Harmonic Dampeners 3

[steve383]

Can any one explain how a harmonic dampener on a V8 works? Is it tuned to the natural frequency of the crankshaft? Does it dampen the torsional twist of the crank from the power pulses? Why does one dampener work for cast/steel cranks? How can sprint car motors run with out one? Thanks!

 

[ivymike]

It's tempting to make a joke about what a "harmonic dampener" might do (spit on you while singing?)

A torsional vibration damper is used to limit the amplitude of the torsional (twist) oscillations of the crankshaft.

The oscillations are excited (caused) by both the firing pulses and inertia loads transmitted to the crankshaft by the reciprocating assembly (and to a much lesser extent by the timing drive and/or fuel system for high pressure injection). These excitations are made up of many harmonic components. The crankshaft+flywheel+rods system has a number of different mode shapes that it would like to take while vibrating, each with its own associated frequency. Crankshaft vibration is greater when one of the harmonic components of the excitation is occuring at the same frequency as one of the crank's vibratory modes (when resonance occurs).

Torsional vibration dampers come in four main varieties:
1) mass-elastic-damper <- typical rubber damper design
2) viscous damper <- seen on large engines, also available in aftermarket for autos
3) visco-elastic damper <- seen on large engines, probably available in aftermarket for autos
4) mass only (absorber not damper) <- seen on aircraft engines, available on aftermarket ("Rattler" brand, for one, I believe)

Type 1 must be tuned (correct stiffness, damping, and mass) to work well with the crank system. It will often be selected to split a big resonance hump in the running range of the engine into two smaller humps, perhaps with one of them above the running range of the engine. It is not unlikely that changing from a cast iron to a steel crank without changing anything else will not have a large enough effect on the crankshaft vibratory mode frequencies to make a huge difference in damper tuning. You could no doubt do better with a custom-tuned damper, but the original might be good enough.

Type 2 is "sized" rather than tuned. It simply absorbs vibratory energy from the crank, and doesn't have the "hump splitting" effect of type one.

Type 3 must be "sized" and tuned, but is more similar in behavior to Type 2 than to Type 1.

Type 4 is a special case - it doesn't really absorb energy the way the other three do - it produces a counter-excitation at a specific engine order. It's very useful when you have one particularly annoying excitation order to deal with, such as when you're running a fixed-speed engine.


There are lots of reasons why some motors can run without the dampers; I don't know which might be the case with sprint cars. If an engine runs at speeds that put it below or above its major resonance humps, then a damper may be unneeded. If the crank system mass has been significantly reduced, it's possible that some of the vib. frequencies would shift out of the running range. If the crankshaft is strong enough that it won't break anyway, then damping the vibrations would be unneccessary (from the point of view of the crankshaft stress, anyway). If the crank life requirement is short enough, then you might allow damaging vibrations to occur with the expectation that you'll replace the crank regularly.

 

[1Bluegrass]

How could I go about some type of determination as to the needs for damping on a sprint car engine?
Can the crank assembly supplier advise on this or other net info be found?
The sprint car is direct drive with an auto type flywheel for starting and use of a damper, all in a balanced assembly.

 

[btrueblood]

The main reason for a torsion dampener is to prevent transmitting a lot of "rattle" down the drivetrain to transmissions, etc., where it may cause trouble. Interestingly, I've worked on a few big V8's with torsion dampeners on the front belt drive pulley too (to keep from getting a lot of vibrations transmitted into the belt driven accessories).

You can also lower the frequency of the drive system to avoid resonances, typically by putting more flywheel inertia on the drive shaft (either in front or back). This can cause other trouble, not least of which is the expense of more metal, and precisely balanced metal at that.

I spent some time working with V-twin engines in the 20-hp range. A few manufacturers made these with so-called "Harley" timing, where both pistons fire on the same revolution. The resulting torque "kick" was more than 10x what we were expecting, and played havoc with pumps & blowers that the motors were to drive. We added dampening couplers and flywheel inertia to avoid resonances with the attached equipment, and managed (eventually) to get reasonable service out of the machines.

