Does driving without propshaft destroy t-case/VC

[jeepruby04]

I've been trying to find a DEFINITIVE answer to this question. There are numerous threads and posts about this and I haven't found one that shows any hard evidence that it will or won't. I'm not looking for "I ran mine without the propshaft and it was fine/not fine when I put it back in after xxxxxxxx miles". I would like to see some technical info that describes how and why damage will occur or not.
The best info I have found is this quote taken from this thread http://www.syty.net/forums/showthread.php?t=26460&highlight=viscous

Quote:
"After spending a day with my nose in the factory service manual staring at the blow-apart drawings of the transfercase and reading the whole chapter on rebuilding one (in a stroke of devine inspiration) I finaly understand how the transfer case works. Here is the low down as it relates to the removal of the prop shaft.

The transfer case alone (with no viscous clutch) functions like an open differential. With no VC, if you were to lift the rear tires off the ground, all of the torque would go to them and the fronts would not turn. Ditto for the front, actually with the open FRONT diff on our trucks, just one tire off the ground would stop the rest from turning. So, without a VC in the transfer case, torque goes to the tires with the least grip. So, if you were to remove the front prop shaft (with no VC) and you try to drive, the front output shaft would spin like crazy and you would go nowhere.

The VC is what transfers all of the torque to the rear. The viscous clutch functions like this (this is straight from the service manual)

"The viscous clutch used in the Borg Warner 4472 (transfer case) is a torque distribution device and is nonserviceable. The internal construction of the viscous clutch consists of alternating plates that are connected to the front and rear outputs of the transfer case. The viscous clutch is filled with a high viscosity fluid which flows through slots in the plates. The resistance to sheer causes the plates to transmit torque."

Like I said in my original post, I was interested in finding out what type of dammage might be caused by the removal of the front propshaft. The book basicly refers to the FLUID transfering torque. How can fluid transfer all of the torque to one output (wont the fluid slip a little)?? What happens after a while?? Does it get worn out kinda like a shock?? What is the service life of the viscous fluid?? The book doesnt really go into detail on the VC so I took it upon my self to find out.

Well, after two days and 14 phone calls to 4 different states, I finaly got ahold of John Barlage (senior production engineer) of GKN Viscodrive. This is the manufacturer of the VC that goes in our trucks. So here is the beef as was related to me.

Basicly the VC progressivly locks the front and rear outputs together when there is a speed difference between the two. The bigger the speed differnece the faster the VC locks the outputs together. The VC is compleatly transparent untill there is a speed difference between the two outputs, at this time the viscous fluid tries to speed up the slower plates transmitting torque to the slower output. As this is happening the fluid is heating up. The fluid has two special qualities, 1 the resistance to sheer and 2 a high rate of heat expansion. The VC is purposly not filled compleatly with fluid. A certain amount of air is left in it to give the fluid room to expand. Once the fluid expands to fill the VC the internal pressure starts to climb rapidly, this actually forces the plates together (can you say metal to metal contact??) This condition is called self induced torque multiplication or HUMP (kinda cool huh)

It takes 110 to 140 degrees C to get to full HUMP (takes me less, hee hee) For those in the states thats 230 to 284 degrees F. At this point, the VC wont heat up any more (no more slipping = no more heat) but, as it cools it will start to slip again.

I asked if the heat would thin the fluid and John said that actually the opposite happens. The metal particles from the metal to metal contact create a condition called jellification. I bet you can guess what that means... If you cant... The viscous fluid ends up solidifying. This equals no speed differentiating between the front and back which equals worn tires or broken transfer case.
"

From this quote as I understand it, it means that the VC is needed to move the truck with the propshaft removed. That wood mean that the VC would need to get to full HUMP to send the power to the rear driveshaft. If operating in this way the VC would become "jelled" and no longer slip.

