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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
CONSTANT
DRIVE SPEED RING QAD
GEARBOX ACCESSORY LINE OIL FILTER
Constant Speed Drive System Component Location
Figure 1
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
2. Constant Speed Drive Unit (CSD) (Fig. 2)
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A. Each engine-driven generator is supported by and driven through an in-line, variable ratio drive unit which transmits the torque to drive the generator at a constant speed from the variable speed accessory drive pad on the airplane engine. The drive unit or constant speed drive (CSD) is attached to the engine with a quick-attach-detach (QAD) device consisting of three interlocking threaded rings. The CSD output end attaches to the ac generator by means of 12 studs. The generator mounting nuts only need to be loosened to remove the generator. The accessory mounted ring of the QAD device is attached to the drive unit with screws. The engine-mounted ring is fastened to the accessory drive pad on the engine with screws. Gaskets, not shown on Fig. 1, make the input spline cavity oil tight.
B. Each CSD consists 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 size, one unit having a variable hydraulic displacement unit and a variable angle wobbler and the other having a fixed angle wobbler and a fixed displacement. The hydraulic units rotate independently and are positioned on opposite sides of a common stationary port plate.
C. The variable displacement hydraulic unit runs at a fixed ratio with respect to the transmission input speed. Bacause the wobbler angle of the variable displacement unit is continuously variable in both directions (from full positive 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 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.
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 loses 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 loss is 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.
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