Tribological Properties of Sapphire under Dry Friction against Chromium and Zirconium Ceramics

The issues of sapphire friction and wear in contact with chrome steel and zirconium ceramics are discussed in this article. The kinetic microindentation method is used to study the mechanical properties of sapphire in frictional contact in modelling experiments on the indentation of a Vickers indent. Taking into account the anisotropy of the mechanical properties of sapphire, the influence of the load and orientation of the indent on the modulus of elasticity and hardness is analyzed. It has been established that, under dry friction conditions it is promising the use of zirconium ceramics in the supports of devices operating under conditions in which the use of lubricants is impossible. The possibility of using a friction couple of sapphire-zirconium ceramics in friction units operating in open space is noted.

Development of a Combined Bulging-Piercing Technique to Reduce Forming Load for a Long Semi-Hollow Stepped Part

This paper aims to develop a new forming technique to manufacture a long semi-hollow stepped part. Traditionally, hot backward extrusion is used. This technique is not suitable, because it requires a very high forming load acting on the die and punch especially at the contact between punch and workpiece. As a result, the service life of the punch is very low. Therefore, a new technique to overcome this problem is needed. A combined bulging-piercing technique was proposed and developed in this research. The main concept of this technique is to bulge the part by upsetting the workpiece between the punch and the counter-punch to generate high frictional contact pressure which will help to restrain the material sliding down to the die cavity during the piercing step. In other words, this technique utilizes frictional force at the die-workpiece interface to reduce the forming load of the punch. Finite element modeling was employed to investigate and determine the suitable level of the bulging which can reduce the forming load without generating any significantly high force to the counter-punch. Only experiments with the minimum forming load were selected and implemented to validate this concept, because other conditions with high load will risk to damage the punch and the machine press of the product line. The results show that this technique can reduce the forming load by almost 40%, and also control a good concentricity of the part and reduce the wall thickness variation.

Microanalysis of Worn Surfaces of Selected Rotating Parts of an Internal Combustion Engine

The present paper analyzes the damage of surfaces at spots of frictional contact, namely, the friction nodes on a camshaft and the connecting rod pins of a crankshaft. The resulting wear of the monitored friction nodes reduces the technical life of the machines, which can lead to the decommissioning of the machine. Wear was assessed by measuring roughness and microhardness and by observing the microstructures of the materials. The results of the experiments show that the rotating parts displayed visible wear on the cams, as well as on the connecting rod pins. The experiments revealed that wear was caused by the heating of the material to a high temperature during the operation of the machine and that there was a gradual cooling and tempering of the material, which led to a reduction in the microhardness of the monitored object. Lower microhardness values can be a cause of greater wear of the monitored objects. When comparing the microhardness of the used and the new camshaft, the hardened layer of the new camshaft from secondary production has a significantly smaller thickness compared to worn cams, which leads to the finding of a different material quality compared to the original parts from primary production. This fact indicates that the wear of a new camshaft as a spare part can contribute to the shortening of the technical life of friction nodes.

A Contact Characteristic of Roller Bearing with Palm Oil-based Grease Lubrication

Grease lubricants are widely used in rolling contact applications to reduce friction between two rolling surfaces. Improper lubrication may cause high contact stress and deformation to the bearings and lead to machine failure The purpose of this study is to investigate the coefficient of friction produced by newly developed palm oil-based grease and to investigate the contact characteristics in lubricated roller bearings. In this work, the coefficient of friction of new greases was evaluated experimentally and the values were compared with the conventional mineral oil-based grease to investigate the friction performance. The friction test was performed using a four-ball tester. The finite element model was developed based on the roller bearing geometry and the simulation was carried out the evaluate the contact characteristic. The experimental result shows that the palm oil grease formulation A had the least coefficient of friction, followed by palm oil grease formulation B, mineral grease and food grade grease. This indicates that palm oil-based grease has the potential to be applied in rolling contact applications due to low friction characteristics. Finite element analysis shows that the maximum von Mises stress and total deformation for frictional contact are higher than the frictionless contact. For the frictional contact analysis with various lubricant COF, similar values were obtained with von Mises stress at 400.69 MPa and 3.4033×10-4 mm deformation. The finding shows that the small difference in grease COF did not affect the rolling contact. The finding also shows that the newly developed biodegradable grease has a similar performance in terms of rolling contact friction and contact characteristic in a condition that the bearing is operating in normal condition.

A QUASISTATIC FRICTIONAL CONTACT PROBLEM WITH NORMAL DAMPED RESPONSE FOR THERMO-ELECTRO-ELASTIC-VISCOPLASTIC BODIES

We consider a mathematical problem for quasistatic contact between a thermo-electro--elastic-viscoplastic body and an obstacle. The contact is modeled by a general normal damped response condition with friction law and heat exchange. We present a variational formulation of the problem and prove the existence and uniqueness of the weak solution. The proof is based on the formulation of four intermediate problems for the displacement field, the electric potential field and the temperature field, respectively. We prove the unique solvability of the intermediate problems, then we construct a contraction mapping whose unique fixed point is the solution of the original problem.