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# Eddy-Current Limited Slip Differential

Use permanent magnets to for maintenance-free limited slip differential
 (+4) [vote for, against]

The typical car is equipped with some device that generates rotational force driving two of the wheels. Because drivers are fundamentally indecisive, cars often change direction*. As the net motion of a car is usually a circle, the wheels on different sides of the car travel different distances. To prevent embarrassing tire noises and strange handling, cars are fitted with a differential <link>. In most cases the problem is solved, it's pretty robust, we can pack it with grease and retire to the pub safe in the knowledge it will last decades.

However, should you get onto a surface with variable traction, snow/ice being the usual culprits, then the wheel with the least friction will spin around uselessly while the wheel with the traction does nothing at all. What we need is a way of allowing some differential movement, but when there's a lot, a way of transferring some of the torque.

Several methods currently exist, the simplest is a clutch <link>. These are somewhat uncivilized and wear out. Torsen<link>, the main competitor here, is good but expensive, so much so, that even most "4wd" cars stick with open differentials. In radio-controlled racing cars which are so light and ludicrously over powered that they have essentially no traction most of the time solve the problem by fitting enormous differentials and filling them with variable viscous oils that act as a velocity-sensitive damper. At full size, this would overheat and require too much maintenance.

So, how do we get the velocity-sensitive damping without the mess? Enter the eddy-current brake <link>. If you try and move a magnet adjacent to a conductor, you will induce a current and produce a force. Examples of this include the dropping a magnet down a pipe experiment <link>. The principle is used as a power-independent brake on roller coasters and on earlier generations of bullet trains. The beauty is, that nothing wears out and it self operates.

To incorporate this, we have the drive shafts fitted with a thin disk, this is close to the differential case. The differential case has powerful neodymium magnets in, when the drive shaft spins quickly relative to the case (one spinning wheel) the magnets will generate eddy currents in the disk transferring torque to the case and therefore, over to the other driveshaft/wheel. There, a relatively mild LSD that should work forever and cost relatively little to make.

Secret bonus, if you ever drop a screw, it will likely be found safe and secure on the diff casing.

*This is something of a scandal, investigation reveals car journeys to be oscillatory in nature, with nested oscillations of various periods. Within a 24hr period, cars leave and return to the same location. Often, the ultimate destination of a car may be the location from which it was purchased. Significant energy savings might be afforded by simply leaving the car stationary after purchase, and having a bloody good think about where you actually want to be, which is unlikely to be at a car dealership.

 — bs0u0155, Sep 21 2020

Limited Slip Differentials https://en.wikipedi...d-slip_differential
[bs0u0155, Sep 21 2020]

Transferring Power through a Magnetic Couple https://digitalcomm...ext=physstud_theses
[xaviergisz, Sep 21 2020]

US5477093 - Permanent magnet coupling and transmission https://patents.goo...atent/US5477093A/en
[xaviergisz, Sep 21 2020]

History and future of a motor at each wheel ... https://spectrum.ie...-evs-more-efficient
... which I expect bs0u0155 has already read. posted for everyone else. [kdf, Sep 23 2020]

It's a Knockout https://youtu.be/Z2t5WP1uYAI
Too highbrow for me [not_morrison_rm, Oct 01 2020]

[link]

There are an awful lot of patents for magnetic clutches, torque converters, and limited slip differentials. Relying mainly on eddy currents in a metal disk might not be a good idea as that generates/wastes a lot of heat.
 — kdf, Sep 21 2020

 Not that wasteful; energy dissipation is algebraically related to speed of rotation, and most of the time the port and starboard shafts are only moving very slowly with respect to one another.

 It's when one wheel spins - something an LSD is meant to prevent - that there's significant differential (ha, ha) velocity.

 A bigger issue might be that the system has a tendency to alternately slip and then latch up, leading to snatching and making torque control problems worse.

