Gobain, when we’re developing our bearings and tolerance rings, we draw a lot from tribological research. Tribology is the name given to the study of friction and wear. The term “Tribology” is derived from the Greek meaning “rub”, and that’s essentially what we’re studying: the frictional properties and wear behaviour of surfaces in relative motion.
The study of friction enables us to better understand the wear processes of materials, and ensures that our components are designed to have the optimum coefficient of friction. As you can imagine, tribology plays a massive part in our research and development.
People have understood aspects of tribology for many thousands of years. Rubbing sticks together to generate heat and make fire showed a crude understanding of friction. Probably the earliest recorded use of lubrication is in ancient Egyptian images, that appear to show people using oil as a lubricant to shift large statues.
It was not until the 15th Century our understanding of tribology made really big steps. It was none other than literal Renaissance man, Leonardo da Vinci, who was one of the first scholars to systematically study friction and advance our understanding of it.
Today, the field of tribology is interdisciplinary, and tribology is embedded in a range of engineering and scientific disciplines. We can broadly divide the subject into three interrelated topics: friction, wear and lubrication. Friction is the resistance between the solid surfaces in relative motion, wear is the result of friction, and lubrication is the means of reducing friction and protecting the contact area by creating wear resistance.
You do not have to work in mechanical engineering to encounter tribology in your everyday life. The soles of your trainers are precision-designed for the best grip, for example. Applications include biotribology in healthcare, where hip joint tribology ensures the best, natural movement possible.
Today, tribological research is going nano and a significant branch of tribological system study is nanotribology. Nanotribologists investigate friction at a nanoscale and use this knowledge in applications such as magnetic storage devices.
At Saint-Gobain, we work with engineers on controlling friction, ensuring that their applications are reliable, robust and have superior longevity. For us, tribological research plays a major part in producing dependable components. For example, we work with our automotive clients to create consistent controlled friction that reduces wear, noise, vibration and corrosion.
Lubrication is a whole sub-branch of tribology. As we’re all aware, lubrication reduces the effects of friction. Besides the obvious lubricants like oil or grease, there are also solid lubricants which can be directly integrated into the material structure of components (an example of this are our NORGLIDE® PTFE Bearings), providing an inherent or self-lubricating property. This short video explains how this mechanism works:
A very small amount of lubrication is enough to drastically change the friction between two surfaces. In cases where a very thin film of lubrication adheres to the surfaces creating a low friction layer, this is generally known as boundary friction.
In some cases, this film is thicker and enough to separate the two surfaces completely from each other. Normally it is created by some kind of movement and is a very similar effect to your car wheel when it is aquaplaning. This is called fluid film friction and generally results in very low friction.
The efficiency of an applied lubricant depends on its viscosity, which can change depending on the operating conditions. Again, tribology is applied to the lubricant’s formulation, finding that crucial balance between reducing sliding friction and too-high viscosity.
Stribeck testing is a standard tribological test that is typically performed on a journal bearing tester using oil as a lubricant. The response of the journal bearing test is used to plot COF as a function of shaft surface velocities and is generally referred to as a Stribeck curve or simply friction curve. Stribeck curves are widely used in research to determine the lubrication regime of the bearing and shaft interface. Typically, there are three lubrication regimes of interest defined by the film thickness between the bearing and shaft interface. The thin and mix boundary regimes operate in very thin oil films and therefore the friction generated is primarily due to rubbing of material asperities. This is referred to as Coulomb friction or solid friction. The hydrodynamic region is defined when shaft and bearing are completely separated by an oil film. During startup, a hydrodynamic bearing must first operate in a boundary lubrication regime prior to operating in hydrodynamic regime. Ideally, an engineer will always attempt to design a bearing to operate in hydrodynamic conditions to reduce wear and minimize friction, however this is not always possible. The conditions to generate a hydrodynamic film may not always be possible; lack of oil or low rpm may drive the bearing system to operate in boundary lubrication.
Typical Stribeck curve used to identify the lubrication regime of bearing / counter face system
Therefore, a low friction material having excellent wear rate characteristics would be desirable. Our HP material was designed for this purpose.
Energy is lost to friction in sliding surfaces. We can reduce our energy consumption by ensuring that our systems are low friction, and tribology research combined with materials science can help to make a difference.
To go back to the automotive industry, recent research into passenger car energy consumption estimated that only 21.5% of the fuel energy is used to power the car – and frictional losses account for a massive 33% (28% if you don’t count braking).
And of course, reducing the wear rate of a component due to friction increases the life of the tribosystem, which in turn, cuts down on disposal and manufacture.
Our commitment to research and development sets us apart from conventional manufacturers. Our bespoke components are based on thousands of hours of research and applied study into surface engineering and tribological systems. If you want to know more, please get in touch with us at Saint-Gobain.