The large girth gear is a critical component responsible for driving the rotation of a ball mill's shell. Its tooth profile design is primarily categorized into two types: spur gears (straight teeth) and helical gears (helical teeth). These two types exhibit significant differences in terms of load-bearing characteristics, operational smoothness, and manufacturing complexity.

With spur gears, the teeth run parallel to the gear axis. Their machining is relatively simple, resulting in lower manufacturing costs. Furthermore, they are less sensitive to center distance variations during installation, which facilitates easier on-site maintenance. However, during the meshing process, the teeth of a spur gear engage and disengage simultaneously across their entire width. This results in significant impact forces, pronounced noise levels, and relatively poor transmission smoothness. Under heavy-load or high-speed operating conditions, spur gears are prone to generating vibrations, which can adversely affect the service life of the equipment.

In contrast, the teeth of helical gears are angled relative to the gear axis. Their primary advantage lies in a meshing process that occurs gradually: contact begins at one end of the tooth and progressively transitions to the other. This gradual engagement mechanism substantially reduces impact and noise, resulting in smoother power transmission and enhanced load-bearing capacity. Additionally, helical gears feature a higher contact ratio—meaning multiple pairs of teeth are simultaneously engaged—which effectively distributes the applied load. Nevertheless, the disadvantages of helical gears cannot be overlooked: their manufacturing processes are complex and demand high levels of precision. Moreover, their operation generates axial thrust forces, necessitating the installation of corresponding thrust bearings to counteract these forces, thereby increasing both structural complexity and overall cost.

In summary, if economic efficiency and ease of maintenance are the primary priorities—and the operating loads are relatively light—spur gears represent a sound choice. Conversely, if the emphasis is placed on operational smoothness, low noise levels, and long service life under heavy-load conditions, helical gears offer distinct advantages. Ultimately, the actual selection of the appropriate gear type requires a comprehensive assessment based on the specific operating conditions at hand.