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  #51  
Old 12-27-2017, 12:39 PM
scsmith scsmith is offline
 
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Default dry clutch torsion springs

I have a question about the springs in a traditional auto clutch disc. Several psru vendors seem to point to these as an example of how autos dampen the torsional vibration.

But I was under the impression that these springs really only do anything at very large low-frequency torque inputs, such as rapid clutch engagement when the car is not moving. I think the spring pre-load is high enough that typical torsional vibration at n/2rev (where n is the number of cylinders) and higher harmonics of that, the springs do not flex and the clutch disc is essentially solid.

Certainly the dual-mass torsion spring flywheels do the job, but I don't think a traditional dry clutch plate does anything for TV.

Am I correct?
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  #52  
Old 12-27-2017, 01:00 PM
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rv6ejguy rv6ejguy is offline
 
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Quote:
Originally Posted by scsmith View Post
I have a question about the springs in a traditional auto clutch disc. Several psru vendors seem to point to these as an example of how autos dampen the torsional vibration.

But I was under the impression that these springs really only do anything at very large low-frequency torque inputs, such as rapid clutch engagement when the car is not moving. I think the spring pre-load is high enough that typical torsional vibration at n/2rev (where n is the number of cylinders) and higher harmonics of that, the springs do not flex and the clutch disc is essentially solid.

Certainly the dual-mass torsion spring flywheels do the job, but I don't think a traditional dry clutch plate does anything for TV.

Am I correct?
I put a torque wrench on some discs I had laying around a number of years back and the strongest springs bottomed out at around 80 ft./lbs. A number of others were closer to 50-60. At these levels you're not going to break anything nor do they offer any useful damping at the typical amplitudes we are concerned with- usually on the order of many hundreds to thousands of ft./lbs. when we reach resonance.

I think this was wishful thinking on the part of some PSRU designers who really didn't grasp TV.

Even DM flywheels may not be up to the task of dealing with TV with a prop/gearbox combination they were never designed for. Again, PSRU designers using OTS auto parts for very different applications and simply crossing their fingers with little or no scientific analysis done.
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  #53  
Old 12-27-2017, 02:34 PM
scsmith scsmith is offline
 
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Originally Posted by rv6ejguy View Post
I put a torque wrench on some discs I had laying around a number of years back and the strongest springs bottomed out at around 80 ft./lbs. A number of others were closer to 50-60. At these levels you're not going to break anything nor do they offer any useful damping at the typical amplitudes we are concerned with- usually on the order of many hundreds to thousands of ft./lbs. when we reach resonance.

I think this was wishful thinking on the part of some PSRU designers who really didn't grasp TV.

Even DM flywheels may not be up to the task of dealing with TV with a prop/gearbox combination they were never designed for. Again, PSRU designers using OTS auto parts for very different applications and simply crossing their fingers with little or no scientific analysis done.
I don't think you can infer how they behave from measuring their static torque to begin flexing or to bottom out. The springs and mass in the clutch disc also is a 2nd order system with its own natural frequency and response curve. At high frequencies, I think it acts essentially rigid, and would transmit extremely high torques with virtually no displacement of the springs. What matters most is where the natural frequency is of the clutch disc relative to the various modal frequencies of the engine-psru-prop system. My assumption is that the clutch natural frequency is well below the primary resonance frequency of the system --- but I admit that may not be true. Thus my question.
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  #54  
Old 12-27-2017, 05:07 PM
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Quote:
Originally Posted by scsmith View Post
I don't think you can infer how they behave from measuring their static torque to begin flexing or to bottom out. The springs and mass in the clutch disc also is a 2nd order system with its own natural frequency and response curve. At high frequencies, I think it acts essentially rigid, and would transmit extremely high torques with virtually no displacement of the springs. What matters most is where the natural frequency is of the clutch disc relative to the various modal frequencies of the engine-psru-prop system. My assumption is that the clutch natural frequency is well below the primary resonance frequency of the system --- but I admit that may not be true. Thus my question.
Not much mass or inertia in a clutch disc compared to the rotating parts of the engine and a propeller. I think you need to look at the purpose and application these parts are designed for. They are really there to smooth initial transfer of torque between the pressure plate/ flywheel and transmission input shaft on slow engagement where the engine is usually not producing much torque.

