Transfer Box & Air Shift x Sprag Operation

[gatdan]

Hi, I'm Daniel, I live in Brazil.
This is my first post on this forum.

I bought a truck manufactured here in Brazil by Terex, here its rating is 5 ton.
I bought it without the drive shaft and without the front wheel drive mechanism.
I'm doing a lot of research here on the forum to understand which parts I need to buy as well as how the traction box works.

Looking at the photos and code 5876w205 is it possible to confirm that it is an American Rockwell T-138 transfer box?

I took the opportunity to attach a photo of the differential axle (very similar to a 5 ton American Military). but this one seems wider at the ends

I saw many posts from members discussing the conversion from Sprag to Air Shift system, many highlighting the consequences of Sprag misuse (truck rolls backwards with first gear in gear and vice versa)

The technical terms are not helping me to understand a more precise difference between "sprag" vs. Air Shift operation.

1 - Does this truck have a Sprag and is it activated by air shift?

2 - Or in this system, there is only air shift (when the piston is activated by air, it moves the fork and there is a permanent transfer (traction of the 3 axes at the same time for forward and reverse?)



Sorry for the many doubts, only 85 units of these trucks were manufactured. Where I bought them all were without the drive shaft .
I was a little afraid of this system having a chronic problem


Thanks
Greetings from Brazil

 

[gatdan]

More images attached
I marked a red circle on the front wheel drive (would it be a piston that goes inside the pneumatic cylinder?)


Thank you so much
Greetings from Brazil

 

[G744]

Yep, that transfer is really similar to those in the G744-series trucks.

Where you are confused, is air is used to pick the sprag rotation direction via a valve on the transmission. It decides which direction you are going, forward or reverse.

The front axle is designed to 'come online' when the rears are slipping at about 6% in relation to the fronts. They will stay engaged until the ratio is the same and then release.When working correctly, you'll never know when it happens, the truck just goes.

Front axle engagement is automatic with the sprag coupling, and you have no positive control over lockup.

As to how it works, the coupling is essentially a bunch of little 'steel pillows' inside two concentric rings. When turned one way, they 'dig' into the rings and effect drive. Turned the other way, they just slip. That is why there two sprag units in there, one for each direction, activated via the air piston as to which way is needed.

A true 'air shift' transfer has no sprag, and uses a larger air piston to lock in the front drive directly. 900 series 5-tons use this topology.

As to the air cylinder question, you already have the piston. You're missing the cylinder housing and end plate.

By the way, your image of the locking device is your light switch, nothing to do with the transfer. That is another mystery for later.

Dennis

 

[gatdan]

Yep, that transfer is really similar to those in the G744-series trucks.

Where you are confused, is air is used to pick the sprag rotation direction via a valve on the transmission. It decides which direction you are going, forward or reverse.

The front axle is designed to 'come online' when the rears are slipping at about 6% in relation to the fronts. They will stay engaged until the ratio is the same and then release.When working correctly, you'll never know when it happens, the truck just goes.

Front axle engagement is automatic with the sprag coupling, and you have no positive control over lockup.

As to how it works, the coupling is essentially a bunch of little 'steel pillows' inside two concentric rings. When turned one way, they 'dig' into the rings and effect drive. Turned the other way, they just slip. That is why there two sprag units in there, one for each direction, activated via the air piston as to which way is needed.

A true 'air shift' transfer has no sprag, and uses a larger air piston to lock in the front drive directly. 900 series 5-tons use this topology.

As to the air cylinder question, you already have the piston. You're missing the cylinder housing and end plate.

By the way, your image of the locking device is your light switch, nothing to do with the transfer. That is another mystery for later.

Dennis

Hi Dennis, thank you so much for your help.

Things are starting to get clear for me.

You're absolutely right, that red button is the headlight blocking device, nothing to do with the transfer. I looked under the dashboard and didn't find anything pneumatic related to the transfer box.
Underneath the truck I found what seems to me to be just a 5/2-way valve, is that right? It's on behind the gearbox (ZF gearbox). Does that mean I don't have an on/off button to activate the sprag/front-wheel drive system? (the 6x6 sprag coupling will have to work permanently)

As you said, the pneumatic piston is used for the gearbox to "communicate" the transfer box to the direction I'm going. Are there only 2 positions (forward and reverse)?

Or is there front, reverse and neutral? And if there is neutral, when is this position activated if I don't have a front sprag/drive shaft on/off control?

Does the pneumatic piston drive work with an input and output (to vary the positions)? (I assume so, as 2 hoses come out of my 5/2-way valve) if so, would it be on the end plate and one on top of the sprag carcass? or is that hole on top of the sprag just a transfercase breather?

Sorry so many questions, this system is totally new to me. I'm studying everything I need to import new parts from the USA to compare everything together

Thank you very much

 

[Superthermal]

To expand this thread past basic engagement and to ensure that I don't blow up my M813's T138 case when I am in the middle of nowhere in a tight spot I have the following questions on operation of the T-138 in wheeling situations
1. Driving forward, rear tandems break free, Front sprag engages and the front assist the loss of traction all wheels are now spinning and eventually the rig no longer is spinning out. If I never lift my foot off the throttle when will the front disconnect from assisting the rears?
A> As soon as I lift my foot off the throttle?(Push in the clutch to say upshift? Let up on the throttle?)
B> As soon as I put the rig into reverse?
C>As soon as the front and rear are moving the same speed? This would seem off meaning it would disconnect over and over and over if in slippery mud/snow.

