SAFARI

4 DLT ENGINE

VISCOUS FAN DRIVE

Operation of the viscous fan drive.

The viscous drive unit for the engine cooling fan

provides a means of controlling the speed of the fan

relative to the temperature of the engine.The viscous

fan unit, see Figure 6. is a type of fluid coupling, which

drives the fan blades by means of 'silicon fluid'.

Operation :

There are two main components of the viscous

fan drive : An input (drive) member (1) consisting

of a threaded shaft passing through a bearing

into the clutch plate and secured to the water

pump, see Figure 7. An output (driven) member

(2) comprises the main body to which the fan

attaches, with the temperature sensing

Input (drive) member.

Output (driven) member.

Running clearance.

Pump plate

Valve (closed)

Sensing mechanism

(bi-metal coil)

Fluid seal

Bearing, input member

Fluid chamber

10. Fluid reservoir

Fig.6

Fig. 7

Viscous unit disengaged (engine at

normal operation temperature)

mechanism (bi-metal coil) (6) and pump plates

(4).

The fan drive has to be engaged only periodically,

between 5% and 10%, during normal operating

conditions because the rest of the time the

vehicle cools itself by ram air cooling.

To engage and disengage the fan drive the bi-

metal coil senses air temperature behind the

radiator.When a pre-determined temperature is

reached, the coil opens a valve (5) which allows

fluid to enter the drive area and due to centrifugal

force circulates to the annular drive area. There

are two sets of annular grooves (3), one in the

drive clutch and the other in the drive body, a

specific clearance being provided between the

two sets of grooves.When this clearance is filled

with viscous fluid a shearing action, caused

by the speed differential between the two drive

components, transmits torque to the fan.The fluid

is thrown to the outside of the unit by centrifugal

force from where it is then re-circulated to the

reservoir (10) via the pump plate 4 adjacent to

the drive member.

If the engine speed is increased the amount of

slip will also increase to limit the maximum fan

speed.

When the air temperature from the radiator drops

sufficiently, the bi-metal coil closes the valve

and prevents fluid entering the drive area, See

Fig. 8 The fluid that is in the drive area will

gradually pump out into the reservoir (10) and

the fan will return to an idle condition.

Fig.8

Viscous unit engaged (hot running temperature)

Bi-metal coil (6) expanded, valve (5) open.

Input (drive) member

Output (driven) member

Sensing mechanism (bi-metal coil)