Crankshaft Dampers, how critical?
Discussion
I have been buggering about with BMW supercharger on A+ engine and have got to the "Need to make a bottom pulley specially for the job" stage. I have looked at dampers and am well aware you can get them w/o pulley. I was wondering how critical the hardness of the rubber is.
I've lightened the crank/flywheel/etc and this will have the effect of upping the resonant frequencies. To what degree, I am unsure. I also did a bit of logical thinking and assumed that the existing damper would have degraded somewhat and therefore be "looser", it would thus be better at absorbing lower frequencies (I think), so having lightened/balanced/etc my bottom end, I am at risk of running an incorrect damper for the job.
How do I calculate what to use, is it important and if so, how important?
As an aside, M Rex, could you direct me to that website with the vibration characteristics of different cranks site?
Cheers Stu
I've lightened the crank/flywheel/etc and this will have the effect of upping the resonant frequencies. To what degree, I am unsure. I also did a bit of logical thinking and assumed that the existing damper would have degraded somewhat and therefore be "looser", it would thus be better at absorbing lower frequencies (I think), so having lightened/balanced/etc my bottom end, I am at risk of running an incorrect damper for the job.
How do I calculate what to use, is it important and if so, how important?
As an aside, M Rex, could you direct me to that website with the vibration characteristics of different cranks site?
Cheers Stu
The problem with A series cranks is 1, couple unbalance not dealt with correctly when lightening crank. 2, torsional vibration, causing fretting of flywheel taper joint. These problems are overcome by lightening the crank paying attention to point 1, and balancing correctly,also making sure the flywheel/crank key is a close fit, also a standard 2 piece damper available from mini spares will be suitable.
Torsional Vibration or TV dampers are critical for component longevity.
When the Jag V8 was being designed we had to diagnose why we had the flexplate break on the back of the engine. It turned out that the TV damper was out of spec which overloaded the flexplate, causing it to fatigue and fail. We ended up redesigning the flexplate system to be 'bulletproof' for durability and in the process learnt a great deal about TV dampers.
As I am sure you are well aware a car company isn't short of a bob or two when it comes to designing a new engine and there is only one sure fire way to tune the TV damper - dyno work.
You have the engine on the dyno and put in a load cell that is sampled at a very high frequency rate. What you then do is then looking at the torque trace.
The engine torque is really a great number of peaks far higher than the quoted torque that when resolved match the quoted torque, and then you sum the TV damper's input. When the TV damper goes into resonance it is absorbing the peaks and displacing them so the net engine torque is the same. But the impulses are lowered so that it doesn't break components.
The next job is to try different TV damper tunings to take out these peaks. Great if you have a big budget, but bad for the home engine builder.
For the home tuner the cheapest way is to put money into improving component robustness by preventing cracks from starting or propogating. Polish everything and shot peen it. This puts the surface into compressive residual stress and makes overloading the component difficult. Use a high strength material wherever possible. Use specialised adhesives that retain components to reduce the chances of fretting.
There are many tuners that say that polishing is a waste of time. Perhaps the dyno figures are the same when you consider either polished ports against rough, or polished components against rough, however, we are not concerned with power but durability.
Read a book on improving fatigue life and it will in essence say what I summarised.
I would design a pulley to mount to the crank that 'packages' and provides pulleys for all the ancilliaries you wish to run and ditch the TV damper part - if it is not tuned it will do no good, and it may do more harm than good.
When the Jag V8 was being designed we had to diagnose why we had the flexplate break on the back of the engine. It turned out that the TV damper was out of spec which overloaded the flexplate, causing it to fatigue and fail. We ended up redesigning the flexplate system to be 'bulletproof' for durability and in the process learnt a great deal about TV dampers.
As I am sure you are well aware a car company isn't short of a bob or two when it comes to designing a new engine and there is only one sure fire way to tune the TV damper - dyno work.
You have the engine on the dyno and put in a load cell that is sampled at a very high frequency rate. What you then do is then looking at the torque trace.
The engine torque is really a great number of peaks far higher than the quoted torque that when resolved match the quoted torque, and then you sum the TV damper's input. When the TV damper goes into resonance it is absorbing the peaks and displacing them so the net engine torque is the same. But the impulses are lowered so that it doesn't break components.
The next job is to try different TV damper tunings to take out these peaks. Great if you have a big budget, but bad for the home engine builder.
For the home tuner the cheapest way is to put money into improving component robustness by preventing cracks from starting or propogating. Polish everything and shot peen it. This puts the surface into compressive residual stress and makes overloading the component difficult. Use a high strength material wherever possible. Use specialised adhesives that retain components to reduce the chances of fretting.
There are many tuners that say that polishing is a waste of time. Perhaps the dyno figures are the same when you consider either polished ports against rough, or polished components against rough, however, we are not concerned with power but durability.
Read a book on improving fatigue life and it will in essence say what I summarised.
I would design a pulley to mount to the crank that 'packages' and provides pulleys for all the ancilliaries you wish to run and ditch the TV damper part - if it is not tuned it will do no good, and it may do more harm than good.
As Gavin stated the crankshaft damper-absorber is to take out the large amplitudes of crankshaft torsional vibration, otherwise the crank wouldn't last very long.
The theory of the absorber is that the natural frequency of the rubber & inertia ring matches the targeted frequency of the crankshaft & rotating components. Targeted frequency being the frequency of the worst torsional excitation in the operative range of engine speed. The natural frequency of the crankshaft system is a function of crankshaft length, crankshaft torsional stiffness, crankshaft stroke, and the moments of inertia of rotating masses of the engine or ancillaries driven by the engine. As you correctly state the lightening of the crank & flywheel will have increased the resonant frequency but the addition of the supercharger will have lowered it. The key is knowing where that worst frequency is, and without all the data and FE models of crankshaft to determine stiffness it is very difficult to calculate. Even then, as Gavin points out, you need to tune it finally on the dyno.
However.....
Apparently though there is a solution for the home engine builder. These are fluid viscous dampers where the damper works over any mode or frequency. It isn't as effective as a properly tuned absorber at a particular frequency, but it does damp the worst out with the correct selection.
I'm probably sounding very pessimistic and the damper you already have maybe good enough. But I'm certainly not good enough to calculate what the differences will be. Perhaps its best not to think about these things too much and JFDI.
Best of luck
Phil
>> Edited by wheeljack888 on Wednesday 9th March 00:09
The theory of the absorber is that the natural frequency of the rubber & inertia ring matches the targeted frequency of the crankshaft & rotating components. Targeted frequency being the frequency of the worst torsional excitation in the operative range of engine speed. The natural frequency of the crankshaft system is a function of crankshaft length, crankshaft torsional stiffness, crankshaft stroke, and the moments of inertia of rotating masses of the engine or ancillaries driven by the engine. As you correctly state the lightening of the crank & flywheel will have increased the resonant frequency but the addition of the supercharger will have lowered it. The key is knowing where that worst frequency is, and without all the data and FE models of crankshaft to determine stiffness it is very difficult to calculate. Even then, as Gavin points out, you need to tune it finally on the dyno.
However.....
Apparently though there is a solution for the home engine builder. These are fluid viscous dampers where the damper works over any mode or frequency. It isn't as effective as a properly tuned absorber at a particular frequency, but it does damp the worst out with the correct selection.
I'm probably sounding very pessimistic and the damper you already have maybe good enough. But I'm certainly not good enough to calculate what the differences will be. Perhaps its best not to think about these things too much and JFDI.
Best of luck
Phil
>> Edited by wheeljack888 on Wednesday 9th March 00:09
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