Driving with hydrostatic drive, function

G55.90-3154-09

1 Engine CAN (Controller Area Network (data bus/CAN bus))

2 Vehicle CAN (Controller Area Network (data bus/CAN bus))

3 Hydrostat pump

4 Intermediate transmission

5 Hydrostat motor

A3 FR (FMR) control unit

A4 Hydrostatic drive control unit

A6 MR (PLD) control unit

A7 Base module (electrical center)

A15 GS (EPS) shift control unit

S42 Hydrostatic drive request switch

S51 Hydrostatic drive oil too hot temperature switch

S52 Hydrostatic drive fan switch

Y44 Hydrostatic drive forward solenoid (proportioning) valve

Y45 Hydrostatic drive reverse solenoid (proportioning) valve

A16 GS (EPS) control unit

B1 FR (FMR) foot throttle actuator

B8 Hydrostatic drive manual operating unit

B8r3 Potentiometer (handwheel)

B8s1 Forward/reverse switch

M14 Hydrostatic drive cooling fan

P2 Instrument cluster

P2h19 Hydrostatic drive ON indicator lamp

P2h20 Hydrostatic drive oil temperature warning lamp

S2 Right steering column switch

After the hydrostatic drive (HYF) has been activated, the clutch has been disengaged and the service and parking brake have been released, the driver must give the driving commands.

The engine speed must be set first and then the direction of travel must be preselected.

Driving with the hydrostatic drive begins from the point at which the ground speed is set. The hydrostatic drive ON indicator lamp (P2h19) remains lit.

Setting engine speed using right steering column switch (S2)

If the right steering column switch (S2) is moved upward, the engine speed increases. If it is moved downward, the engine speed decreases.

The FR (FMR) control unit (A3) processes the electric signals from the right steering column switch (S2) and generates a CAN message specifying the engine speed to be set.

The CAN message is made available to the MR (PLD) control unit (A6) via the engine CAN (1) and to the hydrostatic drive control unit (A4) via the vehicle CAN (2).

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Preselecting direction of travel using hydrostatic drive manual operating unit (B8)

The direction of travel is preselected by actuating the forward/reverse switch (B8s1) on the hydrostatic drive manual operating unit (B8).

The hydrostatic drive control unit (A4) reads in the electric signals from the forward/reverse switch (B8s1).

Setting ground speed

The ground speed can be set using the hydrostatic drive manual operating unit (B8) or the FR (FMR) foot throttle actuator (B1). It is always the higher value that is adopted as the ground speed setting.

Turning the potentiometer (hand wheel) (B8r3) on the hydrostatic drive manual operating unit (B8) changes the voltage applied at the potentiometer proportionally to the ground speed specification. This voltage is acquired by the hydrostatic drive control unit (A4).

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Actuating the FR (FMR) foot throttle actuator (B1) actuates its potentiometer.

The voltage applied at the potentiometer is proportional to the ground speed specification and is acquired by the FR (FMR) control unit (A3).

From the voltage pulse, the FR (FMR) control unit (A3) generates a CAN message that is made available to the MR (PLD) control unit (A6) via the engine CAN (1) and to the hydrostatic drive control unit (A4) via the vehicle CAN (2).

Operation of hydrostat pump (3)

From the point at which the hydrostat pump (3) is activated, the drive shaft of the intermediate transmission (4) drives at a certain speed the pump unit integrated in the hydrostat pump (3) to generate the delivery volume flow, and the feed pump also integrated in the hydrostat pump (3) to generate the feed volume flow within the hydrostat pump (3).

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As long as the hydrostatic drive forward solenoid (proportioning) valve (Y44) or the hydrostatic drive reverse solenoid (proportioning) valve (Y45) is not actuated, the hydrostat pump (3) will not generate any delivery volume flow.