 

[Quickstop [UK]]

DISCLAIMER - this post was only made possible by the contributions of DaveP, 2kwik4u and the others in the thread. I definitely misunderstood it at the start. :tup:

Using the info from above, there are 3 scenarios:

A) Front wheels are not turning, rear are ( so call this a -ve slip)

B) Front wheels are turning at the exact same speed (0 slip)

C) Front wheels are turning faster than the rear (+ve slip)

Look at case B, the normal driven condition. The front and rear wheels are connected to the two out outputs of the VC and they are turning at the same speed. This is because the rear wheels have all the drive power. They push the truck forward and that motion pushes the front wheels. The fronts will spin at the same speed as the rear. There is no slip between the plates. There is no heat generated.

The solidification of the fluid doesn't happen because there is no shear between the plates. The TC is fine.

Look at scenario A. The front wheels are not turning, the rear are. The viscous clutch has to deal with a difference in speed between the two plates. As stated above, the greater the difference between the two, the greater the heat. The greater the heat, the faster the fluid solidifies and the more friction it creates. It becomes more and more solid and as it does so, the resistance to the plates increases, trying to apply more torque to the slower moving front wheels.

Two things can happen. Either the torque being transmitted causes the wheels to start turning at the same speed as the rear,the difference in the speeds diminishes and the viscous clutch cools, or, the second option: the heat builds up, the viscous clutch binds and ultimately fails. It can fuse together or the plates can heat the fluid so much that it is burnt and no longer has the requisite friction properties.

Scenario C is basically the same as A but in reverse. The front wheels are spinning faster than the rear. There is slip. The fluid heats up and the increasing friction means torque is applied to the slower wheels. The power is diverted from the faster wheels to the slower one.

If the rear wheel starts to move, the difference between the two decreases and the fluid cools. If it doesn't, the heat builds and it may fail if power is still applied like in scenario A.

Now, removing the propshaft... What happens...

Well, there is no resistance to the front wheels. So there is nothing causing the prop shaft to rotate at the same speed. The following picture is essential to understand how the TC operates.

The input shaft of the TC goes into the planetary above. This planetary drives the larger ring gear which is connected to the rear wheels. The inner sun gear is connected to one side of the VC. The other side of the VC is connected to the front wheels vie the sprocket and chain but also it is encased in the ring gear.

If the prop shaft is missing, there is nothing to drive that side of the VC at the same speed as the ring gear. The resistance to motion from road friction and aerodynamics means the planetary wants to walk inside the ring gear. Normally it would be stopped by the front prop driving the other side of the VC at the same speed but it isn't there. Here, the planetary is now rotating slightly, thus driving the inner sun gear. The sun gear drives the input side of the VC but the output side of the VC is connected to the ring gear. See the following posts for an explanation.

Ultimately, this means that the residual force is diverted through the VC. Thus, with no propshaft, the VC absorbs quite a bit of power. The more resistance to forward motion there is, the more power the VC absorbs. Further explanation below:

This is a pretty damn good article:

http://shufti.files.wordpress.com/2009/12/vc.pdf

It explains why it is called humping as well (self induced torque amplification or STA).

16:23, check out the planetaries on a 4472 case, same as ours. This is key to the operation of the SyTy and Astro transfer cases.



CONTENTS OF TRANSFER CASE

 

[Quickstop [UK]]

Re: Does driving without propshaft destroy t-case/VC

First off, if you are spinning a front, you are spinning the rears, OR you have an OPEN V-clutch in the T-case. I feel like a broken record on this, but it's fact. You can NOT spin a front, without ALSO spinning the rears IF the T-case is functioning correctly.
Quote:

Originally Posted by THEMADTYPH00N:
Could you explain more about why this is?

It's because of the planetary gear ratio in the T-case. The planet gears are driven by the input shaft. They mesh with the ring gear on the outside (rear output) and the sun gear on the inside (front output). The number of teeth on these two gears determines the ratio between the front and rear output torque split. I've never counted any, but if the 65/35 split is true, the outer gear has 3X as many teeth as the inner. You MUST understand how the power is transmitted through the gear-set, and how the ratio is involved, to "get it" as to how wheels will and won't spin.

Assume that both front and rear axles have equal traction. Because 2/3 MORE power is being transmitted to the rears, they MUST break loose first before the fronts that are only receiving a 1/3 the power the rears are. Remember, both axles have equal traction, or resistance to breaking loose. The rears must break loose first, because they get twice the power to them.

