nf6x
Feral Engineer
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One of the differences between the sprag clutch transfer case and the air-shift transfer case is different front wheel gearing. The air shift transfer case doesn't have a sprag assembly.Ferroequinologist said:So one of the differences between the air operated sprang and the manual sprang is different gearing?
There seems to be a lot of confusion around about how the sprag transfer works. It's sprag (referring to the type of clutch used in the case), not sprang, Sprague, etc.
The sprag clutch is an overrunning clutch. Think of the one-way action of a ratchet wrench, but with smooth operation instead of clicking. The front wheels are driven through one of these one-way clutches. The front drive is geared a little bit differently than the rear, so that most of the time that clutch is slipping and no power gets sent to the front.
When the rears start slipping but the front wheels still have traction, then the front wheels will be turning more slowly than the rears. Then the sprag clutch stops slipping, and starts sending power to the front wheels.
The different front wheel gearing is the key to making it work. If the front wheels were not geared differently, then the sprag wouldn't be slipping when the front and rear wheels were turning at the same speeds, so the front wheel drive would be engaged all the time. That would cause a lot more tire scrub and make the truck harder to steer.
That linkage between the transmission and transfer does not engage and disengage the front wheel drive. It changes the direction of the sprag clutch, much like turning the knob/lever on the back of a ratchet wrench.
The air shift case doesn't need to have different front wheel gearing, because it does not drive the front wheels through a sprag clutch. It just manually engages and disengages the front drive with a small air cylinder that's controlled by a valve on the dash.
I hope this makes things clearer.
What's wrong with the sprag transfer, anyway? The multifuel 5-ton trucks all have them, with no air-shift option.