I was looking for an article describing how the internals of a Quaife differential work. Quaife’s own web site isn’t very helpful.
Rather than using clutch plates as a method of operation, the Quaife ATB uses sets of floating helical cut gear pinions that run in pockets and mesh during normal driving. Should one of the driven wheels start to spin however, the helical gears start to generate a torque bias thanks to the axial and radial thrust of the helical gear pinions in their pockets. The result is a progressive transfer of torque away from the spinning side of the axle to the driven wheel, which is now capable of transmitting a greater proportion of torque.
That’s a summary, not theory of operation.
After reading a few different pages, I started to get a feel for it. I think it goes something like this: Helical planetary gears assume the role taken by pinon gears in a normal diff. Since the planetary gears are helical (at an angle), and are floating in pockets in the rotating housing (”cage”), a torque differential can generate forces that press the planetary gears into those pockets. The friction between the planetary gears and the cage generates lock for the wheel with the most resistance.
I’m still working out the forces. And I don’t yet understand why a wheel that completely slips prevents the diff from delivering power. I’ve read that the differential acts as a torque multiplier. So no torque on the slipping wheel means no torque on the driven wheel. But I don’t yet understand why.
I haven’t digested it yet, but this is the most detailed description I’ve found: Quaife and Guard ATB / TBD Differential Comparison.
A good overview of diffs can be found at How Stuff Works. The article covers clutch based LSDs, but doesn’t really explain why they work. Knowing how the clutches are actuated helps in understanding why custom ramps are awesome. I thought I understood that, but I can’t write a description off the top of my head. So I guess I don’t know it that well.
Don’t get my started on Torsen, which is another kettle of fish. I’m still working on Quaife and clutch LSDs.
Another page on gear-based LSDs.
I’m getting a little more clarity on the unloaded wheel thing. The planetary gears work together. When there’s a speed difference between wheels, they’re moving (as a set) towards the more slowly moving wheel. In doing so, they’re increasing the friction between the slow wheel’s planetary gears and the cage, thereby increasing the torque delivered to that wheel.
This effect works only when there is some resistance from the faster wheel. That resistance means there is force on the interface between the faster wheel’s sun gear and corresponding planetary gears. That drive force translates into lateral force (due to the helical cut), which is an essential element of the side-to-side movement of the planetary gears.
Another diff page.
This served as a reminder for how clutch based LSDs work.
The input pinion drives the ring gear. The carrier (aka “cage”) is attached to the ring gear. The carrier carries the pinion gears that drive the side gears (connected to the half shafts). As torque is applied, the carrier exerts force on the pinion gears’ shafts. They are moving with the carrier, so there has to be some force. In a clutch based LSD, these shafts are called cross pins.
Now suppose the carrier is in two halves, and the cross pins are in ramped slots. As torque is applied to the carrier, the cross pins, acting against the ramps, push the two halves apart. Now suppose that as the carriers are pushed apart they compress clutches that lock the output shafts to the carrier. Instead of moving at different rates, both output shafts will spin at the same speed as the carrier.
For deceleration, a different ramp (or no ramp) can be used at the cross pin/carrier interface. This allows different lockup behavior between acceleration and deceleration.
(I’m sure this would all make more sense with diagrams.)