Elastomeric v Friction Dampers

[evelrod]

We lost an crank damper last month, a ROMAC elastomeric damper unit on our 1380cc Mini Cooper...150hp and rpm range of 5500 to 8500 in normal 30 minute sprint race.

http://www.romac.com.au/

One alternative is the Fisher

http://www.fisherconcepts.net/

However, I am getting a wide variety of advice, pro and con. My tendency is to stick with the elastomeric type damper. Aside dealing with Romac is a PITA, I tend to like them since I'm not to keen on a lot more moving parts than I already deal with.

Rod

 

[MikeHalloran]

The job of the damper is to save the crank. ... not necessarily to save the damper.

The damper's death suggests that it was working. ...hard.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Does anyone still make mercury filled dampers. I think they were used on some popular tractor at one stage and seem extremely durable.

Regards
Pat
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[GregLocock]

Stick with what you know and maybe regard them as a time limited part. Downside of rubber ones is that they do need to be tuned somewhat to your crank frequency (not that that varies much for a given engine layout), and that the rubber has a hard life.

The fluidic ones are brilliant from a vibration point of view, as they don't really need to be tuned (there is a bit involved, but they are wideband in nature). Downside is leaks.

The dry friction ones are pretty robust. I haven't used a commercial one, and the one we built was a bit of a pig to set up, but the sliding velocities are very small compared with a clutch so the thing'll last forever. Downside is complexity (springs and things). Again they are wideband in nature.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

The problem is that when the thing lets go it usually causes something $$$$$ to happen as it generally happen in the heat of battle and I'm not good enough to drive the damn thing and watch the instruments at the same time. It's either win or watch the gauges for me. I built it. I guess it's okay for me to blow it up.

Yes, Mike. The Romac really has done a great job over the last couple seasons. My center main issues went away completely....knock wood!

Good advice, Greg. I think I'll just take it. I have a good deal in the offing for a couple used Fisher's at $200 each, but I have already ordered a new Romac from Oz...assuming it ever gets here. It really is getting more and more difficult to build and race the Mini here in the U.S. and ordering parts can and often is a real bit**.

Pat, Pat, Pat.---I gotta give you a minus star for my environmental compatriots. I would be pilloried for sure if I showed up with anything "mercury"... They might not ever let me out of it. Let's not forget, I live in California ;o(

Rod

 

[Tmoose]

"The dry friction ones are pretty robust. I haven't used a commercial one,.........."

I believe at least some Mercedes 300 SL /gullwing used a friction type of relatively large diameter.

http://i160.photobucket.com/albums/t178/akrikori/Mercedes-Benz-300-SL-Coupe-Engine-1.jpg

The engines were at least slightly "externally balanced" and the damper's large moving outer ring had to be handled as an independent component.

 

[tbuelna]

evelrod,

As GregLocock suggests, a silicone fluid dampener would give the best performance and least potential for dynamic issues in a retrofit situation. The fluid dampeners tend to be pretty reliable since their housings are welded closed.

http://www.fluidampr.com/HOWITWORKS.htm

Friction dampeners tend to have limited lives but are reliable and have very high dampening rates.

Elastomeric dampeners are inexpensive and have limited dampening capability. This is due to the fact that their dampening mechanism is solely through hysteresis in the rubber element. They are mostly used to alter the natural torsional vibration frequency modes in a crankshaft system, and not so much for dampening them.

Part of the reason you are experiencing failures in the elastomeric dampener might be due to changes you have made to your engine. Stock elastomeric dampeners are tuned for very specific engine mass properties and inertias. If you made changes to your engine crank train (pistons, rods, flywheel, clutch, etc.) then you have likely changed the torsional vibration characteristics your dampener was designed for.

Good luck and stand on the gas when you race that Mini!

 

[GregLocock]

"They are mostly used to alter the natural torsional vibration frequency modes in a crankshaft system, and not so much for dampening them. "

Slight disagreement there. Typically the inertia of a elastomeric HD is insufficient to move the mode very far, certainly not out of the operating range for a race engine. Last time I worked on one the engine guys were rather insistent that I tune it for reducing the maximum TV in the entire rev range, rather than what I wanted, more effect lower down.

