Harmonic dampers 5

[yoshimitsuspeed]

I guess while I'm here I'll throw up another idea that's been on my mind. Pendulum style harmonic dampers.

http://www.dragzine.com/tech-stories/engine/tcis-rattler-balancer-tech-review/

There are a lot of solid crank pulleys out there. Often used to reduce weight. There is also a lot of good info out there that suggests it's not a great idea to use them because they can cause added stress, harmonics and on some motors even very rapid failure. Now all the 4 cylinders I deal with are naturally balanced so we don't have to worry about harmonic balancers like some motors do, just the harmonic dampening that helps prevent harmful frequencies from increasing to a dangerous point.
I am developing a pulley that has an integrated trigger wheel and I would like to use a solid pulley as it will be much cheaper to make. I have also read mixed reviews on polymer dampers especially when building high performance motors because from what I can tell they are designed primarily for one frequency range and from what I understand the harmful frequencies can change as you change things on a motor like displacement, horsepower, RPM, Boost, etc.
If I could get a pendulum style damper to work it seems like it would be pretty perfect for what I'm trying to do. I would assume that any weights would help dampen these frequencies but in that article they make it sound like it takes a specific design to work with different motors. It has me wondering if this is a huge issue or something they emphasize to scare people away from adopting it. My main question is whether it would be possible for a design like this to actually amplify frequencies if it was the wrong diameter or weight? Or would it just work less well?
Some day I would love to actually make a few designs and test them monitoring crank speed with a fine tooth trigger wheel or something but that won't be happening any time soon.
Do you guys have any thoughts on this? Would some weights even if not ideal be better than a solid pulley? Or could it be possible for the wrong design to actually be worse than a solid pulley?

 

[140Airpower]

The system shown in the magazine article reminds me of some piston engine aircraft cranks where the weights are fitted into the crank counterweights.

"Or could it be possible for the wrong design to actually be worse than a solid pulley?"

I think it is always possible to make things worse. That's a law of the Universe. Especially with tuned vibration absorbers and dampers. But, there are some non-tuned systems that simply dissipate vibrations through friction. These should always help, but they may absorb a lot of heat.

 

[yoshimitsuspeed]

Yeah from the research I did it sounds like this has been common in aircraft engines but not so much in automotive.

 

[gruntguru]

all the 4 cylinders I deal with are naturally balanced so we don't have to worry about harmonic balancers

Harmonic balancers are used for damping torsional vibrations. "Natural balance" has no effect on TVs. Short crankshaft is the main reason most 4cyls do not suffer.

je suis charlie

 

[Tmoose]

hI yoshimitsuspeed ,

You said -
"Some day I would love to actually make a few designs and test them monitoring crank speed with a fine tooth trigger wheel or something but that won't be happening any time soon.
Do you guys have any thoughts on this? "

==========

Measuring at one location might just show speed variation.
At big throttle an infinitely stiff crank would still respond to Fig 2 here

http://lh6.ggpht.com/_Ii1ukGkfijY/S...6vhgmDknQ/clip_image0026_thumb.jpg?imgmax=800

I believe you'd need to measure at both ends of the crank and precisely consider phase to detect torsional vibration (twist).

 

[yoshimitsuspeed]

gruntguru
I was always under the impression that counterweighted pulleys were just used on motors that were not naturally balanced. I will say I never looked into it much as I have never messed with motors that did use counterweights. It doesn't seem like counterweights would be a good solution for torsional vibration since it will be stronger at some RPM than it will at others.

Tmoose yeah my thought was to just measure acceleration. Yeah you would have the baseline acceleration curve that you would have to work around but my thought was that it should still tell you if one design was better than another or if another was significantly worse. You are right that measuring at both ends would be much better and one thought I have had that would be pretty easy is putting a seonsor on the flywheel teeth. Then a fine tooth sensor on the front of the motor and you would be able to compare positions.

 

[140Airpower]

True you can measure crank accelerations and see some torsional variations, but measuring both ends, and in several other places if possible, can show you more. In most oscillating flexible systems there exists nodes that may show no or very small oscillations.

 

[GregLocock]

The end of a shaft is never nodal. The response at the flywheel end is reduced,but it is not nodal. In practice on automotive engines the first torsional mode is the only one of interest, for which the crank nose is antinodal.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Lou Scannon]

I was always under the impression that counterweighted pulleys were just used on motors that were not naturally balanced.

Not necessarily. On the old school V8s I'm somewhat familiar with, they frequently resorted to counterweighted front balancer and flywheel/flexplate (i.e. external balance) on the large displacement variants of a given family, in order to maintain a common crankshaft with the smaller bore (hence lighter pistons) variants with common stroke, or in the case of longer stroke cranks, to conserve mass and rotating inertia by moving the balance mass out to the ends where you get most bang for the buck.
I think the long stroke crankshaft case is more common than the different piston mass per bore size - I believe it was not uncommon to maintain the same piston mass across different bore sizes (not optimum from a structural efficiency/margin standpoint!), in order to commonize the crank, balancer & front damper regardless of bore.


