Fan Clutch: Description and Operation
DESCRIPTIONFan Clutch:
The viscous drive unit for the engine cooling fan, provides a means of controlling the speed of the fan relative to the running temperature of the engine. The viscous unit is a type of fluid coupling, which drives the fan blades through the medium of a special 'silicone fluid' injected into the unit during manufacture.
OPERATION
The viscous unit consists of two principal components: An inner member 1 which is secured to water pump spindle and is driven by the fan belt. An outer member 2 which has the fan blades attached, houses the working parts and is driven through the medium of the viscous fluid, see illustration below.
The inner and outer members have interlocking annular grooves machined in each, with a small running clearance 3 to allow the silicone fluid to circulate through the valve plate 4. The unit also contains a valve 5 which is controlled by an external bi-metal thermostat 6, see illustration below.
Starting engine from cold
During the time the engine is at rest the silicone fluid drains down, half filling chambers A and B. Thus when the engine is first started sufficient fluid is present in chamber A to provide a positive drive between the members, as is evident by the initial noise of the fan. However within a very short period of time, after starting the engine, the fan speed and noise will decline indicating that the fluid is being centrifuged into chamber B causing the drive to slip, see illustration below.
Viscous unit slipping (Engine at normal running temperature)
1. Inner member (drive in fast)
2. Outer member (drive out slow)
3. Running clearance
4. Valve plate
5. Valve (closed)
6. Bi-metal
7. Fluid seals
8. Ball race
9. Fluid chamber
10. Fluid chamber
If the engine speed is increased the degree of slip will also increase to limit the maximum fan speed.
Hot conditions
When operating in high ambient temperatures or when stationary in traffic, the bi-metal thermostat will operate and open the valve port between chambers A and B. Opening the valve (see illustration below) allows the fluid to circulate between the two members, causing the unit to couple, thus increasing the fan speed and cooling effect.
Viscous unit coupled (Hot running conditions)
Bi-metal expanded, valve (open)