Disengagement of a multi plate clutch

[iamasnowmanboy]

TL;DR Summary

how does a multi plate clutch disengage

I am a student at a mechanical college, current topic of discussion is multi plate clutches.

I understand that the clutch is engaged when the pedal is released, with allows the springs to expand, pushing together the plates, that will transfer the torque in to the drive shaft (if in very brief summary).

However, I don't understand what is the mechanic that disengages or separates these same plates from each other, after they are no longer pressed by the springs, because if nothing releases them from each other, won't they just stay stuck together? Is it the rotational force of the drive shaft, or is there something else in place?

 

[berkeman]

Welcome to PF.

Aren't the ends of the spring extensions attached to the plates? I'd assumed that's how they worked. Can you link to a typical mechanical drawing of such a clutch to illustrate your question? Use the "Attach files" link below the Edit window to upload a PDF or JPEG image. Thanks.

 

[Baluncore]

It is surface friction.
When axial pressure increases, the plates lock together. When the axial pressure is reduced, friction is reduced and the plate surfaces can slide.

Multiple plates multiply the advantage, because the same axial force on all, is used to control several plates, that drag in parallel.

 

[Lnewqban]

Welcome, @iamasnowmanboy !

Even when they stay as a pack, alternately one disc has more torque than the next, and no normal force inducing friction.
Vibration is another thing that tends to release one from each other.
The faces of the metal discs are flat, but the faces of the fiber discs have profiles that avoid complete depletion of the fluid between two discs.

 

[Ranger Mike]

Analyze the force and the individual components.
Steel or aluminum flywheel connected to the crank shaft.
Metal pressure plate with springs that compress.
Clutch made of material made to be shredded off at each engagement.

What happens in this single clutch engagement? Drop the clutch pedal, torque or force is transferred to the tires . If the car weighs too much, the clutch slips and some material is lost, and energy becomes heat because Energy must go somewhere during the process. No problem for the grocery getter and replace the clutch after 100,000 miles.

(Note - torque is the technical term of the force but for ease of understanding, from here on out in this discussion, HP may be substituted for torque because it is easier for me to type HP and the average person to understand.)

Back in the day the slingshot dragsters (drag racing vehicles made to only have maximum acceleration, had the driver positioned like a slingshot with cockpit setting in back of the rear end differential for maximum traction . Maximum weight was placed on the rear drag slick tires. This set up had great advantage until the engine horsepower
was overcoming the drag slick tires traction and the tires smoked big time. Loss of traction by too much HP. Many drag races lost due to lack of traction.

Someone figured out that by reducing the pressure plate spring force, you could hook up the drag slicks and have more traction by intentionally slipping the clutch. This was true but also added a death threat in the fact you create a hand grenade and you literally had a time bomb right under your legs waiting to explode.

Slipper clutches killed a lot of drag racers until Don Garlits went thru this process and lost his foot. Recovering in the hospital, the sage innovator developed the first working rear engine dragster design placing the driver in front of the engine and clutch package.

One evolution as to use multi clutches of smaller diameter to take up the load and disperse the heat over many discs and prolong the fatal clutch detonation. Also you had a lower rotational moment as the discs were much smaller in diameter and helped vehicle acceleration.
So you now see 4 ½ diameter clutch packages compared to the old 12 inch single disc clutch design and more improved clutch material. Asbestos was not too healthy I may add.

Clutch discs do not stick together if there is not force to make them press on the fly wheel if the pedal is depressed. They are made of material to soften the interaction of the dissimilar materials and absorb the impact.

You said -Even when they stay as a pack, alternately one disc has more torque than the next, and no normal force inducing friction.
This may or may not be the case as one disc will wear more than the other but so what?

Vibration is another thing that tends to release one from each other.
Vibration comes from a no balanced situation- not the case with quality parts.

And No, Rotational force separates the disc package when the pressure plate is not engaged.
You do have clutch chatter causes by heat spots on the flywheel from excessive clutch slippage causing an overheated condition.

