Author Topic: Excess vibration  (Read 270 times)

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Offline dieseldoc

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Excess vibration
« on: July 21, 2018, 10:48:13 AM »

I was recently asked to have a look at an RD400 engine for a friend of my son in law. The engine had just been overhauled by a well known respected company, favoured by a number of RD club members.
However, on running the bike the owner said there was a lot of unusual vibration and he sent the engine back to the company to check the alignment. It was duly returned with a note saying all was good and correct.
I first removed the generator assembly and noticed there was some previous damage to the tapered area of the crankshaft (see photo) I then had a look at the tapered bore of the generator rotor and sure enough, there was some damage in the bore each side of the keyway (see photo)
I then removed the clutch assembly for inspection. All okay.
I then removed the top crankcase to set up the dial indicators to check the crankshaft run out. This was found to be within Yamaha's tolerances.
I put some Engineers blue on the crank taper and checked the generator rotor bore for contact area. A bit of a no brainer really as you could feel the roughness in the tapered bore (see photo).
The crankshaft male taper had been dressed up, but the generator rotor tapered bore had not. This in turn stopped the mating surfaces making a snug fit and caused the abnormal vibration.
The probable cause of the taper damage was due to the key breaking up sometime in the past. For what they cost, it is best to replace them at each overhaul.
One other check I did was to weigh the pistons. Their weights differed by 1.5 grams! If the con rods slightly differed in weight, this also would play a part in excessive vibration.
It is surprising how many identical components of the rotating elements differ in weight. Gudgeon pins, big ends, con rods and pistons.
In my crankshaft rebuilds, I have always checked the weights of these and match them up to try and best equalise the weight.
The cost of the scales was not dear. I purchased a set of 0.01g – 500g jeweller’s scales from eBay for under £10.
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Offline 5port

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Re: Excess vibration
« Reply #1 on: July 21, 2018, 11:40:35 AM »
Yes, like your work there Robert!  I clean-up any damage to the taper and lap the rotor onto it with grinding paste.  This works very well and shows where the seating is good.  Not had one fail to clean up, so far.  Were the clutch steel plates correctly "balanced"?  Cheers

5port     
5port

Offline dieseldoc

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Re: Excess vibration
« Reply #2 on: July 21, 2018, 09:11:25 PM »
Thanks Chris. Yes, the clutch steel plates were correctly positioned.
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