1. Fault phenomena
The fracture of atooth is the whole or its partial fracture of one or several teeth, and itsfailure phenomenon is the most serious in the preceding two.
It usually starts fromthe root of the root to produce the crack until the teeth break.
2. Reasons for failure
It is mainly becausethe maximum amplitude or alternating bending stress on the tooth root exceedsthe bending fatigue limit of the material.
There are two cases: asudden break when a short load or shock load occurs.
The other is thefatigue fracture caused by repeated bending.
There are severalreasons for this.
1) the material isdefective.
The non-metallicinclusions as micro - shaped gaps, resulting in stress concentration, reducedthe bending fatigue strength.
2) defects inforgings.
One is that theinternal defects cause the crack in the quenching, and the splitting phenomenonoccurs in severe cases.
The second is thebanded structure, which can easily cause serious mixed crystals. Meanwhile, thesegregation of the components of the microregion can form an uneven hardeningmartensite, the residual austenite and the depth and hardness of thecarburizing layer.
3) the gear density ofpowder metallurgy material is low or uneven, which will greatly reduce thebending fatigue strength.
4) surfacedecarburization during heat treatment.
Surfacedecarburization reduces bending fatigue strength, especially for gears withhigh surface hardness, which will reduce bending fatigue limit by 1/2 ~ 2/3.
It has been provedthat the limit of bending fatigue will be reduced by 10% when thenon-martensite tissues of quenched steel surface contain 5% (volume fraction).
For martensitetissues, good fatigue performance only after proper tempering.
6) the transitioncurve of the root arc is bad or the round Angle is too small, resulting in thestress concentration of the root.
7) hard foreign matteris stuck in the mesh.
1) control themetallurgical quality of steel, especially to reduce non-metallic inclusions.
2) ensure forging andstamping quality and focus on internal defects and banded organization.
3) ensure the qualityof heat treatment, especially to prevent surface decarbonization and control ofmetallographic tissues.
4) increase the powershot blasting process.
The strengthening ofthe root shot is beneficial to improve the bending fatigue strength, becausethe surface can form a favorable residual stress.
The test shows thatwhen the residual stress layer is more than five times the depth of themicro-crack layer, the influence of crack can be eliminated.
5) optimize the toothparameters and increase the radius of the root horn.
6) ensure the sealingof the system and avoid hard objects entering.
4. the instance
A motorcycle, whichhas already been cycling over 7000km, started out with a strange sound,followed by a strange noise, and the idle speed was more pronounced.
A tooth of the primarydriven gear is broken apart, as shown in FIG. 3.
First check theappearance, the tooth surface inorganic machining mark, is the stamping form,belongs to the powder metallurgy gear.
No hard foreign matterexcept broken teeth is not found in the gearbox.
No abnormal heattreatment quality was detected.
In terms of size, thetooth thickness is qualified, but the radius of the root horn is small, theR0.5 is generally required, and the measured results are only R0.1 ~ 0.2.
In order to detect thedensities of the dentures, it was found that the density was not uniform, thehighest point was 6.8 g/cm3, but the lowest point was 6.4 g/cm3 (right near thebroken tooth), and the densities of the tooth density were generally 6.8 g/cm3.
The primary drivengear meshes with the primary active gear on the crankshaft, which has a greatimpact in the beginning, but the powder metallurgy material density is low andthe impact resistance is poor.
Therefore, from theabove test situation, the cause of the fracture of the tooth is thedensification of the tooth density and the low;
The small radius ofthe root horn is also one of the factors.
In conclusion, thereare many reasons for the failure of motorcycle gear damage, and there arevarious reasons. Readers can check and exclude them according to the abovementioned points.