The viscous clutch is a two-element clutch. It consists of alternating discs. Power is transmitted between them by shearing a fluid. One set of elements is spline'd to the front output, the other set to the rear output. Think of it as a "posi unit" between the front and rear outputs. It does NOT "transmit" torque from the rear TO the front. (VW Vanagons and Chrysler Caravans are like this. The clutch is in-line with torque to the rears). You must understand this too: The way the V-clutch works:

With no slippage at either axle, all the elements in the case rotate as a unit with no relative movement between them. It all goes around with nothing changing position relative to each other. IF 65% of the power being produced becomes enough to exceed the available traction at the rears, or 35% becomes enough to exceed the available traction at the fronts, the V-clutch will transmit torque from the 'slipping' axle to the one that isn't. The clutch can transmit a LOT of torque. Hundreds of foot pounds.

You could remove the V-clutch, and you would never know it was missing UNTIL the available traction vs power being produced was exceeded. Almost always the rears will break loose first, as noted above. This what-if is the same as an "open" VC. It is un-able to transfer any torque from the rear to the front. One of the classic symptoms of an open V-clutch in a SyTy is that it chirps, or even spins the rears and fishtails on the 1-2 shift under hard acceleration. It's Because 65% of the power applied breaks them loose, and ZERO percent is transmitted to the front as long as the rears are loose. So they spin.

The scenarios where you could spin the fronts before the rears are really limited. Because you have to do it with only a third of the power that the rears are getting. Like jack the front wheels in the air, and it won't move with an open clutch. The fronts will just spin. Or the auto-x scenario where weight transfer has greatly reduced the traction of a front tire. But this could ONLY happen with an open v-clutch. One that can transmit no, or very limited torque because it has failed.

Sorry for the long-winded diatribe. I hope it helps you understand the workings of the BW T-case. Maybe someone like TJ will put this post in The Vault. Keep it safe, and we know where to find it.

Someone with some experience of this.

 

[2kwik4u]

Re: Does driving without propshaft destroy t-case/VC

.......With the front prop removed the t-case is going to act like an open diff until those plates grab.. .......

This is inaccurate.

There is a fundamental difference in operation between a viscous differential, and a geared differential. The geared differential is by nature a very LOW friction device. It will quickly, easily, and readily transfer movement to the path of least of least resistance. The VC differential is by nature a very HIGH friction device. It will not permit movement unless very high forces are places upon it.

With no load on the front flange, there will be no shearing of the fluid. There is no resisting force to prevent the front shaft from spinning. It will spin the front shaft at the same speed as the rear. Those plates will never be forced to grab, the fluid will never shear, there is no force to create that shear.

I too would like to put a temp probe on a truck and see what happens. I have some thermocouples at work, and some data logging equipment as well. I'll see if, once it warms up, I can get one of the local guys to let me use their truck as a guinea pig. That will be the best answer we can get.

 

[graham1524]

Re: Does driving without propshaft destroy t-case/VC

Someone else had a problem with their VC...

How about this food for thought: We all know the trucks roll very slowly in park with the front shaft removed if parked on a slope. How fast will they roll if the v-clutch is removed? And WHY? (Because the torque that was being applied through the front shaft to the ground to hold the truck on the slope in park is now being applied to the rear wheels through the v-clutch).

Dave, that one was me. I pulled the front prop shaft out and tried to move the truck and it would just rev. I pulled the tc put a new clutch in and its all good now. When I got the old clutch out I could get it to slip by turning each end with my bare hands.

 

[Quickstop [UK]]

Re: Does driving without propshaft destroy t-case/VC

One thing about the torque split ratio. The ring gear looks like it is twice the diameter for the sun gear, so a 2:1 ratio. If it is a 2:1 split then dividing that into 100% torque, you get the 65:35 split.

If the diameter was 3:1 then it would be 75:25.

 

[Quickstop [UK]]

Re: Does driving without propshaft destroy t-case/VC

The way I interpret the case is this...

The input shaft from the trans goes into the transfer case which inputs directly into the middle of the planetary carrier.

This planetary drives the outer ring gear to the rear wheels. The inner sun gear is connected to the viscous clutch output.

