Combining Functions of Harmonic Dampner and flywheel

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ekimneirbo

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While thinking of ways to lighten an auto conversion (LS Chevy in this instance), I'd like some input on the dynamics of using a harmonic dampener to also compensate for some of the flywheel mass and still get the benefits of damping. My plan is to use a short splined shaft inserted in an adapter that is bolted to the rear of the crankshaft. The shaft will drive the propellor and be supported by a housing. The LS engine will have a stock harmonic dampener affixed to the front of the crankshaft as per factory installation. On the rear of the crank an ATI Rattler or Fluidamper would be attached to the flywheel to deal with any harmonics from the propellor. The flywheel would be lightened by appx the weight of the dampener.
Most sources I have seen recommend using a somewhat heavy flywheel to help the engine run smoothly. I also don't want to add unnessesary mass to the drive. The aftermarket dampeners are designed to deal with harmonics thru all rpm ranges while the factory units usually concentrate on one rpm range. Anyone see any conflict problems surfacing if two dampeners are used?
 

rv6ejguy

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While thinking of ways to lighten an auto conversion (LS Chevy in this instance), I'd like some input on the dynamics of using a harmonic dampener to also compensate for some of the flywheel mass and still get the benefits of damping. My plan is to use a short splined shaft inserted in an adapter that is bolted to the rear of the crankshaft. The shaft will drive the propellor and be supported by a housing. The LS engine will have a stock harmonic dampener affixed to the front of the crankshaft as per factory installation. On the rear of the crank an ATI Rattler or Fluidamper would be attached to the flywheel to deal with any harmonics from the propellor. The flywheel would be lightened by appx the weight of the dampener.
Most sources I have seen recommend using a somewhat heavy flywheel to help the engine run smoothly. I also don't want to add unnessesary mass to the drive. The aftermarket dampeners are designed to deal with harmonics thru all rpm ranges while the factory units usually concentrate on one rpm range. Anyone see any conflict problems surfacing if two dampeners are used?
V8 engines don't really need much flywheel inertia to run smoothly. I've idled an LS6 down to 500 rpm with just the basic ring gear- no clutch stuff and it was still smooth. If you use a wooden or composite prop, I don't think you'd have any concern with harmonics from the prop. With the lack of torque reversals on a V8, I wouldn't expect any TV problems with a stiff shaft and front bearing setup. The V8 should be way more benign than a 4 cylinder in this regard and you should be able to keep the rotating components light.
 

Toobuilder

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Two thoughts:

What are you using for a prop?

...and IF you need more flywheel, (big if), you can always attach a steel ring to an auto trans flexplate. Won't add much dead weight, but plenty of inertia.
 

Toobuilder

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one more thought

It would be slick to build a large diameter pendulum dampener that replaced the starter ring gear entirely and also incorporated a splined output. Idealy, you could open the unit up and replace/service/tune the guts for a variety of engine/propeller combinations. I guess the reality is that you would need to do a TV survey on every combination and have the results cataloged somewhere...
 
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ekimneirbo

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V8 engines don't really need much flywheel inertia to run smoothly. I've idled an LS6 down to 500 rpm with just the basic ring gear- no clutch stuff and it was still smooth. If you use a wooden or composite prop, I don't think you'd have any concern with harmonics from the prop. With the lack of torque reversals on a V8, I wouldn't expect any TV problems with a stiff shaft and front bearing setup. The V8 should be way more benign than a 4 cylinder in this regard and you should be able to keep the rotating components light.
I do plan on using a carbon or wood propellor and agree that there is much less chance of TV with them. I will obviously have to run a flywheel of some type and feel that just a flexplate would be too light. I have several LS1 flywheels, so I turned one down a few lbs and took it to the local clutch rebuilder and had them resurface both sides to insure it was flat. I got rid of about a third of the weight. I was thinking that with a little mechanical creativity that I might be able to
use a Fluidamper that I have from an old car project, and attach it to the flywheel (after further lightening) and gain some benefit....kind of a win/win situation. I might be able to just attach it to a flexplate and end up with the same appx weight as a flywheel. That would be pretty simple to do. As you say, its probably not needed. I plan to use a dynovibe to check whatever I do, so maybe I'll try both and see what the result is. Its a ways off right now as I'm embedded in some other projects right now.
 

