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Articles About friction
Minimizing gear losses caused by churning, windage and mesh friction is important if plant operating costs and environmental impact are to be minimized. This paper concentrates on mesh friction losses and associated scuffing risk. It describes the preliminary results from using a validated, 3-D Finite Element Analysis (FEA) and Tooth Contact Analysis (TCA) program to optimize cylindrical gears for low friction losses without compromising transmission error (TE), noise and power density. Some case studies and generic procedures for minimizing losses are presented. Future development and further validation work is discussed.
This article reviews mathematical models for individual components associated with power losses, such as windage, churning, sliding and rolling friction losses.
Friction weighs heavily on loads that the supporting journals of gear trains must withstand. Not only does mesh friction, especially in worm gear drives, affect journal loading, but also the friction within the journal reflects back on the loads required of the mesh itself.
Worm gear speed reducers give the design engineer considerable options, but these gear systems present a challenge to the lubrication engineer. Heat energy generated by the high rate of sliding and friction in the contact zone causes worm gears to be relatively inefficient compared to other gear types. Because worm gears operate under a boundary or near-boundary lubrication regime, a satisfactory lubricant should contain a friction modifier to alleviate these conditions.
Examples from gears in wind turbine, automotive and industrial applications.
The diagnosis and prevention of gear tooth and bearing wear requires the discovery and understanding of the particular mechanism of wear, which in turn indicates the best method of prevention. Because a gearbox is a tribologically dependent mechanism, some understanding of gear and bearing tribology is essential for this process. Tribology is the general term for the study and practice of lubrication, friction and wear. If tribology is neglected or considered insignificant, poor reliability and short life will result.
Transmission errors, axial shuttling forces and friction result in bearing forces that serve as the major excitations of gear noise. This paper will use these factors as well as gear stresses and tribological factors to assist in obtaining optimal gear designs.
Surface measurement of any metal gear tooth contact surface will indicate some degree of peaks and valleys. When gears are placed in mesh, irregular contact surfaces are brought together in the typical combination of rolling and sliding motion. The surface peaks, or asperities, of one tooth randomly contact the asperities of the mating tooth. Under the right conditions, the asperities form momentary welds that are broken off as the gear tooth action continues. Increased friction and higher temperatures, plus wear debris introduced into the system are the result of this action.
Solutions to the governing equations of a spur gear transmission model, developed in a previous article are presented. Factors affecting the dynamic load are identified. It is found that the dynamic load increases with operating speed up to a system natural frequency. At operating speeds beyond the natural frequency the dynamic load decreases dramatically. Also, it is found that the transmitted load and shaft inertia have little effect upon the total dynamic load. Damping and friction decrease the dynamic load. Finally, tooth stiffness has a significant effect upon dynamic loadings the higher the stiffness, the lower the dynamic loading. Also, the higher the stiffness, the higher the rotating speed required for peak dynamic response.
This paper will present data from both laboratory and field testing demonstrating that superfinished components exhibit lower friction, operating temperature, wear and/ or higher horsepower, all of which translate directly into increased fuel economy.
This article describes a method of obtaining gear tooth profiles from the geometry of the rack (or hob) that is used to generate the gear. This method works for arbitrary rack geometries, including the case when only a numerical description of the rack is available. Examples of a simple rack, rack with protuberances and a hob with root chamfer are described. The application of this technique to the generation of boundary element meshes for gear tooth strength calculation and the generation of finite element models for the frictional contact analysis of gear pairs is also described.
For a high-speed gearbox, an important part of power losses is due to the mesh. A global estimation is not possible and an analytical approach is necessary with evaluations of three different origins of power losses: friction in mesh contact, gear windage and pumping effect between teeth.