Hi Grumpygel - I would disagree regarding the non Newtonian fluid, it actually is the case. The thermal effects causing the 'hump' are a byproduct of the shear however the drive is progressive (rapidly) and does not require the 'hump' to operate in the first instance - the 'hump' is simply the stage where the system is - in effect - locked - it is otherwise progressive. I was incorrect in the effects of heating in my description as has been pointed out that the final fluid expansion causes the 'hump' effect - however again, as said, that is not the actual point of operation, only the final locking stage
As a quote from the first article you linked to - more or less the first paragraph -
"When the plates start rotating at different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat and become nearly solid because the viscosity of dilatant fluids rapidly increases with shear"

Note in the above an important factor ! - yes the fluid starts to heat - however - the heat is NOT the factor that causes the system to transfer drive - it is because the "viscosity of dilatant fluids rapidly increases with shear"
The paragraph is simply stating that heat is produced in the friction caused by the action of the non Newtonian effect on the plates - not that heat is the CAUSE - heat is CAUSED.

A 'dilatant' is by nature a NON NEWTONIAN fluid -
https://en.wikipedia.org/wiki/Dilatant

From all the documentation listed the system is clearly operating on the non Newtonian effect with a final lock caused by expansion. This expansion (thermal) is caused by the friction of the cycling of the viscosity (due to non newtonian shear loading) and is the stage where the cycling effectively stabilises.

Interesting stuff though
Joe

edit - also, the vw doc is interesting - see section 3.3 which describes the siloxanes (as used in the coupling) as second order non Newtonian effect as opposed to first order silicone 'oils' which exhibit a FALL in viscosity with shear load - the vw article refers to second order fluids. (with an INCREASE in viscosity as shear rate increases.)

The hump effect is also a necessary mechanism to protect the system from a form of thermal runaway - as well as being a highly beneficial final virtual lock-up. Again, very clever system.

GrumpyGel said:

As for what characteristics a failing VCU exhibits, I'm not sure if there has been any scientific evaluation of this, but my conclusion is that they get to bloody stiff

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Except when it fails in "Mondo" mode - unusuall, but nevertheless a known phenomena.

I only used that first link as it shows where the dyno image came from. The text on that thread is similar to this, ie personal interpretation and perpetuation on either fact or urban myth - we know not - without reference. I have seen the webpage that the dyno graph comes from, its from a rebuilder of VW couplings.

My only point is that ALL the data that I have seen, be it dynos or from GKN or SAE/VW shows that as shear increases the viscosity of a VCU deminishes. GKN describe this as 'The degressive locking characteristics (viscous mode)'. This continues until the tipping point to 'hump' mode which is 'programmed' into the coupling mainly by the air gap that is designed into it.

It may be that the viscosity reduces at a lower rate than the amount of power the shear wishes to transmit - and thus more torque is still transmitted.

My thoughts are that a failing VCU starts from a higher level of viscosity, as can be determined by the 1WUT, and probably retains that same level rather than dropping. That though may be totally wrong and if 'agreed' on by the masses will be a perpetuation of an urban myth!

The Mad Hat Man said:

Except when it fails in "Mondo" mode - unusuall, but nevertheless a known phenomena.

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Sure! Presumably, it would need to lose its fluid for this to happen.

GrumpyGel said:

My only point is that ALL the data that I have seen, be it dynos or from GKN or SAE/VW shows that as shear increases the viscosity of a VCU deminishes. GKN describe this as 'The degressive locking characteristics (viscous mode)'. This continues until the tipping point to 'hump' mode which is 'programmed' into the coupling mainly by the air gap that is designed into it.

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HI GG , Regarding degressive locking characteristics - it is not that the viscosity 'decreases' or that the unit fails to act until the hump is reached -as such - it is simply that the rate of rise in torque transfer increases at a non linear degressive rate depending on the speed difference- - a flattening upward curve. As the unit progressively transfers more torque then the speed differential is less. This is the 'viscous mode' - the heat generated eventually causes the ' virtually locked' hump mode.
The GKN datasheet appears quite clear on this imho.
Regarding the dyno graph I have no idea what is purported to be shown and in what circumstances. The GKN and VW data also appear to confirm the above quite clearly.
Joe

The Mad Hat Man said:

Except when it fails in "Mondo" mode - unusuall, but nevertheless a known phenomena.

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I agree there are reported instances of mondo mode, however, I would tend to agree with GG that this is most likely due to fluid loss via the seals. I have only ever heard of anecdotal instances of such a failure but cannot rule them out.

