Bevel gear module
The module of bevel gears is a fundamental parameter used to measure their size. It directly determines their tooth profile, strength, and transmission performance, and serves as a crucial basis for their design, manufacture, and interchangeability. Similar to cylindrical gears, the module of bevel gears adheres to the national standard (GB/T 1356-2001). However, because the tooth height of bevel gears gradually decreases from the large end to the small end, their module is typically based on the large end module to ensure meshing performance and strength at the large end. Correctly understanding and selecting the module of bevel gears is crucial for ensuring smooth transmission, load capacity, and service life.
The definition and marking method of bevel gear module must be clearly defined to ensure consistency in design and manufacturing. The module of a bevel gear refers to the module of the large end face, which is the ratio of the large end pitch circle diameter to the number of teeth (m = d/z, where d is the large end pitch circle diameter and z is the number of teeth), and is expressed in millimeters (mm). The standard module series specified by national standards include 0.1, 0.12, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.8, 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 50, etc. The first series (such as 1, 2, 3, 4, etc.) should be preferred during design to improve interchangeability and reduce manufacturing costs. The module of the bevel gear must be indicated after the gear model. For example, “Bevel gear 3 module 20 teeth” means that the module of the large end of the bevel gear is 3mm and the number of teeth is 20. For modified bevel gears, the modification coefficient must also be indicated, but the module is still based on the large end.
The selection of bevel gear module requires a comprehensive consideration of factors such as transmitted power, speed, transmission ratio, and installation space, and is crucial for ensuring gear strength. For high power transmission (e.g., greater than 10kW), a larger module (m ≥ 3mm) is required to increase tooth thickness and root area, thereby improving root bending strength and tooth contact strength. For example, bevel gear transmissions in construction machinery typically have a module of 5-10mm. For higher speeds (e.g., greater than 1000 rpm), the module can be reduced (m ≤ 5mm) to reduce circumferential speed and noise, but sufficient strength must be ensured. For example, automotive rear axle bevel gears typically have a module of 2.5-6mm. The larger the transmission ratio (e.g., greater than 5), the fewer teeth the pinion has. To avoid undercutting, a larger module is required. Generally, the pinion should have no fewer than 12-17 teeth (determined by the module and pressure angle). For a pinion with 12 teeth, the module should be no less than 2.5mm to ensure a complete tooth profile. When the installation space is limited, a smaller modulus must be selected while ensuring strength. However, strength verification is required. If necessary, alloy materials or surface hardening treatment can be used to improve the load-bearing capacity.
The module of bevel gears is closely related to other parameters, and the size of the module directly affects gear geometry and meshing performance. The tooth depth parameter is directly proportional to the module. The standard tooth depth h = 2.2m (addendum height ha = m, root height hf = 1.2m). For example, a bevel gear with a module of 3mm has a tooth addendum height of 3mm, a tooth root height of 3.6mm, and a total tooth depth of 6.6mm. The pitch circle diameter d = mz. A bevel gear with a module of 3mm and 20 teeth at the large end has a pitch circle diameter of 60mm. The tooth pitch p = πm. A bevel gear with a module of 3mm has a pitch of 9.4248mm, ensuring pitch accuracy during gear meshing. The pressure angle, typically 20° (standard pressure angle), determines the thickness of the tooth profile along with the module. A larger module results in a more robust tooth profile at the same pressure angle, and thus a greater load-bearing capacity. The tooth width b of a bevel gear is generally (0.25-0.35) d1 (d1 is the small end pitch circle diameter), which is also related to the module. When the module is large, the tooth width can be appropriately increased to increase the contact area. However, an excessively large tooth width will lead to uneven load distribution along the tooth width direction, reducing transmission efficiency.
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