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B.  Each CSD consists essentially of two positive displacement axial slipper piston type hydraulic units and a mechanical differential which performs the speed summing function. The hydraulic units are the same in physical size, one unit having a variable hydraulic displacement unit and a variable angle wobbler and the other having a fixed angle wobbler and, therefore, a fixed displacement. The hydraulic units rotate independently and are positioned on opposite sides of a common stationary port plate (Fig. 2).
C.  The variable displacement hydraulic unit runs at a fixed ratio with respect to the transmission input speed. Because the wobbler angle of the variable displacement unit is continuously variable in both directions (from full positive wobbler angle to zero angle to full negative wobbler angle), the displacement of the variable displacement hydraulic unit is continuously variable from zero to full rated displacement in both directions. The fixed displacement hydraulic unit is driven by oil delivered by the variable displacement hydraulic unit. The fixed displacement hydraulic unit will therefore run at any speed from zero to full rated speed in either direction.
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H20846
AC Generator Drive System Schematic  500 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

D.  The working pressure between the two hydraulic units is proportional to the torque being transmitted to the generator. At the lower input speeds, the variable displacement unit acts as a hydraulic pump to supply flow to the fixed unit which is added to the input speed through the differential. At the "straight through" input speed, torque is transmitted directly through the differential unit and the fixed unit is not rotating. The variable displacement unit wobbler will be slightly offset from the zero angle so that some pumping will be accomplished and leakage losses made up. At input speed above "straight through," the variable angle wobbler is set to allow negative displacement of the variable displacement hydraulic unit. The working pressure, in this case, is manipulated to allow the fixed displacement hydraulic unit to be motored by the differential and subtract from the input speed. The variable displacement unit is acting as a motor. The multiple piston hydraulic unit in the mechanical differential type CSD unit handles only a portion of the power transmitted, therefore it is reduced in size. Since power losses are less in mechanical differentials than for multiple piston type hydraulic units, heat rejection is low resulting in high efficiency. The weight of a drained mechanical differential type drive unit or transmission is approximately 73.3 pounds.
3.  CSD Mechanical Differential and Hydraulic Units
A.  The differential is of the folded type with planet gears in the center and input and output ring gears on the outside to complete the assembly. The planet gears rotate about their own axes and also revolve about the centerline of the planet gear carrier. The planet gear carrier is driven by the drive input. The variable displacement hydraulic unit is also driven by the transmission input. The fixed displacement hydraulic unit is hydraulically coupled to the variable displacement unit and is connected to the differential through the input ring gear. The output ring gear of the differential is connected to the transmission output. Constant speed of the output ring gear is maintained by either adding to or subtracting from the speed of the planet gears by controlling the direction of rotation and speed of the input ring gear. The governor and pumps are driven by a constant speed output gear. Figure 3 shows the transmission epicyclic gearing schematically and illustrates the relationship of the differential with the rest of the power train.
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B.  Differential
(1)  
The differential consists of a carrier shaft, two planet gears, and two ring gears - input and output ring gear. The ratio between the ring gears and the carrier shaft is 2:l.

(2)  
At any speed and load condition, a torque load is imposed on the output ring gear by the output gear. Input torque is supplied by the input gear turning the carrier shaft. (See figure 3.) If there were no torque on the input ring gear, it would run freely at whatever speed would allow the output ring gear to stop. Because the carrier shaft to ring gear ratio is 2:l, the speed of the input ring gear at this condition would be double that of the carrier shaft. Since a given output speed is desired, the input ring gear must be constrained.
　
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