Gear hobbing machine transmission system
Gear hobbing machines are one of the most commonly used devices in gear processing. They create tooth profiles on workpieces through the meshing motion of the hob and workpiece. They are suitable for machining spur and helical cylindrical gears, as well as worm gears. The hobbing machine’s transmission system is central to ensuring machining accuracy and efficiency. It consists of multiple transmission chains, which coordinate and cooperate to achieve the rotational motion of the hob and workpiece, as well as the feed motion. Understanding the components and operating principles of the hobbing machine’s transmission system, as well as its transmission chain adjustment and maintenance, is crucial for proper operation and ensuring gear machining quality.
The gear hobbing machine’s transmission system is complex, primarily consisting of a main motion transmission chain, a tooth-splitting transmission chain, a feed motion transmission chain, and a differential motion transmission chain. The main motion transmission chain drives the hob and consists of a motor, a gearbox, and a drive shaft. Its function is to transmit the motor’s power to the hob, enabling the hob to achieve the desired rotational speed. The gearbox adjusts the transmission ratio of the main motion transmission chain to meet varying cutting speed requirements. Hob speeds typically range from 50 to 300 rpm, depending on the hob material, workpiece material, and module. The tooth-splitting transmission chain ensures proper meshing between the hob and workpiece, ensuring that the hob rotates 1/k times (k is the number of hob heads) for every 1/z revolution (z is the number of workpiece teeth). The tooth-splitting transmission chain consists of a hob shaft, a tooth-splitting gear, and an indexing worm gear pair. The tooth-splitting gear’s transmission ratio is determined by the number of hob heads and workpiece teeth to ensure accurate tooth-splitting.
The feed motion transmission chain drives the worktable, which in turn drives the workpiece in an axial feed motion, enabling the hob to cut the entire tooth width along the workpiece’s axial direction. The feed motion transmission chain consists of the worktable, feed pulley, and feed screw. The feed rate is adjustable via the feed pulley, measured in millimeters per revolution (axial feed per workpiece revolution). The feed rate is typically 0.5-2 mm/rev for rough rolling and 0.1-0.5 mm/rev for fine rolling. The differential motion transmission chain is only used when machining helical cylindrical gears. Its function is to compensate for the additional rotational motion of the workpiece caused by axial feed, thereby ensuring the helix angle accuracy of the helical gears. The differential motion transmission chain consists of the feed screw, differential pulley, and differential mechanism. The transmission ratio of the differential pulley is determined by the workpiece’s helix angle, module, and the helix direction of the hob.
The operating principle of a gear hobbing machine’s transmission system is based on the principle of gear generating. The hob is equivalent to a helical gear with a small number of teeth. During machining, the hob rotates to generate the cutting motion (primary motion), while the workpiece rotates with the worktable (secondary motion), generating the generating motion. This allows the hob’s teeth to gradually form the desired tooth profile on the workpiece. For spur gears, only the primary motion, secondary motion, and axial feed motion are required. For helical gears, since their tooth profiles follow a helical pattern along the axis, an additional rotational motion (implemented by a differential motion transmission chain) is required in addition to the axial feed. This additional rotational motion, coordinated with the axial feed motion, ensures the accuracy of the helical gear’s helix angle. For example, when machining a helical gear with a helix angle of β, the workpiece must rotate an additional revolution for every axial movement of a lead S = πmz/cosβ to form the helical tooth profile.
Adjusting the drive chain of a gear hobbing machine’s transmission system is crucial for ensuring machining accuracy. Different machining requirements require different drive chain adjustments. Adjusting the split-gear pulley is the most critical step. Its transmission ratio, i split, = z cutter/(k × z work), where z cutter is the equivalent number of teeth corresponding to the number of hob heads (typically, when the number of hob heads is k = 1, z cutter = 1), k is the number of hob heads, and z work is the number of workpiece teeth. For example, when machining a gear with 30 teeth z work = 30 using a hob with k = 1 head, the split-gear pulley transmission ratio, i split, = 1/(1 × 30) = 1/30. This can be achieved by selecting an appropriate pulley combination (e.g., a driving gear with 100 teeth and a driven gear with 300 teeth). The feed pulley is adjusted based on the desired feed rate. The relationship between its transmission ratio ifeed and feed rate f is f = ifeed × tscrew, where tscrew is the pitch of the feed screw. The desired feed rate can be achieved by adjusting the feed pulley’s transmission ratio. The differential pulley is used for machining helical gears. Its transmission ratio idiff = sinβ/(πm), where β is the workpiece helix angle and m is the module. Adjusting the differential pulley’s transmission ratio ensures helix angle accuracy. After adjusting the drive chain, a dry run test should be performed to check for smooth movement and abnormal noise. Only after confirmation can trial cutting be performed.
Maintenance of the gear hobbing machine’s transmission system is crucial to ensuring the equipment’s accuracy and service life. This includes regular lubrication, cleaning, inspection, and adjustment. Moving parts in the transmission system, such as gears, bearings, and lead screws, require regular lubrication or grease to reduce friction and wear. The type and amount of lubricant should be specified in accordance with the equipment manual. Generally, the oil level should be checked every 100 hours of operation, and the lubricant should be replaced every 500 hours. The transmission system’s exterior and interior should be cleaned regularly to remove oil, iron filings, and dust to prevent impurities from entering the moving parts and affecting transmission accuracy and service life. Transmission system components should be regularly inspected for wear, looseness, or deformation, such as gear tooth wear, increased bearing clearance, and bent lead screws. Any problems should be promptly replaced or repaired. Transmission system clearances, such as gear meshing clearance and lead screw nut clearance, should be regularly adjusted. Excessive clearance can lead to unstable transmission and affect machining accuracy. Adjusting shims or nuts can reduce clearance.
The performance of a gear hobbing machine’s transmission system directly impacts the quality of gear machining. The higher the transmission system’s precision, the higher the gear pitch, tooth profile, and helix angle accuracy. With the advancement of CNC technology, the transmission systems of CNC gear hobbing machines are increasingly adopting servo motors and ball screws instead of traditional pulley drives, significantly improving transmission accuracy and ease of adjustment. The transmission system of a CNC gear hobbing machine uses a CNC system to control the movement of each axis, enabling digital adjustment of the transmission ratio, reducing manual adjustment errors and improving machining efficiency and accuracy. However, traditional mechanical transmission gear hobbing machines remain widely used in small and medium-volume production due to their simple structure, low cost, and easy maintenance.