In an aircraft application in cruise with the engine producing say 150+ ft. lbs at 150-300hz firing frequency, I'd hazard to say that the springs are fully compressed and out of the equation altogether, therefore of no useful benefit in cruise.

However, at lower mean torque levels and frequencies such as we'd see at ground idle/ taxi conditions, there COULD be some benefits as this is closer to what the components were designed for. Indeed, most PSRU TV issues I've experienced and heard about are down at lower frequencies. Since every system is different though, we certainly can't assume that is always the case. Additionally, even at idle, if we get resonance, peak torque values could be 40 to 100+ times that of the mean crankshaft torque and could still break things.

In the end, mathematical analysis or actual measurement is the best ways to be sure TV amplitudes are not exceeding design limits of the drive system.
Throwing a clutch disc in between the engine and drive with neither is a hit and miss affair.

An engineered coupling/TV absorber would always be a better choice than an OTS clutch disk in my view but as we've seen, many combinations of parts can work well over hundreds or thousands of hours of operation where other combinations can last minutes.
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Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 420 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm



Last edited by rv6ejguy : 12-27-2017 at 09:06 PM.
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  #55  
Old 12-27-2017, 06:43 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by rv6ejguy View Post
I put a torque wrench on some discs I had laying around a number of years back and the strongest springs bottomed out at around 80 ft./lbs. A number of others were closer to 50-60. At these levels you're not going to break anything nor do they offer any useful damping at the typical amplitudes we are concerned with- usually on the order of many hundreds to thousands of ft./lbs. when we reach resonance.
The outer portion of the disc (with the friction elements) is an inertia, modeled as part of the pressure plate and flywheel. The hub and spline shaft is part of a different inertia. The springs are a connecting stiffness between the inertias.

Obviously if the springs will coil bind at a torque less than the normal WOT operating torque, the springs are no longer serving as a soft element at that load. The connecting torsional stiffness is then the (probably higher) spring rate of the shafting. I think that is the point you were trying to make. "Torque to bottomed" is of no use for TV calculations. Need to know ft-lbs per radian, or per degree. I measured some Subaru clutches back in the 90's, but have no idea where to find the data now.

An engineered soft element would accept many times mean torque without reaching an angular displacement limit.

BTW, clutch springs offer no useful damping because they are springs.

In theory, resonant torque can reach infinity, but never does because even the worst system has some small amount of damping. As a practical matter, the prop is a very large anchor inertia against which the rest of the system oscillates....and the rest of the system (the engine) is chock full of viscous friction, an excellent damping force. So, hundreds of thousands of ft-lbs would be excessive for the systems of interest here. Anyway, they would all break instantly at the first pass through a resonant RPM. Reality says we're worried about torsional stress levels well below instant failure, but above a fatigue limit for some component in the train.

Quote:
Originally Posted by scsmith View Post
I have a question about the springs in a traditional auto clutch disc. Several psru vendors seem to point to these as an example of how autos dampen the torsional vibration.

But I was under the impression that these springs really only do anything at very large low-frequency torque inputs, such as rapid clutch engagement when the car is not moving. I think the spring pre-load is high enough that typical torsional vibration at n/2rev (where n is the number of cylinders) and higher harmonics of that, the springs do not flex and the clutch disc is essentially solid.

Certainly the dual-mass torsion spring flywheels do the job, but I don't think a traditional dry clutch plate does anything for TV.

Am I correct?
The springs always flex under oscillating input torque, unless angular displacement due to load causes them to coil bind. Can't be otherwise. The engine output torque always oscillates, at any RPM, and there are no infinitely stiff materials.

Does that make a clutch center a good choice of soft element? Not in my book.
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  #56  
Old 01-06-2018, 12:26 PM
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rv6ejguy rv6ejguy is offline
 
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Mike Talmadge completed the first ground runs on the new gearbox with no problems cropping up. He'll check for excess fuzz on the magnetic drain plug and send out a gearbox oil sample soon.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 420 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm


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