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Hydraulic Theory

Hydraulic Theory. Automatic Transmission Hydraulic Systems. Transmission Hydraulics. The hydraulic system in an automatic transmission: Provides the force to apply the clutches and bands. Provides a means to control these forces. Maintains a fluid flow through the torque converter.

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Hydraulic Theory

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  1. Hydraulic Theory Automatic Transmission Hydraulic Systems

  2. Transmission Hydraulics • The hydraulic system in an automatic transmission: • Provides the force to apply the clutches and bands. • Provides a means to control these forces. • Maintains a fluid flow through the torque converter. • Maintains a fluid flow though the moving parts of the gear train for lubrication and cooling.

  3. Fluids Hydraulics is based on the principle the fluids can be moved through complex passages and take the shape of their container. But they cannot be compressed so they can be used to transfer very high pressures.

  4. Fluid Pressure 50 Lbs If force is exerted on fluid that is trapped in a system, fluid pressure will be created. The amount of pressure is a factor of the area/ diameter of the piston and the amount of force on it. 100 Lbs 150 Lbs Animation: Fluid Pressure

  5. Fluid Pressure, 2 P = F ÷ A A =  D Fluid pressure equals Force divided by Piston Area. Piston Area equals  (3.1416) times Piston Diameter. Area Diameter

  6. Pressure Equalizes Fluids transmit pressure equally throughout a system. There will be an equal pressure at each point in the system.

  7. Fluid Force The amount of output force is determined by the area of the output piston and the amount of fluid pressure. 100 psi pushing on 2 square inches generates a force of 200 pounds. Animation: Pressure at Work

  8. Fluid Force, 2 Fluid force equals Pressure times Piston Area. Piston Area equals  (3.1416) times Piston Diameter. F = P X A A =  D Area Diameter

  9. Simple Hydraulic System This simple system contains a sump to store the fluid, a filter to clean the fluid, a pump to move the fluid and create pressure, a pressure regulator valve to control fluid pressure, a control valve to control fluid flow, and an actuator to do the work. Animation: Simple Hydraulic Shifts

  10. Pressure Regulator Operation When pump pressure gets stronger than the spring, the pressure regulator valve (green) will move upward. Valve movement will open the passage to the torque converter. Further movement from higher pressure will move the valve to allow excess pressure to return to the pump inlet. Animation: Pressure Regulator Valve

  11. Fluid Flow Control Automatic transmissions use spool valves to control the fluid flows. This manual valve is positioned by the shift lever. It determines the operating sequence that is desired by the driver. Animation: Manual Valve: Park

  12. Spool Valves Spool Valves resemble a spool for thread. The valve lands fit closely inside of the valve bore. When the valve slides down a bore, the lands cover or uncover ports in the bore. The valve has grooves for passages so fluid can flow through the valve. Animation: Valve Operation

  13. Shift Valve This shift valve is positioned by a spring that pushes it toward the right, throttle pressure (also pushing toward the right), and governor pressure (pushing toward the left. When the valve is to the right, line pressure flows through to apply the band. When the valve is to the left, band apply pressure escapes to exhaust.

  14. Servos Drum Band Servo Apply pressure moves the servo piston to squeeze the band against the drum to bring it to a stop. This servo can be released fluid pressure in the Release Pressure passage or by a spring (green) when apply pressure is released. Strut Adjustment Screw Release Apply Pressure Pressure

  15. Orifices An orifice is a small opening for the fluid to pass through. It is a restriction, and it will cause a pressure drop as fluid flows through. Orifices are used to slow down clutch or band application. 60 psi 20 psi Orifice Animation: Orifice

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