Why Are Fluid Dampers / Harmonic Balancers Not Good for Balancing the Crank?

[JayTheCPA]

Looking for an educational experience. The engineering / physics piece that I am trying to wrap my head around is: given the fluid HB has a good reputation on a running engine, what is an engine shop doing that causes it to perform differently (or does it perform differently?) such that balancing must occur with an OEM HB? Am not trying to start a ‘flame-war’ of why some engine shops are (or are not) correct with statements to stick with an OEM HB given the crankshaft will not balance with a fluid HB, I am just looking to understand how competing positions are all correct. Did I miss a conversation along the way that covers this topic as a whole and not piece-parts of another topic?


Background to the question:
Have read up on fluid based harmonic balancers (here and other locations / brands) and the general impression / wisdom / experience is that:

> Good for engines (like the 6.5) when sticking closer to stock HP / torque but not necessarily so good if pushing the limits on engines like the DMax. Also recognize that a fluid HB is in the heavy duty OTR setup which implies it is a solid technology.

> When balancing the crank, need to use an OEM style HB as the fluid HB essentially does not play well while trying to balance; this is showing up in the form of multiple engine shops (vaguely referenced here on TTS and specifically when I talk with my mechanic) stating that a fluid HB creates more imbalance than it cures and recommendations to stick with an OEM HB (or put another way, avoid the fluid HB).


Yes, I 'get-it' that those who install a fluid HB (and keep mods / tuning short of drag-strip level performance) report less noise / vibration, no issues with durability, and seem to recall a few comments about a smidgen of better performance / economy (or perhaps the last item came to me in a dream . . .), I just want to understand how everybody is correct.

 

[THEFERMANATOR]

A fluidamper uses a heavy viscous fluid to absorb harmonic vibrations and slight imbalances. This same viscous fluid will actually absorb the imbalances during balancing and mask them so that they cannot be found during balancing. For balancing purposes you have to use a constant weight that cannot absorb or change the balance in any way as they are trying to detect imbalances so they can balance and correct them.

 

[buddy]

The fluidampers are designed to work with stock rotating assemblies. So if you go up in power using stock pistons, cranks, rods, pulley, flywheel, ect... then it should be good to use. If you are going up in power and using lighter pistons and rods or different mass crankshaft then it probably wont be in the tolerance that the Fluidampr was designed for.

 

[3500GMC]

From Fluidamprs website FAQ:

Why can't Fluidampr or Streetdampr be spun on a balance machine?


Balancing a crankshaft with the Fluidampr or Streetdampr is not recommended. The Fluidampr (or its damper ring) and the Streetdampr contain an inertia ring that can rotate inside. This inertia ring is balanced to very close tolerance specifications at the factory but may not be to perfect “zero” balance. When the damper is spun on a balance machine, this inertia ring may rotate inside the damper and the operator may be chasing this small imbalance while attempting to balance the crankshaft. When the damper is installed and running on a combustion engine, the shear forces on the silicone fluid inside the damper from the torsional moments of the engine crankshaft, centers the inertia ring on its bearings. A balance machine does not generate torsional shear forces and the inertia ring may not be completely centered in its bearing clearance gaps. (IT WILL BE CENTERED AND IN BALANCE WHILE RUNNING ON AN ENGINE ) Note: Our new CT Gold Dampers do allow the crankshaft to be balanced while the damper is installed.

 

[millsroger]

