(3)  
If the input ring gear is constrained to zero speed, the output ring gear will run at double the carrier shaft speed. If the input ring gear is forced to rotate in a direction opposite to that of the carrier shaft, the output ring gear will run at a speed more than double that of the carrier shaft. If the input ring gear is allowed to rotate in the same direction as the carrier shaft, the output ring gear will run at a speed less than twice that of the carrier shaft. Thus, the differential is a "speed summer" or "adding" device which is controlled through the input ring gear to add to or subtract from the speed of the engine gearbox to achieve the desired output.

C.  Variable Hydraulic Unit
(1)  
The variable hydraulic unit consists of a cylinder block, reciprocating pistons, a variable angle wobbler, and a control piston. The variable unit is connected to the aircraft engine by direct gearing; consequently, the speed of the cylinder block is always proportional to the input speed and the direction of rotation is always in the same direction.

(2)  
When the transmission is operating in "overdrive," the variable hydraulic unit will function as a hydraulic pump. (See figure 3, sheet 1.) 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.

(3)  
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 or received by the variable unit. At this time the transmission is operating in "straight through drive."

(4)  
When the transmission is operating in "underdrive," the variable hydraulic unit will function as a motor. To enable the variable unit to operate as a motor (receive oil from the pumping unit), the governor ports oil away from the control cylinder causing the wobbler to be positioned so the volume for accommodating oil in the piston bores on the high pressure side is increased; consequently, oil flows from the fixed hydraulic unit to the variable unit.

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D.  Fixed Hydraulic Unit
(1)  
The fixed hydraulic unit consists of a cylinder block, reciprocating pistons and a fixed angle wobbler. The direction of rotation and speed of the fixed hydraulic unit is determined by the volume of oil pumped or received by the variable hydraulic unit. This volume of oil is determined by the angular position of the variable wobbler and the speed of the variable block. Refer to paragraph C, Variable Hydraulic Unit.

(2)  
When the transmission is operating in "overdrive," the fixed hydraulic unit 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 gearbox through the differential, thus maintaining constant output speed. Refer to paragraph B, Differential.

(3)  
As the input speed increases and the need to add speed to the output decreases, the variable hydraulic unit pumps less oil to the fixed hydraulic unit until the cylinder block stops rotating. At this time the transmission is operating in straight through drive.

(4)  
When the transmission is operating in "underdrive," 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 output speed.

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H20828 H20792
CSD Gear Differential and Hydraulic Unit Schematic  500 
　
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