The best solution is probably to drive each wheel independently with a feedback-sensed electric motor. That's where a hybrid drivetrain wins; no need for a gearbox, and for a small vehicle a modestly sized engine (maybe in the 150 - 200kW range) and a small battery means that on startoff, the enitre output of the engine plus the entire battery surge capacity can be dumped into the motors (which will admittedly heat up, but can withstand brief overloads) until the vehicle hits that golden 150km/h moment.
 — 8th of 7, Sep 21 2020

 //alternately slip and then latch up,//

The relationship between velocity delta is proportional, in a very non-linear way. So there's not really a mechanism for that, it's quite similar to a viscous coupling. A clutch-type is more like that in behavior, since the transition from engaged to slipping involves sticktion. There's likely a linear- ish relationship between velocity delta and force once it's slipping but un-sticking is a hill to get over, so there's the propensity for oscillation.
 — bs0u0155, Sep 21 2020

 Yes, the possibility of oscillation is a major issue; just what you don't want, alternate drive/free run.

 The viscous coupling will have quite a nice smooth characteristic - it's the fluid heating issue with prolonged load that's problematic. There are external coolers for ATF (WKTE) but you'd have to couple it to a RWD diff with flexi pipes to keep the unsprung mass down, unless the vehicle has FIRS ...

If the viscous diff is integrated with the gearbox in a FWD design you can patch in the cooler much more easily than for RWD.
 — 8th of 7, Sep 21 2020

 //it's the fluid heating issue with prolonged load that's problematic.//

hmm, place the drive shaft-coupled disks on the outside of the casing, cool them like you might with inboard disk brakes.
 — bs0u0155, Sep 21 2020

//Yes, a motor at each wheel// including the spare, of course!
 — pocmloc, Sep 22 2020

<Sean Connery voice>"The name's Current Limited Shlip Differential, Eddy Current Limited Shlip Differential"</SCv>

This idea appears to add weight and complexity to the car for little gain, solving a problem probably better addressed with the 'motor at each wheel' proposal, so [+].
 — hippo, Sep 22 2020

 //motor at each wheel solves everything neatly//

 This is such a good idea that it has met with universal non-implementation in the automotive industry. There have to be reasons. I'll have a stab at what they might be:

 Torque vectoring is limited: In a proper mechanical 4wd system, 100% of the torque can be sent to one wheel, handy while rock-crawling etc. in the motor per-wheel situation it's 25% + any sort of temporary boost.

 Unsprung mass: If you want to put your motor on each wheel it makes sense to build it right in, only then you have a ~30kg mass on the end of your wishbone/swingarm etc. which will ruin the ride/handling/grip and smash a lot of wheels. The vibration will likely kill motors too. If you move it inboard, then you already concede the need for a transmission, so why not drive two wheels with one motor and do your torque vectoring with an active diff?

 There aren't even any (fun) radio control cars with a motor at each wheel.

Liability: Electric motors are reliable, not infallible, controllers less reliable. By quadrupling the motor & controller count you add 6 additional points of failure. With one motor and a transmission, a failed motor stops the whole car. When one motor of 4 stops, it could easily cause a dramatic change of direction or even for the car to rapidly swap ends on a mild highway bend. If a car stops, the car that runs into the back of it is liable. If a car veers into the opposite lane, then that car/driver is liable. In the case of a motor failure, the company would end up with the bill. It's hard to think of a way to make that situation fail in a safe way.
 — bs0u0155, Sep 23 2020

 It is, isn't it ?

<Evil chuckling/>
 — 8th of 7, Sep 23 2020

<obligatory> Er, who is Eddy, then, exactly?
 — sninctown, Sep 30 2020

 // who is Eddy, then,/

He's the one currently making the differential.
 — bs0u0155, Sep 30 2020

"Eddy's Currants", a homely brand of dried fruit ?
 — 8th of 7, Oct 01 2020

 No connection to Eddie Waring?

See link "It's a knock out"
 — not_morrison_rm, Oct 01 2020

No, thankyou. We are already too familiar with that particular execrable light entertainment feature, i.e. we are aware of its existence, which is actually too much information.
 — 8th of 7, Oct 01 2020

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