All drive is through the rear wheels. When they rotate, they drive the front at the same time. The front therefore spin at the same speed and are connected to the input of the vc.

If the input of the vc and the output of the vc match, no torque is transmitted. All the drive goes through the rear of the truck. If there is a mismatch then there is a speed difference between the two and a force is created which manifests as torque. The resultant torque between the two will split front to rear in a 35/65 split. The rear are driven off the planetary and as the diameter is twice that of the sun gear on the vc, it is the 2:1 split. This is the 65:35 split.

Therefore the difference in the speeds between the front and rear wheels creates a torque which is split accordingly.

With no VC in there, you can see, the planetary carrier has nothing to stop it from spinning freely. A failed VC with no prop shaft in means that there is nothing to stop the planetary spinning in the ring gear = no drive to the rear so the truck doesn't move. If the prop shaft is in then the front wheels are now turning at the same speed so the VC input/output speeds match, the planetary locks and it acts as if the driveline is intact.



I think.

 

[2kwik4u]

Re: Does driving without propshaft destroy t-case/VC

I need to take another one apart. I've clearly forgotten a lot from watching that guy take one apart. SO, I'm softening on my position of knowing WTF I'm talking about in regards to assembly. I will firmly stand by my assertion that the VC is 100% unused when the front shaft is uncoupled.

Here's why:

Look at ~14:20 or so in the video. He pulls the input shaft out. My understanding is that this shaft runs THROUGH the VC and mates with what we'll call the sun gear. Correct me if I'm wrong at any point here. So the sun gear drives the planets, the planets drive the ring. Assuming both outputs are connected, there is no slip between the two outputs, and we move on down the road. It should be noted here that a plantary in this configuration is essentially "locked" in place, and that the planets DO NOT spin as the truck is moving. Similar to a rear differential, and the pinions within the carrier. During straight line driving, the planets are under no need to move.

Now lets examine The VC itself. It has two gears on it. The sun gear (mated to the input shaft), and the external gear (mated to the ring gear). Inside the VC the two gears are separated by some fluid with plates that interchange, some mounted to the outer diameter, others to the inner diameter. This is to takeup the different in rotational speed between the front and the rear tires during turns.

Lets further examine the planetaries (that don't exist), and how they interact with the VC. The ring gear is BOTH the rear output and the front output at the same time. The rear output through direct mechanical connection, and the front output through the outer diameter of the VC. So those two are "locked" so to speak......Well, then how does it slip you ask?....Good question!.....Through the VC to the input shaft. This differential of speeds can "flow" in any number of directions depending on which item is driving, and which is driven at the point in time we examine it. The conclusion to be gained here is that the VC is not induced into any slip when driving the rear wheels, or the front wheels. It is only induced to slip when there is a difference in speed between the front/rear axle.

So far so good?

OK, lets continue. Now, lets look at the internal workings of the VC. It has multiple plates inside that allow some slip when there is a difference in input and output torque applied to the body of the unit. It can be treated as a solid, rigid, body until there is sufficient force applied to it to create the internal shear that allows the slip. Due to the nature of the fluid, this force is high. Due also to the nature of the fluid, the force increases as the torque differential increases. So, we find that without a torque difference between the rear output and the front input there is no shearing of fluid, no heat buildup, and no speed differential created. Keep in mind the VC can be treated as a rigid body, until that torque differential presents itself.

Moving forward we should consider what forces are in effect when we have a truck without a front propshaft installed, and it is attempting to move itself forward. There is an input torque from the engine, and a resistive torque from the rear driveshaft. There is also a VERY VERY small resistance from front flange, and the small amount of torque required there to spin it's associated bearings, and mass. having worked through the last paragraph we see that there is a TINY torque differential between the rear wheels and the front wheels, as both are being driven by the input shaft, not driven against one another. The "flow" of power in this scenario is that 100% will be delivered to the rear wheels, or to be more exact, perhaps 99.9% rear and 0.1% to the front to spin the bearings and mass of the front plate. There are no other forces at play here.