The faces of the metal discs are flat, but the faces of the fiber discs have profiles that avoid complete depletion of the fluid between two discs.

What fluids?
If you ever pulled apart a race car clutch package the whole thing is covered with clutch disc dust.

Don't overthink this. it is not rocket science but straight physics.

 

[berkeman]

Slipper clutches killed a lot of drag racers until Don Garlits went thru this process and lost his foot. Recovering in the hospital, the sage innovator developed the first working rear engine dragster design placing the driver in front of the engine and clutch package.

I did not know that! "Necessity is the mother of invention"...

 

[Ranger Mike]

Don Garlits saved a lot of drag racers lives doing this. All racers are innovators at heart. Always looking for new wrinkles and race edges for an advantage.
Most all do not have college degrees but there is no limit in a free society to their innovation.
Kind of gives you hope on future of mankind, huh?
https://www.hemmings.com/stories/20...ketching-his-ideas-for-a-rear-engine-dragster

 

[berkeman]

https://www.hemmings.com/stories/20...ketching-his-ideas-for-a-rear-engine-dragster

Holy crap!

 

[Ranger Mike]

This was in 1970 with a 1600 horsepower engine. Today the dragsters have 7000 horsepower engines. Material evolution of clutch material and multi disc packages had to evolve.

 

[Ranger Mike]

I know this is off topic from the Op but consider the spark plugs on a fuel dragster are completely shot1/2 way thru the race. The engine has more fuel consumption than a 747 airliner. It is on the verge of hydrostatic lock. See below..just for your information
TOP FUEL ACCELERATION PUT INTO PERSPECTIVE

Top Fuel dragsters are the quickest accelerating racing cars in the world and the fastest sanctioned category of drag racing, with the fastest competitors reaching speeds of 335 miles per hour (539 km/h) and finishing the 1,000 foot (305 m) runs in 3.64 seconds. Here are some fuel facts. * One Top Fuel dragster 500 cubic-inch Hemi engine makes more horsepower (11,000 HP) than the first 4 rows at the Daytona 500. * Under full throttle, a dragster engine consumes 11.2 gallons of nitro methane per second; a fully loaded 747 consumes jet fuel at the same rate with 25% less energy being produced. * A stock Dodge Hemi V8 engine cannot produce enough power to merely drive the dragster's supercharger. * With 3000 CFM of air being rammed in by the supercharger on overdrive, the fuel mixture is compressed into a near-solid form before ignition. Cylinders run on the verge of hydraulic lock at full throttle. * At the stoichiometric 1.7:1 air/fuel mixture for nitro methane the flame front temperature measures 7050 degrees F. * Nitromethane burns yellow. The spectacular white flame seen above the stacks at night is raw burning hydrogen, dissociated from atmospheric water vapor by the searing exhaust gases. * Dual magnetos supply 44 amps to each spark plug. This is the output of an arc welder in each cylinder. * Spark plug electrodes are totally consumed during a pass. After 1/2 way, the engine is dieseling from compression plus the glow of exhaust valves at 1400 degrees F. The engine can only be shut down by cutting the fuel flow. * If spark momentarily fails early in the run, unburned nitro builds up in the affected cylinders and then explodes with sufficient force to blow cylinder heads off the block in pieces or split the block in half. * Dragsters reach over 300 MPH before you have completed reading this sentence. * In order to exceed 300 MPH in 4 seconds, dragsters must accelerate an average of over 4 G's. In order to reach 200 MPH well before half-track, the launch acce leration approaches 8 G's. * Top Fuel engines turn approximately 480 revolutions from light to light! * Including the burnout, the engine must only survive 900 revolutions under load. * The redline is actually quite high at 9500 RPM. * THE BOTTOM LINE: Assuming all the equipment is paid off, the crew worked for free, & for once, NOTHING BLOWS UP, each run costs an estimated $1,000 per second. 0 to 100 MPH in .8 seconds (the first 60 feet of the run) 0 to 200 MPH in 2.2 seconds (the first 350 feet of the run) 6 g-forces at the starting line (nothing accelerates faster on land) 6 negative g-forces upon deployment of twin ‘chutes at 300 MPH An NHRA Top Fuel Dragster accelerates quicker than any other land vehicle on earth . . quicker than a jet fighter plane . . . quicker than the space shuttle. The current Top Fuel dragster elapsed time record is 3,628 seconds for the 1000' track (2018, Clay Millican). The top speed record is 336.57 MPH as measured over the last 66' of the run (2018, Tony Schumacher). Putting this all into perspective: You are driving the average $140,000 Lingenfelter twin-turbo powered Corvette Z06. Over a mile up the road, a Top Fuel dragster is staged & ready to launch down a quarter-mile strip as you pass. You have the advantage of a flying start. You run the 'Vette hard up through the gears and blast across the starting line & pass the dragster at an honest 200 MPH. The 'tree' goes green for both of you at that moment. The dragster launches & starts after you. You keep your foot down hard, but you hear an incredibly brutal whine that sears your eardrums & within 3 seconds the dragster catches & passes you. He beats you to the finish line, a quarter-mile away from where you just passed him. Think about it - from a standing start, the dragster had spotted you 200 MPH & not only cau ght, but nearly blasted you off the road when he passed you within a mere 1000 foot long race! That's acceleration!

