A typical single-rotor power tram system (Figure 5-1) consists of a main transmission (main gearbox), a main drive shaft, and a series of tail rotor drive shafts with two gearboxes. The main transmission includes input drive with freewheeling provisions if no clutch assembly is required, output drive, and main rotor mast. The main drive shaft between the engine and main transmission drives the main transmission. A series of tail rotor drive shafts with two gearboxes (transmissions) -- intermediate and tail rotor -- between the main transmission and tail rotor drive the tail rotor.


The main drive shaft (Figure 5-2) transmits torque from the engine to the main transmission. The shaft is a hollow, statically balanced tube. In addition to required fittings, bolts, nuts, and washers are provided with flexible splined or rubber couplings for installation between the engine and transmission. On systems using a clutch assembly, the main shaft is attached to the clutch on one end and to the transmission input drive on the other end. The clutch assembly provides freewheeling (Figure 5-3). On systems not requiring a clutch assembly, the shaft is attached to an adapter on the engine output shaft on one end and to the freewheel coupling of the transmission input drive assembly on the other end.

Clutch Assembly

The clutch assembly allows for a smooth engagement of the engine to the power train system. The clutch is used to stop possible blade damage and shaft shearing due to sudden torque loading. Some clutches are designed to let the engine start and run without the rotor turning. This is very useful for warm-up and maintenance procedures. Due to the free power system in all gas turbine engines used by the Army, a clutch assembly is not needed on aircraft with gas turbine engines.

The centrifugal clutch assembly is used only with engines of low horsepower output. When the engine speed is increased, centrifugal force throws the clutch shoe against the inner surface of a drum, completing the drive to the rotor. This type of clutch, because of its slippage at low and medium speeds, generates heat, which is harmful to the life of clutch parts.

Freewheeling Unit

All rotary-wing aircraft have a freewheel unit located between the engine and the main rotor or rotors. Three basic types of freewheel units are roller, sprag freewheel unit is to free the power train drive system from the drag made by the dead or idling engine. By doing this the freewheel unit makes autorotation possible. This allows an aircraft to land safely without engine power. All types of freewheel units generally work in the same manner. They provide a positive lock of the power train drive system to the engine at any time engine speed equals rotor speed. When rotor speed is faster than engine speed, the freewheel unit unlocks the power train drive system clutch, and overrunning clutch. The purpose of the from the engine.


A typical main transmission performs a number of functions (Figure 5-4). It --

The main transmission is mounted in a variety of ways according to a particular manufacturer's design. Some transmissions contain a support case. The case is an integral part of the transmission mounted directly to the transmission deck. The transmission may be secured to the transmission deck by a system of tubular support assemblies. In one power train system, the transmission is secured to the main rotor mast support structure. In the power train system of a reciprocating-engine-powered, observation-type helicopter where neither shafting from engine to transmission nor drive angle change is necessary, the main transmission is mounted directly on the engine.

Input Drive

Engine torque is transmitted through the main drive shaft to the input drive, which drives the main transmission gear trains. On systems not using a clutch assembly, a freewheel coupling is provided in the input drive assembly, which automatically engages to allow the engine to drive the rotor or disengages the idling engine during autorotational descent. On dual-engine, single-rotor power train systems, the transmission has two input drive assemblies.

Tail Rotor Drive

The tail rotor is mounted on the end of the main transmission and is driven by the accessory gear train. A flexible splined coupling provides a means of attaching the tail rotor drive shaft.

Generator Drive

The generator drive is driven by the main transmission accessory gear train. The generator is driven off the main transmission so that, when the helicopter goes into autorotation and the engine is idling or stopped, enough electrical power will be left to operate instruments, radio, and electrical fuel pumps.

Main Transmission Oil System

Most main transmissions are lubricated by a wet sump oil system which is separate from the engine oil system (Figure 5-5). However, the engine oil cooler and transmission oil cooler may be mounted close together so that they can use the same blower system to cool the oil. Oil supply from the transmission sump is circulated under pressure from a gear-driven pump through internal passages and a filter to the sump outlet. From this outlet external lines are routed to an oil cooler with a separate thermal bypass valve, then to a manifold on the transmission main case. This manifold is equipped with a relief valve to regulate system pressure and distribute oil through jets and internal passages. This lubricates bearings and gears inside the transmission where the oil drains back to the sump.

Oil temperature and pressure gage readings are shown by a thermobulb and a pressure transmitter. Most transmission oil systems provide a heat switch and a pressure switch which will light caution lights on panels lettered XMSN OIL HOT and XMSN OIL PRESS (low pressure) if such conditions occur. Servicing and draining provisions are provided in the transmission oil system. Oil level sight gages are provided on most transmissions; others use the dipstick method. Chip detectors used in the transmission oil system are similar to those used on engines.

Rotor Tachometer-Generator Drive

The rotor tachometer RPM indications are provided by the rotor tachometer-generator. The tachometer-generator drive is driven by the main transmission accessory gear train.

Hydraulic Pump Drive

The hydraulic pump drive is driven by the main transmission accessory gear train. The hydraulic pump provides hydraulic pressure for the flight control servo system. Some helicopters use two separate flight control servo systems completely independent of each other. One system is the primary servo system; it gets hydraulic operating pressure from a hydraulic pump driven by the main transmission. A secondary servo system gets hydraulic operating pressure from a hydraulic pump driven by the engine.


The main rotor mast assembly is a tubular steel shaft fitted with two bearings which support it vertically in the transmission (Figure 5-6). Mast driving splints engage with transmission upper-stage planetary gear, providing counterclockwise rotation (viewed from above). The upper bearing retainer plate has an oil jet fed by an external oil hose. Splints on the upper portion of the mast provide mounting for main rotor arid control assemblies.


The purpose of the tail rotor drive shaft is to transmit torque from the main transmission to the tail rotor gearbox (Figure 5-7). The shaft is made up of a series of hollow tubes with provisions for statically balancing and coupling attachments on each end. Flexibility in the shaft is provided by splined or rubber couplings. The tail rotor drive shaft is supported by a series of support bearings and support hanger assemblies.


An intermediate gearbox is located on the tail boom of the helicopter (Figure 5-8). This gearbox provides a specific degree change in direction of the tail rotor drive shaft with no speed change. The gearbox assembly consists of a case with flexible coupling provisions for attaching onto the tail rotor shaft fore and aft. The gearbox is splash-lubricated, and the case is fitted with an oil filter cap, a vent breather, an oil level sight gage, and a drain plug equipped with a magnetic insert. The magnetic insert collects metal particles coming from inside the gearbox. When there is a requirement, the metal particles can be collected and analyzed to determine the condition of the gears and bearings in the gearbox.


The tail rotor gearbox is located on the extreme aft end of the tail boom -- in some cases on top of the tail boom vertical fin (Figure 5-9). The gearbox is splash-lubricated. It consists of mating input and output gear assemblies set into a case provided with a vented oil filter cap, oil level sight gage, and a drain plug with a magnetic insertplug for collecting metal particles. By analyzing these metal particles, the condition of the gearbox gears and bearings can be determined. Flexible couplings are provided for attaching the tail rotor drive shaft onto the input end of the gearbox. The tail rotor gearbox provides a specific degree change in tail rotor drive shaft direction and a specific speed reduction between input shaft and output shaft on which the tail rotor assembly is mounted.

When troubleshooting or working on a specific single-rotor power train system or component, always refer to the applicable maintenance manual.