Rotax MMH-912 Series Maintenance Manual page 117

Effectivity 912/914 Series
Edition 1 / Rev. 2
BRP-Rotax
Maintenance Manual
The speed at which the pitting increases depends on the lubricant used
and the load. The pitted area can become so large that the remaining,
unaffected tooth flank can no longer transmit the load safely. Further
operation of the components in this condition will result in the total
destruction of the gear-tooth system.
◆ NOTE: The most common area where pitting forms is on the
base of the gear tooth on the drive gear. For this
reason, inspect the drive gear first.
Pitting is damage caused by material fatigue. The latest insights reveal
that its formation is influenced by exceeding the permissible Hertzian
stress for the material, tangential stress on the surface (friction) and
temperature strain. This means that in addition to the material and heat
treatment of the material, the surface quality and structure, the surface
treatment and the lubricant (viscosity at operating temperature and the
additives used) are also important.
Pitting can be divided into the following categories according to the size
of the pitted area, the type of pitting involved and the number of areas
affected.
– minor pitting (breakin pitting)
– advanced pitting
– spallation (large zones of the tooth surfaces have broken off)
Minor Pitting: See Figs. 72-44, 72-45 and 72-46.
Characteristics:
Isolated small areas of pitting (up to approx. 0.5 % of the flank area)
or porelike zones of pitting, usually only found at the root of the flank.
The formation of this type of pitting may stop once the gearbox wearin
period is over.
Causes:
High local pressure on new gears which have not yet been broken in can
cause isolated pitting. With increased wear, the stress on these areas
is relieved and the pitting may stop. In the same way, changes in the
operating conditions can stop the progress of pitting formation.
Conclusion:
This type of pitting does not prevent safe operation. The gear set can
remain in use.
◆ NOTE: Figs. 72-44 to 72-46 show gear flanks with minor
pitting.
72-00-00
page 45
July 01/2008