 

[Ranger Mike]

Jack action, do you know of an engine dynometer capable of measuring this type engine? And thanks for the nice note.

And thank you and welcome to this forum iamasnowmanboy

 

[jack action]

However, finding a dynometer that could even begin to manage a Top Fuel engine's enormous fury and torque has been a problem, so racing teams have always relied on calculations based on g-force data to come up with an estimated horsepower figure.

However, for Don Schumacher Racing (DSR), there's no more guessing. Thanks to AVL Racing, DSR has implemented a torque sensor into the coupling between the clutch packs and differential.

Basically, the steel coupling is magnetized by electromagnets until it holds a strong magnetic field on its own. The torque sensor then uses a pick-up coil around the coupling that measures changes in the magnetic field that are caused when engine torque twists the coupling as it passes through to the differential. It turns these changes in the coupling's magnetic field into an accurate measure of engine torque, which is recorded by a Racepak data logger. As we all know, the Racepak records a wealth of vehicle data, so this torque figure can be calculated against engine rpm to produce an accurate engine horsepower measurement.

​

 

[Baluncore]

I note that the title of this thread was "Disengagement of a multi plate clutch", not "Disintegration of a multi plate clutch".

 

[Ranger Mike]

excellent point. Balun..and a catastrophic failure will disengage...permanently!
Jack great post I learned something!

 

[Dullard]

For the record: Not every multi-plate clutch is in a dragster. There isn't any dust on the the clutch in my Yamaha FZ-1 (it's a wet clutch).

 

[Nik_2213]

Is this similar tech to the lift-fan drive on F-35 VTOL variant ??

 

[berkeman]

​

Wow, 11,000bhp. That's crazy!

 

[Juanda]

Welcome, @iamasnowmanboy !

Even when they stay as a pack, alternately one disc has more torque than the next, and no normal force inducing friction.
Vibration is another thing that tends to release one from each other.
The faces of the metal discs are flat, but the faces of the fiber discs have profiles that avoid complete depletion of the fluid between two discs.

I was having trouble understanding why the number of discs multiply the torque as if they're working in parallel, because in my head I always saw them as being in a chain. The video you shared helped me understand the difference.
Thanks for posting.

Kind of embarrassing as a mechanical engineer, but this is how I thought it worked, so it made no sense for the torque to be multiplied by the number of discs.