In almost all instances of VCU failure I would theorise that the biggest and almost universal failure is caused by constant and progressive overheating of the fluid eventually destroying it's physical properties and in some cases damaging or contaminating the plates leading to a situation where the unit is never in the 'free to operate' mode. once a breakdown of fluid properties starts, then it rapidly will progress as heat will be constantly generated leading to an early onset of torque transfer or hump and wind up.
The most likely cause of such is tyre sizes / pressures - again imho.
The simple test that BELL suggests (running on a fairly straight road at normal speeds and then checking temperature of the vcu )(should be cool) is a very very good indicator of early problems me thinks.
It really is a fascinating subject.
Joe.

Joe_H said:

HI GG , Regarding degressive locking characteristics - it is not that the viscosity 'decreases' or that the unit fails to act until the hump is reached -as such - it is simply that the rate of rise in torque transfer increases at a non linear degressive rate depending on the speed difference- - a flattening upward curve. As the unit progressively transfers more torque then the speed differential is less. This is the 'viscous mode' - the heat generated eventually causes the ' virtually locked' hump mode.
The GKN datasheet appears quite clear on this imho.
Regarding the dyno graph I have no idea what is purported to be shown and in what circumstances. The GKN and VW data also appear to confirm the above quite clearly.
Joe

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To further clarify - one could also say the 'degressive locking characteristics' are basically proportional to a diminishing delta due to increasing locking effect of the unit (the more the unit is starting to lock the lower the delta of speed difference) - aka - degression - which - as gkn says - can be altered via the "fluid viscosity, number and size of plates, and fluid filling percentage"

The truth is probably somewhere between what we describe.

Your description of digressive looks good and is shown as the green line in this graphic...

However most people believe that the blue Progressive line is how the VCU operates. As born out by your description of low slippage making the Freelander "effectively front wheel drive" but when wheels start slipping this causes "lock up via the increase in viscosity". This isn't true, Freelander always supplies quite a reasonable level of torque even at no/low slippage and viscosity won't lock up the transmission.

Once again it is a common belief that the VCU fluid is non-Newtonian in that it shear thickens. I want to believe that, but I don't! I believe it is true that it is non-Newtonian - but because it shear thins not thickens - hence the curve flattens and only continues to go up because of the increased torque being put in to make the shear greater. This is born out by the following graphic from PDSM manufacturers (Freelander is 'believed' to use 100K CST)....



Credit : http://www.dowcorning.com/content/discover/discoverchem/weight-vs-viscosity.aspx

Viscosity of the fluid will also reduce due to the heat shear produces.

So going by the digressive curve, I may be wrong in saying that torque transmission reduces as slip increases. However, I'm pretty confident that the viscosity of the fluid reduces with slip and that if torque does continue to rise with slip, it will only be because of the extra torque put into the VCU (needed to make that slip greater) that gets transferred to the back axle - eg 100 'torques in' gives x slip resulting in 80% viscous transmission resulting in 80 'torques out' - where as 200 'torques in' gives x * 2 slip resulting in 70% viscous transmission resulting in 140 'torques out'.

These discussions go on for ever because nobody can describe a definitive answer!

Hi GG,
This is an interesting article -
Please read (section 4.3.1)
https://books.google.co.uk/books?id...EIMTAF#v=onepage&q=degressive viscous&f=false

Best description I have read to date and perfectly logical when you think about it.
Joe

Hi Joe,

I saw that yesterday. It is a good concise description. However I chose not to put a lot of emphasis on it for 2 reasons (1) its by Borg Warner (or someone who works for Borg Warner) and while they obviously have a huge amount of knowledge in vehicle transmissions, I don't associate them with viscous couplings and (2) he says that hump mode only kicks in after several seconds of plates slipping - all the info I've read to-date indicates it happens much much much quicker than that.

I suspect that BW tested with VCUs that were using a larger plate and/or air gap than in the GKN units - so it will indeed take a lot longer for hump mode to kick in - which will explain why some Freelander rebuilds are naff - because they don't put enough fluid in them (obviously they reuse the same plate setup).