RJW

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Using lighter and stiffer stuff will simply shift any resonance to a higher frequency (as we have discussed here many times). I don’t recall details but an analysis worked out a couple years ago using a wood prop and a reasonable guess for the stiffness of a SBC crank put first order resonance in the operating range. The point of the exercise was to see if lightest stiffest stuff would put resonance above the operating range of the motor. Unfortunately it didn’t. If one is guessing and testing, it is probably best to go heavy with the flywheel and prop, and use a soft system to put the first order below the operating range. Second order will then probably end up somewhere in the operating range so you will have to avoid that RPM or provide a damper for it. Higher orders have much less amplitude and "generally" do not cause problems.

Let us know what your tests show.

Rob
 

Midniteoyl

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I was thinking of using a TCI Rattler.. been a long time since I thought about it/looked it up, but I believe the TCI would be better suited.
 

rv6ejguy

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I think you'll find F1 with a wood prop and super stiff shaft down in the 300-400 rpm range which is transitional, not operational. MMOI would not be markedly different than with the standard flexplate and torque converter in place.
 

RJW

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I think you'll find F1 with a wood prop and super stiff shaft down in the 300-400 rpm range which is transitional, not operational. MMOI would not be markedly different than with the standard flexplate and torque converter in place.
I’ll dig out my notes and run though the process again. I need to sharpen up anyway. So the implication is that F1 on a regular SBC or similar V8 fitted with stock flexplate/torque converter or flywheel/pressure plate occurs at 300-400RPM? It seems like the crank would be pretty rubbery if that were true. I thought the stock dampers were there to handle F1? Aren’t the stock dampers tuned to about 3200RPM? It’s very possible that I am remembering all of this wrong. Is the damper there to handle higher orders? I really don’t know. Anyway, if the implication is true, then use the lightest flywheel, pretty much any prop you want, find out where the full sine wave occurs, damp and avoid.

Rob
 

rv6ejguy

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I’ll dig out my notes and run though the process again. I need to sharpen up anyway. So the implication is that F1 on a regular SBC or similar V8 fitted with stock flexplate/torque converter or flywheel/pressure plate occurs at 300-400RPM? It seems like the crank would be pretty rubbery if that were true. I thought the stock dampers were there to handle F1? Aren’t the stock dampers tuned to about 3200RPM? It’s very possible that I am remembering all of this wrong. Is the damper there to handle higher orders? I really don’t know. Anyway, if the implication is true, then use the lightest flywheel, pretty much any prop you want, find out where the full sine wave occurs, damp and avoid.

Rob

I couldn't say what ranges the factory engineers were aiming to tame on the automotive application and how well it does the job over the full rpm range (good enough that the crank has basically infinite life in stock form).

I'm going to try to plug some MMOI estimates for this engine into my spread sheets assuming a very stiff shaft and simple 2 element model and a 15 lb, 80 inch prop. A torque converter filled with oil is not exactly light.

The safe bet might be to leave room and use the same bolt patterns on the extension shaft setup to be able to insert a Lovejoy type damper, instrument the shaft with WBs in solid form, then if you have bad TV shown by the instrumentation, you'd be able to pick a suitable damper from the catalog. We've got some math models and actual test info for the Subaru engines and the M300 gearboxes now and they agree quite closely. This was preceded by a actual MMOI measurements and stiffness measurements of all the components. Many hours of calcs but this was a far more complex model.
 

rv6ejguy

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Looks like I was pretty wrong with my post above. It pays to work the numbers.

I spent a couple hours estimating MMOI of the crankshaft, rods, OEM damper, flexplate and a 15 lb., 80 inch wooden prop. Plugged these numbers into my Holzer spreadsheet and varied the coupler stiffness. If my estimates and calcs are correct (a big IF), it looks like if you bolted the prop right to the flexplate with an uber stiff, metallic adapter plate (3750 lb/ft/deg), resonance with firing frequency on a V8 would occur at 2948rpm.

With a super soft coupling (100 lb/ft/deg) resonance at 150 rpm.

With a medium hard coupling (800 lb/ft/deg) at 430 rpm.

Since most starters turn the engine at 200-300 rpm, it might be good to avoid this range, either above or below this and below the idle range of say 600-800 rpm.

This is more for illustrative/ discussion purposes, showing possibilities rather than solid numbers which would require actual MMOI measurements of the parts via bifilar suspension tests.

