Titanium Fretting – May 26, 2015
May 26, 2015
NOTICE TO ALL SAFARI OWNERS REGARDING INSPECTION OF MAIN TRANSMISSION AND TAIL ROTOR TRANSMISSION OIL
Within the transmissions of the Safari Helicopter, there are several parts made of Titanium. Titanium is an alloy that is not magnetic, so any particles of Titanium that are present in the gear oils will not attach to the magnetic plugs and produce an alarm. Those safeguards are very effective in detecting the presence of steel debris from bearing and gear wear, but not useful to detect Titanium particles.
Because of the tensile nature of Titanium, it does not usually chip off as steel does. Instead, it will “fret” which is essentially scrubbing off tiny particles. Those minute particles will cause the oil surrounding it to blacken.
Examine the oil in both transmissions through the sight glasses. If the oil appears to be darker than normal, action should be taken immediately. If the oil is blackened, it may be an indication that there is wearing on a Titanium part. The oil should be drained and analyzed using spectrometric analysis.
In the case of the main transmission, darkening of the oil may indicate wear on the main shaft at the area of the keyways and gear to shaft contact face. If the keys are loose in the slots or the gear is loose on the shaft, it may allow movement of the key against the shaft or gear which could result in fracturing over time. The main transmission shaft should be inspected at the area of the intersection of the bull gear and shaft if titanium is found in the sample.
If the oil in the tail rotor transmission is darkened, it may indicate wear on the tail rotor shaft where it intersects gears. Again, movement of loose keys can result in fatigue fractures. The tail rotor transmission should be disassembled to inspect the area of the keyway on the output shaft if titanium is found by the analysis.
Regardless of the current condition of the oil in the transmissions, an analysis should be performed at the next oil change to establish a base line for future comparisons. We recommend that an analysis be included as part of your annual condition inspection or at 50 hours, whichever comes first.
In aviation, it is common practice to test engine oil regularly for metal particles that may be an early indication of failure. In rotorcraft, there are additional areas of moving parts that are bathed in oil, which can be examined for the same reasons. This safeguard is not limited to the Safari. All rotorcraft manufacturers recommend or require periodic testing of this type.