Design Differences in Aftermarket vs OEM Harmonic Crankshaft Damper 2

[Mechie10]

I am considering a replacement of the OEM Harmonic Crankshaft Damper with a aftermarket damper because of the lower cost. The aftermarket company claims to meet OEM specs, but the aftermarket design is different. The center hub that attaches to the crank has less about 0.5 Kg less mass and a 5 mm larger diameter. Without knowing the details of harmonic damper design, it seems possible to design inner and out hub of the system with the OEM same frequency tuning by lowering the elastomer spring rate to compensate for the lower mass inertia. Or to keep the moment of inertia of the inner hub the same by increasing the diameter. But, there will still be less mass attached to the crankshaft. It is possible for the aftermarket damper design system to have the frequency tuning as the OEM and be an effective damper?

 

[GregLocock]

Yes, it could quite easily be as good as the OEM part as a TV damper. Unfortunately without measuring the performance of both you'll never know.

Bear in mind that the OEM may have additional constraints (such as NVH, and bending modes, and durability) that the A/M part does not address.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Mechie10]

Thanks for the input Greg. From what I researched, it is fairly involved and specialized test equipment is needed to accurately measure the performance of TV damper. But, is it possible to get a qualitative comparison between the two dampers by marking the position of the inner and outer hub, sweep the engine through the rpm range and use a timing light to "freeze frame" the relative positions of the hubs? Is the torsional deflection too small or crank phasing to inaccurate to use this method?

Are the bending modes you mentioned crankshaft bending modes? I would think the reduced mass of the A/M design could affect this and might place additional stress on the crank. It looks like the OEM design added mass,especially to the center hub.

On a similar issue, some manufacturers of performance crankshaft pulleys (ie "underdrive" or "lightened") have eliminated the concentric hub elastomer design to reduce the mass at crankshaft to increase the power. The manufacturers claim the concentric hub elastomer design is for NVH only and not for TV damping, since some engines have a one piece pulley. My thought would be that you can't draw that conclusion, since TV is the result of a system design of crank and its assemblies. Maybe the crank in the engines without the damper was designed to have a resonant frequency out of band or can withstand the stress. Is there a damper designed only for NVH and if so, is there a way to tell it apart from a TV damper?

 

[SomptingGuy]

"to reduce the mass at crankshaft to increase the power"

You may get a different (better/worse) throttle reponse, but no changes to the TV damper will affect power.

Measuring instantaneous flywheel speed is not a new science. There are a few ways to do it. Targetting inductive probes at gear teeth is a very cheap and simple method, which can give remarkable results.

- Steve

 

[Mechie10]

"You may get a different (better/worse) throttle reponse, but no changes to the TV damper will affect power."

Yes, true enough - part of mfrs marketing I guess. I would think the lighter pulley "indicates" a power increase on an inertia dynamometer, so they feel they can make the claim. I am not sure about the OEM concentric hub elastomer design is for NVH only, though. Someone with design experience would have to comment.

 

[GregLocock]

"The manufacturers claim the concentric hub elastomer design is for NVH only and not for TV damping" That claim is frankly rubbish. A race engine may not operate at the resonant speeds long enough to fail, but if you remove the TV damping function from a TV damper a production crank may well fail - it just so happens that production redline speeds are around the typical resonant speeds at firing order.

You can't measure TVs with a timing light - a crank will fail at 3 degrees peak to peak which is inside the scatter of atypical timing light. The cranknose toothed wheel is a great place to measure TVs.







Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[tbuelna]

I'm not a shaft dynamics expert, but I would argue that the primary use of a crankshaft dampener is to mitigate troublesome crank structural torsional vibration modes, and not so much for NVH issues. If all you want to do is smooth out the peak-to-mean instantaneous torques at the crank for improved NVH, then this can be done more easily and cheaply by increasing flywheel inertia. Torsional dampeners are useful because they can reduce the max combined stress levels in a crankshaft structure at certain frequencies, but unfortunately they also may exacerbate the situation at other frequencies. The additional cost of the dampener can be offset by allowing the use of less costly materials for the crankshaft.

The various vibratory modes (linear, torsion, simple bending, S bending, whirling, chucking, etc.) of an engine's crankshaft system are very complex and can be affected by many variables. Determining how and where these modes occur, as well as the magnitude of their impact on the structure's loading and fatigue life, is no simple matter.

As others noted, simple crank torsional displacements can be measured real time using a trigger wheel and hall-effect sensor with a suitable sampling rate in your data acquisition system. But performing a torsional survey of your engine with sufficient detail will require lots of test points and dyno hours. For both your baseline engine configuration and the modified engine configuration. Characterizing other structural modes such as bending or whirling would require some sort of accurate, non-contact displacement sensor, such as a prox sensor. And of course, the ideal instrumentation would be a series of strain gauges on the relevant parts during test.

I find it doubtful that any aftermarket product would be both better performing and less expensive than the OEM device. The OEM's have the benefit of mass production on their side with regards to cost. Plus I doubt any aftermarket manufacturer would commit the necessary engineering and test budget required to properly develop an improved torsional dampener. The aftermarket guys will claim their product is better, but who's going to bother to put it to the test?

Regards,
Terry