NOW.....lets discuss the 65/35 split, or whatever the numbers might actually be, that is unimportant. The split in power comes from the planetaries (that don't exist), and what happens when one tire spins, and power must be transferred through the VC to the front that has resistance. This is when the gear (and torque) multiplication occurs through the planetary gearset occurs. In this scenario BOTH input and output shafts have a resistive torque, and the power must flow through the planetaries due to a mismatch in speed between the front and rear.

The takeaway from my rambling here in this post is this. There is NO significant torque differential when the front propshaft is removed to create the necessary slip in the VC. Sure, there is some shock loading that would produce slip, however there will be no appreciable slip in normal operation, even when full power is applied. The planetary provides us a mechanical link that will drive the truck regardless of the VC being present or not. It will be far more detrimental to the VC to run mismatched tires than it would be to run without the front shaft attached.

Again, I need to take another one apart and make sure I have my components mating properly. I think that I do, however a cursory double check is a good idea I think.

 

[Quickstop [UK]]

Re: Does driving without propshaft destroy t-case/VC

Not quite...

If the VC is not there, there is nothing to hold the planetary in place so the truck will just spin that planetary inside the ring gear. If the front prop is gone, the VC is not actually transmitting any torque - it is merely acting as a locking differential.

With the VC present, if the planetary tries to spin, there is a differential speed because the prop is out and the front output isn't driven. The VC sees a difference and then the force generated, albeit very small, catches up and the two shafts rotate at the same speed. (By acting as this locking diff component) But if the VC has failed and doesn't provide enough resistance, the planetary can spin freely.

You need that VC intact to lock the diff so power can get from the input shaft, through the locked planetary to the ring gear. No VC = spinning planetary = no drive.

 

[mattw]

Re: Does driving without propshaft destroy t-case/VC

Not quite...

If the VC is not there, there is nothing to hold the planetary in place so the truck will just spin that planetary inside the ring gear. If the front prop is gone, the VC is not actually transmitting any torque - it is merely acting as a locking differential.

With the VC present, if the planetary tries to spin, there is a differential speed because the prop is out and the front output isn't driven. The VC sees a difference and then the force generated, albeit very small, catches up and the two shafts rotate at the same speed. (By acting as this locking diff component) But if the VC has failed and doesn't provide enough resistance, the planetary can spin freely.

You need that VC intact to lock the diff so power can get from the input shaft, through the locked planetary to the ring gear. No VC = spinning planetary = no drive.

This is EXACTLY how I understand it to work...

 

[Snoman002]

Re: Does driving without propshaft destroy t-case/VC

How much power does it take to push a SyTy down the highway at 70 MPH? Using fuel consumption as a rough gide to how much power is being consumed I came up with 40 HP at 2100 engine RPM. This is 100 ft lbs of torque. What an easy number. 35 ft lbs, or 35% of the torque required to move the truck will be carried by the v-clutch to the rear wheels.

Will a continuous 35 ft lbs harm the clutch on a prolonged basis? Define "harm". Will this "harm" occur in two trips, or twenty years? I have no idea. Each individual will have to make their own assessment of the risks. The worst case is you have to replace the v-clutch sooner. Or it breaks, and the truck stops moving leaving you stranded.

To each, their own. Good Luck.

Close, but not quite.

The 35% thing is not a gear reduction, or a set number. You can't just say .35 and call it a day. And this is where the rub is, with no front driveshaft your asking the VC to transmit 100% of the engine power to the rear wheels. No front driveshaft and no VC, the truck doesn't move, kinda like an open diff with one tire in the air (actually very much like that). No front driveshaft and a VC installed and now its like a diff with one wheel in the air, except it has a limited slip. The truck is going to move forward, but all the force to do so is on the clutches. In the end the front wheels don't provide 'only' 35% of the driving force, its something kinda like that, or no more than 35% of peak HP stock, or... Well you get the picture. But in the end a fluid coupling like the VC doesn't provide a set number like a gear reduction system would.

Will driving without a propshaft kill the t-case? Not right away, and maybe not at all. BUT, it IS putting a lot of stress on the VC.

But here is the question, is the force on the VC without a front propshaft more than is on it if you have a shaft in and run different sized tires? Its not actually, the force of different tire sizes can build up tremendous force, not 'force' so much as a cumulative pressure maybe.

