Since the meshing line coincides with the common normal line of the tooth profile, and it is a fixed line, the positive pressure direction between the tooth profiles is always the same (along the direction of the meshing line N1N2) during the meshing process of the involute tooth profile, which is very beneficial to the stability of the gear transmission.
In addition, the involute profile is widely used because of the advantages of simple cutting tools and mature technology. But there are also some inherent defects, mainly as follows:
1) Because of the limitation of the involute profile, the radius of curvature of the profile curve is small, and it is difficult to improve the load-carrying capacity of the tooth greatly under the condition of a certain size.
2) Because of manufacturing, installation error and deformation, the load of involute gear transmission tends to concentrate to one end, which reduces the bearing capacity of gear.
3) Because the relative sliding speed between the tooth surfaces is different when the tooth profiles of two gears are meshed in different positions, the wear of each part of the tooth profiles is not even.
Because of the above shortcomings of the involute tooth profile, people have been studying the new tooth profile curve. The circular arc gear is widely used. It overcomes the shortcomings of involute tooth profile, mainly including the following points:
1) The tooth profile is the meshing of convex and concave tooth surfaces, and the comprehensive radius of curvature is large. Therefore, under the same size and material, the bearing capacity of circular arc gear is about 1.5-2 times that of involute gear.
2) Because the circular arc gear changes into line contact along the direction of tooth height after running in, it is not sensitive to manufacturing error and deformation.
3) Since there is no undercut problem for circular arc gear, the minimum number of teeth is not limited by undercut, so the radial dimension can be smaller.
The circular arc gear also has the following disadvantages:
1) If there is center distance error in circular tooth profile transmission, the bearing capacity will be significantly reduced, so the accuracy of center distance is required to be higher.
2) Since the end face coincidence degree of the circular arc gear ε a = 0, to ensure the transmission coincidence degree, this kind of gear must be made of helical teeth and have enough tooth width, so the axial dimension is large.
3) The convex and concave tooth surfaces of gears need to be machined by two cutters.
In order to overcome the above shortcomings, a kind of double circular arc gear has been adopted in recent years. In the transmission, the tooth top of a pair of gears that mesh with each other is convex circular arc, while the tooth root is concave circular arc. Large and small gears only need one tool to process. At present, double circular arc gear has been widely used in high-speed and high-power gear transmission. Because the research of the circular arc gear transmission is still in the primary stage, many aspects of the theory is still blank, so this paper only introduces the circular arc gear.