The hump mode kick in comes back to your problem with the Portuguese VCU recon. You were on a slippery slope with front wheels spinning and not enough power was transmitted to the back to get the car moving. Obviously, the VCU never got into hump mode - it was just using viscous mode and even though you probably "went large" with the Go peddle, it still didn't move. If viscosity increased with slippage (ie dilatant), even with 30K CST fluid eventually it should have had enough slippage to shift the car. But it didn't. Hopefully a Bell's recon with 100K CST would have shifted it in viscous mode with low slippage/high viscosity and if that didn't work, then hump mode will kick in almost immediately to get it moving.

If I were to be blind focused in my belief that torque transferred drops with slippage, I should rave over this article as the torque transfer graph clearly shows that!

Hi GG,
My take on all I have now read and discussed so far is -
I think BW are are experts in transmissions of all types, including viscous couplings, however, a book is a book and is the content is up to the author / publisher. I do tend to agree with their synopsis I must confess.
But, as has been observed by the various documentation, there are many features of vcu operation that can be 'tuned' and altered so sometimes one can sometimes be comparing the characteristics of apples to apples yet one is made into cider
The hump mode is interesting as the thermal expansion has to work in conjunction - and - as a by product of the heat generated in the slippage. If the hump mode caused a thermal lockup only then it would be impossible for the heat to rise in less than a few seconds and also to dissipate and allow normal operation. The dissipation would take far far farrrrr longer. (see below re coefficient of thermal expansion and possible reasons the above is incorrect)
Even with a 'completely filled unit due to thermal expansion' the vcu must be able to operate normally in an (effectively) instant manner as soon as the torque loading via spin is reduced when normal conditions return.
I would tend to agree with the description in the book that the 'viscous mode' is the 'normal' method of operation and that the 'hump' mode is only entered after a sustained period of slippage - ie - when the viscous mode torque transfer is unable to provide the desired reduction of spin speed. If - as spin increases then torque transfer reduces - Because all high spin incidents start with low spin incidents, the torque transfer at low spin (above normal driving conditions!) incidents should rapidly feed in a sackfull of torque to attempt to control the spin - which the graphs and description show. However, if the torque transfer characteristics are not enough to reduce the spin then obviously the spin increases (or remains constant!) and the hump mode is triggered (which also follows the books description). In most slip conditions I believe that the viscous mode would be perfectly suited to the needs, however, as the article say, in a period of constant slip (the inability of viscous mode to control the spin speed, then heat is generated causing the thermal expansion, if the viscous mode is effective in controlling the spin (on snow / ice / loose surface etc) then the hump mode will never be entered - and does not need to be.

Regarding my horrible portuguese 're-con' unit - the unit offered NO low down torque transfer even on wet grass - only if you gave it 'large' did transfer take place.
With the Bell unit - this is not the case - the unit works from even the gentlest of spins and also - measuring the temp of the vcu after a few minutes use in a mild slip condition (wet grass / wet surface underneath) indicates that the temperature of the vcu is little changed from normal. That would definitely indicate to me that at no stage at all was 'hump' mode engaged.
I must confess though - I have no idea what the thermal expansion rate of the fluid medium is.... however, I doubt it is in the order of 10's of degrees C to fill the chamber and cause plate to plate friction. Conversely though the same reduction in expansion of the fluid HAS to occur extremely rapidly when normal drive conditions are encountered which appears counter intuitive to the laws of thermodynamics.
The differing ratios of the front and rear may well be a reason for rapid unlock in even high thermal hump mode as the force of effort on the plates would effectively be removed and ever so slightly reversed (if my brain is functioning this morning- which it may not be! ) - that could actually explain a hell of a lot... I realise the v6 is geared differently, but I believe there is still bias - also, plate design could also favour far different characteristics in spin direction this would also tend to explain the above and also why normal driving does not overheat the vcu even with a small overdriven bias - it would also explain the extremely sensitive issue of tyre sizes which will cause issues with bias reversal if sizing is incorrect leading to the vcu operating in it's 'far more sensitive' direction bring on a rapid increase in Torque in viscous mode which is extremely undesirable in normal conditions and can and will cause damage eventually- Hmm, me needs to ponder that as it would definitely explain a heck of a lot!!! - hmmmmmmmmmmm. -

Again, all good fun -
I have PDF copy of the above book if you want me to upload it to my cloud. ? it is rather excellent. (in fact it is a most amazing book and includes all math where needed. It covers a massive range of subjects and is very up to date !) - definite 'dump enlightenment' lol

I find these subjects fascinating and it is good to hear differing and thought provoking arguments.
Joe

ps GG - yikes !!!! and fookin 'ell.... !!!!!! - check the RRP of the book in it's 2015 edition !!!! - £1595 !!!! - Amazon Prime are offering a 'tremendous' deal on it - for the incredibly low price of - wait for it - £1311.30 - WTF ???????