 

[GT6Steve]

This is a topic I've been stewing on for a while. I'm no expert and will ask more questions than provide answers.
I understand the concept of harmonics and have some ideas about how it can be measured. sadly, none within my scope. I race a non Chevy Non V-8 so it's probable that a proper study has never been done.

So, Thinking about it in terms of a spring/damper equation I came to realize that the inertial ring is precisely that. Essentially analogous to a fixed ground plane in the spring /damper calcs.
So, if I'm on the right track there, is it safe to say that the weight of the inertial ring is not really a factor in damper selection as long as it is heavy enough to provide the inertia for the reaction between crank and rubber damper? I haven't yet sorted out the reaction from too much weight but I'm still thinking.

Where I'm going is that I need to select an off the shelf Ford/Pontiac based damper to suit my six cylinder application and trying to comprehend the variables of that selection.

I think my question is, can I be overdamped with too heavy an inertial ring and am I approaching the problem from the correct direction?
Thanx in advance, Steve

 

[GregLocock]

GT6Steve

The inertia of the damper controls the frequency difference between the two resonances, and also controls how much energy can be absorbed. A rule of thumb is that the interia of the absorber needs to be 10% of the interia of the mode you are trying to control. That rather begs the question of how we work out the modal mass - for a crank take the rotational inertia of about half the crank.

"The main reason for a torsion dampener is to prevent transmitting a lot of "rattle" down the drivetrain to transmissions, etc., where it may cause trouble. Interestingly, I've worked on a few big V8's with torsion dampeners on the front belt drive pulley too (to keep from getting a lot of vibrations transmitted into the belt driven accessories)."

No. The main reason is to stop the crank breaking from fatigue. The NVH benefits are secondary to that on most production cars.

If you think about it it is obvious that a tiny dynamic abosrber won't have much effect on a large system.

The other part of he tune is the stiffness of the rubber in the damper, and its damping characteristics.

Try and pick a damper that comes off an engine with the same length and weight of crank as your engine - that might be close enough.

Cheers

Greg Locock

 

[btrueblood]

Sorry Greg, you're the expert. I was thinking more of torsion dampeners that are added on (by OEM/users) to commercial engines, for the purposes of reducing torsional vibration inputs to attached equipment (pumps, blowers, generators, etc.)

 

[WilliamH]

The way we describe crank harmonics (vibration) in a simplified manner is thus:

Imagine a thin section shaft welded to a large diameter and relatively high mass "flywheel". If momentary torsional loads are applied at four locations along this shaft the far end of the shaft will experience a twist relative to the flywheel end. This twist is a helix with a starting point at the flywheel. Depending on the applied loads and stiffness of the shaft/natural frequency of the shaft, etc., a considerable displacement can occur. (Angular velocity difference from end to end).

If you observe the flywheel and pulley of say a SBC V-8 on a dynamometer at WOT, full load, from say 4500 RPM to 9500 RPM with a strobe light at each end (one on the flywheel one on the pulley) one can see how much oscillation is occuring at each vibration node at each end.

A heavy flywheel will exibit little vibration as compared to the opposite end of the crank. Almost all of the crank failures we have seen occur at the flywheel end of the crank.

In a sprint car with no flywheel the relative end to end crank twist (torsional amplitude) is much reduced. Hence, maybe a harmonic dampener can be eliminated.

Will

 

[steve383]

Thanks guys!

 

[patprimmer]

A sprint car still has a load that tends to keep the output end of the crank at constant speed, whereas the other end is only restrained by the stiffness of the crank, and the inertia of the water pump.

Maybe its the lack of mass in the drive train and the flex in the tyre walls that dampens the crank in a sprint car, or maybe the crank life is not so critical as performance.

The mass of a damper will decrease acceleration performance due to the inertia of the extra rotating mass.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[DesignOutsourcing]

Hi Steve,

I have a '69 GT6+ and did 120,000km around the US in it. One of my favorite cars!

