Hertzian Contact Area Research Articles

Surface Damage of Engineering Ceramics in Rolling Contact

Discs of typical engineering ceramics such as silicon carbide, alumina, zirconia and SiAlON were tested in the ball-on-disc type rolling contact fatigue mode using steel balls under lubricated conditions. The fatigue life of the ceramics increased in the order described above. By calculating the maximum tensile stress generated on the edge of a Hertzian contact area in the ceramic discs and comparing with the inherent strength of the material, it was possible to estimate whether the fracture mode depends on the Hertzian cracks or not. Although the cracks occurred in silicon carbide and alumina, no Hertzian cracks appeared in SiAlON materials in both the pressureless sintered and HIPed states for a stress level of 5.88GPa. It is also clarified that HIP treatment improves the fatigue life two orders of magnitude.

Contact of surface asperities in wear

Stresses have been examined under the elliptical Hertzian contact area, which appear during a real contact between two asperities in the process of sliding friction. The stresses are found according to the Mises-Hencky criterion and characterize the ability of the material to change its shape. Relationships have been found of such a maximum stress under the surface of the contact area depending on the geometric parameters of the contact area, on the friction coefficient and the maximum pressure in the centre of contact area. The processing of the calculation data was conducted according to a special program on a computer and yielded an approximate relationship to calculate the maximum reduced stress. The distribution of dangerous stresses under the contact surface has been shown. The most critical points may be both within the surface layer of the material and on the contact area itself.

Oil Drop Formation at the Outlet of an Elastohydrodynamic Lubricated Point Contact

The loss of lubricant from grease-lubricated bearing surfaces is determined by a number of different mechanisms such as side-flow out of the contacts, evaporation, drop formation, oxidation etc. Some of the drops formed behind a rolling element in a bearing will be onrolled by the next rolling element but some will be lost from the bearing. This loss of lubricant is dependent to a high degree on the speed of the air flow through the bearing and is one of the reasons for early failure of grease lubrication in bearings operating in strong, axial air streams. To evaluate the most important parameters in drop formation, the droplets being thrown out from the outlet of an elastohydrodynamic contact were analyzed using picture processing and statistical methods. An empirical model has been built to describe mathematically the principal factors involved in the volume flow thrown out into the air at the outlet side of the Hertzian contact area. For example, drops in a ball bearing will contribute to the inlet feed of oil for the next ball if it is not lost by sideways flow out from the bearing.

Oil drops leaving an EHD contact

Many machine elements are lubricated with oil or grease. The flow of lubricant depends on different parameters such as surface roughness, surface energy, temperature, etc. A known problem with rolling bearings is that lubricants have to be refilled more frequently if a gas stream passes through the bearing. It is therefore important to know the behaviour of the lubricant in order to predict, e.g. bearing fatigue life due to a gas stream. By studying the droplets thrown out into the air after passing an elastohydrodynamic contact, the most important parameters for oil drop formation have been determined. The analysis is based on picture processing and statistical methods. The experiments show that an increase in temperature, viscosity or surface velocity will increase the total drop volume. Decreased contact pressure and pure rolling also increase the total drop volume. The volume flow, passing the Hertzian contact area, varied between 18 and 30% of the air born oildrop flow and it is reasonable to assume that increased flow of air-borne droplets will decrease the film thickness due to insufficient replenishment.

Speed Ratio Control of a Half-Toroidal Traction Drive CVT.

A continuously variable power transmission has potential for the improvement of fuel economy and cruising comfort as an automotive propulsion system. A traction drive CVT changes its speed ratio by the control of side slip force on the Hertzian contact area. This mechanism has the feature of equal torque transmission of every rolling element and low wasting power for the speed ratio change, but sometimes falls into self oscillation at high rotating speed. The paper shows the principle of the speed ratio changing mechanism of the half-toroidal CVT and highlights a digital compensation method for stabilization of the electrohydraulically operated speed ratio control mechanism.

Modelling of wear under partial elastohydrodynamic lubrication conditions

Wear under partial elastohydrodynamic lubrication (PEHL) conditions was studied on an Amsler disc machine utilizing hardened EN 31 and unhardened mild steel disc pairs. This work is an extension of previous work reported on the wear mechanisms under PEHL conditions. In the present paper further evidence of fatigue wear is presented. The wear was successfully modeled on the basis of the fatigue wear model of Jain and Bahadur modified for PEHL conditions. Jain and Bahadur considered that the normal stress responsible for wear was the stress acting at the surface. In the present model, the normal stress responsible for fatigue was considered to be the overall asperity stress acting in the contact area which is defined as the ratio of the total asperity load to the hertzian contact area.

