Geoff,
No I do not have an electronic copy of the HARP Report (CAP 491 - 1984) but, courtesy of Adobe Acrobat and OCR, here is section 7.7 - Gear box and Transmission:
The most complex element of a helicopter, after perhaps the engine, is the transmission gearbox, taking the drive at high speed from one, two or more engines generally in a horizontal plane, and gearing it down by 80 or 100 to 1, or so, to a single vertical shaft carrying the rotor head, with another drive going rearward to a tail rotor (or another rotor head system), the engines having free Wheels, and accessory drives being taken off for generators and oil pumps.
These gearboxes are remarkable examples of the art of the mechanical engineer. Although simplicity is a good starting point in the conceptual stage, complexity is inevitable, and it is not surprising that each designer has found his own solutions. The panel has seen spur gear trains, epicyclics, bevels; it has seen normal straight teeth, 'conformal' tooth profiles; it has seen ball bearings, rollers, tapered rollers, shafts with separate ball races and shafts with rollers running direct on them; we have seen aluminium and magnesium alloy cases. The only common element noted was in the gear material, a traditional low carbon nickel-chrome case hardening steel, either carburised or nitrided, and invariably made of high purity vacuum melted stock.
Notwithstanding the exceptional facilities for gear production we were shown, and the great attention to quality control being paid we are bound to record that in our view it will never be possible to eliminate completely the possibility of failure of some part of so complex a mechanism in service, between specified inspection or overhaul periods.
Although minor surface damage to a gear or bearing is unlikely to be instantly catastrophic, and should be detectable, and although gearboxes are tested to run for a period such as 30 minutes with the oil supply failed, the possibility of a major failure within the box is ever present.
Since with present knowledge a main rotor cannot be duplicated although a tail rotor gearbox might (conceivably a helicopter may survive the failure of the latter) we now come up against the fundamental difference between normal aircraft and rotorcraft -the inability to guard against a possible defect by duplication.
The Panel believes that, while gearbox development and testing should be as stringent as practicable, other means must be introduced to monitor the condition or "health" of the gearbox, preferably in flight. Elementary condition monitoring is already practiced (e.g. oil chip detectors), but much more attention should be paid to this. We make our detail recommendations on this subject later in para. 8.5.
There have been failures of transmission systems taking the drive to the tail rotor, or coupling twin rotor heads. We believe it may be possible to achieve a degree of redundancy here, or at any rate 'damage tolerance'. On the other hand it may be simpler to have some other method of cancelling main rotor torque to use in an emergency (bleed gas jet, rudder surface •••••• )
The text in paragraph 8.5 (referred to above) contains recommendations for 'condition monitoring'; methods include: chip detectors; vibration monitoring; thermal detection or imaging in flight; oil sampling; telemetry (i.e. instantaneous download of HUMS data); and usage monitoring.
Jim