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Old 03-18-2004, 10:15 AM   #1
Orson
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Default Detailed description of Viscous Coupling behavior

This post will describe the physical behavior and mechanism of a viscous coupling. It is based primarily on a recent SAE paper found in "Transmission and Driveline Symposium 2004" publication SP-1817. That paper primarily addresses fluid and friction modeling, but its summary of viscous coupling behavior is a good source of basic information. The sample data shown are not for a Subaru VC, but some other experimental sample, so take the information in a generic sense.

There has been lots of misinformation about viscous couplings, owing to the lack of any real information. I hope that this post will rectify that.

Let’s start with what most people already know. A viscous coupling comprises two sets of plates. Each is attached to a shaft. For example, one set of plates is attached to the left drive shaft and the other set of plates is attached to the right drive shaft. As one wheel spins (loses traction), the speed difference between the left drive shaft and the right drive shaft increases. This speed difference causes the silicone (viscous) to shear. The fluid’s natural inclination is to resist the shearing. Thus, the fluid resists the speed difference, causing a net transfer of torque to the spinning wheel. Hence, the limited slip function.

Let’s run an experiment. We will take a VC with solid plates (plain solid discs, no holes drilled, no slots), and constantly subject it to a 100rpm load. That is, we will hold the left wheel still and spin the right wheel at 100rpm (which is not very fast – less than 1.5 rotations per second). We will measure the temperature of the viscous coupling. We will also measure the torque that is required to maintain 100RPM – in other words, the torque transfer ability of the VC. The following data is what we get:



Note that as the fluid heats up, the VC action decreases.

Note that I said this experiment was run with a VC that has solid plates with no perforation or slots of any kind. In practice, no one makes a VC this way. In practice, it has been found that if you put holes and slots in the plates in a particular way, you get a very interesting behavior called “Self Torque Amplification” (STA). Here’s a picture of what the plates in the rear WRX VC differential look like (courtesy of “petrol”):



Let’s run the same experiment, but now with plates that have holes and slots.



So as the fluid heats, up, it loses torque transfer ability. But when the fluid gets hot enough and the pressure increases enough in the fluid, the torque transfer actually skyrockets. This STA is also called “humping”. (The “humping” name comes about because if you graph wheel speed difference data from a VC vehicle, it starts at zero, then there is a momentary burst of wheel speed difference before STA happens, and then the wheel speed difference goes to zero as STA occurs – the graph has a hump in it.)

The reason for this “humping” or STA is that the fluid, having been heated up and become highly pressurized, have caused the plates to deform and come in contact with each other. In other words, the viscous coupling becomes a mechanical clutch. The evidence for this is wear marks when a VC is operated in the STA mode. So this is why slots and holes are so important – without them, you cannot easily deform the discs to come in contact with each other. But with slots, you have essentially made the disc into a bunch of tabs. These tabs can now easily twist and bend. And that is exactly what happens during STA – the tabs bend towards each other and then slide against each other. Interestingly enough, there is little understanding of why the tabs bend (after all, the fluid exerts the same pressure on both sides of the tab, presumably). It’s a case of a happy engineering accident and lots of trial and error.

In the above experiments, the speed difference was only 100RPM, and so it takes 400 seconds for this experimental VC sample to reach STA. In practice, VC units are tuned more aggressively. In addition, when you spin up a tire from low traction, the difference escalates to 300RPM and above very quickly.

In practice, I have seen VC reach what appeared to be STA in 0.2 seconds on uniform snow. I have also seen STA in 10 seconds in what appeared to be a malfunctioning unit. I have no experimental data on the WRX.
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Last edited by Orson; 03-22-2004 at 03:49 PM.
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Old 03-18-2004, 10:16 AM   #2
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Old 03-19-2004, 01:09 AM   #3
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This is a good article, but it's a bit far afield from normal operating conditions for a VLSD system. Is there not a general description of VLSDs in here already? Here's some discussion...

The temperature issue is not directly related to how well this sort of differential works in regular use. They ran it for several minutes with one axle locked, and the temperature got really high--150 degrees celsius is pretty darn hot. So their experiment was to see what happened to the system's behaviour with really hot fluid.

But this isn't normal use. The point of a differential (for cars on pavement, at least) is to adjust the locking ratio between the wheels, and except in improper drag race starts where one wheel sticks and the other spins, the differential doesn't turn very fast. The axles may be spinning at 1000 rpm or more, but the DIFFERENCE between the axle speeds is what counts, and that is mostly quite small.

