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 Viscous Couplings

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Posts : 1285
Join date : 2010-12-30

PostSubject: Viscous Couplings    Sun Apr 29, 2012 8:53 pm

Viscous Couplings

Viscous couplings are a very simple and effective way to save energy and cool any vehicle, particularly a 4X4 as they are much more durable then electric fans and only work when they are needed, unlike a permanently driven fan, and they save energy and improve fuel consumption.

Viscous couplings work on a very simple principle, heat, to do this they contain a Bi-metallic spring which opens or closes a valve directly in proportion to the heat, this does this by the spring being connected to the valve and the heat expanding the spring forces the valve open or closed. Once the valve opens it allows a silicone based oil into the drive mechanism which is basically a simple torque converter as you would find on any automatic vehicle, and contains two impellors, a driven one and a receiving one. Once oil is allowed into the driven impellor the rotation of this impellor throws the oil in a directional flow onto the receiving impellor which is then turned, and the whole assembly turns the fan which is connected to it.

Problems arising from a defective unit can easily be avoided by a little simple maintenance as there are only three moving components, the Bi-metal spring, the valve, and the driven impellor and any failures of a viscous coupling can be easily repaired by virtually anyone with a little skill. Prevention is better than cure, so the best way to prevent failures is to maintain them as most problems arise from springs clogged with dirt, preventing them from expanding/contracting, and corrosion of the spring/valve which seizes the valve in the open or closed position.

Every 6 months ensure you clean any debris from within the spring, this is basically a large coil like a watch spring, and cleaning it allows it to work, once fully cleaned it needs a little lubrication by spraying with a silicone based thin penetrating oil to soak into the valve. This cleans and lubricates the valve and allows it to turn, thus opening and closing, so spray liberal amounts of silicone based oil into it, once fully lubricated it needs corrosion protection which is as simple as applying a liberal coating of silicone grease to the coil spring/valve assembly. Now you have a fully maintained viscous coupling for a couple of minute’s work cleaning and lubricating.

Fault Diagnosis

Fault diagnosis is simple on such a simple item, but as many people don’t know how they work they don’t know how to diagnose faults.

Checking the valve rotates is the first thing to do as these are prone to seizure from lack of maintenance, using a pair of snipe nosed pliers you simply unhook the spring from the valve and hold it slightly away from it, but don’t let it go, or become detached from the other retained end. Using a very small screwdriver in the slot of the valve, turn this in either direction to see if the valve is operating or seized, if its seized it may be freed with the application of a little silicone penetrating oil, if not you need to release the spring. To release the spring you simply uncoil it, but count the number of full turns it takes as when you refit it to the valve you need to wind it up the same number of turns to correctly tension it so it works correctly. If you can free the valve then fine, your coupling will work and you can reconnect the Bi-metal coil spring.

Testing the Bi-metal spring is easily done with a source of heat, do not use any naked flame as it will be too hot and damage the mechanism, use a hair dryer as this is hot enough to activate the Bi-metal spring. Blow heat from the hair dryer onto the spring, you will see it expand or contract and the valve should begin to turn, if the spring expands or contracts and the valve can be seen turning this is fine, if not, and the valve turns the failure is with the Bi-metal spring which can be replaced.

With the Bi-metal spring and valve assembly working it’s now time to check the fan assembly, this is simply done by spinning the fan blades and checking for resistance, if resistance it felt then the units fine as its full of oil, if no resistance is felt then the most likely problem is an oil leak. You can make a comparison of resistance by checking the resistance of viscous couplings of others vehicles of the same make and model as yours.
Over time many viscous fans lose oil, this is normal, but why scrap a perfectly good unit when it can be refilled with good silicone oil for a couple of pounds and about 15 minutes of your time.

1/ Remove the viscous coupling assembly from the vehicle.
2 Strip any drive pulleys/flanges from the coupling.
3/ Remove the fan blades from the viscous coupling.

You should now be left with the aluminium housing of just the coupling, this needs cleaning thoroughly as from now on cleanliness is essential, with a clean housing you can identify how the two halves are attached, most are with nuts and bolts or Pozi drive screws. Undo these, but beware of those with Pozi-drive screws as these will be extremely tight and you don’t want to chew the heads up on them; then gently prise the two halves apart, these are a machined fit so take your time, and be careful not to damage the aluminium.

With the two halves separated you need to find the deeper of the two halves, three quarters fill this deepest half with thick silicone oil, under no circumstances use any other types of oil as they aren’t designed for it as they can generate lots of heat during their operation. Measure the amount of oil you have put into the unit for future reference by noting the amount of your oil bottles contents and deducting it from what you have left in the bottle.
Place the remaining half of the coupling on top of the filled half and slowly screw them back together, and pull then the screws up slowly and with an even torque, finally tighten them and check for any leaks. Reassemble any dismantled components such as the fan blades and any drive pulleys, and refit to the vehicle, run the vehicle and test the operation of the viscous coupling/fan assembly by running the vehicle up to temperature and checking the coupling engages and drives the fan blades.

Some viscous couplings are riveted together, where they are riveted they need grinding out very carefully and replacing with nuts and bolts of the same diameter.


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