clutch type ltd. slip diff. (function of)

[Artsi]

Can anyone please shed light into very essence of conventional clutch pack LSD?
I have been wondering just what makes it tick.
I was surprised to find all-metal clutch discs instead of discs with some type of friction surface on them when I disassembled one.

Metal discs in question had an array of spiral grooves machined onto them. Pondering this arrangement awhile, I came into conlusion of grooves making oil being worked by the grooves. I guess it's safe to assume this generating some amount of heat as a byproduct of primary action/function.
I did not realize to inspect which way the spirals would push the oil; off center towards outer edges of disc, or other way round.
Here's a pic a friend took during assembly. (Note completely smooth disc being wedged between spiral grooved discs.)

http://celica.samoth.net/projektit/pera_110803/kuvat/lukon_kitkalevyt.jpg

The rear diff had about 120thou km's/75thou miles on it. Clutch mating surface had absolutely minimal wear on it, round about at 0,02mm/0,001" ,or less (on one side).

Question is; how does oil grab adjacent spiral grooved clutch disc to tag along with another disc, in order to provide some torque to driving wheel that is on firm non-slippery ground?

Browsing the net has taught me that conventional diff oil will make LSD unit to chatter and lock up violently. LSD oil has 'friction modifiers' in it. (Alternatively LSD additive can be added to conventional diff oil.)
How do these friction modofiers work?


-A.

 

[Metalguy]

The reason these things work is because of the "spreading force" between gears under load. In this case the diff. pinions (the small gears on the cross-shaft) press the diff. side gears apart, and thereby squeeze the clutch plates together. The actual movement is very small, and you can make the LSD work better by keeping the no-load clearance between the plates/discs to a minimum. Either find and use slightly thicker plates or use shims to reduce the clearance to the min. I've found that as long as you can turn the assembly by hand you have enough clearance.

The oil is just for lubrication/wear.

 

[drwebb]

Oils may contain some molecules with dipoles attached to one end that stick themselves ('adsorb') to the metal surfaces and thereby reduce metal-metal friction. Conversely the molecules may contain dipoles at both ends, (tethered by long oily segments), that stick to two metal surfaces, or can link to other molecules that are stuck, thereby increasing metal-metal traction without the wear of actual contact. While these interactive forces are weak, with several quintillion molecules acting in concert it adds up to measureable changes in metal-metal friction coefficients.

Such chemistry has been used in traction fluids before, and is of keen interest to increasing torque capacity for the various CVTs now under development.

 

[shabba]

Never heard of the diff Metalguy describes, but have seen a few *torque-dependant* lsd's which use a clutchpack with the application pressure resulting from the pressure angle of the pinion and the driven gear. There are other types of diff which work with synthetic (mostly silicone based I believe) oils. These use the fact that some fluids change their viscosity when put in shear, and therefore the drag between closely running plates increases as their relatice speed changes. i.e. the more one wheel spins wrt the other, the more resistance in the disc pack.

Cheers

 

[mikergp]

The force between the 2 "side" gears caused by the outward force of the angled "pinion" gears can be of great use. I have developed a "LSD" kit that can be installed in a front wheel drive transaxle that uses this outward pressure to engage paper friction plates. This turned out to work quite well and I have sence been testing putting spring plates between the side gears preloading the clutch packs. Not sure about the life of the clutches as of yet but it works much better so far.
The front wheel drive LSD actually got rid of all the "torque steer" that comes with a 450HP FWD car

Mike
http:\\

http://www.c-ya-racing.net