When a pair of helical cylindrical gears are engaged, the tooth profiles of the two gears contact along a straight line inclined to the axis because the gear teeth are not parallel to the gear shaft. In the process of meshing transmission, the contact line of tooth surface changes from short to long, and then from long to short until it is out of contact. Therefore, the load on the gear teeth is gradually from small to large and then from large to small, which makes the transmission more stable, less impact, vibration and noise, and improves the stability of the transmission.

The characteristics of spur gear transmission are compact structure and strong bearing capacity.

The transmission is used where speed change is needed, or to transmit motion and power as needed. Parallel shaft transmission adopts cylindrical spur gear, and the gear engages and exits simultaneously along the tooth width, which is easy to produce impact, vibration and noise.

Extended data

design criteria

Different failure forms of gear transmission can't happen on a pair of gears at the same time, but affect each other. For example, pitting corrosion on tooth surface will aggravate tooth surface wear, and serious wear will lead to gear tooth breakage.

Under certain conditions, gear tooth fracture and pitting corrosion are the main failure forms. Therefore, when designing gear transmission, the possible main failure forms should be analyzed according to the actual working conditions to determine the corresponding design criteria.

For closed gear transmission with soft tooth surface (hardness ≤350HBW), the lubrication condition is good, and pitting corrosion on tooth surface will be the main failure form. In design, it is usually designed according to the contact fatigue strength of tooth surface, and then checked according to the bending fatigue strength of tooth root.

For closed hard tooth surface (hardness >; 350HBW) gear transmission, with strong pitting corrosion resistance and large gear tooth breaking capacity, is usually designed according to the bending fatigue strength of the tooth root and then checked according to the contact fatigue strength of the tooth surface.

The main failure form of open gear transmission is tooth surface wear. However, due to the complex wear mechanism, there is no mature design and calculation method at present, and it can only be calculated according to the bending fatigue strength of the tooth root. By increasing the modulus of 10% ~ 20%, the tooth thickness can be increased to make it have a longer service life, so as to consider the impact of wear.