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inline 6 order of vibration

inline 6 order of vibration

(OP)
i have an inline 6 vibration damper from bmw engine s52. stamped on it is 390-430hz. what order and rpm would this be tuned to? 3rd would mean 7800-8600rpm by my calculations but the engine never sees that rpm. so would it be the 6th and thus 3900-4300rpm?

RE: inline 6 order of vibration

You are talking the front pulley? If so, that frequency relates to the torsional resonant frequency of the crankshaft. That will relate to its torsional stiffness (this is the typical weakness on an inline 6) relative to its moment of inertia. You have an ignition firing somewhere in the engine three times per revolution, and you have a firing at the front cylinder (furthest from the flywheel) once per two revolutions; there are all manner of sources to excite that torsional vibration at all sorts of multiples of the engine RPM.

The frequency that damper should be best at absorbing, is the frequency near the torsional resonant frequency. This will depend on the torsional stiffness, inertia, etc which are pretty hard for the average backyard mechanic to figure out.

If you are dealing with an unmodified BMW S52 engine then don't modify it and use that damper unmodified, and don't change the flywheel inertia too much, either. Trust BMW's engineering, unless there is strong evidence that you shouldn't.

If you are hacking parts together from different engines or if you are lightening the crankshaft or doing some other modification to the crankshaft or using a lightened flywheel, good luck.

RE: inline 6 order of vibration

390 Hz = 23400 cpm.

With 3 power pulses per revolution, I'd say "first order" would be 3 X crank rpm.

23400/3 = 7800 rpm = first order, but 3rd order for 2600 rpm.

It is not unusual for higher orders to be the bigger problems for crankshaft torsional vibration.
https://encrypted-tbn0.gstatic.com/images?q=tbn:AN...

Even when not tuned precisely for a problem frequency, it seems like rubber-in-shear dampers offer some useful amount of damping at other frequencies as well. ( Arm chair / keyboard expert observation )

RE: inline 6 order of vibration

Tmoose you are confusing the terminology. First order in engine is once per rev, also known as 1E. In a 4 stroke I6 third order is firing order or 3E.

IC engines individual cylinders fire at 0.5E, and since they are spaced along the crank they have different effects on the torsionals. Their harmonics are at all integer multiples of 0.5E.

So, there is plenty of force available at .5,1,1.5.....9.5......etc order.

Luckily the human ear can only create a harsh sound from neighbouring harmonics if they are within a certain frequency range, roughly 1/3 octave.

So in order to suppress harshness typically you look for reasonably strong harmonics within 1/3 octave, exciting a crankshaft resonance -bending or torsional. Then tune the damper go that. I6s typically need both a bending and torsion damper, one pulley can do both.

This is very easy to do by ear if you've done it once.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: inline 6 order of vibration

(OP)
So i do know that the "vibration damper" located on snout of the crank is tuned for the torsional natural frequency of the system. lets use 400hz somewhere in the middle of the 390-430hz range a nice number to use. what order and rpm is this likely to be tuned for?

for a frequency of 400hz the rpm would need to be the following correct?

1E = 24000 rpm
2E = 12000 rpm
3E = 8000 rpm
6E = 4000 rpm

if someone was to delete the crank what rpm would the lack of damper cause issues most likely? i am NOT going to delete the damper but are bmw trying to dampen out the 6E at 4000rpm or the 3E at 8000rpm or something else?

the s52 engine cast iron block has vibration issues above 7000rpm shaking the hell out things, breaking oil pumps etc when used on track for extended periods in this rpm, this might be a resonance ? or could it just be a amplitude (rpm^2) things coming into play? the cranks dont break but ive heard some say that they do develop some cracks that stop the cranks ringing when you knock it with a hammer which suggests a slight amount of resonance.

the same exact damper is used on the short stroke engines e.g. 75mm & 84mm vs 89.6mm each of the cranks use same main and rod size so have same overlap

yet the exact same crank in the m54 engine (alloy block)uses a damper 275-305hz. Is this likely because of different reciprocating and rotating mass? the stiffness would be the same since the crank p/n is the same. it has worse vibration issues in the same rpm band

im thinking that retaining the higher freq damper might be better than using the lower frequency one even after changing the different reciprocating and rotating mass e.g. conrod, piston , flywheel to lighter versions.

RE: inline 6 order of vibration

Your maths is correct. BUT you don't necessarily tune a TV damper to the exact torsional frequency of the crank.

There are a couple of reasons to run a TV damper. Some engines will fall apart due to TVs. Some won't. However the crank resonances tend to cause a harsh sound (aka combustion harshness) which we like to get rid of. Without actually measuring what is going on this is just hand waving.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

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