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Cam and pawl differential, continued...

Cam and pawl differential, continued...

Cam and pawl differential, continued...

thread78-313320: Cam & Pawl Differential

As it happens I've been researching this type of diff as a variation is used as the center differential in the 2006 through 2012+ 4WD Suzuki Grand Vitara, I have a '10 model and per my usual habit am trying learn every technical detail about it, as I do with every vehicle I own.

There is some, but not too much information on the web about this. It was invented by ZF in 1930 and used on certain VWs. It was intended as a limited slip axle differential and later appeared on some low volume UK vehicles. These applications were known to have high wear. The path gets blurry but it appears to have been developed further by Automotive Products LLC (called APTRAC) then passed on to Eaton and was once known as a Suretrac, before that name was transferred to a more conventional clutch-pack LSD design. It was used on the 2004 Subaru WRX (at one axle only) but received bad reviews.

OK, enough of that. The main thing is to clarify that it's nothing like a Detroit Locker. No slippage occurs nor is there any ratcheting action. It's more like a harmonic drive. The cams have a differing number of teeth on each side so the pawls can slide axially in-between the cams. There is a lot of axial sliding going on for very little speed difference so the friction provides an "LSD" effect. A Subaru technical description can be found here.

It's interesting that there is an inherent uneven torque split, in my case 47/53 as a center diff, but this did not seem to discourage axle diff applications. I have not discovered where Suzuki sources this unit (pic below) but the geometry has been refined from earlier versions, presumably for ease of manufacture. When I do a U-turn I can hear (feel in the steering wheel) the pawls sliding back and forth. Because it doesn't work that well as a limited slip Suzuki adopted brake-type traction control from 2009 and possibly even revised the cam angles in the unit to reduce noise and wear yet still provide an open center diff function in a small package.

RE: Cam and pawl differential, continued...

It's axial cam and pawl diff, but I'm intrigued a bit about opting for wedge profile for cams and pawls... I would think it's supposed to give some 'preload' i.e. to increase torque difference between wheels required for diff to unlock (I'm still of the opinion I think I've expressed in older thread that the natural 'state' of those diffs is locked).

Thanks for the technical description- will look into it.

RE: Cam and pawl differential, continued...

Why is there an inherent uneven torque split? It appears to be symmetrical in every respect.

RE: Cam and pawl differential, continued...

RossABQ, the reason for this assumption is the different number of lobes on each of the cams (IIRC the radial diffs typically had 13 lobes on inner cam and 15 on outer- so this type would have different number of lobes on left and right cam)- but the way I see it cam & pawl diff does not work like a gear, so the different number of lobes or 'teeth' does not come into play. I'd say, although I've seen a few mentions of uneven torque split, I do not subscribe to that theory. With diff in 'normal' operation it's locked and transmits torque evenly, and should it begin to slip the torque split will be a complex function (position dependant) and the friction will, when it's designed properly, prevent it from excessive wheel speed difference (not allow it to function like open diff).

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