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Influence of textures on friction
Research on textures to reduce friction is of interest for components such as bearings that are subjected to necessary friction.

Influence of textures on friction

When two surfaces move over each other, friction occurs. The force and wear are affected, among other things, by the roughness of these surfaces. Because of the wide variety of friction conditions and the complexity of the problem, this is a challenging area of research. New technologies, such as texturing with a femtosecond laser, allow small structures to be introduced into the surface to influence friction behavior.

EKU blog 5 plain bearing
The plain bearing or the shaft (with flywheel on end).

Controlling friction in industrial applications is an opportunity. Just think of energy loss or shortened service life due to friction or problems creating sufficient grip. Therefore, surface texturing to control friction and wear has been the subject of numerous studies, over the last few decades. Despite the interest, understanding the underlying causes of the effects of surface texturing under certain contact and lubrication conditions remains a challenge.

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An example of a texture: topview.

Reduction to 62% possible

Research at the Department of Mechanical Engineering at Imperial College London showed that under certain friction conditions, structures vertical to the direction of motion resulted in a reduction in frictional force.

The effect of surface textures on friction was investigated experimentally for a convergent-divergent bearing operational under different lubrication regimes. Textured patterns, consisting of cavities of varying shapes and orientations, were evaluated for their ability to reduce friction losses. Tests were conducted with a variety of lubricants of different viscosities and normal loads. The results were compared with those from a smooth reference surface and showed that a friction reduction of up to 62% was possible. 

Research into these effects is also being conducted at Sirris. Recently, it was demonstrated that with a certain texture, the speed of a sliding bearing can be increased by 20%. Now a broader study is being set up with the support of the steering committee of the COOCK project SURFACESCRIPT.

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Optimized demonstrator for measuring textured shafts, where rotational speed is measured via sensors.

Approach to research

When optimizing a process, there are often several parameters that can be set. How do you go about finding the right settings? Which values do you all want to try and how many tests can you do? And what about the question of "how is the result affected if you turn two knobs at the same time? In the search for a surface modification that can reduce friction, the literature teaches us that several aspects can be important. What form of structures will you put in the surface? How large and deep will these structures be, and finally, how much of the surface will you treat this way? These are the four factors (inputs). The measured frictional force is the response (output). The next experiment looks for the texture that gives the lowest friction force. Two to three values are chosen for each factor, in accordance with what is already available in the literature. In this case, the microstructures are chosen in the following way:

- Round holes or long lines (1 mm)
- A width of 20 µm or 80 µm
- A depth of 10 µm or 50 µm
- A surface coverage of these structures of 5, 15, 50%.

EKU blog 5 bushing in oil bath with rotating shaft
The bushing in the oil bath in which the shaft rotates.

Setup

Textured shafts are mounted in a bushing that was in an oil bath. Driven by a falling weight (200 g - 1 kg), the shaft will rotate. The speed from standstill until a constant rotational speed is obtained will be measured via sensors that monitor notches on the flywheel behind. Once the measurements are made, the DOE technique (design of experiments) allows the influences of the various factors (effects) to be quantified. A mathematical model allows finding the optimal spot or trend within the parameter variations that have been tried. The results will be published in a later blog. 

Do you have your own case around friction in your company or would you like to know more about our research or activities? Contact Olivier Malek and Eddy Kunnen at Sirris!    

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