I just fell on the floor...........................

lol, yeh it is because nobody (on here at least) 'really' knows how it works - or even the actual fluid (or fluid mix) that goes into a LR spec GKN unit that no-body can say for sure exactly how it works. You think you fully understand it, and then another 'characteristic' (of the coupling or supposed fluid) is brought up and you rethink it all again

Its why these threads go on and on. Its all pretty academic and just for interest anyway - I don't think we're going to make the Freeander's transmission any more reliable by knowing exactly how it works.

I think we can categorically and absolutely say that the fluid is PDMS at 100K CST - I'm totally 50% convinced of that!

As such PDMS is chosen obviously because of its viscosity and 'widget' characteristic that puts it into hump mode (or is it the plates reacting to it!). I've also read that it is chosen because it does have a high rate of fluid expansion - so it will pressurise the unit into hump mode with the 'minimum' of heat rise - whether that's 10 or 1000 degrees I know not - but that initial graph indicates that in the VW tuned units its at about 125 degrees C. But of course the outside of the unit doesn't have to reach that temp - just the fluid - so if a rapid shear puts it up to 125 degrees in 1/4 of a second, that's not gong to permeate through the casing before traction is gained and it all cools down. As you mention though, once it hits that magical number and locks up, there is no shear so immediately cools and goes back to viscous - which if it hasn't gained traction in that small fraction of a second while it was locked (and once again I quote a figure that I have no understanding of!) will immediately put it back into hump/locked. This 'equilibrium' of viscous/hump condition is well known about.

With traction gained in a second or 2 of equilibrium, I'm not sure how much heat would permeate through the casing. If you've got a naff tyre and the coupling is constantly slipping at a rate that's not high enough to reach hump mode (say 50 degrees), then yes, you would/should feel that on the outside of the casing. If you have a failing overly tight coupling then its viscosity has risen so high that all/most slippage is through tyres, not the VCU, so it stays cold.

I've also seen discussion about PDMS that it has an elastic-solid type behaviour at rest - only once there is shear does it become a fluid (with the viscous traits). This could be seen to aid the VCU operation (as you say to stop slippage before it starts) - but of course there will always be 'some' shear through IRD gearing and slight tyre size differences - so one could probably discount this trait from a motor vehicle VCU. Incidentally I think this is what gives 'Silly Putty' its main characteristics.

Will the 'book' in the 'cloud' not get a bit soggy? I'm not sure if I really want to red that book - I'm sure it would be fascinating and I'd love to read it, but I should really fix my guttering or project Freelander first! Mind you, if I could flog it for that silly $1,595 price, I could buy another Freelander

GrumpyGel said:

I think we can categorically and absolutely say that the fluid is PDMS at 100K CST - I'm totally 50% convinced of that!

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LMAO !!! I love that quote
I am also 50% sure that I believe 100% of what I am saying about 20% of subjects ....

Nodge68 said:

Changing the VCU bearings isn't going to fix the VCU. If there's sufficient torque passing through the VCU to rattle it in its rubber bushings, the VCU is f.....d.

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utter bull****. on mine the bearings and the rubber bushes were shot to bits, new bearings sorted it

Its an interesting debate, The principle of the vcu works is a similar way to an auto gearbox. Bear with me on this one.
I had the luck/missfortune to to work on a forklift truck when I was on holiday in Australia (long story, don't ask).
The drive on a forklift and the auto gearbox is almost identical.
Anyway, I went with my mate to an autobox repairshop to set up the drive clutches for the forklift.
Basically we had to set the clutch with a 3-4 thou (inches) gap, The drive is provided when the gearbox oil is forced into the clutch assembly and it locks giving you drive. when the assembly wears beyond about 10 thou you get a very lazy gear change and it may start to slip as the oil pressure cannot provide enough pressure to lock the assembly.
In the vcu you have the same arrangement of inner and outer drive plates. what you dont have is a pressure delivery system. I can see the temp increase doing the same job as a pressure pump, but I have to say the manufacturing tolerances of the parts and the filling of the VCU must be pretty tightly controlled to get consistant results.

I suspect it's more about the laws of physics and a fluid only being able to be displaced at a certain rate.
This principle is used in laser shock peening on metals. I introduced this on components I work on. I need more time to explain than I have today, but google MIC and laser shock peening.