We have a number of 4 and 6 cylinder DD auto engines with many flight hours turning lightweight props with "solid" metallic couplers. These may be more successful than a V8 DD because of the firing frequency differences. Generally, resonance with firing frequency would be well above the operating range of a DD prop on a four and most sixes as well but maybe not on the eight.
 
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ekimneirbo

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Looks like I was pretty wrong with my post above. It pays to work the numbers.

I spent a couple hours estimating MMOI of the crankshaft, rods, OEM damper, flexplate and a 15 lb., 80 inch wooden prop. Plugged these numbers into my Holzer spreadsheet and varied the coupler stiffness. If my estimates and calcs are correct (a big IF), it looks like if you bolted the prop right to the flexplate with an uber stiff, metallic adapter plate (3750 lb/ft/deg), resonance with firing frequency on a V8 would occur at 2948rpm.

OK, so in a direct drive V8 situation this would appear to be exactly what I would need (assuming 2700 rpms)..........?????? Is that what you are saying? Can you give some examples of what you mean by soft coupling vs medium or hard coupling? Since I only partly understand some of this stuff, it would seem to be an awfully large variation in the rpm ranges. What I mean is that instead of saying there are certain zones which begin where the previous rpm zone ends..........it seems as though you are saying that one zone exists at a very low rpm and the next harmonic zone is much high in the rpm range. I realize that I'm probably incorrect here, so can you expound a little more on the subject? Thanks for the time you have invested in trying to provide some tech info.
 

rv6ejguy

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Looks like I was pretty wrong with my post above. It pays to work the numbers.

I spent a couple hours estimating MMOI of the crankshaft, rods, OEM damper, flexplate and a 15 lb., 80 inch wooden prop. Plugged these numbers into my Holzer spreadsheet and varied the coupler stiffness. If my estimates and calcs are correct (a big IF), it looks like if you bolted the prop right to the flexplate with an uber stiff, metallic adapter plate (3750 lb/ft/deg), resonance with firing frequency on a V8 would occur at 2948rpm.

OK, so in a direct drive V8 situation this would appear to be exactly what I would need (assuming 2700 rpms)..........?????? Is that what you are saying? Can you give some examples of what you mean by soft coupling vs medium or hard coupling? Since I only partly understand some of this stuff, it would seem to be an awfully large variation in the rpm ranges. What I mean is that instead of saying there are certain zones which begin where the previous rpm zone ends..........it seems as though you are saying that one zone exists at a very low rpm and the next harmonic zone is much high in the rpm range. I realize that I'm probably incorrect here, so can you expound a little more on the subject? Thanks for the time you have invested in trying to provide some tech info.
I only looked at engine firing frequency resonance as this is usually the most powerful exciting factor.

What I listed above is basically showing the effect of no damper/ very stiff coupling of the crankshaft and propeller and two scenarios with a soft rubber damper between the crank and prop and third, a very stiff rubber damper. You can see how it changes the resonant frequency of the system.

You might look at adapting one of these couplers to your design: Lovejoy, Inc. : Products : Couplings & Power Transmission: Torsional Couplings The typical automotive "harmonic balancer" is not a good device to couple a crankshaft to a propeller and absorbing/damping the peak torsional vibration forces likely to be present. The LF30 has some possibilities here for this application or there are spline type models which might suit what you are doing more.

The proper way to design something that will be safe is to instrument the shaft with strain gauges and run it through the whole rpm range while logging the frequencies and amplitudes, then you can easily select the proper damper/ coupler. A second, though less accurate way would be to measure the inertias of all the parts and calculate the stiffness of the parts, then use the math models to calculate the resonate frequencies and amplitudes. Having gone through the second way on my setup and the staggering amount of calculation, I'd have to recommend actual measurement with strain gauges which is the most accurate method.

If my calcs are in the ballpark, it looks risky to rigidly bolt the prop to crank on a V8 so I'd design your propeller bearing support with one of these couplers in mind- probably pick a model which has the same bolt pattern but a variety of different stiffnesses available within that model.

As one more data point, if we double the inertia of the model prop in the case of the medium hard coupler, resonant rpm drops from 430 rpm to 367 rpm.

Really, throwing any old damper on the system without any idea could make things a lot worse.
 
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