Investigating how and optimizing the polishing process of ceramic balls (Si3N4) used in bearings by magnetic float polishing method.

Mechanical Engineering

Manufacturing and Production Group

Master's Thesis

Introduction

High speed rolling bearings have many applications in rotary machines whose task is to separate rotating parts from fixed parts It is a car. Accuracy is very important in machine tool applications and bearing corrosion resistance in chemical machines. In aerospace equipment, the factors of reliability and long life of bearings and other used components are very important. With the advancement of technology in the field of space exploration, the conditions and characteristics that are expected from bearings have gone beyond, and we are faced with working conditions such as very high vacuum (6-10 Torr), very high temperature (-150 to +230 c?), long life (both in terms of fatigue and wear, which is usually 10-15 years without the need for repairs) and low production friction force.

Steel rolling bearings

Currently, most of the rolling bearings for general and normal use are made of high carbon steels that contain high chromium and have the ability to work hard. AISI 52100 steel can be mentioned from this group. For air and space applications, bearings are made of VIMVAR (vacuum induction melting and remelting by electric arc method in vacuum) M50 tool steel, which has relatively long fatigue and wear life due to high alloy elements and low oxygen content. rtl;">Very high rotational speeds such as the rotational speeds of gas turbine rotors and high-speed machine tool spindles [1] make the use of conventional bearings difficult. This problem is caused by the introduction of centrifugal forces at the point of contact of the balls with the outer guide groove [2]. One of the severe effects of rotating at a high speed is the effect of centrifugal forces on the rotating components, which itself leads to the introduction of additional loads to the contact point of the outer guide grooves with the components. The intensity of this load increase has a quadratic relationship with the rotational speed and a linear relationship with the density of the pellets, which leads to changes in the contact angles in the internal and external grooves. Any difference between the contact angles in the mentioned grooves causes a relative rotation in these contact points, which will cause the balls to slip or slide. The highest sliding speed in this place is obtained by the rotation speed parameters and the Hertzian curve at the contact location [1]. As a result of such sliding, the heat generated in metal bearings at high speeds is significant. If the amount of slip is high, the phenomenon of erosion also occurs. All the effects caused by high rotational speeds lead to failure of steel bearings. Normal steel bearings lose their hardness at temperatures above 300°C and special steel bearings at temperatures above 500°C [1]. Even the properties of advanced lubricating fluids are reduced at such temperatures. In such temperatures, solid lubricants such as MoS2 are usually used. But it is not possible to form a complete layer between the surfaces in contact with these lubricants. Such lubricants cannot play a role in transferring the heat produced on the surfaces in contact, unlike liquid lubricants, and this causes the bearing temperature to increase even more. Ceramic bearings were suggested. These types of bearings, which are all ceramic and hybrid, have many advantages over the steel type. The conditions described for steel bearings in the previous section are better for all-ceramic or hybrid-ceramic bearings. Because ceramic bearings have a lower density at high rotational speeds, the centrifugal force produced is low, so the amount of contact angle changes is small and less slip will occur.. The heat produced in such [p>

bearings will be about 30 to 50% less than the heat produced in steel bearings in high periods [1].

Another advantage of using ceramic bearings is their ability to work in high temperature environments. Hot environments such as adiabatic engines and air and space applications require bearings that maintain their physical and mechanical properties at such high temperatures. Sodium nitrate is a material resistant to microwear and shows better performance in high heat and vacuum.

The coefficient of friction between Si3N4 and Si3N4 ceramic components in the bearing is 0.17 and between M50 and Si3N4 components is 0.15. While the coefficient of friction between M50 and M50 components is equal to 0.54]1 [.

As it is evident from the comparison of the above friction coefficients, the amount of friction in all-steel bearings is much higher than all-ceramic or hybrid ceramic bearings. However, solid lubrication of ceramic bearings is unavoidable in high temperature applications to prevent wear on the guide grooves. The low friction coefficient in ceramic bearings automatically causes the low starting torque of such bearings.

In addition to the items mentioned in precision lathes or center machines, ceramic bearings can be used for positioning the spindle with high bearing rigidity, high rotational accuracy, low bearing temperature rise and long maintenance period with minimal maintenance operations to maintain accuracy. Machine tools must be used. Due to the high corrosion resistance of ceramic materials, such materials are used in chemical industries, where the corrosion resistance factor is of great importance.