Not real good on sprags, but I think it is trying to break.
BC
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You should never let a sprag equipped deuce roll backwards while it is in a forward gear, or roll forwards in reverse gear. Tends to break things on hard pavement. Other than that, sounds like it needs adjusted, search sprag adjustment and should be some info.
I was describing the issue with a friend who has a sprag and he just wanted to know why it does it. Technically why it does it. We do know not to roll it in reverse of what gear you have it in. I guess I was just trying to impart the importance of why you shouldn't, beside just because "it breaks it".
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The sprag transfer cases have a 6% overdrive on the front propeller shaft. When you get 6% slip on the rear, the sprag will engage and hold till the front shaft returns to overspeed. This makes them VERY effective on pulling the front through turns and maintaining control in slippery conditions. They can also provide reliable 6x6 operation on hard surfaces when slippery conditions are variable, because of this unlocking feature. BUT, the 6% overdrive means that when rolling reverse to the sprag selection that you have a mis-match of speeds and the transfer attempts to lock-up. In the 2-1/2 ton t-case the sprag is the weak link, what you hear is the sprag outer race popping over the "sprags" . (Not to be confused with a roller clutch that does the same thing.) Yes this will either lock-up the sprag or brake it. The air shift case does not have the overdirve and will not experience the lock-up, but can not be left engaged on hard surfaces due to no over-run provision.
Hope this answers the question.
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The diagram shows the number of teeth of each gear, so you can do a little math yourself easily...
For each full turn of the rear output shaft the front only turns .926 of one revolution. That is the same as saying the rears have to be slipping 8% to start getting any traction from the front (going straight ahead).
OR... (while in any forward gear, clutch depressed and letting her roll backwards) the front axle stays engaged and now wants to turn the rears - 8% faster!! A serious windup does occur..... good if the sprag lets loose - with the usual loud bang - before something expensive breaks!
Note, the 1965 TM says the "wind-up" tension creates harder transmission shifting conditions, which may be relieved by coasting in neutral. There is no warning of breakage, which leaves me wondering how many sprag failures are due to adjustment issues vs broken metal?
Sprag pop rolling backwards.
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