Tooth thickness upper limit deviation and tolerance
The upper limit deviation and tolerance of tooth thickness are important parameters in gear precision design, which directly affect the meshing performance, transmission smoothness and load-bearing capacity of the gear. The upper limit deviation of tooth thickness refers to the difference between the maximum allowable value of tooth thickness and the nominal tooth thickness, and the tooth thickness tolerance is the difference between the upper limit deviation and the lower limit deviation of tooth thickness. The two together determine the actual tooth thickness range of the gear. In gear transmission, appropriate tooth thickness deviation can ensure reasonable side clearance when the gears are meshing, avoid jamming caused by thermal expansion and contraction, and manufacturing errors, and reduce transmission shock and noise. The determination of the upper limit deviation and tolerance of tooth thickness needs to be combined with the operating conditions of the gear, transmission accuracy requirements and processing technology capabilities to achieve a balance between functionality and economy.
The upper limit deviation of tooth thickness must be set to meet the gear mesh backlash requirements. Backlash during gear meshing refers to the gap between the non-working tooth surfaces of a pair of gears. It primarily serves to retain lubricant and compensate for thermal deformation and manufacturing errors. The smaller the upper limit deviation (i.e., thinner the tooth thickness), the greater the backlash; conversely, the smaller the backlash. For power transmission gears (such as reducer gears), sufficient backlash is required to prevent seizure. A value of -0.05m (m represents the module) is acceptable. For precision transmission gears (such as machine tool feed gears), backlash must be strictly controlled to ensure transmission accuracy. A value of -0.02m is acceptable. Backlash calculation considers various factors, including center distance deviation (generally using an IT7 tolerance), thermal elongation of the gear shaft (calculated based on the difference between the operating and ambient temperatures), and bearing clearance. Ultimately, the required backlash value is achieved by adjusting the upper limit deviation of tooth thickness.
The determination of tooth thickness tolerance is closely related to gear machining accuracy and process capability. The tooth thickness tolerance reflects the allowable manufacturing variation in tooth thickness. Larger tolerances reduce manufacturing difficulty, but also increase the range of backlash fluctuations. Smaller tolerances require higher machining accuracy, increasing manufacturing costs. Tooth thickness tolerances are typically determined by the gear’s precision grade. GB/T 10095.1-2008 categorizes gear precision into grades 1-12. Grade 6 gears typically have a tooth thickness tolerance of 0.015mm-0.03mm, while grade 8 gears have a tooth thickness tolerance of 0.03m-0.06m. For example, a grade 6 gear with a module m = 3mm can have a tooth thickness tolerance of 0.05mm; for grade 8, it’s 0.1mm. In actual production, the tooth thickness tolerance must also match the processing method. The tooth thickness tolerance of gear hobbing is generally 10%-20% larger than that of gear shaping, while gear grinding can achieve a smaller tolerance value (such as within 0.01mm). Therefore, a reasonable choice must be made based on the company’s process capabilities.
The marking and inspection of the upper limit deviation and tolerance of tooth thickness must comply with standard specifications. On gear part drawings, the upper and lower limit deviations of tooth thickness are usually marked after the tooth thickness dimension. For example, “s -0.02/-0.05” indicates that the nominal tooth thickness is s, the upper limit deviation is -0.02mm, the lower limit deviation is -0.05mm, and the tooth thickness tolerance is 0.03mm. Tooth thickness inspection methods include normal line length measurement, tooth thickness vernier caliper measurement, and measuring cylinder measurement. Normal line length measurement indirectly reflects the tooth thickness by measuring the normal line length across k teeth, which is suitable for rapid inspection in mass production. Tooth thickness vernier calipers can directly measure the tooth thickness at the pitch circle with an accuracy of up to 0.01mm, making them suitable for single-piece and small-batch inspection. For high-precision gears, a gear measuring center is required to perform a full tooth thickness scan to ensure that the tooth thickness of each tooth is within the tolerance range and the measurement uncertainty is controlled within 0.002mm.
Tooth thickness deviations and tolerances for special operating conditions require targeted design. For gears operating in high-temperature environments (such as automotive engine gears), the impact of material thermal expansion on backlash must be considered. The upper limit deviation for tooth thickness should be 20%-30% greater than at room temperature. For example, a tolerance of -0.03 μm at room temperature and -0.04 μm at high temperatures. For gear transmissions requiring forward and reverse rotation, the tooth thickness tolerance should be smaller (e.g., 10%-15% smaller than for unidirectional transmissions) to minimize switching shock and ensure uniform backlash. In applications with high vibration and impact (such as construction machinery gears), the upper limit deviation for tooth thickness can be appropriately reduced (i.e., thickened) to increase the tooth contact area and improve load-bearing capacity. However, the minimum backlash must be maintained at no less than 0.01 μm to prevent seizure. Through refined design tailored to actual operating conditions, the upper limit deviation and tolerance for tooth thickness can be fully utilized to ensure the reliability and economy of the gear transmission.
