(a) Overdrive

If the input speed supplied to the drive is lower than that needed to produce the required output speed, the transmission hydraulically adds the necessary speed to the speed of the engine gearbox through the differential. When the drive is adding speed hydraulically it is operating in overdrive. When the transmission is operating in overdrive, the variable hydraulic unit will function as a hydraulic pump. To enable the variable unit to pump oil, the governor ports control oil to the control piston, which in turn positions the wobbler so oil will be compressed as the pistons are forced into the rotating cylinder block. This high pressure, (working pressure) oil is ported to the fixed hydraulic unit, which now functions as a hydraulic motor. High pressure oil pumped from the variable unit, forces the fixed unit pistons to slide down the inclined wobbler face thus causing the cylinder block to rotate. The block's rotation forces the input ring gear to turn in a direction opposite to the carrier shaft rotation and adds to the speed of the engine gear box through the differential thus maintaining constant output speed.

(b) Straight through Drive
If the input speed supplied to the drive is sufficient to produce the required output speed, the transmission drives the generator directly through the differential. As the input speed increases and the need to add speed decreases, the governor will port less oil to the control cylinder until the variable wobbler is in a position approximately normal to the pistons. When the face of the variable wobbler is approximately perpendicular to the pistons, no oil, (except that required to provide for power losses due to leakage), is pumped by the variable hydraulic unit. At this time the cylinder block stops rotating and the transmission is operating in straight through drive.

(c) Underdrive

When the transmission is operating in underdrive, the variable hydraulic unit functions as a motor and the fixed hydraulic unit functions as a pump. The variable wobbler in the variable hydraulic unit is positioned so the variable hydraulic unit can receive oil from the fixed hydraulic unit. The fixed hydraulic unit's pistons are forced into the cylinder block as they slide up the inclined wobbler face thus pumping high pressure oil to the variable hydraulic unit and allowing the cylinder block to rotate in the direction opposite to that of overdrive operation. The opposite block rotation allows the input ring gear to turn in the same direction as the carrier shaft rotation and subtracts speed from the speed of the engine gearbox through the differential, thus maintaining constant speed output.

Speed Governor

 This magnetically produced torque then works in conjunction with the centrifugal torque to position the valve stem. The magnetic trim provides a means of introducing electrical trimming signals to the transmission with out any additional parts above those already in the governor.

During normal operation the governor accepts oil under pressure, which is distributed through the flyweight-operated valve to a pressure actuated speed switch, keeping its electrical contacts open. Charge oil pressure through the governor stem valve allows free passage of oil to the wobbler control cylinder.

If the output speed falls below the prescribed limit, spring force becomes greater than centrifugal force on the flyweights, resulting in the displacement of the stem valve to cause drainage of oil from the pressure (speed) switch through the governor housing to the sump. Release of pressure on the speed switch results in completion of the electrical circuit which sends an under-speed signal to the generator control unit.

If the CSD output speed exceeds the prescribed limit, centrifugal force on the flyweights causes the displacement of the governor stem valve whereby charge pressure is cut off from the speed switch and is ported through the governor stem valve to the sump. The drop in pressure in the switch allows the contacts to close, causing the generator circuit breaker to trip. Also the oil from the control cylinder is allowed to drain to the sump. 

Since charge oil is always present on the side of the control cylinder piston which has the smaller cross sectional area, removing pressure from the side with the larger area causes the piston to actuate in such a direction that the variable displacement unit wobbler moves through the vertical position and comes to rest against a stop pin. 

Therefore, if the output speed of the CSD exceeds prescribed limits, the action to the governor also causes the transmission assembly to assume minimum speed ratio. The CSD will continue to be at minimum speed ratio as long as charge pressure is available to keep the control cylinder from resetting to its normal operating position. Normally, sufficient charge pressure will be available during flight to prevent the wobbler control cylinder from resetting. A reset is affected automatically when all input speed to the CSD ceases.

From the above it is evident that the governor performs two protective functions. First, centrifugal force causes the governor to assume its over-speed position and second, as the input speed drops the governor valve spring puts it in the under-speed position. A third protective function is performed by the fail-safe spring should the governor drive mechanism fail. When this happens and the governor stops rotating, the fail-safe spring takes over for the motionless flyweights and moves the valve sleeve against a stop, equivalent to a maximum under-speed position for the governor. This then provides the desired under-speed response to a mechanical failure in the governor gears.

Hydraulic system
The hydraulic system consists of the charge pump, scavenge pump, charge relief valve, all attitude reservoir and air separator, and two filters.
The charge pump is located in the hydraulic circuit between the all-attitude reservoir and the transmission. The charge pump supplies oil to the cylinder blocks, governor, control piston, and the lubricating system.

The scavenge pump is located in the hydraulic circuit between the transmission sump and the external oil cooler. The scavenge pump picks up lube oil and internal leakage and pumps it through the external oil cooler into the all attitude reservoir.