 

[Quickstop [UK]]

Re: Does driving without propshaft destroy t-case/VC

Snoman...

The 35:65 thing is absolutely a fixed constant. The ratio of the planetary to the sun gear is 65:35 because of the diameters. They are linked by the VC at the input end and output end.

The force created is due to the slip. Torque is force x distance. Each force must be opposed by an equal and opposite force (Newton). So F1 = F2. R1 is the radius of the sun gear, R2 is the radius of the outer gear.

The Forces are the same but the radii are different in the 1:2 ratio (R1:R2)

The two torques are therefore: F1*R1:F2*R2 = 35:65.

Because the forces are equal and opposite, 35% of the torque will apply in the opposite direction of the other 65% and as the sun drive is in opposition to the planetary due to the movement, the power gets split to the wheels to drive them in that ratio.

 

[2kwik4u]

Re: Does driving without propshaft destroy t-case/VC

Of course not. The v-clutch is transmitting 35% from the front to the rear. 35+65=100.

The sun gear and the ring gear MUST rotate at the same speeds. You taught me this today. The forces on these two elements must be equal for the two speeds to be the same, and all three elements to rotate at equal speeds..

Input to the t-case = 100%
35% to front output, to tires, into pavement
65% to rear output., to tires, into pavement.
35+65 = 100% of input applied to T-case transmitted through the front and rear outputs to pavement.

Remove front prop shaft to run on Dyno.

Input to t-case = 100%
35% to front output, through v-clutch, to rear output, into rollers.
65% to rear output, to tires, into rollers.
33+65 = 100% of input into rear output into rollers.

Note that the forces on the sun gear and ring gear are equal in both scenarios. They have to be. On pavement the outputs are equalized through the pavement. On the rollers they are equalized through the v-clutch.

I can't give up.

I'm glad you're not giving up.....we're almost there. You've presented the information in a new way, I get what you're saying......But we still have a disconnect.

For the 35%/65% rule to apply, there must be relative movement between the sun and the ring. Otherwise there is no gear reduction. If the VC is in a locked (or near lock due to the high force required to the shear the fluid) then there is no relative movement between the sun and the ring. So no torque division or multiplication can occur unless that relative movement exists. The only way for that relative movement to exist is if the VC shears the fluid (or has failed "open"). The only way to shear the fluid is to have a mismatch between any two (or all three) operating pieces of the planetary (that still isn't there).

I think the confusion stems from the typical thought process of a differential being "open" and only locked at certain times, where as the center diff here is always "locked" only under certain conditions is it allowed to "unlock". This lock attempts to keep all 3 portions of the planetary (that still doesn't exist) rotating at the same speed relative to one another. Until forces that are VERY large come into play, that lock can be thought of as a rigid body.

This leads to a bit more confusion, as without a front propshaft there is ZERO force on that input to the system. So it can neither multiply nor divide any torque in any direction into or out of that input. This is a similar problem with the "Zexel" gear locking rear differentials. Torque can only be transferred from one side to another if there is an opposing force. Put one tire on ice, and you still get a "one wheel peel", until there is a small amount of torque to present (created by the opposition to movement), there is nothing to transfer. Same holds true here, without an opposition to movement on the front propshaft, there are no forces to be transferred through that outlet, so ALL power in from the engine MUST leave from the rear shaft, or be turned into heat.

I see your side of things Dave, I really do. The input shaft CAN spin while the rear tires are held stationary, and that would shear the heck out of the fluid in the VC and spin the front output shaft. VERY quickly that shear would lead to a full lock (due to the aforementioned "humping" phenomenon), and then the planetary would lock again and 100% power flow to the rear wheels occurs. The truth of the matter is nobody really knows how much the VC will hold as it shears, and how quickly it achieves full lock. My postulate here is that the tires loose traction before this every occurs, as the force to turn over a set of tires is less than that of the force created by the difference in diameter while turning......make sense?

 

[Snoman002]

Re: Does driving without propshaft destroy t-case/VC

Sheesh, this is like the argument that a locking differential transmits 50% of the power to each wheel.

