What are the characteristics of helical gearing compared to spur gearing? Why is there a difference between normal and end face for the module and pressure angle of helical gears? Why should the normal face parameter be taken as the standard value?
Answer:
(1) Compared with spur gear transmission, the main advantages of helical gear transmission are: ① Good meshing performance. In helical gear transmission, the contact line of the gear teeth and the gear axis inclined straight line, the gear teeth begin to mesh and disengage from the mesh are gradual, so the transmission is smooth, low noise, at the same time, this mode of meshing also reduces the impact of manufacturing errors on the transmission. ② The degree of overlap is large. This reduces the load on each pair of gear teeth, thereby relatively improving the gear's load-bearing capacity, extending the service life of the gear, and making the transmission smooth. ③ The minimum number of teeth of helical standard gears that do not produce a root cut is less than that of spur gears, so a more compact mechanism can be obtained by using helical gear transmission. Compared with spur gear transmission, the main disadvantages of helical gear transmission are: Due to the existence of helix angle, the transmission will produce axial thrust Fa, and with the increase of helix angle increases.
(2) Since the tooth surface of helical gear is involute helical surface, the tooth shape of its end face and the normal tooth shape perpendicular to the helical direction are different, and thus its parameters are divided into the end face parameter and the normal parameter.
(3) Since the manufacturing of helical gears is often done with rack-type cutters or disk gear milling cutters, and the cutter is fed along the helix of the gear teeth when cutting the teeth, it is necessary to select the cutter in accordance with the parameters of the gear's normal plane. Therefore, the parameters on the normal face of helical gears (module, index circle, pressure angle, top height coefficient, etc.) are specified as standard values in engineering.