Sequential Gearbox Failure


Project Info

It was reported that a failure of the internal components of a motorcycle sequential transmission resulted in a rear wheel lockup. The disassembled gearbox and its components were photographed and the images provided to RTI for analysis.
A sequential manual transmission is a type of a manual gearbox in which the gear ratios can only be selected in sequence and does not mechanically allow the operator to skip gears during a shift. There are five major components in the sequential transmission. They are:

  • Main shaft
  • Counter shaft
  • Output shaft
  • Dog gears
  • Selectable gears

 

In this particular application, it is a five forward speed gearbox with no reverse gear. The main shaft is connected to the engine through the clutch and serves as the input for power delivery to the rear wheel. The first four gears are engaged by manipulating the combinations of the dogs and selectable gears on the main and counter shafts. Rotation of the main shaft in these gear combinations causes counter rotation of the counter shaft which in turn causes rotation of the output shaft in the same direction as the input shaft. The top gear is set by locking the main shaft to the output shaft with a dog. It is important to understand that this particular gearbox is designed with coaxial main and output shafts where the output shaft is hollow to allow a portion of the main shaft to be inserted.

There are two sets of needle bearings in between the outer surface of the main shaft and the inner surface of the output shaft to reduce friction and free play while allowing separate rotation speeds in first through fourth gears at 2.688, 1.850, 1.433, and 1.181 ratios. The identified dog gear locks the main shaft to the output shaft to complete the top gear earning the term direct drive with a gear ratio of 1.

Examination of the photographs revealed a disintegrated outer needle bearing on the main shaft/output shaft interface. The surface on the main shaft in contact with the bearing was severely damaged as well. These bearings are typically cooled and lubricated by the oil in the transmission case but it was determined that the transmission was either dry, or had very little oil available. The oil seal at the output shaft was found to be compromised and allowed the oil to leak out. This particular motorcycle uses separate oil systems for the engine and transmission. The lack of transmission oil does not inhibit the function of the engine. It is unlikely that any large pieces of the damaged needle bearing could migrate into the case and interfere with the gears to cause a hard lock up due to the presence of a second, inner needle bearing. The inner needle bearing did not exhibit damage that would result in loss of function nor were any pieces of the damaged bearing found inside the case.

From the lack of lubrication and cooling, the outer bearing disintegrated due to heat and friction and created a situation where the loose rollers and the bearing cage began to cause drag between the main and the output shafts. In top gear, the power loss due to friction would not be evident as the two shafts are rotating at identical speeds. However, in any of the lower four gears, the power loss could be significant and in neutral, power would be partially transmitted to the rear wheel resulting in undesirable forward movement unless the clutch is disengaged.

While RTI did not find conclusive evidence of a hard lock up, it was determined that the disintegrated needle bearing had severely affected the safe operation of the motorcycle. The damage to the needle bearing was a result of lack of lubrication and cooling that would normally be provided by the transmission oil in this particular motorcycle due to a compromised seal.

Categories

  • Mechanical Engineering
  • Metallurgical Engineering
  • Product Liability
  • Visual Media