The Analysis Of Motorcycle Gear Tooth Broken

Jul 20


Jack Leeyaso

Jack Leeyaso

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There are many kinds of motorcycle gear, such as master and driving gear, change gear, start gear, rear driving gear, balancing gear, timing gear, tachometer gear and fuel pump gear. Here is some analysis about some common gear damage, like gear tooth face wear, gear fatigue and gear tooth broken.


We have discussed about the tooth face wear of motorcycle gear and gear fatigue in last two articles.Here is the analysis on gear tooth broken.

  • Gear fracture

What is worst,The Analysis Of Motorcycle Gear Tooth Broken Articles one or several teeth broke from the gear.It generally starts from the root of crack until the broken teeth.It happens mainly because the bending stress on the tooth exceeds the bending fatigue limit. There are two kinds of fracture, sudden fracture due to the short-time overload and fatigue fracture caused by repeated bending.The specific reasons are:

  1. Defect material. The nonmetal impurity will lower the bending fatigue strength.
  2. Defect forging parts. Micro cracks happened when quenching caused by internal defects, even worse, there may appear gear splitting. Banded structure may easily cause serious mixed crystal.
  3. The density of gear powder is low or uneven, which will greatly reduce the bending fatigue strength.
  4. Surface decarburization during heat treatment. Decarburization on the surface will reduce bend fatigue strength. Especially those motorcycle gear with high hardness, the bend fatigue limit will be lowered at half or more.
  5. The transition curve of motorcycle accessories aren’t standard or rounded corners is too small, which will result in stress concentrated on gear tooth.
  6. There is hard impurity inside the mesh of motorcycle gear.



  1. Control steel quality, especially reduce the non-metallic impurities.
  2. Ensure the quality of forging and stamping, especially internal defects and banded structure.
  3. Ensure the quality of heat treatment, especially prevent surface decarbonization and control the microstructure.
  4. Enpower the shot peening process. It can improve the bending fatigue strength.
  5. Optimize parameters of gear tooth and increase the fillet radius.
  6. Guarantee the sealing of cheap brakes so as to avoid hard impurities.


  1. Riding a motorcycle after 7000 km, when started it, there is suddenly a loud sound and then the sound is carry on there, more obviously when idling. Open the gear box, and then found a primary driven gear tooth broken.
  2. Check the appearance, the rectifier regulator is stamping formed. There is no hard impurities inside the gear box except the broken tooth. Heat treatment quality is fine, size and thickness is qualified, but fillet radius is a little small, only R0.1 to 0.2 while the standard should be R0.5. Besides, the tooth density is uneven, the thickest point is 6.8 g/cm3 while the thinest point is only 6.4 g/cm3 and the broken teeth is just near the thinest point. In a word, the general thickness should be more than 6.8 g/cm3.
  3. Primary driven gear will mesh with the primary driving gear on the crankshaft and there will be a very large impact force when starting. The impact resistant ability is very poor if the density is low or uneven, which will also cause the gear tooth brake, besides, if the fillet radius is small, it will also cause the damage of gear tooth broken.

In conclusion, there are different kinds of motorcycle gear damage faults as well as its reasons, you can detect and test your motorcycle gear with the above points. Hope these will help you know more about your motorcycle gear.Thanks for your time and attention.

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