2.2.2 Transmission System
The helicopter transmission system is a set of shafts and gearboxes which receives the power from the engines, and forwards it to the rotors as well as equipment such as cooling fans, hydraulic pumps and power generators. Helicopter transmission systems are characterized by high exchange rates, high power and low weight, making it critical high-precision machinery.
Shaft Unbalance
Unbalanced rotation is caused by bent or otherwise damaged shafts. This fault types is especially critical for the high speed shafts between the engines and the main gearbox, due to the amount of force generated by even slight unbalance. Shaft unbalance is easily detectable by measuring the energy corresponding to the shaft rotating speed.
Shaft Misalignment
Bad shaft coupling can cause one shaft element to become misaligned. This is a critical point for engine shafts, and for rotorcraft where the tail drive shaft is made up of concatenated segments. Shaft unbalance is easily detectable by measuring the energy corresponding to twice the shaft rotating speed.
Localized Gear Damage
Bent or broken gear teeth are critical faults in helicopter transmission sys-tems, and require immediate retirement of the damaged components. A localized damage to a gear surface typically generates an irregularity in the vibration waveform whenever the damaged region interfaces with another gear.
Gear Hub Crack
A gear hub / web crack occurs, like localized damage, due to excessive load. This fault type will change the gear shape from perfect circular to some-what oval. An oval rotating track results in the vibration signature changing periodically, with period equal to the gear rotation.
Distributed Gear Damage
Distributed gear damage, or fretting, occurs as .ne scratches across the gear tooth pattern. This fault type typically accompanies localized damage and hub cracks, as unbalanced rotation or rough tooth edges on one gear tend
2.2. ROTORCRAFT FAILURE MODES
to damage its interfacing gear. Distributed gear damage alters the vibration signature just slightly, and is inherently di.cult to detect using traditional vibration monitoring.
Epicyclic Carrier Cracks
Several rotorcraft gearboxes have one or two epicyclic stages just before the output to the main rotor. Due to high torque, the epicyclic planet carrier is in some cases prone to crack propagation. Cracks from the carrier rim toward the center results in one or more planets interfacing with di.erent force than the others. This will again result in a periodic change in vibration signature, with period equal to the carrier rotation.
Bearing Race and Roller Cracks
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