Questions on sprag / airshift t-case

[roger-wilco-66]

I understand that the sprag t-case has to be "prepared" for reverse into forward operation by shifting rev into the first gear and vice versa, hence the linkage between the transmission and the transfer-case.
But I see that with the airshift-type there is no mechanical linkage between the transmission and the t-case. It is not clear to me how this operates: how does the airshift t-case switch from forward /reverse operation?
Another unclear thing to me is what happens with the sprag transmission when I start in the second gear from reverse, omitting the first gear and therefore drive forward without putting the t-case in to forward operation. ''


Thanks for the answers,
Mark

 

[M543A2]

Mark:
Hello! The air shift transfer is similar in operation to transfers in pickup trucks where you lock in the transfer then shift and drive normally, forward or reverse. No matter what direction, the transfer is locked in and driving. Most pickups, especially the older ones, have a transfer shift lever on the floor board you engage to go to "lock". The air shift "(better to call it an air lock) does the same thing, only using a small air cylinder on the transfer case to shift to lock instead of a mechanical linkage. The case must be shifted to "unlock" before any dry road operation just like the pickup truck.
Transfers that use the mechanical linkage connected to the transmission have a two sided over-running, or sprag clutch in them. These sprags can only drive one way, freewheeling the other, so one is needed for forward and one for reverse. The gear ratios are such that the rear wheels have to slip a certain percentage before the sprag is activated to engage all wheel drive. When the rear wheels are not slipping, the gear ratio difference causes the sprag to freewheel and not engage for normal driving. This creates an "automatic" front drive engagement with no need for driver actions. The shift rod that moves with the transmission gear shift selects which sprag is activated. If you watch the shift rod to the transfer as a helper shifts the transmission (motor OFF for safety), you will see that it shifts the transfer each way no matter which forward gear you go into from reverse, or go into reverse from any forward gear. For that reason, you do not need to make any special shifts to "prepare" it. The same is true on the 5 tons with the air cylinder that selects the sprag.
The air lock cases have a gear ratio that is matched for front and rear axles like in the pickup truck example.
I personally wish all of the trucks had the air lock transfers and eliminated the sprags. It must have been kind of a "some are dummy drivers" factor.
Regards Marti

 

[roger-wilco-66]

Thanks, Marti, seems like I had some misconceptions there.
I thought that the air-lock transfers also only really engage when the rear wheels start to slip. This is partly because a friend whitnessed the rear wheels of a deuce slip with engaged air-lock, and it took about half a wheel-turn until the front wheels started to turn. That's, as said before, why we figured that the front-wheels are only engaged when the rear wheels slip. But there must be another cause to this effect then.

 

[emr]

I would think it is as simple as there is alot of drive train between the 2 ends of the vehicle, and its not so tight that it would be instantainious. ...Randy

 

[Recovry4x4]

Great explanation Marti!