Say you go around a pretty sharp 90 degree corner, 100 foot radius. The outside wheel goes about 157 feet (pi/2 * 100), and the inside wheel goes about 150 feet (pi/2 * 95). Say you're going 60 miles per hour, 88 feet per second (which is pretty fast for that sharp of a corner). So it takes something over a second to go around the corner, and the wheels go a difference of only 7 feet--something over one revolution. In this extreme case, the axles are spinning at a difference of only about 1 rps, or about 60 rpm--comparable to their experimental 100 rpm. Differential overheating is not going to be a problem unless you're spinning the wheels or running around on a skid pad for minutes at a time.

The neat thing about VLSDs is that they are NOT locked up by the fluid heating up (except in the special overheating case in your article). The silicone fluid has the property that its viscosity rises as the shear increases, which is the opposite of "normal" fluids like oil. This happens instantaneously, and the fluid doesn't wear out. It's complicated physical chemistry, but the result is quite amazing: Smooth take-up and locking that's adjustable by changing the characteristics of the plates--holes, slots, how close they are, etc., as you describe.

There's a lot of incorrect information about this subject on the net, much of it spread by manufacturers of mechanical locking differentials. The operation of the VLSD doesn't have anything to do with heating up the fluid, and it happens instantaneously so there's no delay involved, unlike what you will find claimed in some sources.

Another example of this effect is when you walk on the beach right at the edge of the sand, where it's damp but not quite wet. Sometimes you'll notice that there's an area maybe 6" around your foot where the sand changes color as you step on it, and that the sand is very firm to walk on. That's the same effect as the VLSD is using, in this case the result of the mixture of sand and water...

Silly Putty (a slicone compound) has the same property, although its viscosity is quite a bit higher. If you pull it slowly, it flows. If you pull it quickly, it gets stiff and breaks.

Systems based on the VLSD concept have been used in plenty of racing cars to adjust the understeer or oversteer balance under acceleration and braking. It's not quite so useful in a 4WD truck, though, because you don't get 100% locking.

I suspect that what you saw in the snow was just the normal operation of the VLSD.

Last edited by Dougie01; 03-19-2004 at 01:21 AM.
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Old 03-19-2004, 07:28 AM   #4
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Keep in mind that this experiment was done on a custom-built unit for experimental purposes. Although not specified, I am guessing that they filled the casing with viscous fluid on the low side so that the events would unfold is a slow way, so that the sharp-onset of STA could be studied closer. A production differential is probably filled more generously and thus achieves the pressure for STA more quickly when the wheel speed difference builds.

From your post, I think that you inferred that the point of the paper was to study a failure mode. This is not true. Studying STA is not about studying a failure mode, but a performance-enhancement mode. STA (humping) is desired by the manufacturer. If they wanted only the viscous fluid behavior and to eliminate the STA mode, all they would have to do is put in smooth discs. This is both cheaper and more optimal for the viscous fluid to work (more shear area). In fact, early VC units utilized smooth discs, according to the paper. However, the plates in recent VC units (and in the WRX) have gone through the expense of machining to make the holes and slots to create the STA mode. And lots of papers have evidently been written to understand how the humping phenomena works and how to optimize it.

In working with fully instrumented vehicles, I had always wondered how it was possible for a VC to fully lock. That is, how can the left and right wheels be exactly equal in speed when you are spinning up the wheels? When speed difference is zero, viscous action should disappear, thus encouraging wheel speed differences again. But I saw this again and again. The wheel speed difference plot consistently looked like this:
__/\___________________

STA explains how this is possible.
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Old 03-26-2004, 03:10 PM   #5
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I have had an opportunity to speak with experts in VC design. In fact, the people that I spoke with work for FFD who originally invented the VC and, according to them, do ALL viscous engineering work for racing vehicles (they own the patent and it is still active; they sold the rights for production vehicles to another company).

The humping phenomena is indeed the principal operating mode that creates limited slip operation. That is, older VC units relied on the viscous fluid to transfer torque. No more. Modern VC units rely on the mechanical friction between the plates. The viscous fluid is now merely a "first strike" weapon to transfer torque, but with the intent of eventually creating STA. And yes, the intent is always to cause STA within a couple tenths of seconds.

So to summarize, yes a VC can and will lock, although its operation may vascillate between slipping and locking, and the primary operating mode of a VC is as a locking clutch!

This would seem to go against almost every popular understanding of viscous couplings, but I do not dare doubt the engineers of a company that pioneered viscous couplings. As I mentioned, this finally explains all the data that I had seen from test vehicles that showed VC units locking.
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Old 03-26-2004, 04:19 PM   #6
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What's the name of the company or organization?
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Old 03-27-2004, 01:42 PM   #7
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"FFD". It stands for Furgeson something something. In any case, it doesn't matter - they were purchased by Ricardo.

