This is a drastic oversimplification, but try and visualise the inside of the transfer case as having 2 rotating plates facing each other separated by s tiny gap.
The front plate (we will call it the driven plate)is connected through the front driveshaft, to the front differential and front wheels.
The rear plate is connected to the transmission and rear wheels via the transmission output shaft (we will call the rear plate the drive plate).
Visualize the drive plate (rear plate) as bring on a splined shaft which allows it to slide fore/aft, just enough that it can can be forced forward along the splined shaft, and into contact with the front shaft.
For this explanation, it's necessary to visualize relative clock positions of the two plates (the plates clock positions relative to each other). Let's say that when driving in a strait line, on pavement, the rear wheels are turning and pushing the truck along the highway at x speed. The roadway is causing the front wheels to rotate at the same speed as the rear wheels, as such, the two plates are rotating at the same speed relative to each other. They are maintaining the same relative clock positions.
The ballramp/torsion spring assembly is basically a big, heavy duty ratchet. Like a recluse motorcycle clutch. Imagine it as being similar to a starter Bendix as well. Picture the drive plates shaft as having helical splines, Wich the drive plate rides on. Like a nut on a screw. When the shaft the drive plate rides on is turned, the torque applied to the shaft forces the drive plate tighter against the driven plate. The higher the force applied to the shaft, the higher the force the drive plate applies to the driven plate.
If the truck begins to turn, the front wheels begin to track around a larger radius than the rear wheels. This takes the pressure off the drive plate and allows the driven plate to turn faster than the drive plate. This could be viewed as "slip", but not the kind of slip that causes wear. It's more like the slip that a brake pad undergoes, when the brakes are-not in use.
When the rear wheels start to lose traction, this allows the drive plate and shaft to advance it's clock position relative to the driven plate, which, because of the drive plates helical splines, forces the drive plate against the driven plate and rotates the driven plate as well.
The torsion spring is the part that holds the drive plate and the driven plate apart.
The electromagnetic clutch , when energized, overcomes the strength of the torsion spring and forces the plates together.
So, in 2wd, the electromagnetic clutch is never activated and the drive/driven plates are never in contact.
In 4 auto, when the computer detects wheel slip, it energizes the electromagnetic clutch, it overcomes the force of the torsion spring forcing the plates together. When the plates are in contact with each other, the ball ramp (helical splined shaft) forces the plates together using whatever amount of torque the engine is producing. If you start to go around a turn, the driven shafts clock position advances relative to the drive shaft and the ball ramp releases, allowing the torsion spring to separate the plates. If your rear wheels slip while in the turn, the drive plates clock position advances relative to the driven plate, and the ball ramp mashes the plates back together. About 90ish degrees of advance in the drive plates clock position, relative to the driven plate, is required to force the plates together. When the computer senses no more slip, it deactivates the electromagnetic clutch, allowing the torsion spring to separate the plates.
It is important to note however, that the engines torque transmitted through the helically splined shaft and drive plate, greatly exceed the strength of the torsion spring. If under engine load, computer deenergization of the electromagnetic clutch will not cause the drive plate to release the driven plate.
In 4 lock, the electromagnetic clutch remains energized. The clutch plates remain in contact. Any time the driven plates clock position advances relative to the drive plate, such as while turning, the ball ramp (like a ratchet) allows the driven plate to advance it's clock position relative to the drive plate. Any time the drive plates shaft (helical splined shaft) attempts to advance it's clock position relative to the driven plate, it forces the plates tightly together.
In 4lock, while turning, the driven plate is advancing it's clock position relative to the drive plate, so technically, it is in fact slipping. But, it's slipping only a few degrees and only overcoming the applied force of the electromagnet clutch, which at 12volts10amps, is very small. The only force capable of any significant effect is the engine torque, via the helically splined shaft, forcing the plates together.
Now, all that said, it is a simplification for the purpose of easing explanation. There is no helically splined shaft. A big heavy duty assembly of balls and ramps takes the role of the helically splined shaft.
The 2 clutch plates are actually a couple dozen plates mounted in a big splined basket.
The clutch basket assembly is rated for about 1700 ft lbs of CONTINUOUS use, and for intermittent loads of much much more. Any slippage occurring as the driven plates advance clock position relative to the drive plates do-not occur under load.
Any time the drive plates attempt to advance clock position relative to the driven plates, there is significant load. The engines torque force the plates together. The more throttle you add, the more the torque, the more the force driving the plates together.
If your in 4 lock or 4 auto, and the transfer case clutch pack is allowing the rears to turn but not the front, then there is something severely wrong.