Using a control current, the hydrostatic drive control unit (A4) actuates the hydrostatic drive forward solenoid (proportioning) valve (Y44) if forward travel is selected, and the hydrostatic drive reverse solenoid (proportioning) valve (Y45) if reverse travel is selected. This determines the direction of the delivery volume flow.

The strength of the delivery volume flow is proportional to the control current.

The actuated hydrostatic drive forward solenoid (proportioning) valve (Y44) or the hydrostatic drive reverse solenoid (proportioning) valve (Y45) switches to a switch position proportional to the control current.

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The feed volume flow that is fed by the feed pump is changed corresponding to the switch position.

The feed pressure of the inflowing feed volume flow becomes the control pressure of the outflowing control volume flow. The control pressure of the control volume flow is ultimately proportional to the control current of the hydrostatic drive control unit (A4).

 If the feed pressure deviates from the specified value of 17.5 0.2 bar, this has a direct effect on the control pressure and, therefore, on the delivery rate of the hydrostat pump (3).

The control volume flow actuates other components inside the hydrostat pump (3) hydraulically so that the swivel angle of the infinitely variable swash plate of the pump unit of the hydrostat pump (3) is then set.

The swivel angle of the swash plate determines the strength of the delivery volume flow, which

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is ultimately proportional to the strength of the control current of the hydrostatic drive control unit (A4).

Operation of hydrostat motor (5)

The delivery volume flow of the hydrostat pump (3) drives the hydrostat motor (5) inside a closed circuit. The direction of the delivery volume flow determines the rotational direction of the

hydrostat motor (5).

The absorption volume of the hydrostat motor (5) can be steplessly adjusted hydraulically and determines the rotational speed of the output shaft of the hydrostat motor (5). When the hydrostat motor (5) is in its basic position, the maximum absorption volume and, therefore, the lowest output speed are set.

To allow stepless adjustment of the absorption volume, the hydrostat motor (5) is actuated hydraulically by means of a control pressure. The incline angle of the swash plate in the pump unit of the hydrostat motor (5) is adjusted proportionally to the control pressure.

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The stroke of the axial pistons attached to the swash plate changes accordingly, thereby changing the absorption volume flow that can be processed by the hydrostat motor (5).

If the control pressure increases linearly within a predefined range, the absorption volume of the hydrostat motor (5) decreases. As a result, the output speed of the hydrostat motor (5), and therefore ultimately the ground speed of the vehicle, can be adjusted steplessly.

Changing gear

In order to change gear with the hydrostatic drive activated, the potentiometer (hand wheel) (B8r3) on the hydrostatic drive manual operating unit (B8) and the potentiometer of the FR (FMR) foot throttle actuator (B1) must be set to 0 % beforehand, and the service or parking brake must be applied.

The gear can now be preselected again by the driver via the GS (EPS) shift control unit (A15), whereby only gears 1 to 5 are available.

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After the gear has been shifted by the GS (EPS) control unit (A16), the service and parking brake must be released again and, to allow the journey to be continued, the ground speed must be set again.

Automatic activation of hydrostatic drive cooling fan (M14)

If the temperature of the hydraulic fluid rises above 65 5 °C, the hydrostatic drive fan switch (S52) is actuated.

This switches the hydrostatic drive cooling fan control relay (K32) and the hydrostatic drive cooling fan (M14) is supplied with power. The hydrostatic drive cooling fan control relay is supplied with power from the base module (electrical center) (A7).

The activated hydrostatic drive cooling fan (M14) supports cooling of the hydraulic fluid until the temperature of the hydraulic fluid drops back below the threshold temperature.

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Indicating high hydraulic fluid temperature

If the temperature of the hydraulic fluid rises above 90 5 °C, the hydrostatic drive oil too hot temperature switch (S51) is actuated and switches the electric current through to the hydrostatic drive oil temperature warning lamp (P2h20) in the instrument cluster (P2) and to the hydrostatic drive control unit (A4).

The hydrostatic drive oil temperature warning lamp (P2h20) lights up.