Incidentally, FFD (now Ricardo) also makes the transmission for the Jaguar XJ220, the MacLaren F1, and the upcoming Bugatti Veyron. Like I said, I do not doubt anything these people say when it comes to drivelines.
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Old 03-29-2004, 09:42 PM   #8
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Someone educate me, but from what I know the rear viscous LSD used in the early legacy's and the current have the same operation and construction.

If that is the case, I have performed a simple test of my own that resulted in no hump behaviour.

I locked the center differential so that the wheel speed couldnt be diverted to the front. The car had all wheels rotating freely from the ground.

I used a needle torque wrench and attached a 32mm socket to the axle nut. I measured the torque and it was instantly applied, and only increased a small amount (if counted as a percentage of the original torque) as I held it in place. The speed was low (15-20mph at the speed) so maybe I didnt reach HUMP velocity.

Edit: I should mention the LSD in question was from a 93 or 94 JDM turbo wagon, which I converted from 4.44:1 to 4.11:1 and installed in my legacy.

Last edited by ciper; 03-30-2004 at 12:38 AM.
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Old 03-30-2004, 12:17 AM   #9
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Very interesting information. Thank you.
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Old 03-30-2004, 07:08 AM   #10
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Quote:
Originally posted by ciper
Someone educate me, but from what I know the rear viscous LSD used in the early legacy's and the current have the same operation and construction.

If that is the case, I have performed a simple test of my own that resulted in no hump behaviour.
First of all, I know nothing about the Legacy VC unit. It is old enough that it may be of the "smooth plate" variety with no humping behavior.

Second, even if it was designed for "humping" with slotted plates, I am not sure what test you were performing and whether it is valid or not. For humping to occur, the wheel must spin up very quickly.

Also, as I noted, the current theory on humping is still not widely agreed upon. One other theory of how humping occurs is that the torque imbalance (one wheel sticks and delivers lots of torque, the other wheel slips and delivers almost no torque) causes the plate deformation. Therefore, in the case where you have the car up in the air, there is no significant torque difference. Under the "Torque Imbalance" theory of humping, a hoist test is completely invalid.

As I said, it is interesting that the "humping" phenomena is still not understood. Engineers know that it works, but do not understand why.
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Old 03-30-2004, 10:24 AM   #11
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Confirmed information:

The viscous coupling was invented by Ferguson Formula Developments (FFD). FFD has since been purchased by Ricardo, a British engineering consultancy.
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Old 03-30-2004, 12:43 PM   #12
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Quote:
Originally posted by Orson
Let?s run an experiment. We will take a VC with solid plates (plain solid discs, no holes drilled, no slots), and constantly subject it to a 100rpm load. That is, we will hold the left wheel still and spin the right wheel at 100rpm
I don't suppose the paper gave any details about the experimental setup? In particular, is there anything to suggest what differences (if any) it might have from ciper's "hoist test"?
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Old 03-30-2004, 08:35 PM   #13
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The paper did not give a lot of information on the set-up. Only that they were running at a constant RPM and were measuring torque. One obvious difference is that the experiment was run at 100RPM while the "hoist test" was presumably run much slower.

Keep in mind that the experiment appears to have been run to show the lock-up in slow-motion. Also keep in mind that the point of the paper was to prove a certain theory of STA. Many other theories have apparently been offered with varying degrees of satisfaction.
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Old 03-30-2004, 10:12 PM   #14
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Would that be 100 rpm to the input shaft or 100 rpm resultant speed by having one wheel stopped (50rpm input shaft)?

What rpm is 20MPH with 205 50 16 tires?
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Old 03-31-2004, 01:20 PM   #15
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100RPM on one drive shaft and the other drive shaft locked. Remember that this is a test with just the VC - there is no differential in this test, just a VC in isolation.

I calculate 270RPM for 20mph. Check my work:
radius of 205/55R16 is .31595m
20mph = 8.94 m/s

therefore, 8.94 / (2*PI* 0.31595) * 60 = 270RPM
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Old 03-31-2004, 08:08 PM   #16
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Back of envelope calculation:

60 mph is 88 fps, so 20 mph is about 30 fps.
One rev is about six feet, so 30 fps is about 5 revs/sec, or 300 rpm. Your number looks good..
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Old 03-31-2004, 08:09 PM   #17
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The "is it working properly" test for the early Miata VLSD is exactly what was described above: Use a torque wrench and verify some given resistance while turning the axle slowly.
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Old 03-31-2004, 09:36 PM   #18
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Quote:
Originally posted by Dougie01
The "is it working properly" test for the early Miata VLSD is exactly what was described above: Use a torque wrench and verify some given resistance while turning the axle slowly.
If you are speaking from the perspective of a 1993 Miata and it is advertised as having a viscous, it may be an early version with no STA/humping. I don't know, just speculating.
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Old 03-31-2004, 09:44 PM   #19
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I just thought of a way to clarify further.

