Machinery's Handbook, 31st Edition
Standards for Spur and Helical Gears 2253 Application Factor allows for load fluctuations from the mean load or loads in the load histogram caused by sources external to the gearing. Dynamic Factor allows for load fluctuations arising from contact conditions at the gear mesh. Load Distribution accounts for the increase in local load due to maldistribution of load across the face of the gear tooth caused by deflections, alignment tolerances and helix modifications. Minimum Demanded and Actual Safety Factor The minimum demanded safety factor is agreed between the supplier and the purchaser. The actual safety factor is calculated. Geometry Factors allow for the influence of the tooth form, the effect of the fillet and the helix angle on the nominal bending stress for the application of load at the highest point of single pair tooth contact. Sensitivity Factor allows for the sensitivity of the gear material to the presence of notches such as the root fillet. Surface Condition Factor accounts for reduction of the endurance limit due to flaws in the material and the surface roughness of the tooth root fillets. ISO TC/600.— The ISO TC/600 Standard is similar to BS 436, Part 3:1986, but is far more comprehensive. For general gear design, the ISO Standard provides a complicated method of arriving at a conclusion similar to that reached by the less complex British Standard. Factors additional to the above that are included in the ISO Standard include the following: Application Factor accounts for dynamic overloads from sources external to the gearing. Dynamic Factor allows for internally generated dynamic loads caused by vibrations of the pinion and wheel against each other. Load Distribution makes allowance for the effects of non-uniform distribution of load across the face width, depending on the mesh alignment error of the loaded gear pair and the mesh stiffness. Transverse Load Distribution Factor takes into account the effect of the load distribution on gear tooth contact stresses. Gear Tooth Stiffness Constants are defined as the load needed to deform one or several meshing gear teeth having 1 mm face width, by an amount of 1 μ m (0.00004 in). Allowable Contact Stress is the permissible Hertzian pressure on the gear tooth face. Minimum Demanded and Calculated Safety Factors The minimum demanded safety factor is agreed between the supplier and the customer. The calculated safety factor is the actual safety factor of the gear pair. Zone Factor accounts for the influence on the Hertzian pressure of the tooth flank curva ture at the pitch point. Elasticity Factor takes account of the influence of the material properties such as the modulus of elasticity and Poisson’s ratio. Contact Ratio Factor accounts for the influence of the transverse contact ratio and the overlap ratio on the specific surface load of the gears. Helix Angle Factor makes allowance for influence of helix angle on surface durability. Endurance Limit is the limit of repeated Hertzian stresses that can be permanently endured by a given material. Life Factor takes account of a higher permissible Hertzian stress if only limited durabil ity is demanded. Lubrication Film Factor refers to when the film of lubricant between the tooth flanks influences the surface load capacity. Factors include the oil viscosity, pitch line velocity and roughness of the tooth flanks. Work Hardening Factor takes account of the increase in surface durability due to mesh ing a steel wheel with a hardened pinion having smooth tooth surfaces. Coefficient of Friction’s mean value depends on the lubricant, surface roughness, lay of surface irregularities, material properties of the tooth flanks, and the force and size of tangential velocities.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online