The dangerous harmonic for the GT6 crank is around 7200rpm. My cam ran out of breath at about 6500 so I never bothered changing the damper. Some people have fitted Ford 5.0 fluid dampers which seem to work. There is a guy in the UK (John Wood) who made dampers for the Triumph 6 but I just looked up his site

http://www.classicracecraft.com

and got nothing. I have an extra Weber dcoe manifold set if you need one.

cheers, derek

Join us at

http://groups.yahoo.com/group/opensourcecars

 

[GT6Steve]

Hello Derek,
Will you contact me direct at GT6Steve@aol.com? Steve

 

[kmb1949]

Is tuning of a dampener necessary on diesel engines that have a max rpm of 3500? Is any good rubber dampener okay?

 

[GregLocock]

I don't think you'll absolutely have to have one as a diesel's crankshaft is already a fair bit stronger than a petrol engine's crank.

Just to be sure find a similar sized engine to yours and have a look.



Cheers

Greg Locock

 

[ivymike]

but Greg, diesel crankshafts are still usually "near the edge" regarding stress, aren't they? That's why they often come with the (expensive) viscous or visco-elastic dampers?

 

[GregLocock]

Oo er, perhaps I made an assumption there that I shouldn't have. When I saw a 3500 rpm red line I assumed it was automotive size, in which case the first crank resonance is likely to be well above the red line (unless it is a V12! or a two stroke).

Typical crank torsional is 300 Hz plus.









Cheers

Greg Locock

 

[kmb1949]

This dampener will be used on a new diesel engine design. It will have a new sectional crankshaft with HD needle bearings on the mains and rod journals.

 

[ivymike]

Greg, sounds like both of our assumptions may have been wrong - I was figuring that the engine in question was likely an I6, 6L to 7L engine, which was being "tuned" for high power output (500hp+). It's been my experience with such designs (I've only seen a couple, though) that a hi-perf damper is critical for keeping crank stress acceptably low.

kmb - I'm not sure that I understand what you mean when you say "sectional" crankshaft. Would you mind elaborating on that? Also, how many cylinders will this engine have? Just one or two? The proper way to determine the damping requirements, if there is any question about whether the crank will survive, is to perform a fairly rigorous analysis (or have it done for you). I would suggest using a detailed FEA with both kinematic + dynamic crank loading applied to check crank stresses (@fillets, webs, oil drilling breakouts, etc). If crank stress is acceptably low without a damper, or with a "generic" damper on the cranknose, then don't worry about it - otherwise, run a few torsional vibration analyses to help select a damper that will give acceptably low crank vibration, and/or strengthen the crank. If you can't do this in-house, then there are plenty of consultants out there who can do it.

 

[kmb1949]

This crank will be used on a three, four and six cylinder in-line engine application. The crank is sectional. The wings are billet machined so that all are the exact same. The main journals as well as rod journals are also billet machined. The parts are put together by the use of induction heating or pressing. The journals are designed with a flat side that fits against a flat side in the wing. Kind of a D shaped joint. There are no welds or oil gallies. All the parts are 4340 alloy. These designs are patent pending. The bearings will be heavy duty needle bearings with inner races that are pressed over the machined journals. With the inner races pressed over the journals the needle bearing slides over the journal before the wings are pressed on the journals. The engine will use a standard main cap and rod cap arrangement. Since all parts are CNC machined,I hope the finished product is already in balance. I still want to use a dampener however, one with a serpentine belt groove for the alternator. We will be using a gear driven coolant pump.

 

[ivymike]

the finished product is already in balance. I still want to use a dampener however

Note- the function of a torsional vibration damper has nothing to do with engine balance.

If you want to size and select a TV damper, do the TV analysis and the crank stress analysis, and assess your needs.

If you'd like to have a consultant do that analysis work for you, there are a few good ones in the US. I'd recommend Ricardo, Inc., because of my favorable impression of the experience level of their engineers.

http://www.ricardo.com/portal.asp

Many of their competitors would also be able to perform this analysis adequately, I assume.