At present, the vast majority of internal combustion forklift gearboxes adopt hydraulic transmission
forms (Power Shift) - Hydraulic Torque Converter Transmission (Most Common)
This system replaces the mechanical clutch with a fluid coupling and uses hydraulic pressure to shift gears.
A. Torque Converter Assembly (The Fluid Coupling)
1. Impeller or Pump: Connected to the engine flywheel. It converts engine mechanical energy into kinetic
energy of the hydraulic fluid.
2. Turbine: Connected to the transmission input shaft. It is driven by the fluid from the impeller and converts the
fluid energy back into mechanical energy.
3. Stator: Located between the impeller and turbine. It redirects the fluid flow to multiply engine torque,
It is mounted on a One-Way Clutch (Overrunning Clutch).
4. Torque Converter Housing: The sealed unit that contains all the above components.
B. Gearbox and Clutch Assembly (The Power Shift Unit)
- Input Shaft: Transmits power from the turbine to the gear train.
- Output Shaft (Main Shaft): Delivers power from the gearbox to the drive axle.
- Gear Train: A set of constant-mesh gears for different speeds (e.g., Forward 1, Forward 2, Reverse).
- Wet Clutch Packs: The core of power shifting. Each gear range has a dedicated multi-disc clutch pack
engaged by hydraulic pressure.
- Clutch Piston
- Friction Discs & Steel Plates
- Clutch Plates (Friction Material)
- Clutch Drum & Hub
C. Hydraulic Control System (The "Brain")
1. Transmission Pump: Provides pressurized oil for the control system and lubrication. It is usually driven by
the engine or the converter.
2. Trans. Control Valve Assembly (Body): Contains a network of precise directional control valves and pressure regulation valves.
It directs oil flow to the appropriate clutch pack based on the operator's command.
3. Shift Lever: The lever in the cabin (F-N-R) that sends a signal to the control valve body.
4. Inching Valve: A unique and critical valve in forklifts. When the inching pedal is depressed, it bleeds off clutch pressure,
allowing partial engagement for very slow, precise movement.
D. Auxiliary Components
- Oil Filter: Filters metal particles and contaminants from the hydraulic fluid.
- Oil Cooler: Cools the transmission fluid, which gets very hot during operation. It is often located next to the engine radiator.
- Shaft Seals and Gaskets: Prevent fluid leaks at various points.
- Sensors: Monitors output shaft speed for the dashboard or control module.
Usually, maintenance and replacement are carried out around the torque converter, Transmission charging pump, oil cooler,
oil seal and sensors.
