The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact

Laser surface texturing (LST) is an emerging effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were conducted with a pin-on-disk apparatus at sliding speeds in the range of 0.015–0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two oils with different viscosities (54.8 and 124.7 cSt at 40 °C) were used as lubricants. The test results showed that laser texturing expanded the contact parameters in terms of load and speed for hydrodynamic lubrication, as indicated by friction transitions on the Stribeck curve. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.     

Elastic–plastic spherical contact under combined normal and tangential loading in full stick

The behavior of an elastic–plastic contact between a deformable sphere and a rigid flat under combined normal and tangential loading with full stick contact condition is investigated theoretically. Sliding inception is treated as a plastic yield failure mechanism, which allows static friction modeling under highly adhesive conditions. Several contact parameters such as: junction tangential stiffness, static friction force and static friction coefficient are extensively investigated. The phenomenon of junction growth and the evolution of the plastic zone in the contact region are briefly described. It is found that at low normal dimensionless loads the static friction coefficient decreases sharply with increasing normal load, in breach with the classical laws of friction. As the normal load further increases the static friction coefficient approaches a constant value that is about 0.3 for many material properties combinations.     

Vaporization in Face Seals Operating with Liquid Mixtures

The problem of vaporization in face seals operating on mixtures where the minority liquid is discontinuous and has the lower boiling point is analyzed. A model based on several simplifications is presented and used to solve a numerical example for water-in-oil mixtures. It is shown that even the smallest amount (in the order of 1 %) of water in the oil may considerably alter the seal performance and result in seal failure.     

Techniques for assessment of wear between human cartilage surfaces

Osteoarthritis (OA) is a disease of joints, affecting a large number of people worldwide. One of the symptoms of OA is wear of articular cartilage; it is thought that among other factors this may be due to failure of lubrication. Injection of bio-lubricants into a joint may remedy this problem. Wear of cartilage and its prevention is a focus of much interest. The present paper describes wear tests performed using human cartilage on cartilage under various working conditions. Several techniques assessing wear are described, such as changes in surface morphology using optical profilometry and variation in the content of collagen and proteoglycans (PG) in the lubricating solution. Of all these techniques the PG content analysis was found to be the most efficient one.     

The Effect of Dwell Time on the Static Friction in Creeping Elastic–Plastic Polymer Spherical Contact

A theoretical model is developed to study the effect of dwell time on the junction growth and static friction of a creeping polymer sphere in contact with a rigid flat under full stick contact condition. A rapid normal loading into the elastic–plastic contact regime is followed by a rest period during which creep takes place causing contact area growth, and stress relaxation that can completely eliminate the plastic zone in the sphere. At the end of this rest time, an increasing tangential loading is applied to the flat till sliding inception occurs. During this loading step, further increase of the contact area and reappearing of a plastic zone in the sphere take place. An increase in static friction resulting from the dwell time during the creep stage is clearly demonstrated and explained.     

Electrical Conductivity and Friction Force Estimation in Compliant Electrical Connectors

The electrical conductivity and friction force of a compliant curved beam loaded by a rigid flat surface which simulate electrical connectors in high-tech applications is analyzed. Both the elastic deformation of the curved beam and elastic-plastic deformation of the asperities are considered. The effects of the applied load, the surface hardness and roughness as well as the beam geometry on the electrical conductivity and friction are investigated. It is found that the electrical conductivity is insensitive to changes in the beamís geometry like its angular span and slender ratio. Surface roughness and hardness affect both electrical conductivity and friction.     

Performance of the Coned-Face End Seal with Regard to Energy Conservation

The effect of a face coning on seal performance, especially energy losses as indicated by a torque, were studied by comparing torque, face temperature, leakage, and wear of a conventional, widely used flat-face seal with three coned-face seals. Experimental evaluation was performed at rotation speeds to 8000 rpm, with pressures to 2758 kPa (400 psig), using a petroleum-base turbine oil. Axial movement of the mating seal parts was recorded using a digital data acquisition system. The height of the convergent-type cone on the face of the tungsten carbide primary ring ranged from 0.51 μm (20 μin) to 5.6 μm (220 μin) over the 3.175-mm to 6.36-mm (0.125-in to 0.25-in) face width. The torque of the coned-face seal, balanced lo 76.3 percent, was an average 42 percent lower, the leakage eleven limes higher than that of the standard flat-face seal. Reducing the balance to 51.3 percent by decreasing the face width of the coned-face seal resulted in lowering the torque by an additional 44 percent and increasing the leakage 12 to 230 times, depending on the seal shaft speed. No measurable wear was observed on the face of the coned seals. All four seals operated in the stable region of the stability map.     

Experimental Observation of the Dynamic Behavior of Noncontacting Coned-Face Mechanical Seals

The dynamic behavior of a coned-face noncontacting seal is experimentally observed by means of three proximity probes monitoring the motion of the flexibly mounted stator. The various tilts of the stator are analyzed and the relative misalignment between stator and rotor is found. The effects of both the rotor runout and the flexible support on the relative misalignment are discussed. The tests demonstrate both stable and transition to unstable modes of seal operation. Reasons for failure and other phenomenon during stable seal operation are explained.     

Analysis of Surface Textured Air Bearing Sliders with Rarefaction Effects

A numerical model is developed to study the effect of texture on air bearing sliders for large Knudsen numbers. The effect of texture location, texture size, and density on the pressure generation is studied. First, a textured plane slider parallel to the disk surface is investigated, and the texture parameters are determined that result in optimum pressure generation. Then, a plane inclined slider is studied using optimum texture parameters found in the parallel slider case. Thereafter, the effect of texture on the steady state flying characteristics of an actual magnetic recording slider is investigated. Finally, the flying height modulation, pitch, and roll motion of a textured slider (pico and femto form factors) are determined numerically by exciting the slider using a step on the disk. Comparison of the results for textured and untextured sliders is made. It is found that textured sliders show better dynamic performance compared to the untextured sliders in terms of stiffness and damping.     

A Model for Mechanical Seals with Regular Microsurface Structure

A mathematical model is developed to allow performance prediction of all-liquid noncontacting mechanical seals with regular microsurface structure in the form of hemispherical pores. Seal performance such as the equilibrium face separation, friction torque and leakage across the seal are calculated and presented for a range of sealed pressure, pore size and pore ratio of ring surface area. An optimum pore size is found that depends on the other variables and corresponds to maximum axial stiffness and minimum friction torque. Also, a critical pore size is found above which seal failure is possible.