The Stovebolt Site Forums The Parking Lot Big Bolts tandem axle power divider

Probably an unusual question, but I am sure some on here would know. On a tandem axle truck, don't both axles get the same amount of torque applied to them when the power divider is unlocked? On another forum there are some people that say the only time both axles get power is when the power divider is locked, but I think this is wrong.

Here's how I understand it...
In a power divider with an inner axle differential how much torque each axle gets depends on how much traction to some extent.
With the power divider unlocked both are driven, but if one axle comes off the ground it'll spin, the other won't and you won't go anywhere.

The differential in the power divider acts same as the differential in the rear axle differentiating between two wheels, only it differentiates between the front and back axle.
With it locked both front and rear axles will turn together.

Grigg

Hy Crabbyappleton and guys, Grigg has describbed the modern inter axle differential operation fairly well. Both axles get power only when the inter axle diff. is locked. The power divider in the 56 through 59 series Chevy tandem trucks functions a little differently. In "normal" operation only the forward axle recieves power, the power divider has two operating levers, the lever farthest to the right shifts the unit between underdrive, "puller", and direct ratios. The lever to the left of the range shift lever is the "declutching" lever, this lever engages drive to the rearmost axle, and is automatically shifted when "puller" ratio is selected by the range shift lever. The declutching lever can engage or disengage the rear axle in all ranges except "puller" range, I hope that clarifies power divider operation in the 56 through 59 series Chevy trucks.

I'm not sure we are sure if crabby has specified what kind of twin screw he has. The setup 3B describes was a short lived option on medium duty Chev for a few years. Rockwell already supplied an inter axle differential at that that worked similar to modern twin screws. The axle with the least traction gets the power. A power divider lock locks this interaxle, but cannot be used on hard surface roads. It is only to be used in emergencies. Lockers are also available that lock the individual differentials. If he will post a picture or a model number, I can tell him how it works.

Actually, this was not a question about one of the old Chevrolet trucks. It is about the tandem axle trucks we have today and maybe 20 years ago. I had a discussion on another forum and I maintain that both axles receive the same amount of torque when the power divider is unlocked and the other gentleman maintains that the only time the rear axle receives torque is when the power divider is locked. I am about 95 per cent certain that I am right, but I was wrong in 1974, so it is possible. Just wanting to get some opinions.

Hy Crabbyappleton, if your asking about the modern inter axle differential, then Grigg described it perfectly, when not locked whichever axle is easier to turn get the power, the least traction gets all the power. When you lock it some power goes to each axle.

The power divider is necessary also because it is almost impossible to have tires with the exact same rolling diameter. The power divider compensates for slight differences in tire diameter in a similar manner to an automotive differential compensating for differing distances traveled by a car turning a corner. If one wheel on each axle has no traction, even with the power divider locked in, the truck will not move.

Some of the newer BIG trucks have the standard power divider as well as a differential lock, which locks the differentials side to side. All power is divided equally between all four drive hubs.

Crabbyappleton, I have found that the rear drive tires on a semi tractor wear faster than the front axle drive tires. I am not sure if that is due to the rear axle tires scuffing in a turn or if the design of the power divider unintentionally allows more power to get to the back axle.

I think the rear drive axle tires wearing has something to do with the suspension. In the 60's until the mid 70's I ran Hendrickson walking beams and all 8 tires wore evenly. In the mid 70's is when I started phasing in 4 spring suspension and the rear wear first appeared. In the mid 80's when I started phasing in air ride, it became very apparent. I now try to rotate the drivers at 50k miles. It may also be due to the fact that on my tractors that pull bulk trailers, I center the 5th wheel on the forward axle to get my 12k on the steering axle. 40 years ago we didn't worry about axle weights. It could be that the forward axle gets more traction and the rear one slips a little.

Now throw in the old Timken worm drives that ran together all the time, you had to run the same size tires all the way around or risk damage to the bronze ring.

Hi all. I'm new here, posting because the "mental puzzle" aspect of this is something I've always appreciated. My short answer is that the original poster is totally correct about the "torque" being equal on both sides of an open differential (whether an inter-axle differential or any other of an open design). Note that this is not the same as power distribution, and all statements about "power" distribution are actually not addressing the question. Since an inter-axle differential is functionally the same as the differential between right and left wheels, let's use the differential in a single-axle truck to decribe what's going on.

Example #1:
Say for example that a single-axle truck is stationary and 450 pounds of "push" from the wheels is necessary to get it moving. One wheel is on hard ground and has far more traction than what is needed, but the other wheel is on snow or mud and can only generate 200 pounds of traction when spinning "uselessly". In this case, the truck is stuck and it won't move. While the one wheel spins, what's the other wheel doing? It may not be spinning, but it IS supplying traction, 200 pounds worth to be exact, just like the spinning wheel. The nature of an open differential is that the torque output on both sides is always the same, no matter what (ignoring the small amount of friction losses within the gear system). That's why it's misleading to talk about where the "power" goes. Sure, the only place power is being used in this case is where the spinning tire contacts the ground (the power in this case is only being used to generate heat, rather than move the truck), but the total tractive effort is 400 pounds, shared equally by the right and left wheels. Since the truck needs 450 pounds of traction to move, a person pushing on the back is all it takes to get it going. However, IF the stationary wheel were indeed accomplishing nothing, as so many people believe, the truck would be stuck much worse instead of "just barely".

In summary, the wheel that doesn't turn in this situation DOES have torque applied to it, and it's the same amount of torque that's applied to the wheel that's spinning on account of having too-little traction. BOTH wheels are pushing even if only the one with poor traction is spinning. Their combined effort in that case just isn't great enough since the wheel with good traction can never push any harder than the wheel with poor traction, or more accurately, both wheels always push with the same force because the TORQUE on the two axles is always equal.

Example #2:
Imagine a truck with a locking differential with one wheel on very slippery ice and the other on hard ground. A truck with an open differential would be terribly stuck in this case, but when the right and left axle shafts are locked together by the locking diff, both wheels turn together and the truck gets moving easily. In THIS case, the torque applied to each wheel is not the same (there's almost no torque applied to the wheel with almost no traction), but contrary to what most people think, the power is not shared equally between them either. Nearly all the work being done to move the truck is performed by the wheel with good traction. Power is mathematically related to the work being done, so where the work gets done is ALSO where the power is expended (to illustrate, note that you can't measure the horsepower of an engine unless you provide an opposing torque). Thus, if traction is unequal between the two sides of a locked differential, power application is unequal as well. It is incorrect to say that the power is distributed equally when the differential is locked.

Very informatave and welcome to the Stovebolt!

Mike B