"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Lou Scannon]

Speaking of harmonic dampers, I had a 1955 Olds 324 that I rodded with all the parts I could get my hands on, incidentally raising the peak operating speed to more than 2000 rpm higher than intended in factory tune. In hindsight, I probably needed an upgraded damper, not to mention full prepping of the cast iron crank. I chalk these oversights up to ignorance/inexperience; anyway, the end result was a fatigue failure in the web between cyls 2 & 3.
Had I known better, I would have desperately sought a steel crank and upgraded damper that must surely have been part of Old's contemporary "special heavy-duty" or "special export" parts list which existed in support of NASCAR racing, in which Olds was a strong contender from 1949 thru 1955, and had a brief resurgence in winning the 1959 Daytona 500, if I'm not mistaken.
In the event of failing to locate the factory racing parts, an alternative approach would have been to improvise the best I could, e.g. full prep on the crank, and adapt a herky aftermarket damper.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

A "counterweighted pulley" and a "harmonic balancer" are two different things.

je suis charlie

 

[yoshimitsuspeed]

Okay well these motors don't use counterbalanced pulleys so what about damping? For now would it be safest for me to design a solid pulley with no damping? Or would there be any way to incorporate this pendulum style into my build being confident that it would be at least better than having none at all without doing a ton of R&D on it?

 

[Panther140]

I'm not even sure what kind of vibrations you are getting. Your priority right now should be illustrating the vibrations to us with data.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[yoshimitsuspeed]

Panther140
The stock crank pulley has a bonded rubber damper.
I haven't studied these harmonics myself but there seem to be enough to make Toyota and all other OEMs I have seen choose to do this over a solid pulley.
Now on the 4AGE many people have run solid aftermarket pulleys without any issues that could be directly attributed to it so that is comforting. I have heard of some motors that seem to be affected much worse much sooner. The question still remains whether it might have any long term effect. Maybe they had bearing failure at 150k miles when the motor would have otherwise gone over 200k. That is not the type of thing that anyone would really tend to link when their motor lets go.
My plan is to make this solid crank wheel with integrated trigger wheel but if there is anything that I can do without investing a ton of time or money into it that would help then I would like to do that.
I do realize it would be hard to do something guarenteed to help without that large amount of R&D but that's why I'm here. To see if there is anything that I could do relatively quick and easy that should at least be better than nothing.

This article does a pretty good job of summarizing my concerns with solid pulleys.

http://www.atiracing.com/products/dampers/damper_dinan.htm

 

[Panther140]

Oh I see now. A pendulum type dampener would help greatly with the issue of torsional vibration in that case.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[BigClive]

Some straight fours do suffer quite badly from TVs - with "hotted-up" Morris A-Series 1100s you were advised to stay under or over about 6,000 RPM - but keep away from 6,000RPM itself where the TVs could break the crankshaft.

 

[yoshimitsuspeed]

Panther140 I do agree but the big question is whether there is a safe way of implementing something that should be better than nothing or if there is a chance that without a lot of R&D if it would be possible for the design to actually be worse than nothing.

BigClive Do you know if anyone ever managed to eliminate the issue with any type of dampening or other solutions?

 

[BigClive]

The Cooper S and other higher performance models used a conventional rubber-and-metal damper.

 

[FeX32]

Pendulum dampers are actually absorbers.
They cancel the torsional vibration of an order. That is the frequency they counteract is proportional to the speed of the shaft.
They have been used extensively in aerospace. Recently we have used them in Dual Mass Flywheels.

 

[EHudson]

The steel inertia ring on rubber is a spring (the rubber or polymer) and a mass (the inertia ring) set up that is tuned to absorb torsional vibrations of a specific frequency.

The pendulum damper uses the centrifugal force owing to the rotation around the crankshaft axis as a “restoring force” returning the pendulum mass to the point furthest from the axis. This force varies with the square of the rotating speed. This variation in restoring force contrasts to the rather constant spring or restoring force of the rubber mounted ring.

FeX32 is correct, a pendulum damper is tuned to absorb energy at a multiple of the crankshaft speed. This multiple is commonly referred to as an ‘order’. (The third order is three times the number of crankshaft revolutions.)

The presence of damper on the original engine strongly suggests that the design engineers determined that there was as speed at which the combination of drive train inertias (not just the engine components) and the relative flexibility of those components in torsion resulted in excessive crankshaft torsional vibration or twisting.

By excessive torsional vibration we usually mean a twisting moment that results in a fatigue failure of the crankshaft or some component in the driveline.

Please know that the specifications of the damper were not chosen arbitrarily. A random selection of components is unlikely to result in an improvement in vibration damping. The calculations require considerable amounts of data and measurements not easily performed.

Without the damper, the engine will rev faster and some racers may choose to remove the damper for this reason. If the engine does not spend much time in the critical rpm they may get away with this practice.

The most comprehensive reference is “A Handbook of Torsional Vibration” published by Cambridge University Press in 1958. It is a collective work by the British Internal Combustion Engine Research Assn. (B.I.C.E.R.A.). Luckly for engineers everywhere, the book has been reissued recently in paperback and is available for about 40% of the original price from the same publisher. Regrettably, the only thing not included in the reissue was a nice foldout with a compilation of formulas for pendulum dampers.