2. Driving up a hill forward motion stops as the rears break loose, the front sprag engages the front all 6x6 spinning. Now the truck moves forward or digs some holes for all 6 tires. In normal wheeling (not in my M813) I would now let off the throttle and push in the clutch and allow the rig to roll backward in my holes to the top of the holes I just dug and then throttle and let off the clutch and rock the rig forward and up the hill. Rocking in this way what will happen to my front axle engagement?
A>As soon as I push in the clutch the front will disengage causing me a loss of 6x6 so when I dump the clutch to power back into the holes the rears will need to spin again before I get any front end assistance?
B>Transfercase blows to pieces as you cannot go the opposite direction, in this case rolling backwards any amount with transfer case sprag engaged.

Very interested in the answers to these questions.
As far as a sprag replacement kit where it is replaced with a manual lever. I would very much like one to eliminate this potential headache. If anyone has one let me know how it worked for you and what you like or don't like about it.

 

[G744]

Good questions, possible I can relate some good answers.

The sprag system will allow up to a bit over 5% rotational slip before it locks in the front axle. 6% is the target for action.

It really doesn't care what you are doing as far as driving goes, all it looks for is the loss of rear traction. If you have to shift down, do so.

What will make it unhappy is going backwards in forward gears on hard surfaces. (Or vice versa).

In off-road situations, it seldom causes problems, as you are already sliding around.

Dennis

 

[Superthermal]

I am particularly interested in what causes the sprag to disengage. Is it simply when power is removed by an up-shift or downshift or simply letting off the gas? Or is it mechanical ratio when the front is able to keep all 6x6 moving at the same speed allowing the slightly higher geared front moving at a higher ratio than the back eventually unbinds the two regardless of letting off, it just "Turns OFF" with regular driving forward "when the time is right"...LOL.

If anyone has a bit of practice with this and can shed some light on the questions in my last post above that would be cool of you to shed some light on this for us.
I would also like to know if I was to be on a hill still in just 4x6 going forward and I push in the clutch and roll backwards say 30 feet or so is that going to mess anything up.

 

[G744]

The sprag works kinda like a "Chinese finger puzzle": The harder you pull, the tighter it gets.

And yes, it slips when there is no need for it, as when the rears and front are at the same speed.

Remember, there are two sets of them in there, one for forward motion, the other for reverse.

The normal state is forward, until it gets a signal from the transmissions air valve to select the reverse set. It is sort of a type of ratchet, but without real teeth.

It uses a set of steel bits inside two concentric rings that grab one way but not the other any time there is a rotation difference between the rings. All of this in a bath of gear oil.

The TM for that transfer case has a good blow-up diagram of how it looks inside.

Coasting down a hill backwards with the last use in forward will lock it up and really not be good. Better practice to stop and put it into reverse and back down.

DG

 

[MyothersanM1]

Or is there front, reverse and neutral? And if there is neutral, when is this position activated if I don't have a front sprag/drive shaft on/off control?

Yes. On the sprag (de-clutching) cylinder air pressure delivery is as follows...

-rear port = forward

-front port = reverse

The sprag cylinder will default to a neutral position if air pressure is completely removed from both forward and reverse sides. I used a Bendix TW-1 air control valve to do this. When the air is cut in the sprag will function as normal. It will still be a limited slip type operation and not full time.
I keep it cut out most of the time. That way I can eliminate any unnecessary wind up in the wrong direction.

 

[Superthermal]

Yes. On the sprag (de-clutching) cylinder air pressure delivery is as follows...

-rear port = forward

-front port = reverse

The sprag cylinder will default to a neutral position if air pressure is completely removed from both forward and reverse sides. I used a Bendix TW-1 air control valve to do this. When the air is cut in the sprag will function as normal. It will still be a limited slip type operation and not full time.
I keep it cut out most of the time. That way I can eliminate any unnecessary wind up in the wrong direction.

View attachment 873051View attachment 873052

OK, With the sprag, in a neutral position, I can see there is the same spring (parts #21 below) to push the piston (part #26 below) to a "middle" or "neutral" position. Can you enlighten me to understand "It will still be a limited slip type operation" means. I thought I had read that it was a bad thing to leave this in "neutral" and it would wear out or something like that if left that way for long periods of time. I think that thread was discussing leaving the front drive shaft OFF when not needed or something like that.
I am curious to know what the driving experience would be for daily driving in "neutral". I am assuming that leaving the air OFF for all on-road driving would be positive as it would remove any "daily driver error"that could blow the sprag to bits, and only supplying air when going off road where the sprag engagement is needed.
Did you notice better fuel milage when in "neutral"? Like 50 to 60 MPG? ... LOL? Sorry, added some extra zero's there .

I do have a question in addition to this if you don't mind. When I was out wheeling this past weekend it seemed that the front end sometimes locked in very fast and other times it took its sweet time when the rears had broken loose. Any thoughts?

 

[msgjd]

I have the following questions on operation of the T-138.. When will the front disconnect from assisting the rears?
A> As soon as I lift my foot off the throttle?(Push in the clutch to say upshift? Let up on the throttle?)
B> As soon as I put the rig into reverse?
C>As soon as the front and rear are moving the same speed? This would seem off meaning it would disconnect over and over and over if in slippery mud/snow

the answer to your question is the one choice you did not list.. D> All of the above !! .. And when on slippery road such as mud, ice, or snow, yes you will feel the front axle engage many times over and over , especially on a long slippery hill whenever you put your foot into the pedal, let off slightly, and come on again.. I have often felt the fronts of M39-series and M809-series kick in when above 25mph in slippery conditions.. Conversely, if you are just crawling along in low gear you usually will not feel it kicking in and out, unless you are turning sharp, then you might feel the pull when it happens