Reduction of Friction Losses Due to the Vortex Flow of the Magnetic Fluid Caused by the Additives of Carbon Nanotubes

Abstract

Magnetic fluids are promising lubricating material, in particular, in sliding bearings. With the aid of the magnetic system the magnetic fluid is held in the gap of friction that simplifies the design of the lubrication system sufficiently. It is known that when conventional lubricants (mineral oil, water) flow, with increasing of speed of rotation of the inner cylinder the transition of laminar flow in a vortex takes place. This dramatically increases the viscous friction losses. The friction losses in a wide range of speeds and possibilities of their decrease due to the vortex flow of the magnetic fluid in the gap between the cylinders are experimentally studied. It is revealed that when the dimensionless speed – number of Taylor equal to 41.2 – is reached, the slope of the curve of friction torque sharply increases, viscous losses also increase, i. e. there is a change laminar flow to a vortex one. The average temperature in the layer of the magnetic fluid reaches 60 оC. This factor leads to increased evaporation of the carrier liquid (water, mineral oil), which reduces the service life of the lubricant i.e. the magnetic fluid. In order to reduce viscous friction when a vortex flow of magnetic fluids takes place, carbon nanotubes, which are cylinders with a diameter of 5.0 nm and a length of about 0.1 mm, are brought into the magnetic fluid. Carbon nanotubes demonstrate elasticity under transverse bending: they curve under the impact of load, and after its removal they restore their original shape. They are also able to elongate along the axis by 16 % and to return to its original position after removal of the load. The effect of reducing friction (about 30 %) with a vortex flow of magnetic fluid by the introduction of carbon nanotubes in a magnetic fluid is experimentally obtained. The likely mechanism of friction reduction is the ability of nanotubes to deform under the influence of pressure pulsations and the velocity of the swirling flow, and to absorb partially a part of their energy. As it was experimentally demonstrated, there is an optimum weight concentration of the additive of nanotubes in the magnetic fluid (~10–4) that is associated with the maximum effect of reducing friction by 30 %. Thus, the insertion of carbon nanotubes in the lubricant (magnetic fluid) makes it possible to reduce the viscous friction and, consequently, to increase the range of operating speeds, to strengthen the online lubricant site.