Here's some nice graphs, the bottom one in particular (tho note they are only looking at firing order, tut tut, it is the total twist that kills the crank)

http://fluid-damper.com/fluidampr-fluid-damper/why-use-fluidampr-harmonic-flud-damper.htm

This has a nice graph... but the results for the competing technologies look a long way out.

http://www.atiracing.com/products/dampers/testing/index.htm

And here's some more salesmanship, but hey, data is data

http://www.popularhotrodding.com/enginemasters/articles/ford/smallblock/0306mm_windsor2/index.html

Now, I'd argue with the 0.4 deg pk-pk, but that is probably material dependent. I've seen production cranks run a hundred hours at 1 degree without failing, it depends on your stress raisers and material.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tbuelna]

GregLocock,

"Slight disagreement there. Typically the inertia of a elastomeric HD is insufficient to move the mode very far, certainly not out of the operating range for a race engine. Last time I worked on one the engine guys were rather insistent that I tune it for reducing the maximum TV in the entire rev range, rather than what I wanted, more effect lower down."

I might disagree with you, but my knowledge of crankshaft torsional dynamics is pretty limited. However, I don't believe it is normally possible to reduce the maximum amplitude of a given structure's modal response to an excitation force over a wide range of frequencies as your engine guys requested. A complex dynamic system like an engine's crank train has lots of structural modes whose orders and magnitudes are the result of a wide range of excitation forces. The reason torsional dampeners are used is to minimize the additional stresses imposed on the crankshaft structure due to the structure's natural modal response. The crankshaft system has many natural frequency modes of various orders. The best result one can usually hope for when designing a torsional dampener is to alter the system's lower order (ie. usually the most energetic ones) structural modes enough so that they do not couple with the excitation frequencies produced by the engine's firing order throughout its normal operational range. They can be shifted, but not eliminated.

In short, the elastomeric dampener is a TV "tuning" device and not so much a "dampening" device. In order to be an effective dampener, the elastomer element would need to be capable of absorbing (through hysteresis)and dissipating a significant amount of energy solely through conduction. Most of the elastomeric dampeners I've seen have the thin elastomeric element thermally isolated between two metallic rings (the inner hub and the outer inertia ring). And that elastomer usually has a pretty low temperature capability. So it would quickly fail thermally if it had to dissipate any significant amount of energy.

I mostly agree with your statements about how much torsional deflections a crankshaft can endure. It's a function of material fatigue capability, Kt's, degree of load reversals, and torsional stiffness characteristics of the structure. A long, 6 pin V12 crank will naturally have more torsional deflection end-to-end than an in-line 4 crank, right? As for that article you linked, I quit reading it after the author claimed that his dampener both absorbed less power and also provided greater dampening. I don't believe the two results are mutually compatible; greater dampening would also imply greater energy absorption.

Best regards,
Terry

 

[GregLocock]

They wanted the maximum amplitude reduced, they didn't care about frequencies.

In the running range of any engine I've dealt with there is only one torsional mode of interest - -the node is just in front of the flywheel, and the antinode is at the cranknose. That is THE mode.

The article was a bit of a muddle, the claim is that reducing TVs improves the cam timing accuracy. Maybe, maybe not. I find it hard to believe that 1 degree of crank timing is worth 15 hp or whatever it was.

Incidentally you can tune these things by ear, but the instruments make it easier. Crank torsion adds a very characteristic harshness sound.











Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

I quit reading it after the author claimed that his dampener both absorbed less power and also provided greater dampening. I don't believe the two results are mutually compatible; greater dampening would also imply greater energy absorption.

seven years ago when I last did TV analysis on a regular basis, that result was not uncommon. The most effective damper commonly did not dissipate the most heat - they often dissipated less heat than those which were less effective at reducing vibrations. It's a bit fuzzy now, but I think that result was common with elastomeric or viscoelastic dampers, and not so much with viscous ones. Heat dissipation of elastomeric dampers is a design-limiting factor, and their damping effect is definitely important. In modeling of crank vibrations, a believable adjustment in the damping of the rubber element can make the difference between meeting or completely missing your end-to-end twist bogey.

As Greg mentions, the one-node mode is the most important on automotive-size cranks. A two-node mode is also important on longer cranks.

 

[evelrod]

I'll just stick to my problem for now. I used a couple different dampers and the Romac lasted the longest, four seasons (I know, I shoulda changed it), and it was the first damper that totally eliminated all my main bearing problems as well as, apparently, adding a couple hp, on dyno tests.

My crank is a billet with an 8lb. clutch/flywheel in a 1300 GT block. 7800 rpm normal max.(rev limiter) 115hp/100ft.lbs at the wheels in 1400lb '63 Mini Cooper.

http://files.engineering.com/getfil...cc2-8c6e-e9fa9af65c85&file=1380_bottonend.jpg

Rod

 

[patprimmer]

Rod

That's exactly why I asked if they were still made. Last one I saw was 30 years ago. It had been adapted to a SBC in a 5 litre class circuit race boat. It seemed to work well in that very demanding application.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[tbuelna]

evelrod,

I looked at the photograph of your engine's bottom end and it became apparent why your Romac elastomeric "harmonic balancer" was helpful. Most likely it's because your in-line 4 cylinder engine has a 3 main bearing crank.

