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It all has to do with the viscosity of the fluid in the alloy case. When hot, it be come runny. When cold it gets sticky. So, when the engine is hot and needs cooling, the fan is hot and the oil is runny. So the clutches engage and cause friction. When cold the fluid is stickt and thick and the clutches dont grab.
The same also works with a bimetal spring, like on a thermostat. I am quite sure the clutch is viscous...
It has an inner (bolted to the fan pulley) and an outer, bolted to the fan blades. Between them is a viscous fluid that when cold allows the fan outer to slip rather than be solidly connected to the inner. So when cold the fan outer does NOT turn as fast as the pulley is rotating.
As the fluid gets hot, as a result of heat coming though the radiator on the air flow, it gets more and more solid, so when hot enough, the fan outer turns pretty much as fast as the pulley driven inner, thus providing decent airflow.
The system is similar to viscous coupled differentials, which lock up when the rotational difference between one output shaft and the other gets sufficiently great. The diff system was invented by Harry Ferguson of tractopr fame, and first used on the Jensen Interceptor FF which was the first road car to have anti-skid brakes and incorporated a four wheel drive system using viscous diffs. A similar diff was used on Range Rovers for the central diff, and for all I know still is. It obviated the need tom lock the centre diff (as you had to do on RRs for many years) as it would lock automatically.
The idea of the system on radiator fans was to save fuel and noise when the engine was cold and did not need the fan to run, and to automatically "turn on" the fan when the engine needed extra cooling. All a bit hit and miss, but widely used on all cars for many years, from the humble Vauxhall Viva upwards.
The electric fan has overtaken this system on just about all newer cars, as it can be more efficiently switched on and off by a temp sensor, and unlike any engine driven fan, can deliver full cooling at rest in traffic, when the engine driven fan (like the engine) would be doing its lowest rpm - just when you needed it most!
Testing is not easy, but it should be easy-ish to turn the inner while holding the outer still when the thing is cold. Leave it in the sun for a bit to get hot (or put a hot water bottle on it wrapped in a blanket for 15 minutes) and it should be harder.
Greg
Last edited by Greg in France; Jul 6, 2016 at 11:57 AM.
It has an inner (bolted to the fan pulley) and an outer, bolted to the fan blades. Between them is a viscous fluid that when cold allows the fan outer to slip rather than be solidly connected to the inner. So when cold the fan outer does NOT turn as fast as the pulley is rotating.
As the fluid gets hot, as a result of heat coming though the radiator on the air flow, it gets more and more solid, so when hot enough, the fan outer turns pretty much as fast as the pulley driven inner, thus providing decent airflow.
The system is similar to viscous coupled differentials, which lock up when the rotational difference between one output shaft and the other gets sufficiently great. The diff system was invented by Harry Ferguson of tractopr fame, and first used on the Jensen Interceptor FF which was the first road car to have anti-skid brakes and incorporated a four wheel drive system using viscous diffs. A similar diff was used on Range Rovers for the central diff, and for all I know still is. It obviated the need tom lock the centre diff (as you had to do on RRs for many years) as it would lock automatically.
The idea of the system on radiator fans was to save fuel and noise when the engine was cold and did not need the fan to run, and to automatically "turn on" the fan when the engine needed extra cooling. All a bit hit and miss, but widely used on all cars for many years, from the humble Vauxhall Viva upwards.
The electric fan has overtaken this system on just about all newer cars, as it can be more efficiently switched on and off by a temp sensor, and unlike any engine driven fan, can deliver full cooling at rest in traffic, when the engine driven fan (like the engine) would be doing its lowest rpm - just when you needed it most!
Testing is not easy, but it should be easy-ish to turn the inner while holding the outer still when the thing is cold. Leave it in the sun for a bit to get hot (or put a hot water bottle on it wrapped in a blanket for 15 minutes) and it should be harder.
Greg
Hi Greg
Thanks for that very Clear and Detailed explanation, as I was thinking the Fluid in the Fan Clutch got Thinner as it got Hotter, so now I know.
Could I put it in a container of Hot Water, or would the Water get inside it, and mess the whole thing up.
Just so I can Test it, to see if it works as it should.
I'll just add that, with a typical thermal controlled fan clutch, that little bi-metal spring in the front of the clutch reacts to temperature and actually opens/closes a valve inside the clutch which allows the viscous fluid to enter the area between the two segments to form the fluid coupling.
Older Jags (and others) used a non-thermal clutch. With those, the clutch was engaged until xxx-rpm ...at which point they allowed slippage of the fan. On Jags these are easily identified by the single bolt thru the center of the clutch.
I use the spin test for checking. Get the engine as hot...a bit hotter than normal if possible. Turn the engine off. Now spin the fan by hand. You should feel some resistance and the fan should spin no more than one turn.
It all has to do with the viscosity of the fluid in the alloy case. When hot, it be come runny. When cold it gets sticky. So, when the engine is hot and needs cooling, the fan is hot and the oil is runny. So the clutches engage and cause friction. When cold the fluid is stickt and thick and the clutches dont grab.
The same also works with a bimetal spring, like on a thermostat. I am quite sure the clutch is viscous...
Hi Daim
According to Greg the Fluid gets 'Thicker' as its gets 'Hotter' which seems to make a lot more sense to me.
Just wondering if you unintentionally got it the wrong way round.
I'll just add that, with a typical thermal controlled fan clutch, that little bi-metal spring in the front of the clutch reacts to temperature and actually opens/closes a valve inside the clutch which allows the viscous fluid to enter the area between the two segments to form the fluid coupling.
Older Jags (and others) used a non-thermal clutch. With those, the clutch was engaged until xxx-rpm ...at which point they allowed slippage of the fan. On Jags these are easily identified by the single bolt thru the center of the clutch.
I use the spin test for checking. Get the engine as hot...a bit hotter than normal if possible. Turn the engine off. Now spin the fan by hand. You should feel some resistance and the fan should spin more than one turn.
Cheers
DD
Hi Doug Cheers
When you said the Fan should spin more than one turn, just wondering if you meant to say 'no more than one turn'
OB, Other posters
I may well be wrong in as much as maybe the fluid engages owing to a valve being opened by the bimetal strip as the thing gets hotter, rather than the fluid actually getting thicker. But either way, as that great video shows, as the thing warms up, the fan coupling somehow engages the fan outer hub. So heating it will still show whether the coupling is working properly or not, as it should be harder to turn the outer relative to the inner when hot. The things do not have a great record for longevity though, that is for sure.
Greg
How does the 'Fan Clutch' work on an XJS V12
