The Hydrostatic and Mechanical Travel Drive

Category: Hydraulics Training

The Closed Hydraulic Circuit Combined with a Transfer Case

Input at the hydrostatic gearbox: The vehicle engine delivers torque to the transmission input shaft.
Switching/Disengagement mechanism: allows switching between hydrostatic drive mode (low, finely adjustable speed/high torque) and mechanical drive mode (maximum speed, more efficient).
Mechanical path: a direct gear/shaft connection for the mechanical drive for high travel speeds.
Hydrostatic path: a hydraulic pump flanged to the gearbox and a hydraulic motor (on the output side) form a closed hydraulic circuit - the pump conveys oil, the motor converts oil pressure back into torque.

The engine power drives the variable displacement/feed pump. Through displacement/pressure, the pump generates an oil volume that flows to the hydraulic motor.
The flow rate (and thus the motor speed/drive speed) is infinitely regulated - by adjusting the pump displacement. This achieves very slow travel movements with high torque (e.g., sweepers, sewer cleaners during operation).

The mechanical connection is engaged, or the hydrostatic side is disconnected. Drive power flows via gears/transmission - significantly more efficient for high speeds and lower continuous loss.

Speed control is achieved via pump adjustment (electrically proportionally controlled) - resulting in very sensitive accelerator pedal or joystick response.
Thanks to the connection to the truck CAN interface, the hydrostatic travel drive is easy to integrate into the body/system and parameters can be adjusted.

Advantages: infinitely variable control, very high starting/tractive force at low speed, simple direction reversal, good response behavior during work operations (e.g., sweepers, airport sweepers).
Disadvantages: hydraulic losses (efficiency < mechanical drive at high load/long distance), heat generation (requires oil cooler), greater installation space requirements and weight.