A VC cannot 'lock' without slippage. Slippage is needed to cause the fluid to resist sheer. It doesn't 'lock' it slips less. It also doesn't Resist greater force. Cruise down the highway and there will be slippage, thee has to as that is what activates the fluid resisting sheer, but jump on the throttle hard and it will slip more. It will slip a bit, the sheer fluid will 'activate' and further resist more slippage. But its still slipping, not much, but a little more than it was before.

Look up the walking on corn starch videos, its the same thing.

The locking 'pin' of the system is the VC, its what is holding the gears to one another. It is supporting the load. It has the pressure of the engine on it to drive the rear wheels. The transfer case is an open differential without the VC. Take the VC out and its like breaking an axle. The VC is like the limited slip clutches. Break an axle in a limited slip and it will still move forward, but now the clutches are supporting the load.

 

[Snoman002]

Re: Does driving without propshaft destroy t-case/VC

I think the confusion stems from the typical thought process of a differential being "open" and only locked at certain times, where as the center diff here is always "locked" only under certain conditions is it allowed to "unlock". This lock attempts to keep all 3 portions of the planetary (that still doesn't exist) rotating at the same speed relative to one another. Until forces that are VERY large come into play, that lock can be thought of as a rigid body.

This leads to a bit more confusion, as without a front propshaft there is ZERO force on that input to the system. So it can neither multiply nor divide any torque in any direction into or out of that input. This is a similar problem with the "Zexel" gear locking rear differentials. Torque can only be transferred from one side to another if there is an opposing force. Put one tire on ice, and you still get a "one wheel peel", until there is a small amount of torque to present (created by the opposition to movement), there is nothing to transfer. Same holds true here, without an opposition to movement on the front propshaft, there are no forces to be transferred through that outlet, so ALL power in from the engine MUST leave from the rear shaft, or be turned into heat.

I see your side of things Dave, I really do. The input shaft CAN spin while the rear tires are held stationary, and that would shear the heck out of the fluid in the VC and spin the front output shaft. VERY quickly that shear would lead to a full lock (due to the aforementioned "humping" phenomenon), and then the planetary would lock again and 100% power flow to the rear wheels occurs. The truth of the matter is nobody really knows how much the VC will hold as it shears, and how quickly it achieves full lock. My postulate here is that the tires loose traction before this every occurs, as the force to turn over a set of tires is less than that of the force created by the difference in diameter while turning......make sense?

The t-case cannot be 'locked' and then unlocked unless there is a mechanical locking system in place. There is not as far as I know. The VC is the lock between a difference in rotation of the planitaries.

As for your last point. Sheer is what creates the 'locking' force. That sheer creates heat. So, there has to be sheer to 'lock' the vc, the vc is what keeps the planitaries from spinning relative to another, if there is relative motion only one shaft spins...aka, sheer is needed to lack the plantaries, the planitaris need to be locked to transmit all power to the rear wheels. This meens there is sheer in the VC to drive the truck down the highway, this sheer is creating heat in the VC. Its the only way the VC keeps 'locked' and preventing the front output from spinning wildly.

 

[2kwik4u]

Re: Does driving without propshaft destroy t-case/VC

The t-case cannot be 'locked' and then unlocked unless there is a mechanical locking system in place. There is not as far as I know. The VC is the lock between a difference in rotation of the planitaries.

As for your last point. Sheer is what creates the 'locking' force. That sheer creates heat. So, there has to be sheer to 'lock' the vc, the vc is what keeps the planitaries from spinning relative to another, if there is relative motion only one shaft spins...aka, sheer is needed to lack the plantaries, the planitaris need to be locked to transmit all power to the rear wheels. This meens there is sheer in the VC to drive the truck down the highway, this sheer is creating heat in the VC. Its the only way the VC keeps 'locked' and preventing the front output from spinning wildly.

Correct.....sort of. The idea that it is "locked" always is that the force required to shear the fluid is great than the differential forces between the input and the output shafts when in RWD operation. While I understand that there is not a physical lock, the premise of the super high force required to shear the fluid essentially makes it locked at any torque level below the shear force.

My use of quotes appears to not be as effective as I thought. My apologies.

