harmonic balancer
- Thread starter PeteyAce
- Start date
Michael Yount
Mustang Master
Actually, it's a common misconception that the harmonic balancer balances the rotating assembly. It doesn't. The rotating balance is provided by matching sizes/weights on the rotating bits like rods/pistons/pins, and then adjusting the counterweights on the crankshaft so that there is some semblance of balance for the rotating parts. A custom balance (machine shop) does this even more carefully - and they'd often like to have the flywheel and clutch as well.
There is another important source of vibration in the rotating assembly however. And it's the fact that power isn't input into the crank in one smooth motion. It's hammered in everytime combustion occurs forcing a piston down. The hammering of those power strokes sets up a torsional vibration in the crank that is critical at certain rpms - depending on the engine, number of cylinders, angle between banks, 2 stroke or 4 stroke, length of crank, how many main bearings, etc. We tend to think of the crank as perfectly stiff - but of course it's not; it actually twists slightly each time the rod pushes on the throw and springs back once it passes through bdc and the rod stops pushing on the throw. This torsional vibration is entirely different than the rotating vibration that would be caused by - for example - one piston being noticeably heavier than another. The harmonic balancer is designed to cancel out/minimize those crankshaft torsional harmonics at the critical rpm. There are both fluid dampers and elastomer dampers. Most folks on our engines use an elastomer damper. There is a thin rubber ring separating an inner and outer ring of steel. It's designed so that the outer ring of steel twists the rubber - the rubber acts like a spring. The ring twists on the rubber to cancel out crankshaft torsional vibration. Some of the energy is transferred as heat in the rubber, but the rubbers's main function is to store the rotational energy of the outer steel ring and return it to the crankshaft at just the right time to suppress torsional vibration. Fluid dampers do the same thing but they generally use a silicon fluid with temperature dependant viscosity to provide the damping instead of rubber.
There is another important source of vibration in the rotating assembly however. And it's the fact that power isn't input into the crank in one smooth motion. It's hammered in everytime combustion occurs forcing a piston down. The hammering of those power strokes sets up a torsional vibration in the crank that is critical at certain rpms - depending on the engine, number of cylinders, angle between banks, 2 stroke or 4 stroke, length of crank, how many main bearings, etc. We tend to think of the crank as perfectly stiff - but of course it's not; it actually twists slightly each time the rod pushes on the throw and springs back once it passes through bdc and the rod stops pushing on the throw. This torsional vibration is entirely different than the rotating vibration that would be caused by - for example - one piston being noticeably heavier than another. The harmonic balancer is designed to cancel out/minimize those crankshaft torsional harmonics at the critical rpm. There are both fluid dampers and elastomer dampers. Most folks on our engines use an elastomer damper. There is a thin rubber ring separating an inner and outer ring of steel. It's designed so that the outer ring of steel twists the rubber - the rubber acts like a spring. The ring twists on the rubber to cancel out crankshaft torsional vibration. Some of the energy is transferred as heat in the rubber, but the rubbers's main function is to store the rotational energy of the outer steel ring and return it to the crankshaft at just the right time to suppress torsional vibration. Fluid dampers do the same thing but they generally use a silicon fluid with temperature dependant viscosity to provide the damping instead of rubber.
very well put Michael.Michael Yount said:
I did not know about the elastomer 'storing' energy. so even if one's balancer 'looked alright' but there was a crack or two in the elastomer, that could mean it needs to be swapped, right? I'm thinkin that the elastomer is 'tuned' and if its strutural integrity is compromised (via a crack, etc), then it does not function just exactly as it should. many folks might have a crack or two and not replace the balancer until further symptoms or wear traits show up, but the balancer might not be working as well as it should......? sound right?
Michael Yount
Mustang Master
I think you're correct JT. Analogous to dry rot cracks in old tires. It may still hold air and roll just fine - but you're on borrowed time once the rubber starts to break down.
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