Broken Crankshaft Bolt/Balancing

[twoqu]

Hello,

Has anybody experienced a broken/sheared crankshaft bolt. The one holding the pulley on.
Also what is the consensus of opinion on balancing. Am I right in thinking that this can only be static as opposed to dynamic on the five cylinder? What effect does balancing have on harmonics?
Apologies for the technobabble!

[Pisobiker]

Using the appropriate methods you can always dynamically balance the cranckshafts. In the case of our engine, balancing must be done with the bobweights, crank pulley, flywheel and clutch pressure plate on, 'cause the counterweights themselves don't have enaugh mass to compensate for each corresponding throw + the rotating and alternating masses of conrod and piston.

Marco

[Pisobiker]

.....

[twoqu]

Thanks Marco,
A picture really does say a thousand words!
The reason I am asking these questions is that someone has had a crank bolt shear and the engine builder has put this down to harmonics. I thought the vibration damper (pulley) purpose was to cancel or minimise these effects.
So if you had an engine built and balanced, and then replaced the crank, the whole assembly would need to be balanced again?
Also is it likely that the material the crank is made from would vary significantly on the turbo engines?

[Pisobiker]

Well, to be honest, before attempting my crankshaft balancing I've tryed to collect some infos from whoever had experience/know how on the IL5 crankshaf balancing, whitout too much luck. So I did my study and calcultations myself, hoping I did my homework right. My friend who owns the machineshop didn't have any experience on 5ers so he just gave me the use of the machinery letting me be free of messing things up on my own....

Anyway, to make complex things simpler, from my analysis the IL5 72 deg. throws crank could be dynamically balanced by its own only if the counterweights of each cylinder, with theyr mass, balance the weight of the respective throws. This way each "throw-caunterweight" assembly balances itself, therefore the whole assembly is balanced.

BUT

When you add the wight of conrod+bearing's rotating mass and the weight of conrod+piston's reciprocating mass to each rod journal, things don't match anymore. To balance singularly each "throw-counterweight", extra mass should be added to the counterweight, therefore increasing dramatically overall crank weight thus inertia (engine responce will be affected). So the balancing is partially accomplished by the contribution of "unbalance" of the other "throw-counterweights", and for the rest by adding the correct amount of unbalance at both sides of the crankshaft (therefore on harmonic balancer and flywheel-clutch pressure plate).

So the crank bolt, which holds in place the harmonic balancer+cambelt gear assembly, is actually getting some univen radial load by these, which might cause a bit of "hammering". Of the 5 or 6 5ers I've disassembled so far (20V turbos and TDIs) I've noticed all of them showed radial hammering signs on the crankshaf tip where harmonic balancer fits, making therefore the assembly a little loose, but I've never found broken or sheared bolts.

About replacing parts, answer is YES:

unfortunatly on those engines any time you change pulleys, flywheel, crankshaft, and you put conrods and pistons significantly different in weight, you need to rebalance the whole lot.
At least these are the conclusions I took while analizing the theory.


Marco

[twoqu]

Thanks again Marco,

I knew you would take a complex subject and explain it in a way that is easier to understand. You are an invaluable resource!

[Pisobiker]

[Graham]

Of the 5 or 6 5ers I've disassembled so far (20V turbos and TDIs)

Dissassemble Johnny Five --No dissassemble Johnny five (Short Circuit)

[Pisobiker]

Took a few seconds to get it....

problem is in Italy we use to have foreign movies all translated in our language, so very seldom it's possible to make a direct link to the original one. In this case "our" Jonny Five was "Numero Cinque", which translates into "number five".