Scavenge oil pumped through the cooler returns to the transmission reservoir via the swirl chamber. This return oil, which is highly aerated, enters the swirl chamber at high velocity through a tangential inlet, causing a swirling action, which creates a vortex within the swirl chamber. Air entrained in the entering oil, having a lower density than the oil, moves to the center of the vortex and escapes to the case. The oil, relieved of its entrained air, moves along the wall of the swirl chamber and out into the reservoir.

As described above, return oil is always de-aerated and ported to the reservoir, regardless of transmission attitude. The inlet suction port is located in approximately the center of volume of the reservoir, and the volume of oil contained within the reservoir is such that, regardless of transmission attitude, the inlet port will always be surrounded by oil. Static pressure in the reservoir is approximately the same as case pressure.

The charge relief valve regulates the operating pressure of the charge oil system. The valve accomplishes this function by metering the discharge of oil from the charge oil system to maintain the preset charge pressure. The charge pump draws oil from the reservoir and delivers a constant volume of oil to the charge relief valve cylinder. The piston in the relief valve cylinder is moved back by oil pressure compressing the spring. 

Oil is bled to the sump as determined by spring pressure against charge pressure acting on the piston. The oil under charge pressure is used for control in the governor and control valve and to replenish the oil used in the multiple piston hydraulic units through the stationary port plate. When charge pressure reaches a determined value, the electrical contacts on the charge pressure switch open, interrupting a circuit to extinguish a CSD oil pressure indicator light on the cockpit instrument panel.

The filters are used to filter the oil in the hydraulic system. The charge filter, functionally located in the charge pressure line is equipped with a bypass around the filter element to ensure a flow of oil in case the element becomes clogged. The other filter is in the scavenge line between the scavenge pump and the reservoir.

Case pressurization is required when the constant speed drive is operated at higher altitudes, or at higher than ambient temperature. This pressurization is maintained by use of a vacuum break check valve. During normal operation, when case pressure is equal to or above atmospheric pressure, the vacuum break check valve remains closed. In this way, atmospheric air that is entrapped at low altitudes is retained within the transmission case until the constant speed drive develops a negative case pressure, or vacuum. During this condition the vacuum break check valve opens and allows atmospheric air to enter the case replenishing the air supply. As vacuum is relieved and case pressure equalises, the vacuum break check valve closes.

A temperature and pressure sensitive valve, or thermal bypass valve, located in the transmission allows oil to bypass the cooler until it reaches a temperature of 100 degree F. at this temperature the valve closes and the oil is directed through the cooler. The valve will also open at a preset pressure, to allow oil to bypass, if the cooler should become damaged or clogged.

Generator Drive Disconnect

The disconnect is an electrically actuated device which decouples the input shaft from the input spline shaft in the event of a transmission malfunction. When the disconnect solenoid is activated by moving the drive disconnect switch on the cockpit panel, a spring loaded pawl moves into contact with threads on the input shaft. The input shaft acts as a screw in a threaded hole, and input rotation causes the input shaft to move away from the input spline shaft, separating the driving dogs on the two shafts. When the driving dogs have been separated, the input spline shaft, which is still being driven by the aircraft engine, spins freely in the transmission without causing transmission rotation. Reset may be accomplished only on the ground following engine shut down, by pulling out on the reset handle until the solenoid nose pin snaps into position.

Generator Drive Cooling
Heat produced by the CSD transmission, is absorbed by the hydraulic fluid in the unit and dissipated in the oil cooler. Hot oil is first piped through a filter then to the cooler. The CSD oil cooler is placed usually into the fan duct, exposed to the fan air stream. Normal temperature rise of the oil through the transmission is about 10-Celsius degree, at continuous full load, with an inlet oil temperature of 80-Celsius degree, at normal input speeds. The oil in the CSD transmission serves as a lubricant, as a coolant and as a hydraulic medium between the pump and motor of the drive.

An oil temperature rise indicating system shows abnormal operation of the CSD, whereby the opportunity is afforded to allow corrective action, such as disconnecting the CSD, before total failure occurs. Temperature bulbs, mounted in the scavenge filter downstream from the scavenge pump and in the integral tank, sense the temperature of the operating oil before and after it is subjected to cooling. The oil IN temperature bulb in the tank and the oil OUT bulb in the scavenge line are connected to form a bridge circuit with a CSD OIL TEMP indicator in the cockpit's instrument panel. The difference in temperatures sensed by the two bulbs is indicated by a meter movement on two ranges of the indicator. 

In the event that the oil cooler becomes clogged or damaged a cooler bypass valve opens at a preset value and directs return oil around the cooler.

References :

Boeing Aircraft Maintenance Manuals

Airbus Aircraft Maintenance Manuals