1. TH-57 Transmission / Hydraulic Review

2. The transmission is placed approximately at the midpoint of the travel.
The two-stage planetary transmission system provides a gear reduction of
The transmission also drives an accessory drive gear that powers the , the , and the
center-of-gravity
15.22:1
transmission oil pump
hydraulic pump
Nr tach-gen

3. Main Drive Shaft
Failure ?

9. Note
During cold-weather operations, temporary conditions of psi maximum engine oil pressure and/or psi maximum transmission oil pressure are allowable following an engine start.
NATOPS
150 70

10. Flight Profile ?

11. IMMINENT TRANSMISSION FAILURE

12. TRANS OIL PRESS / OIL TEMP CAUTION LIGHTS

14. HYDRAULIC SYSTEM

15. Hydraulic fluid is gravity fed from the reservoir to the pump assembly, which pressurizes the fluid and sends it through the pressure regulator valve. This valve regulates system pressure to approximately psi.
600 +/-50

16. The hydraulic pressure switch, located downstream from the filter, continually monitors system pressure and will close if the pressure falls below psi. A hydraulic pressure light illuminates when the switch closes and goes out when the pressure rises above psi.
300
400

20. Sequence valve
Differential relief valve
Pilot valve

21. Sequence Valve
Hydraulic pressure on the sequence valve pushes down on the poppet valve and spring, thereby opening a return port for fluid to return to the reservoir.
If pressure is lost, the sequence valve closes, trapping fluid in the servo which will continue to dampen main rotor feedback (vibrations).

22. Differential Relief Valve
The function of the differential relief valve is to cause fluid to be routed to the return line if back pressure exceeds system pressure.
Heavy main rotor loads are one possible source of excessive back pressure

23. Pilot Valve
The pilot valve is mechanically connected to the flight controls and receives input from movement of the controls. This causes the pilot valve to move and allow fluid under 600 psi pressure to enter the actuator, which then moves the flight controls. When the flight controls reach the desired position, the pilot valve centers and fluid flow to the actuator is stopped.