There have been many papers reporting that the oil film thickness in the low speed region becomes thicker than theoretically calculated values in grease lubrication at rolling contact. In their reports, the main reason for the formation of thicker oil film was due to adhesion and deposition on the rolling contact surface of thickener and/or increase in viscosity because of concentration of thickener. The effect of existence of thickener to the oil film thickness is quite large in any case, but its mechanism is still unclear. In this study, in order to clarify the mechanism of thicker oil film formation especially focusing on the adhesion and deposition of thickener to the rolling contact surface, intermolecular interaction between thickener and surface of steel and shear strength of thickener were quantitatively investigated. Then, the five kinds of greases that consist of different thickeners, three kinds of urea and two kinds of lithium soap ones, were prepared as samples. First, the effect of chemical structure of thickener on the adhesion and deposition properties to steel surfaces was evaluated from the viewpoints of ʻadhesion workʼanalyzed by contact angle measurement. Second, the shear strength of each thickener was evaluated from coefficient of friction of the surface of thickener pellet measured by a ball-on-disk friction tester. Finally, the relationship between the obtained properties, adhesion work and shear strength of thickeners, and the oil film thickness formed in a two-roller testing machine was discussed. It was concluded that both of intermolecular interaction between thickener and steel surface and shear strength of thickener affect to the adhesion and deposition of thickener on steel surface, resulting in changing the oil film thickness in grease lubrication.

Lubricating grease shows the typical rheological properties, such as yield stress, shear thinning and thixotropy. These properties are believed to be originated by changes of thickener structure of grease, though the relationship between the structural change and grease rheology is not fully understood. This study investigates this relationship using lithium 12-hydroxy stearate greases and lithium stearate greases with different thickener concentration, penetration, base oil type, and prototype methods. Rheological property with two pre-shear conditions was evaluated, and thickener structure of greases were observed using an optical microscope, SEM and AFM. Orientation of thickeners and dispersion of thickener aggregations induced by shear were observed using AFM. In addition, it is shown wettability of base oil on thickeners depends on thickener types.

The dynamic characteristics of the shock absorber, which is a component of the suspension, have a large influence on the dynamic performance such as the steering stability and riding comfort of the automobile. The damping force of the shock absorber which is the deciding factor of the exercise performance is the sum of the hydraulic damping force generated at the valve portion and the frictional force generated at the sliding portion between the piston rod and the oil seal or the piston and the cylinder. So far, it has been required to reduce friction as much as possible to the sliding parts of shock absorbers. However, recent studies have revealed that reducing friction is not necessary for improvement in the performance. When the suspension deforms, vibration damping force is generated as the piston of the shock absorber moves, but a time difference occurs before the large hydraulic damping force rises. On the other hand, the frictional force in the shock absorber is thought to play an important role in generating effective damping force against deformation of the suspension, since its generated force itself is small but its response is high. Although the frictional force in the shock absorber occurs in reciprocating motion in various speed ranges from minute amplitude of 1mm or less to large amplitude of several tens of mm, its behavior changes dynamically and complicatedly and sufficient understanding has not been advanced. Therefore, in this study, we developed a device that can measure the speed dependence of friction in reciprocating motion with high accuracy with the aim of grasping dynamic friction characteristics in shock absorber. We investigated the evaluation index for extracting the features of the dynamic friction characteristics from the measured data.