Oscillation damper

[SailingDog]

Hi All,
Can some knowledgable soul explain what an oscillation damper does apart from dampen oscillation, can I assume it smoothes wobbles between the engine and the gear box and drive shaft.
What are the symptoms if the OD has broken or warn out ?

Thank you SD

 

[kashurst]

when an internal combustion engine is running every time an ignition point happens the crank tries to accelerate faster, however as the other pistons etc are all at different points in their part of the cycle the crank bends slightly as its is rotating to absorb the shock of the ignition burn and the resultant force. At certain loads and speeds there is a risk that the crank shaft will hit a resonant frequency (like twanging a ruler) as it rotates, the bending gets bigger and ultimately the crank can snap.
the oscillation damper on the crank (and on big engines on the cam shaft), changes the resonant frequency of the whole crank conrod piston asssembly so that resonant state should never happen.

If the oscillation damper is missing or breaks up the crankshaft can snap and will probably wreck the engine

 

[BruceK]

To add to K's explanation a similar process also happens in a driveshaft with a single UJ when the two shafts are not exactly lined up either side of the UJ. One half actually has to spin faster to make up the turn than the other


edit. This explains it https://www.learnengineering.org/understanding-universal-joint.html

 

[SailingDog]

Kashurst, thank you for your speedy response, I can understand your explanation. Does this mean there little relationship between the OD and the gear box.

Thanks

 

[kashurst]

The oscillation damper is there to protect the engine and smooth the high power running. Between the engine and the gearbox there is often a drive plate assembly which is a disc with springs or rubber inserts in to isolate the harmonic vibrations of the engine from the gearbox and vice versa. The drive plate also acts as a mechanical fuse if the gearbox locked up. Ordinarily any issue with the oscillation damper should not affect the gearbox, nor the gearbox the engine.

 

[SailingDog]

Thank you for your explanations, much appreciated.
SD

 

[SailingDog]

Thank you, for your explanation.
SD

 

[scottie]

As I recall there are several types with varying sophistication depending on power usually fitted on the front end of the crankshaft.
On many commercial craft it was common to have driveline calculations carried out as part of the design.
it was not uncommon to have the dampers checked periodically as they did definitely fail (the effective element was fluid ) I don’t recall if they were repairable

 

[spannerman]

The damper works in two directions, if you monitor the rotational speed variation on a diesel engine with an oscilloscope you will see the engine slow down as it compresses the air in the cylinder, then after combustion it speeds up until the next cylinder approaches compression. This causes chatter on anyting connected to the crankshaft, if you have ever stood behind a boat With a Mercruiser 1.7 diesel With an Alpha drive you will think the drive is knackered until you raise the revs. This then reduces the time interval between the cylinders firing and it smoothes out.
Interestingly back in the 80's Lycoming tried to produce a diesel version of their Aircraft engines and couldn't understand why prop blades kept breaking off. The lenght of an aircraft prop blade being constantly accelerated and braked as the engine went through its firing cycles stressed the blade root causing failure, which never happened on the petrol version as its a smoother combustion cycle.
Thats why you hear a chatter on some shaft drive engines with a worn damper plate until you put it in gear which then loads the damper plate.