Clutches, Single, Twin, Triple, how do they work???
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Clutches, Single, Twin, Triple, how do they work??? - 1/31/2007 9:24:17 PM
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EMonz57
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Joined: 5/13/2005
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Clutches are pretty simple devices that can be made very complicated by misunderstanding and misinformation. I will do my best to explain how a basic clutch works, and then go on to how multiplate clutches work. Hopefully you will have a better understanding of why one type is better than another for certain applications.
Clutches and Brakes, believe it or not, work on the exact same principal, except that when you're driving along, the brakes are freely spinning and the clutch is locked up. When the clutch pedal is depressed, the clutch disk spins freely and when stopped, your brakes are locked up. However, both rely on friction for that area in between, when you're gently applying the brakes or starting your car from a stop and slipping the clutch.
The Single Plate Clutch:
A single plate clutch has 4 major components. The Flywheel, the Clutch Disk, the Pressure Plate, and the Throwout Bearing. The Flywheel is directly bolted to your Engine, no matter what it will always spin at the same RPMs as your engine (unless something mechanically breaks, then you have bigger problems to worry about  ). The Pressure Plate directly bolts to the flywheel, so it too will always spin at the same RPM as the engine.
Imagine the flywheel and pressure plate are one big, round, continuous brake caliper. (Work with me here)
The Clutch Disk is sandwiched in between the Flywheel and Pressure Plate. It is connected to the transmission, and no matter what, it wants to spin the same speed as the transmission input shaft.
Imagine the Clutch Disk is a brake rotor. (again, work with me)
The Flywheel and Pressure plate constantly have the Clutch Disk clamped down tight and locked up. The clamping force (in lbs) of the Pressure Plate is distributed over the clutch disk (area) to get a force over an area (psi, pounds per square inch) The psi is then used with the friction coefficient of the disk material to find out the sliding force the clutch can hold before it starts to slip. This force, multiplied by the radius of the clutch will determine the torque that a clutch can hold. (remember this later)
The Throwout Bearing (TOB) simply actuates the Pressure Plate (PP). When you push on the clutch pedal the TOB pushes (or pulls, as in the case of the Evo) against a set of fingers on the PP. When these fingers are actuated by the TOB, the pressure plate and flywheel release their grip on the clutch disk.
Why does this matter? When you are pushing on the clutch pedal, the Crankshaft in the Engine and the Input Shaft in the Transmission are allowed to spin at different speeds, when you are not pushing on the pedal they must spin at the same speed. When you are stopped at a light and the car is IN gear the input shaft wants to be stopped, since it is connected to the wheels through the transmission and the wheels are stopped. That is why you must push in the clutch pedal, it disconnects the engine from the transmission and prevents your engine from stalling.
When you want to start from a stop, you put your car in first gear (again, the input shaft does NOT want to spin since the car is stopped) and slowly let out the clutch. The Clutch Disk slides between the PP and Flywheel until they match speeds and the clutch pedal is fully released.
That my friends, is how a basic clutch works.
Before I get into multiplate clutches, I want to talk about the clutch disk more...
The Clutch Disk (CD from now on) has a tremendous impact on the way a clutch behaves, moreso than any other component in my opinion.
There are many types of CDs. Full Face, Puck, Spring, Unsprung, bi-metalic, organic, semi metalic, carbon, full metalic... etc.
Stock CDs are generally Full Faced, Organic, Sprung, with a Marcel Spring. What does this mean?
Full Face - The clutch disk is a solid round disk.
Organic - The friction material if of a non-metallic compound.
Spring - The friction material is separated from the input shaft spline by a set of springs that are oriented tangent to the input shaft.
Marce Spring - The two friction surfaces (one on each side of the disk) are separated by a wavy potatoe-chip like sping.
Why are they like this??? Full face disks give the highest surface area, and therefore have the most material to wear away as the CD gets old. The downside is that the clamping force of the PP is distributed over a large area, reducing the pounds per square inch of force on the disk.
Organic disks are more slippery than metallic materials making it easier to slide the clutch when starting from a stop. They also tend to wear themselves down rather than the Flywheel and PP.
Center Springs reduce the shock transmitted into the transmission when the clutch pedal is released quickly. They help save the transmission and syncros.
The Marcel Spring has the greatest effect on ease of driveability. This allows the clutch disk to compress as the pedal is released. This increases the "slip to grip" zone in the pedal making the clutch easier to modulate off of a stop.
Puck Clutches reduce the clamped surface area, which increases the psi force on the disk. This is only good if you have the disk made of a material that requires a greater psi to not slip (metallic clutch materials are like this) When this is combined with a grippy metallic material, the holding torque (the maximum about of torque the clutch can hold) is increased, without changing the material it would be decreased.
Clutches without sprung centers are lighter and transmit the force into the tranny quicker, it makes for slightly quicker on/off power with a sacrifice in drivetrain life.
Clutches without marcel springs also engage quicker, but are significantly harder to drive off a stop. (they tend to chatter a lot)
Different friction materials simply have different charecteristics when hot/cold and have higher or lower friction coefficients (they used the pressure applied by the PP better).
VERY IMPORTANT - What you should indirectly get out of what you've just read, is that clutches hold a certain amount of TORQUE only, not HP!! I hate it when clutch manufacturers market clutches to hold "X" HP, because I then simply ask "at what RPM?"
Why do I ask that??
HP = (Torque x RPM)/5252
HP is just another way of expressing torque, that is why every dyno graph has HP and Torque cross at 5252RPMs (mathematically they have to) and every car makes more torque than HP below 5252RPMs (diesel anyone?) and more HP than torque above 5252RPM. A clutch that can hold 300HP on a 15k RPM street bike will NOT hold 300HP on a diesel, period!!!
END RANT
The Multi-Plate Clutch:
Simply put, a multi-plate (twin disk, triple disk) clutch has more than one clutch disk. Why? It increases the surface area that is being clamped.
If you want a clutch to hold more torque you must change certain things. Increase the force the Pressure Plate exerts (makes the pedal harder to push), increase the diameter of the clutch, increase the surface area, increase the friction of the disk material.
If you add a second disk into the equation you double the surface area. That means that you can hold twice as much torque with the same pedal pressure!
How do they add in another disk?? The add a second flywheel too! This second flywheel floats between the two disks and is connected to the flywheel that is bolted to the engine. When you push in the clutch pedal you release the grip on both disks just like a single plate clutch.
Upsides:
- Increased torque capacity.
- You can decrease the diameter with the same holding capacity (this reduced rotational inertia and helps the engine rev quicker)
- More torque capacity without a stiffer pedal
Downsides:
- The disks must be thinner since they occupy the same space, to make them thinner the marcel spring is removed and the friction material is thinner. They chatter and don't last as long
- The floating flywheel rattles around when the pedal is pushed in
- Having two disks connected to the tranny increases the weight on the input shaft, increasing the load on the syncros
- Typically harder to drive than a single plate clutch
- Expensive!
Borrowed from NE-Evos.net written by: SuperHatch
< Message edited by EMonz57 -- 1/31/2007 10:06:37 PM >
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1993 Renn Red Supra Turbo 6spd
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