912 Dynamo and shims

[leewiffen]

Recently my pulley wheel on the end of the dynamo shattered and jammed up
It jammed pretty bad and possibly damaged the dynamo
However I removed the damaged pulley wheel and replaced it, thinking it was just an old part.
The dynamo is pretty new and appears to be running true when turned by hand, but obviously at revs may be imbalanced
After fitting the new pulley I was getting a bad clunking noise which appeared to be where the rear pulley was touching the dynamo
Removed and put a shim to space it, but noticed the rear pulley was already showing signs of damage where it fits over the hub ( see image )
Any ideas what is going on ?
Maybe the dynamo is out of line and not spinning on a true axis, although when you turn it by hand there is no noise or movement
However when I refitted the new pulley and ran the car, there was a vibration through the fan housing, but when I turn it by hand, nothing is touching on the fan at the rear
Also, should the terminals on the dynamo be that close to the oil filler?
Maybe it is not the correct dynamo, or I have mounted incorrectly on the fan?
See pictures
https://www.flickr.com/photos/95118177@ ... 1317826693
Ta
Lee

[SeanP]

Your pulley is knackered and you should replace the hub and have 10 shims. The vibration is the pulley moving on the hub

Sean

[fetuhoe]

The damage to the pulley is typical of what you seen when the alternator/dynamo shaft is bent.

I think you need to replace the dynamo and pulley.

[leewiffen]

Interestingly I can feel the vibration in the fan housing.
If we are right and assume it is coming from the shaft being bent, despite it not catching or vibrating when I turn it by hand with the belt removed, is it because even the slightest distortion to the dynamo spindle will cause the rear fan to distort and vibrate?
The dents on the new rear pulley do tend to point to the hub causing a problem, again despite it not seeming to be damaged
That damage appeared the first time I ran the car after fitting the new rear pulley
Sean, how do you set your shims?
I have two between the rear pulley and hub, another three between the two pulley faces and the remaining five in the front behing the nut

[fetuhoe]

Sadly when a 'bowed rotor' is subjected to asymmetric loading such a belt drive at one end it is more likely to suffer from a torsional vibration than a 'straight rotor'. The torsional vibration may vary in magnitude with angular acceleration and applied torque.

This may feel like a simple out of balance as there will be a measurable vibration but it will also mean that due to the difference in inertia of the pulley and the shaft there will be a force generated that will overcome the clamping force provided by the nut.

The torsional vibration will cause the pulley to 'shuffle' and the corners to wear as they will then see the torque generated by the dynamo.

Without any torque being applied to the shaft the effect will be minimal and the degree of 'bow' required to cause the problem could also be difficult to measure

The nut will still appear tight what stationary as this effect is simply due to the rotordyamic behaviour of the bowed shaft. The math is quite complex but well established.

We recently had an identical issue with a 911 alternator that was destroying outer pulleys on a routine basis and wasn't resolved until the alternator was changed.

[leewiffen]

Fantastic explanation fetuhoe
I do love it when engineering and rotordynamic behaviour come into play
Many Thanks
I'll replace tomorrow and let you all know
L