Cutting the rough with the smooth

Ride-on mowers and lawn tractors take all the hard work out of keeping large areas of grass in trim. For maximum peace of mind, Honda fits all its ride-ons with a special start-up safety feature. This ensures that the engine will only start when the engine is not in gear, the blade is idle, the handbrake is on, and someone is sitting in the driving seat. Once the machine’s engine is started HST (hydrostatic transmission) makes even the largest lawn tractor easy to manoeuvre, as there is no need for a clutch, and a single pedal controls forward and reverse movement during driving.

The primary feature of HST is to affect stepless change of forward/backward speed through a simple control lever while the driving engine turns at a stable speed and at the most appropriate power and torque curve.

In a basic HST system, the vehicle engine drives an adjustable hydraulic pump which is interconnected by pipes, hoses, or a distributor plate, to a fixed-capacity hydraulic motor. There are two types of hydrostatic transmission: closed and open. The version used by Honda is the closed type, where a pump and motor are linked and fluid circulates in a continuous loop.

The engine power is converted into a static high pressure, low speed hydraulic medium the oil which drives the hydraulic motor. The torque conversion in this type of transmission can be adjusted by the delivery capacity of the hydraulic pump.

The pump consists of a control yoke and swash plate which is able to change its tilt angle, and a cylinder which contains seven spring loaded pistons in contact with the swash plate.

When the engine starts, the engine power is transmitted to the pump shaft which in turn rotates the pump cylinder. While the control yoke and swash plate are vertical and centrally positioned around the pump shaft, the pistons do not stroke and oil does not flow between the pump and the motor, so the motor is hydraulically locked. As movement of the control yoke and swash plate begins to introduce an angle of tilt to the plate, the pistons in the cylinder begin reciprocating.

Fluid is drawn into the piston in the oil pressure pump via the suction port and on the opposite side of the swash plate output through the discharge port to the motor. As the angle increases the volume of oil drawn into the piston increases. Thus the amount of fluid discharged varies according to the control yoke angle.

The oil is output to the fixed capacity hydraulic motor, which consists of a bearing set at a fixed angle and the motor cylinder containing seven pistons. Transmitted oil pressure is directed through the suction port to the pistons. The force applied through the pistons onto the angled bearing is converted into sliding motion. As the pistons are part of the motor cylinder, the sliding motion is converted into a force turning the cylinder.


Whenever the engine is running, the pump cylinder turns. Movement of the pedal in a forward direction inclines the swash plate causing oil to flow to the hydraulic motor.

The fluid discharged from the oil pressure pump rotates the oil pressure motor and returns to the oil pressure pump.

Any fluid leaking through clearances in the hydrostatic circuit is compensated for with fluid from a charging pump combined with the main pump flowing into low-pressure intake line of the main pump.

When the gear change pedal is moved to the reverse position, the control yoke turns the swash plate in the opposite direction, which exchanges the suction port and discharge port of the oil pump, resulting in reverse oil flow and reverse motor cylinder rotation.