After a period of operation high speed turbo gears may exhibit a change in the longitudinal tooth contact
pattern reducing full face width contact and thereby increasing the risk of tooth distress due to the decreased
loaded area of the teeth.
The phenomena may or may not occur. In some units the shift is more severe than others and has been
observed in cases where there is as lttle as 16,000 hours of operation. In other cases there is no evidence of
any change for units in operation for more than 1 70,000 hours. This condition has been observed primarily in
helical gears. AlI recorded observations have been with case carburized hardened and ground gear sets.
This document describes the phenomena observed among some of many installed high speed gear units in
field operation which have been inspected. The authors have not found any written material describing this
behavior and upon further investigation suggest a possible cause. Left unchecked and without corrective
action, this occurrence may result in tooth breakage.