 

[Snoman002]

Re: Does driving without propshaft destroy t-case/VC

OK, you lost me on your last post.

Under normal operation (front shaft in, tires the same size) the t-case is not locked. In fact the both sides of the VC would spin freely (hold with me a moment). In this case the VC is doing nothing and is in effect unlocked. Theoretically one could hold it in your hand and spin one end freely. Except that attempting to do just that creates sheer, that sheer causes the fluid to react and resist sheer. It doesn't resist sheer until there is some. The t-case itself Doesn't transmit power directly to the rear wheels, so its split between the front and the rear. It is in effect and open differential. When driving normally, there is no speed difference between the front and rear, and no attempt to sheer the VC. It isn't until there is a difference that the VC starts to resist the sheer (as the sheer itself creates the resistance).

Normally the planitaries travel together because the front and rear wheels are traveling at the same speed. But remove the front driveshaft and now the front wheels are not spinning the front driveshaft, which was spinning the output of the t-case, which was keeping the planitaries in 'alignment' if you will. Now if you remove the propshaft there attempts to be a difference in speed of the planitaries, but to have this happen the input and output of the VC would need to be different. In trying to be at a different speed this created sheer in the VC, which the VC resists. If it didn't the front output would spin wildly (which has already been proven to happen in the case of a failed open VC). Because the VC is resisting a difference in speed in the planitaries the front driveshaft spins at the same speed as the rear even though it is transmitting no power. In the end though its the VC is keeping the speed the same by resisting relative motion between the front and rear output of the t-case.

It is only 'locked' because the engine power and gears in the t-case are trying to spin the outputs at different speeds. If other forces keep the speeds the same the t-case, the VC specifically, is 'unlocked'.

 

[Quickstop [UK]]

Re: Does driving without propshaft destroy t-case/VC

I think I get it. Dave's right.

The VC sun gear is driven by the planetary. The VC output has a gear which drives the chain but it also a gear which sits in the ring gear.

If the resistance to the planetary drive is high, say 70mph on the freeway because it is pushing a brick, the planetary will want to rotate inside the ring gear. This causes the sun gear to rotate.

But...

The sun gear is connected to the VC and the other half of the VC is fixed in the the ring gear. So the VC is absorbing that power. That is why the truck rolls with no prop and why the VC will absorb a certain proportion of the power to the rear wheels.

 

[Couger1968]

Re: Does driving without propshaft destroy t-case/VC

I am a little lost on what keeps my truck from rolling on a hill, if the vc is not pretty much locked normally. What keeps the output shafts from counter rotating inside the case and slipping against the vc. It wouldn't matter if both shafts were in or not. I have very little knowledge of how the vc actually works. The way I read these posts one person seems to be saying it is tight, but allows a slipping for speed differences , and the other makes it sound they are loose at first then tighten as the shear (speed diffences) increases. If it was loose wouldn't it be like the front output shaft was disconnected when parked. Having it tight seems to be right and would mean that you need the front shaft to keep everything turning the same speed. Otherwise the vc would have to transfer all the load to the rear wheels. If it slipped a little it would heat up and cook it. If any of this makes sense.

 

[mattw]

Re: Does driving without propshaft destroy t-case/VC

I am a little lost on what keeps my truck from rolling on a hill, if the vc is not pretty much locked normally. What keeps the output shafts from counter rotating inside the case and slipping against the vc. It wouldn't matter if both shafts were in or not. I have very little knowledge of how the vc actually works. The way I read these posts one person seems to be saying it is tight, but allows a slipping for speed differences , and the other makes it sound they are loose at first then tighten as the shear (speed diffences) increases. If it was loose wouldn't it be like the front output shaft was disconnected when parked. Having it tight seems to be right and would mean that you need the front shaft to keep everything turning the same speed. Otherwise the vc would have to transfer all the load to the rear wheels. If it slipped a little it would heat up and cook it. If any of this makes sense.

Your front and rear tires would have to spin in opposite directions to allow the truck to move.. Under almost all circumstances this will not happen. Think of it like an open rear diff when the VC is not locked.. Major difference being the 65/35 split instead of 50/50 like a diff would be..