Viscous Couplings
By Scott Moll

Scot Stockton writes: Viscous couplings - The only ones I really know about were put behind engines of about 200 HP. There was one used on Summits QuadraDuece that brags about 500HP, but I've not seen a reliability report on that. An AWD torque monster, I like it, although I'm not sure how I'd run it.

Ok, you threw out the line and it's too tempting for me to resist...I'll bite. Hang on. Horsepower is of little or no consequence to an automotive driveline engineer. Maximum available engine torque, and the potential spike torque when a manual transmission are involved, are primary factors to consider, but the most basic parameter is the vehicle's "skid torque" (also known as slip torque). Driveline components are designed based on the torque they will be required to transmit, this torque is limited by the available traction, for example: you have an engine capable of producing 600ft.lbs. of torque and your trans has a first gear ratio of 2.66:1 and the rear end ratio is 4.10:1, this means than any driveline part after the ring and pinion must potentially withstand 6,543 ft.lbs. torque. However, if you slip the tires when only 800 ft.lbs. is reached, the drive line components only need to be able to withstand that much. Now, it should become clear why manufacturers say to use "only the recommended parts" if you don't, you take the risk of exceeding the design limits. Should we all stop modifying our, safety factors are designed in also, to protect for circumstances not forseen by the engineer...when you break parts you know you've reached that limit. What factors effect the skid torque? Well, here's the equation for a 2W drive vehicle:

skid torque= {mu*GAWR*Rad} / {Reff*e}

mu: coffecient of road/tire friction commonly .7 to .8 for concrete/radial
GAWR: Gross Axle Weight Rating, vehicle weight supported by the axle in question
Rad: rolling or static loaded (depending on case) radius of the drive tires
Reff: Effective gear ratio, any ratio's multiplying torque prior to transmitall to the drivleline component in question
e: driveline efficiency .85 is commonly used

Studying this equation we know why the Summit Quadradeuce didn't blow the Borg Warner - GMC Cyclone (YUCK) transfer case all over the tarmac. Even though they put those monster tires on there (and effectively increased the skid torque and consequently driveline stress) the chassis was only around 2500lbs (greatly decreasing the skid torque). The torque capacity of the coupling, however, depends on the diameter and the number of friction elements, something that Borg decided based on the skid torque of the Cyclone. Does that mean that if they get tired of smoking all four tires and decide to make it hook up really well, they might not have a Tcase anymore? Quite possibly. Some of the turbo people can relate to this, I'm sure. They increase boost and torque to insane levels, don't upgrade the driveline components and go out smoking the tires everywhere. Then one day they bolt on some sticky tires and get some great launches...until the trans pukes, or the halfshafts blow, etc. Not to pick on turbo people :), it just makes a good example (my 440 + Nitrous four speed Roadrunner laying waste to 8-3/4"s all the time is another example :P )

Now, viscous couple tech. Just because your Tcase has a viscous couple you don't necessarily have AWD. Tcases equipped with only viscous couples are usually part of an On Demand system whereby torque is transmitted to only two tires (typically the back) until there is tire slip. Once slip occurs, torque is progressively applied until the couple locks and you have maximum torque to the front (limited by the torque split), or no more slippage exists and you return to 2WD drive. This is why some may not like their newer, '95 up, NV249OD Grand Cherokee nearly as much as their freinds older one. You see, the couple requires time to react...time that can be spent fishtailing or sliding into things :( instead of going straight. In some systems, the viscous couple is used as a biasing device for a differential, the older (pre '95) Jeep Grand Cherokees had such a system. The viscous unit is used with a planetary style diff creating a limited slip differential rather than an "open" one. This is/was a true AWD system, torque is constantly being applied to all 4 wheels, even if no tire slip exists, then when slip does occur, the torque is biased to the axle with traction. Another true AWD system would be one with a center (read: in the Tcase) differential only (hence an "open" diff.), the NV242 used in the Jeeps and the AMG Hummer is an example. In this case, though, there is no device to bias the torque when slip occurs, the driver must select "4W lock" which mechanically couples the front and rear drives just like a part time system.

What are the principals of operation of a viscous couple? Well, the couple consists of a housing, drum, steel discs splined to the mainshaft (transmitts torque to the rear), friction discs splined to the housing (transmitts torque to chain and thus the front axle), and a highly viscous silicone fluid, the steel discs are spaced between the the friction discs. Now, when the steel discs rotate relative to the friction dics (tire slip at front or rear), fluid shearing forces develop which resists the relative motion, thus torque transmits to the axle with traction. However, the silicone fluid is a non-Newtonian fluid. This means that the fluid viscosity does not vary lineraly with the shear rate, allowing the steel plates to lock to the friction plates at a certain point called the "hump temperature". The hump temperature for the silicone fluid depends largely on the amount of fluid fill and whether any contamination is present, clean assembly conditions are a must.

Scott Moll
Product Development Engineer
New Process Gear
East Syracuse, NY