INPUT TORQUE (T_in)
                 |
                 V
    ---------------------------
    |   Friction Interface 1  |  <--- Limit: T_max. If T_in > T_max, it SLIPS HERE.
    ---------------------------
                 |
                 V  (Full torque is not transmitted further)
    ---------------------------
    |   Friction Interface 2  |
    ---------------------------
                 |
                 V
    ---------------------------
    |   Friction Interface 3  |
    ---------------------------
                 |
                 V
            OUTPUT TORQUE

I now understand it's more like this instead so the $n$ factor makes sense.

     CLAMPING FORCE --> | | | | | | <-- CLAMPING FORCE
                        V V V V V V
  +-----------------------------------------------------------------+
  |    OUTER BASKET (INPUT - Rotates with Engine)                     |
  |                                                                 |
  |   +------------------------+                                    |
  |   |     Friction Disc      |----(T1)---->+-------------+         |
  |   +------------------------+             | Steel Plate |         |
  |   (Engages w/ Basket)                    +-------------+         |  <-- INNER HUB
  |                                          (Engages w/ Hub)        |      (OUTPUT)
  |   +------------------------+----(T2)---->+-------------+         |
  |   |     Friction Disc      |             | Steel Plate |         |
  |   +------------------------+             +-------------+         |
  |                                                                 |
  |   +------------------------+----(T3)---->+-------------+         |
  |   |     Friction Disc      |             | Steel Plate |         |
  |   +------------------------+             +-------------+         |
  |                                                                 |
  +-----------------------------------------------------------------+

  TOTAL TORQUE CAPACITY = T1 + T2 + T3 + ...

 

[sophiecentaur]

TL;DR Summary: how does a multi plate clutch disengage

I don't understand what is the mechanic that disengages or separates these same plates from each other, after they are no longer pressed by the springs,

I think you could ask how 'perfect' is the model that the above arguments would apply. IMO it would be less likely that the plates would all separate at the same time

If there is any change of axial force between a pair of plates then the torque will change. The friction pads are not rigid, they will deform slightly so the contact force (hence torque) will be (sort of) proportional to that force.

I suggest that it is much more likely that the deformation will not be the same over each disc. A bit like the argument that gives Coulomb's law of Friction; the tangential force increases as the normal load increases because the actual area of contact increases due to deformation. The area of contact of each plate will increase as the axial force increases and all the plates will behave like one large plate.

I remember I had an Oil Filled Clutch on my Excelsior Twin Engine on my Barclay Three Wheeler (1960 ish). (Don't laugh!!!) The oil would flow / circulate (like in a torque converter and transfer momentum amongst the plates without needing the sort of forces that you get in a dragster.

 

[Baluncore]

IMO it would be less likely that the plates would all separate at the same time.

I think that is certain. It is likely that the first plate to release will be the one at the controlled end of the stack. Then as each plate releases, the torque presented to the remaining engaged plates increases, forming a cascade that results in full disengagement. The rate of disengagement will be determined by the time it takes a thicker film of oil to enter between the plates.

Wet multi-plate clutches can stick engaged if the unit is left engaged and unused for some time. The oil is then squeezed out from the boundary layers between the discs, and corrosion of the disc face can mate with the friction material surface. To release the clutch, it is then necessary to get oil back between the plates. Since the stack of clutch plates is inside a keyed or splined housing, a mechanical force cannot be applied between plates to encourage separation.

The failure to release after a long period of stagnation is not restricted to wet multi-plate systems. The common dry hand-brakes, and single-plate clutches used on road vehicles, will often stick if not used for several months. Those systems can usually be freed, by the creative application of a torque impulse.

 

[Averagesupernova]

For the record: Not every multi-plate clutch is in a dragster. There isn't any dust on the the clutch in my Yamaha FZ-1 (it's a wet clutch).

Indeed. Top fuel dragsters aren't the only place multi-disk clutches are used. I suspect most multi-disk clutches are in fact run in oil. I'd say drag racers are in the minority in placement of multi-disk clutches.

 

[sophiecentaur]

The common dry hand-brakes, and single-plate clutches used on road vehicles, will often stick if not used for several months.

Yep. I've heard a scary "bang" after releasing the handbrake a second time after a long period of non-use.