Wear Mechanism of Ceramic Materials in Dry Rolling Friction

Wear tests in dry rolling contact were carried out at room temperature on five ceramic materials, such as silicon nitride, silicon carbide, cermet, titania, and alumina. The results showed that wear rate of silicon nitride was smaller than any of the other ceramic materials and bearing steel. Observations of worn surface and wear debris revealed that ceramic materials have two types of wear, one related to real contact area and another related to Hertzian contact area. It was also found that brittle fracture dominates the wear process of ceramic materials in dry rolling contact. Based on the experimental results, wear rate of ceramic materials was expressed with a new nondimensional parameter.

Model rough surfaces in elastohydrodynamic lubrication

Model rough surfaces have been produced on steel balls with a high (1 microin) surface finish by (a) sputter deposition of chromium through a stencil-type mask and (b) laser milling of the steel surface. The technology for the chromium coatings has been developed to achieve sufficient adhesion to survive lubricated rolling and sliding contact at a pressure of 100 000 lbf in -2. The laser milling technique was refined such that grooves of 10 microm width, spaced 10 microm apart, could be cut under microprocessor control. The main elastohydrodynamic lubrication work has been on a machine in which the steel ball bears against a glass disc. The oil film interference colours in white light in the hertzian contact area provide a measure of film thickness and the effect of grooves of different spacing and orientation has been examined for a range of loads, speeds and oils.

Stress Concentration in the Vicinity of a Hole Defect Under Conditions of Hertzian Contact

Two-dimensional photoelastic stress analyses were conducted for epoxy resin models containing a hole defect under the conditions of Hertzian contact. Stress concentrations around the defect were determined as a function of several parameters. These were hole diameter, its vertical distance from the contact surface, and the horizontal distance from the Hertzian contact area. Also determined was the effect of tangential traction (generated by a friction coefficient of 0.1) on the stress concentration. Sharp stress concentrations occur in the vicinity of both the left and the right side of the hole. The stress concentration becomes more distinct the larger the hole diameter and the smaller the distance between the hole and the contact surface. The stress concentration is greatest when the disk imposing a normal load is located at the contact surface directly over the hole. The magnitude and the location of stress concentration varies with the distance between the Hertzian contact area and the hole. Taking into account the stress amplitude, the area which can be involved in a process of rolling contact fatigue seems to be confined to a shallow region at both sides of the hole. The effect of tangential traction is comparatively small on the stress concentration around tile hole.

Wear of Rail and Tire Steels Under Rolling/Sliding Conditions

Prediction of the wear rate of rails on curves is becoming an increasingly important requirement as maintenance costs rise. One possible method of achieving this aim is to relate wear behavior to contact forces and creepages which can be evaluated using existing theoretical analyses of wheel/rail contact. Such a relationship can only be derived from laboratory experiments and this paper describes the results of some dry wear tests in which a rail/tire steel combination was tested over a range of creepage (1–10 percent) and contact stress (500–1300 Nmm-2) conditions. A particular feature of the experimental technique was the use of an air blast to cool and clean the specimens. It is shown that in the regime of wear most similar to that encountered in service, wear rate can best be expressed in terms of creep force, creepage and Hertzian contact area.

Study of Surface Fatigue of Steel Rollers by Rolling-Sliding Contact Method : 2nd Report, A Basic Concept of the Strain Hardening of the Hertzian Contact Area and Its Subsurface Layer in a Run

Study of Surface Fatigue of Steel Rollers by Rolling-Sliding Contact Method : 2nd Report, A Basic Concept of the Strain Hardening of the Hertzian Contact Area and Its Subsurface Layer in a Run

A study of dark etching area (D.E.A.) type structure modification of material and hertzian contact area induced by ball bearing type motion

The paper presents the results of a study of the influence of load and load duration on DBA (dark etching area) type structural modifications of bearing material by the rolling and sliding motion of ball bearings. Load has a significant influence. There is a correlation between the DEA zone length and the larger axis of the Hertzian contact area. The DEA sickle may be used for the investigation of elastic contact during motion particularly under heavy loads when some of the assumptions on which the Hertzian calculation is based are not valid.