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Gear Machining
Published in David A. Stephenson, John S. Agapiou, Metal Cutting Theory and Practice, 2018
David A. Stephenson, John S. Agapiou
The basic geometry of spur gears is shown in Figure 17.4. Gear tooth profiles are designed to ensure smooth power transmission by minimizing speed variations and vibrations and to minimize internal stresses due to cyclic contact loads. The most commonly used tooth profile for spur gears is the involute profile, based on the involute of a circle, the curve drawn by a point on a taut cord unwinding from a base circle [9]. (Most gears do not use a true involute profile but incorporate modifications for clearance or load distribution.) For gear sets with involute tooth profiles, the common normal to the tooth profiles at contact points throughout the mesh cycle passes through a fixed pointed on the line of centers, the pitch point, and the velocity ratio of the gears is constant through the mesh cycle and equal to the ratio of the gear diameters. The pitch point also determines the pitch diameter of the gears. The line of action passes through the pitch point and is tangent to both base circles. The pressure angle is the angle between the line of action and the common tangent to the pitch circles. Standard gears are manufactured with fixed pressure angles, with 14.5° and 20° being common values.
Asymmetric Gearing Limits
Published in Alexander L. Kapelevich, Asymmetric Gearing, 2018
The vast majority of gears are designed with the standard symmetric tooth proportions. One of the main tooth proportion parameters is pressure angle. Most gears are designed with the standard 20° pressure angle. The old standard pressure angle of 14½° is still in use. In some industries like, for example, aerospace, the 25° and 28° pressure angles are used [13, 172]. The term pressure angle, in this case, is actually related not to the gear mesh, but to the basic or generating rack that is utilized for gear design or as a cutter profile, respectively. A gear involute profile angle varies from the root form diameter to the tooth tip diameter. This section describes transverse operating pressure angles, which are defined for a pair of mating asymmetric tooth gears as follows:
Gear Types and Nomenclature
Published in Stephen P. Radzevich, Dudley's Handbook of Practical Gear Design and Manufacture, 2021
The most common pressure angles used for spur gears are 14.5°, 20°, and 25°. In general, the 14.5∘ pressure angle is not used for new designs (and has, in fact, been withdrawn as an AGMA standard tooth form); however, it is used for special designs and for some replacement gears. Lower pressure angles have the advantage of smoother and quieter tooth action because of the large profile contact ratio. In addition, lower loads are imposed on the support bearings because of a decreased radial load component; however, the tangential load component remains unchanged with pressure angle. The problem of undercutting associated with small numbers of pinion teeth is more severe with the lower pressure angle. Lower pressure angle gears also have lower bending strength and surface durability ratings and operate with higher sliding velocities (which contribute to their relatively poor scoring and wear performance characteristics) than their higher pressure angle counterparts.
Influence of tip modification on performance characteristics of involute spur gears
Published in Australian Journal of Mechanical Engineering, 2020
Wasiq A.M. Abdul, Timothy L. Krantz, Iqbal Shareef
For this study, a spur gear design previously used in gear durability research at NASA Glenn Research Center is used as an example design, for which pressure angle was 20°. The gear pair is a 1:1 ratio gear pair for a power recirculation gear tester. The gear design specification is provided in Table 1. The most commonly used pressure angles for spur involute gears are 14.5°, 20° and 25°, although other values are chosen for certain gearing applications. In general, the higher the pressure angle, the greater is the gear load carrying capacity, and a smaller number of teeth can be adopted without undercutting. But the higher the pressure angle, the greater is the separating force and the potential for larger dynamic effects and resulting noise. On the other hand, with lower pressure angles, one can reduce or eliminate undercutting and improve gear meshing while reducing dynamic stresses and noise. But lower pressure angles require higher number of teeth to avoid undercutting and decrease the maximal power transmission capacity of optimised gears. In general, a 20° pressure angle in involute profiles reduces risk of undercutting, reduces interference due to increased pressure angle compared to 14.5° and the tooth becomes slightly broader at the root. A pressure angle of 20° is a commonly selected good compromise design choice for many, but not all, gear applications as discussed in Dudley (1984).
Reduction of tooth wear on asymmetric spur gear through profile correction factors
Published in Australian Journal of Mechanical Engineering, 2022
Several studies have been carried out on asymmetric spur and helical gears to investigate the tooth strength under static and dynamic loading conditions. In asymmetric spur gears, loading and meshing are not similar in drive and coast sides of the gear tooth. The tooth drive side carries more load than the coast side so as to enhance the tooth load capacity of the drive side. Spur gear with asymmetric tooth is designed by providing two unlike pressure angles at the pitch surface of the coast and drive sides. The pressure angle is one of the key parameters in spur gear design which greatly affects the tooth contact load capacity of involute gears. Increasing the drive side pressure angle more than the coast side pressure angle increases the tooth thickness and radius of curvature which improves both bending and contact load capacity of the drive side teeth (Kapelevich 2000; Senthil Kumar, Muni, and Muthuveerappan 2008; Costopoulos and Spitas 2009; Prabhu Sekar and Muthuveerappan 2014, 2015). Thomas et al. (2018) developed a new analytical method to identify the maximum bending stress position in the fillet region when the asymmetric spur gear was loaded at the highest point of single tooth contact and the calculated results were compared with the adopted ISO and finite element methods (FEM). Recently, the bending fatigue strengths, tooth form factor and stress correction factors for asymmetric polymer gears were determined experimentally as well as numerically by Pandian, Gautam, and Senthilvelan (2020). Their analysis indicated that the tooth form factor had a greater influence on fillet stress than the stress correction factor.