How Exactly does the QT diff work?

[Frustrated]

I went to an auction the other day and bought 2 QT transfer cases. I have one in my Jeep that is converted to part time and it is pretty easy to see how that one works. My question is how does the full time QT differential work? I tore one of my cases apart and have the brake cones and spider gears laying on a bench and it doesn't add up to me.

I have visited RiverBeast's site and checked out the pictures, which were helpful (nice job RB) but not quite complete enough (for me anyway!).

I have often heard the QT compared to a limited slip diff, does anyone have a good way to explain this. Better yet, does anyone have a website that explains how limited slips work.

This post is intended to induce thoughful conversation and help me understand some engineering principles, thanks in advance.

[Rogue]

hey Frustrated - I'm originally from Warren PA about an hour from ya! ok to keep it simple - it is a limited slip differential - it works just like a limited slip in the axle - if the front driveshaft spins faster than the rear then the diff will "lock up" transferring power to the axle with traction, this can be bypassed with the vacuum switch ( e drive switch in glovebox ) which will mechanically lock the front and rear driveshafts together

[Crazy_Jeepman]

Close but not quite. It transfers power to the axle with the least amount of resistance, thus being the axle with the least amount of traction. This means in normal mode your rig could be front wheel drive or rear wheel drive but not both, unless equal traction on both axles. E Drive locks both front and rear axles together giving both axles power. ;)

[wanablikemike]

What is the advantage of having more power where there is less traction? That doesn't sound right to me.

[Crazy_Jeepman]

There is no advantage, but that is Physics and that is how all Differentials work. Ever notice when stuck its always the wheel that is in the Mud or snow that does the spinning? Limited slip, possi trac, etc all transfer a portion of the power to the wheel with traction, but unless like new it will react like and open diff and the wheel with the least amount of traction will still spin. Q-Trac is the same way it sends some power to the axle with the traction but in most cases it is the one with the least amount of traction that gets the power.

[Rogue]

that makes no sense TO ME whatsoever Pete why would there be a limited slip differential in the t case if it transferred power to the axle that is spinning? why not just use an open diff if you wanted to give power to the wheel with the least traction?

http://home.off-road.com/~jseries/BWQT.html

check this site out ( only cause it agrees with me, hee hee ) it gives a pretty decent explanation

[Rogue]

i will agree though it doesn't actually work as well as intended

[Crazy_Jeepman]

Here is further info on this. LOL I knew this would get interesting. Q-trac and other types of Posi Diffs all have a break point. Meaning it will take say 150 LBS of torque to break away from the power. Complicated now, when new no wear and in condition all dream of, it will send power to the axle with the traction, untill it breaks over 150 lbs of torque, so if the rig will not move with in the clutches holding power the axle/wheels with the least amount of traction spins once again. I am useing 150 Lbs of torque (Clutch Pressure) as an example some are more most are less. ;)

[Rogue]

i forgot about the breakaway stuff redface.gif oh well ;) once again i bow smile.gif

[Crazy_Jeepman]

No need to bow!!!! :D Just get STUCK MORE OFTEN!!!!!! :D :D :D

[Rande]

So, if your QT is in good condition and your axles have equal or nearly equal traction, you will have 4 wheel drive. Or...actually you won't. Because you have open diffs in your axles So you will have 2 wheel drive. Unless you install limited slip diffs or lockers in the front and rear axles.
Also, if you have one axle driving and the other doing nothing, that is where the Edrive switch comes into play. Engaging Edrive locks the two axles together.When you exit the slippery stuff that has you spinning your wheels, you need to disengage it.

[Crazy_Jeepman]

Rande has been stuck before :D :D :D :D

[Bob Barry]

The breakaway torque on a good QT is 75 ft/lbs; that means that if the *difference* in the traction available front to rear is 75 ft/lbs or more, power will be sent to the axle with less traction.

Since most conditions don't involve absolute-zero traction (except glare ice or lifting a wheel), most of the time all tires are getting some power.

Of course, any time you're driving a QT on a surface that you otherwise would engage 4wd on a D20 or NP208-equipped rig, you should lock it in E-drive for true part-time 4wd.

[Frustrated]

Excellent discussion so far but I am looking for something a little more in depth.

How does the limited slip mechanism actually work? What purpose do the washers and break cones serve?

Also, when you put the QT in E-drive mode, how does it lock the two shafts together. The collar engages the splines of the rear shaft and the splines of the drive gear. Since the drive gear is always spinning (due to the drive chain, which is attached to the tranny output), I can see how that would "lock" the rear output splines. But, how does this "lock" the front since its power still has to go through the center diff?

Complicated yes, but impossible, no!

[Pete1111]

I was reading through these posts just for my own general knowledge about my '82 QT unit and now I am really confused! :rolleyes:

RogueStar had a link a few posts above this to an off road site. I read that and it says EXACTLY the opposite of what people are saying in here. Does anyone know how the QT units REALLY work and what their benefits are??? Seems everyone is disagreeing with one another yet there can only be one right answer. And as we're on it, the off road site says after '79 the QT unit is completely different. Anyone know the differences besides that mine can take Auto Tranny fluid :D ???

[Crazy_Jeepman]

The Chain drives the center Diff, not the rear output. Then the power is transfered to the Pinion Mate Gears, (Fancy name for spider gears) These then power the Side Gears, to front and rear outputs. However power from the side gears to the output shafts is done through the Clutch cones. The torque on these clutches in the BW1339 should Breakaway between 80 to 170 FT LBS (Jeep Shop Manual) When you engage the E-Drive, the Clutches are not able to slip because the E Drive Lock Collar slides into place and locks both front and rear output shafts together. I hope this helps. Here is a Blow up of the BW1339 if it looks like it will help anyone, save it to your hard drive as I will take it down in a few days/weeks, whenever I get to needing the space :D :D Q-Trac Blow Up (http://images.andale.com/f2/116/106/4651773/1026916896677_Q_Trac.JPG)

[ July 17, 2002, 08:55 AM: Message edited by: Crazy_Jeepman ]

[Rogue]

Pete1111 - the "quadratrac" in your 82 is a completely different unit than the one we are discussing, as far as the discussion goes we are all right, the quadratrac unit will deliver power to the wheel that slips up until the breakaway torque of the limited slip unit, once the torque demand exceeds the limited slip unit's capacity than it no longer functions as intended, this can be bypassed with the Emergency drive switch that mechanically locks the front and rear driveshafts together - kinda like putting a spool in your rearend ( err, since we're being technical the Jeep's rearend, lol ) i can't help you with your t-case i don't have the first clue what model it is or how it works, i think that is the style though that had a viscous coupling ( mini torque convertor )inside of it.

FRUSTRATED - when one driveshaft spins faster than the other, this will force the brake cone into the side gear not allowing differential action to be taken by the spider gears, the spider gears are what enables a differential to allow the drives to turn at "different"(ial) speeds

[ July 17, 2002, 09:13 AM: Message edited by: RogueStar ]