A rotating shaft structure can have many different types of modal response to an excitation force. A particular order structural response can take the form of an "S" bending shape, it can be a simple "U" bending shape, it can be a whirling shape, it can be a pure axial mode, it can be a pure torsional mode, etc. The most damaging modes are the ones that produce the most dynamic loading and stresses. The lower orders are usually the most energetic, and if they couple with the forcing frequency, they can be especially harmful.

Most modern engines have well constrained crankshafts supported by closely spaced main bearings. With these engines, torsional modes are usually of primary concern. But your engine doesn't have a crank that is well supported by main bearings, so it is more susceptible to bending due to structural modal responses. I'm just guessing, but it appears that your elastomeric dampener was very helpful in mitigating a bending mode in your crank, and when it began to fail your engine's main bearings were subjected to additional misalignment loads due to the increased crank bending.

That's my theory anyway.

Regards,
Terry

ps. By the way, according to Romac's website, that elastomeric dampener can be rebuilt by replacing the elastomeric element.

 

[evelrod]

Thanks for that, Terry.
I think it is a foregone conclusion by the majority of the folks here that the BMC/Leyland "A" engine is a bit of an anachronism. Designed in the 30's and still used in Minis into the 21st Century. Yes, crank is very much akin to a piece of bent spaghetti. Yes it needs to be constrained and dampened a LOT to make it live. As you can see, the crank is a billet as is the center main cap. However, that is just about as good as it gets as the RULES don't allow for further enhancements. Bummer. Also the cylinder head is a FIVE PORT design, limiting the hp to about 150 max. Some of the racers are getting a bit more and twisting the little darlings to an astonishing 9500 rpm. Not me. My limit is 7800, electronically limited so I don't screw it up.

Rod

 

[NickJ67]

Very interesting thread. Terry's post above strikes a particular chord with me. In years past I've broken a couple of cranks on Triumph inline 4s (Spitfire engine). This being very similar to the A series in having a small, flexible crank and ony 3 mains. Unlike the A series it does not have a torsional damper as standard. Both my cranks broke across the front web, between no 1 big-end and the front main. At the time I blamed poor radiusing of the fillet edge during re-grinding, which probably was a factor, but maybe not the root cause. I know several others who have experienced breaks in the same spot. My engines saw the wrong side of 7000rpm quite regularly but only for very short periods. Both lasted around 20k miles before failure. I now suspect that a torsional damper of some kind would have helped. It might also have a helped another common problem on those engines which is rapid wear / damage to the centre main.

I've since moved on to the Triumph 6 cylinder (inline) engines which have a heavier crank but only 4 main bearings - not ideal, but at least they do have an elastomer type harmonic damper as standard. However, as the newest of these are now 30 years old and some as much as 50, it seems unlikely that many of them still perform as intended.

To add to the various types of damper mentioned above, does anyone have an opinion on the TCI Rattler

http://www.tciauto.com/Products/Rattler/

Must admit I can't quite picture how this is meant to work from their description!

I also have a vague memory of reading something about a damper which was effectively a sand filled device but googling that just now didn't turn anything up so maybe memory is faulty?

Regards

Nick

 

[evelrod]

Yeah, Nick. My old '60 Sprite 948 would break the crank at the front web on a regular basis. One time I drove it for several hundred miles with it broken. Lot of noise, but it made it with no other problems...It was just one of those times when I needed to keep going. Flew in a used crank and bearings from California...replaced the crank in my dad's driveway (El Paso) and continued on. No lift or other fancy tools either...Picked that little dude out by hand!

Photo of our Lotus twincam bottom end. I have never used a damper on it and it gives NO problems. Rev limit is 9500 rpm.

Rod

 

[NickJ67]

I guess the Lotus benefits from the steel crank but mostly the 5 mains, keeping whip in check. Short stroke probably helps too. Still, 9500 without a damper....!

I think you said the Mini crank was steel too - certainly looks like a nice piece in the photo. A good friend of mine is in the process of building an A-series bottom end with a BMW K-series bike head on it (surprisingly straight forward). That's just with standard crank and rods though they've seen quite a bit of fettling. This to go in a an Austin A30 which is a Sprite saloon more or less.

Cheers

Nick

 

[evelrod]

Lots of ways to skin a cat, Nick. I will admit to using stock BMC/Leyland rods in a 130+ hp engine using 7200 as a rev limit.......However, our 150 hp engine benefits from custom rods as well as the custom crank and bearing caps. I think, considering the cost v risk factors in a A-series bottom end, I think I would opt for, at least, an EN-40 crank and a set of custom rods.

Rod