Discussion on Measures to Strengthen Gear Contact Force

Increasing the number of gear teeth and the modulus can reduce the contact stress, and the number of teeth and the modulus cannot be amplified. The tooth width is increased, and the contact stress can be reduced theoretically. However, the tooth width should not be too wide due to the support stiffness and gear manufacturing precision. Generally, the tooth width coefficient = b/m is recommended to select the tooth width, and the gearbox gear = 57; Gear = 911. Table 1 compares the tooth widths of the final drive gears of several models. The increase of the meshing angle can also reduce the contact stress. The relationship between the meshing angle and the tooth angle (pressure angle) is: =arccos(mZcos/2a) where / is the tooth angle/engagement angle; Z is the number of teeth; distance.
Several models final drive gear width comparison model parameters FIAT45-66 Shanghai-50 Dongfeng-50 Jianghuai-50 harvest -35 Dongfanghong-30 engine power P/kW3736372622 gear module m/mm4.565.565 gear tooth width b/ Mm4554585048 can be seen from the above formula, the tooth angle is proportional to the meshing angle, so the increase of the tooth angle can reduce the contact stress. For low-speed heavy-duty gears that take into account strength and have low requirements on noise and vibration, it is feasible to increase the tooth angle to reduce the contact stress. At present, many of the final transmission gears of foreign tractors have adopted 25-tooth angles. However, if the tooth angle is too large, the tip of the tooth will become sharp, and the life of the tool tip will be shortened.
Increasing the contact fatigue strength of the allowable contact stress gear is directly related to the hardness of the tooth surface. Under the condition that the tooth tip or the tip angle is not brittle, the hardness of the tooth surface should be increased as much as possible. The hardness of the tooth surface of the carburized steel and carbon-nitrogen co-infiltrated steel should not be lower than HRC56, the heavy-duty gear should not be lower than HRC59, and the allowable contact stress is obviously increased.
AGMA and ISO allowable contact stress at different hardnesses. Heat treated tooth surface hardness allowable contact stress AGMAISO quenching and tempering HB240758.3680HB300878.9760HB3601051.3845 induction hardening HRC501240.91160HRC541275.41210 carburizing and quenching HRC551309.81350HRC601464.91475 improve the viscosity and reduction of lubricating oil The tooth surface roughness can improve the lubrication state of the gear and increase the allowable contact stress. For low speed, heavy duty final drive gears, high viscosity lubricants should be used. The tooth surface roughness is directly related to the lubrication state of the tooth surface. Under the condition of a constant viscosity, the ratio of the minimum oil film thickness hmin to the arithmetic mean value Ra of the roughness is the film thickness ratio. =hminRa film thickness ratio <1, the contact surface is the boundary lubrication state, the meshing surface can not establish the elastic fluid dynamic lubricating oil film; 1<<3 is the mixed lubrication state, can form the oil film, but not enough to separate the two meshing tooth surfaces; >3 hours for full oil film lubrication. Therefore, under the same conditions, the reduction of the tooth surface roughness can be improved, thereby improving the lubrication state of the gear. Recommended tooth surface roughness based on usage requirements and process conditions. Foreign tractors have strict requirements on tooth surface roughness, such as: US JD tooth surface Ra = 2m; Italy FIAT tooth surface Ra = 1.2m. The gear must be well worn before use, and the tooth surface roughness can be guaranteed to be 0.8m after running.
In order to ensure that the gear tooth surface metal is not welded, extreme pressure additives are required. Extreme pressure additives are, in nature, active chemicals such as sulfides, phosphides and chlorides. When the intermetallic contact occurs, the microscopic convex spots on the surface of the gear and the rubbed metal particles react with the additive molecules to form a metal compound to re-adsorb to the metal friction surface, and automatically fill the microscopic concave portion of the gear surface under the contact stress. A protective film is gradually formed on the tooth surface to prevent the metal from being directly contacted and welded. Many foreign automobile tractor drivetrain lubricants contain a small amount of extreme pressure additives.
For the heavy-duty final drive gear, the method of increasing the pressure angle, increasing the hardness of the tooth surface, reducing the roughness of the tooth surface, fully running after use, and improving the contact strength of the gear by using high-viscosity lubricating oil are sometimes added to the lubricating oil. Some extreme pressure additives are an effective and economical solution.

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