What if the bearings in the wheels of your future car/bicycle/motorbike was doing more than the normal bearing job? What if it could not only bear the load with minimum friction, but also insure other functionalities? To be honest, it is already the case as wheel hub bearings also sometimes include a speed sensor unit. But what if it could also contribute to braking or even make any other brake system useless?
Murashima et al.  created and investigated a bearing mechanism that allows for actively controlling the friction. Whereas it is not a proper bearing that can be directly integrated in a mechanism, it is possible to vary the friction coefficient of this lubricated sliding bearing between 0.03 and 0.19 for the same operating conditions.
But how is it possible? Murashima et al. have actually varied the conformity of the contact, enabling to dynamically change between conformal and non-conformal contacts. In their experiment, the contact occurs between a glass disc and a thin metal film (see Figure 2) fixed on a metal base (the morphing surface specimen). The metal base holding the film is hollow and the relative pressure inside it can be varied between positive and negative values (see Figure 3). This results in a controllable morphing of the surface.
As Murashima et al. explain, the morphing has an influence on the behaviour of the contact. When the metal base is pressurised, the metal film locally shows bumps. The contact with the glass disc occurs at the tip of the bump and the sliding allows for oil collecting and hydrodynamic pressure to build up. Even if it is a pure sliding condition, the rotating glass disc drags lubricant towards the contact area: because the distance between the metal film and the glass surface decreases as the lubricant approaches the contact area, the oil pressure increases. This results in a lifting of the mating surfaces and at least a partial solid separation.
On the contrary, when the metal base shows a zero or negative relative pressure, a flat or convex shape is respectively produced. Together with the rest of the metal sheet, the flat or convex areas show a very wide contact area with the glass disc. As the contact area does not show anymore a shape prone to generate hydrodynamic pressure, the solid separation is bad. As a result, solid to solid contact occurs and increases the friction coefficient. Because less lubricant remains in the contact, the lubrication condition is considered poorer.
In the convex case, the device runs under boundary lubrication conditions at low speed but reaches hydrodynamic domain for the largest speeds. This results in a low friction coefficient. The concave/flat case does not show a clear evolution and remains in the boundary lubrication domain. Together with the large contact area and the poor lubrication, the boundary lubrication occurring in the contact results in a larger friction coefficient. By varying the pressure applied to the membrane, the device can show very different behaviours and vary the contact friction for the same operating conditions.
You may think that this device is a laboratory bearing and cannot be applied in an industrial mechanism or in a consumer product. Besides, the load applied on the bearing is rather low. While all this is true, it is the first time that the mating surfaces of such a device are made of hard material. This makes the device more durable and more resistant. This pressure mechanism will hopefully inspire future bearing and mechanism developments. Together with other leads (such as actively controlled ionic liquid lubricants), this device shows what can be done, and what could be the ways to achieve it.
Let us know in the comments what you think about this device, and feel free to read the full article for more details!
 M. Murashima, Y. Imaizumi, R. Murase, N. Umehara, T. Tokoroyama, T. Saito, M. Takeshima, Active friction control in lubrication condition using novel metal morphing surface, Trib. Int., 156, 2021, 106827