In an "old school" VC with smooth plates, the only mechanism for torque transfer is the shearing of the viscous fluid. This mechanism is immediate - as soon as you have a wheel speed difference, you get torque transfer.

A "new school" VC is very different. The shearing of the viscous fluid actually contributes very little. After all, you plates have holes and slots in them! The fluid does not have lots of good surface area to work with. So in a modern VC, the torque-wrench method may be a very poor test, because until you get to STA/humping, a "new school" VC performs very poorly.
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Old 03-31-2004, 10:26 PM   #20
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I dont agree on one of the things you said Orson.

With the holes and slots in the new designed VC the amount of fluid shearing should be greater than the smooth plate design. Meaning more imediate torque transfer AND humping.

Im still fairly sure that in regards to Subaru once they switched from clutch plate rear LSD to viscous units a single design has been used (the slotted unit). Especially considering the same early subaru viscous LSD is more widely available in other regions and they describe the same behaviour.
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Old 04-01-2004, 07:28 AM   #21
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Quote:
Originally posted by ciper
I dont agree on one of the things you said Orson.

With the holes and slots in the new designed VC the amount of fluid shearing should be greater than the smooth plate design. Meaning more imediate torque transfer AND humping.
Not true according to the paper. The SAE paper actually does discuss this briefly. For maximum shearing action, you want the maximum surface area. That is, you want smooth plates. Holes and slots reduce surface area and thus reduce the viscous action (but adds humping).

I can kind of see how you might be thinking, but "cutting" fluid is not the same as "shearing" fluid.
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Old 04-02-2004, 05:15 PM   #22
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Thanks for taking the time to post this information. I certainly learned from the read. The pictures helped me as well. My R160 diff in my WRX recently failed. The problems started because I was overloading it with power. The first symptom and the first thing to fail was the coupling action of the VC. At WOT runs up into 3rd gear I would get some rear wheel spin. Normally one rear tire.. The diff would fight this and send power to the other rear tire. You could feel this action and had to counter steer.

The Side to side distro of power started to become slower and slower and the diff allowed the spinning wheel to spin longer as days went on and more WOT runs where made. Then, one day on a WOT through second gear I got some bad wheel spin on the right rear tire. As the diff tried to send the power to the left rear tire I heard a Very fast and high pitched Clicking sound.. Almost like a ratchet type noise..... Almost like a rip saw hitting hardwood... The sound stopped as the spinning wheels power was reduced and traction was gained. Over the next few days this scenario continued.. It was a pretty bad click/high pitched Ratchet type noise as the one wheel would spin. I guess now after reading this post that this was the plates making contact with one another? The Fluid was probably burnt and not acting as designed?

A few more days of abuse and I got an extremely loud high pitched Click/ratchet/ whine type sound as a wheel let lose and lost traction. Then the noise subsided. I was left with an open diff.. Just like driving a truck in the rain or snow with an open diff. Once the one of the rear wheels started to spin it would simply spin faster and faster until I would let off the gas to gain traction.

About a month later I went WOT in first gear, Got some wheel spin pretty bad for about 2 seconds, Then the tires hooked along with some slight wheel hop in the rear, the car pulled forward pretty sharply. At that moment I heard a series of very loud teeth rattling BANG! noises. About 4 or 5 of them. I guess these where teeth stripping off the ring gear. After the 4th or 5 the BANG! I heard a low and gutteral BOOOOM followed by the revlimiter and a loss in power. I then heard loud metalic crushing noises, a lockup of the rear end, some skidding and then a release.. I coasted off the road and towed the car to the shop.

When the diff was removed we found that the drivers side axle was snapped at the Splines outboard. The theory is that metal locked the diff up after the explosion and the axle was loaded hard (car traveling 40mph or so) which snapped it.

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Old 04-02-2004, 06:38 PM   #23
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I feel bad for you, but why did you keep driving it for months with it making nasty sounds?
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Old 04-05-2004, 03:39 PM   #24
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Why not??

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Old 06-23-2004, 07:00 PM   #25
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I anyone aware of a test for the VC units in the WRX?
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