Ferm is right on, let me expand it a bit. When you balance the crank on the machine it is a static balance for that rotating mass. To achieve static balance you need to have all rotating elements installed. It is assumed that the elements are static in nature, in that, they do not change position or weight. To achieve full static balance you then match the rods, bearings, pins and pistons for weight. On the machine, the dynamic nature of the fluid damper makes it impossible to achieve true static balance because it is changing as it rotates. Where you can have a problem is balancing a crank with the original damper and then removing it. What you install in it's place must also be in static balance or you start out with vibration. IMHO, not all fluid dampers include static balance as a starting point--most companies assume that the fluid damper will compensate for the static imbalance, while a good concept statement, not always true. What destroys cranks is not static imbalance as that is usually minor, it is dynamic imbalance caused by harmonics. Consider the crank a solid beam with eight hammers in a row along the beam, start hitting the beam with the hammers in the sequence of the firing order. At certain rates, (RPM), the vibration created by one hammer will syncronize with another--that's harmonics. The beam flex is amplified by the harmonics. These harmonic waves run from end to end and hammer to hammer, if they add up at a main bearing web, it can crack it, or beat the bearing up. There are "orders" of harmonics, as in, how many cycles of one does it take to syncronize with another, and a whole science has evolved to get these orders reduced within the RPM range of a given engine. The truth is that the damper just reduces power impulses from getting to the belt so the accessories do not start shaking in sympathy. It is at the end of the beam and cannot remove internal vibration or harmonics. When the damper fails it allows the location of the static balance weight to shift location, that throws the crank WAY out of static balance--that will destroy the crank very quickly, usually not just the crank. Some engines do not have a balance weight in the damper, when those fail it just lets the pully slip, these engines do.

 

[millsroger]

An addendum, advantage of a fluid damper is that they do not wear out or slip-at least withing the life of the rest of the engine.

 

[Twisted Steel Performance]

Jay, go find my build thread and look at the balance specs for my motor, I watched them do the balance job, my pistons,pins,clips,rings,bearings, were all close enough that nothing needed to be done to them.... the rods were another story all together... they were very bad off in weight on both ends from the lightest to heaviest, I don't remember what the off weight was but it was very bad, and that was with a GEP motor.... I think that every motor should be balanced, then with a good HB it will be a smooth motor...

 

[6.2 turbo]

The stock damper has the balance weight cast into the hub. If the outer ring slips it would only change the timing mark.

 

[schiker]

A simple way to think about some of it is compare and think about a tuning fork, drum, or other like instrument. Hard metal you can ring with a tap it takes vibe energy through the metal to make noise. Drum when its tight etc. But if you add soft mass that can absorb energy of the vibrations are dampened. That is kinda what a harmonic dampener does. So the mass helps in function but you couldn't tune a piano if you deadened the tuning fork so you can't use it while balancing - it messes up the tuning.

 

[millsroger]

Nicely put schiker, just consider a drum mounted in a rubber ring, the drum still sounds but the stand it is mounted to will not. The harmonic balancer is outside the end support of the crank and has little effect on internal crankshaft vibration. It DOES dampen rotating surges and pulses. And, per 6.2turbo, the static counter weight still needs to be part of the fluid damper and be exactly the same as stock. And, as quoted by 3500 GMC, on a balancer the fluid damper may not center up and stay put, as part of that same quote it referred to an "inertia ring"---that functions only in rotation, not sideways---torsion, not axial.

 

[JayTheCPA]

Chris, saw all the specs on your build and definitely good information:hihi: I decided to start a higher level discussion on this topic to put all of the piece-parts discussions into one dedicated thread and respectfully try to pull together how two opposing viewpoints are both correct. Personally, I am coming at it by a different angle of replacing the OEM HB with the fluid HB as a maintenance item rather than part of a build and probably will not remove the crank for re-balance; this was deliberately left off the original question to get a free-flow of information and learn something along the way.

And so far, it looks like a great discussion :thumbsup:

 

[3500GMC]

Also important to note that a Fluidampr for the 6.2/5 is roughly double the weight (18 lbs. IIRC) of an OEM piece. More stored energy, mass and inertia to counter the hamonicas.

 

[millsroger]

Jay, highest and best use of a fluid damper is as a one time maintenance item--should last the life of the engine. Will also extend the life of the belt, tensioner and accessories.

 

[Twisted Steel Performance]

That's right, the Fluidampr is a one time thing, if you sell a truck, change the balancer and keep the Fluidampr for the next one... they don't wear out... and it is a bit heavier than stock...