Imbalanced Flywheel 6

[stragenism]

Apologies for asking a possibly common flywheel question: my car forums of choice are vastly under-qualified to help. My question is twofold: How likely is it that a 23 lb (mild steel?) flywheel turned on a lathe (improper tool) or ground (proper tool) would become out of balance significantly enough to cause the entire driveline to vibrate? How much imbalance would be necessary to cause this situation?

The car is a 2.5L H4 AWD (Subaru WRX).

I recently had my clutch changed at a shop which sublet the flywheel resurfacing to another shop of unknowable quality. The service manager inadvertently informed me that no shops in the area are capable of balancing flywheels of "that size." I assume they must have ground the surface because the clutch does not slip.

I've done a great deal of googling trying to understand the issue better. I don't have access to a vibrations text, only my notes from the class. So I have the equations of motion, but lack the experimental capability to characterize the constants. Ultimately, I'd like to use Shigley's to make some prediction on the increased fatigue life of the crank. (I've yet to apply ISO 1940/1 which was suggested in another forum for predicting amplitude of vibration of a pump rotor.)

Thank you for any information!

 

[Tmoose]

tipping a well balanced flywheel on the crank would induce some "couple" unbalance, that would have a relatively small effect on the longish crank/flywheel assembly unbalance. If the shop that refaced the flywheel did so with swarf between the table and flange, they'd remove more material from one side and create static or single plane unbalance right at the flywheel plane.

 

[thruthefence]

Man, that's a great flow chart, fer sure.

looks like it goes right to here:

"If clutch work recently done,
problem could be related to
the clutch. Verify proper
clutch was installed."

 

[GregLocock]

I can't remember the exact spec for engine assembly out of balance, but from memory it was of the order of 10-30 g cm for both ends of the crank.

Now, that is for an engine built from a balanced crank and a balanced pulley and a balanced flywheel, and we still ended up balancing the flywheel plane for EVERY engine after assembly.

So, fully machined parts assembled to a balanced crank can and will cause detectable vibration. Whether they would cause destructive levels of vibration is another thing entirely.

Cheers

Greg Locock


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

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

 

[stragenism]

Thanks Greg, this just reaffirms the old "you get what you pay for" adage. Soon, a brand new OEM FW should be installed, and my hope is that this will restore the balance.

Can you explain why so many people manage to get away with machining their FW without noticeable issues? In my case, I would absolutely venture that the fully machined part was fully machined improperly considering that the vibration has proven destructive: a recent development is a pretty loud intermittent squeaky sound in low gears in low rpm ranges. It's probably a bearing. Since I don't know which and subsequently don't have the part, I expect this will become a problem in the future. I can only guess at what will happen when it fails.

 

[gruntguru]

"Can you explain why so many people manage to get away with machining their FW without noticeable issues?"

It actually takes fairly sloppy machining technique to get it wrong. Any machinist who cares more than a jot, will observe the sound at the beginning of the grinding operation. Any runout will be evident as an intermittent sound.

 

[Tmoose]

"Especially when the ring gear flies off because it was, primarily, secured to the Al flywheel by the six 5/16" cap screws that mounted the pressure plate !!!!!!"

Plenty of cars' flywheels are retained and secured by shrink fit and a shoulder to resist the initial slam of the bendix.
No pressure plate involvement at all.

Vintage US -

http://www.2carpros.com/how_does_it_work/images/clutch_flywheel.jpg

Anglophile -

http://drc

http://www.uvt.nl/~bogaard/manual/cluttran/exppress.gif

Not sure about this trendy mod dual mass set up -

http://www.neatcarparts.co.uk/ekmps/shops/neatautos/images/dual-mass-flywheel-2.jpg

mark twain said something about a cat that having once sat upon a hot stove would never risk sitting on a cold one either

 

[stragenism]

Grunt, I was directing my query to "Now, that is for an engine built from a balanced crank and a balanced pulley and a balanced flywheel, and we still ended up balancing the flywheel plane for EVERY engine *after assembly.*
So, fully machined parts assembled to a balanced crank can and will cause detectable vibration."
What you say is what I understood to be true previously. I'm interested to hear Greg's reasoning.

Tmoose, I'm not sure how Rod's situation would come to pass, either. My flywheel is a single machined steel body bolted onto the crank.

 

[Tmoose]

Should have been "Plenty of cars' RING GEARS are retained and secured to the flywheel by shrink fit and a shoulder to resist the initial slam of the bendix."

Dan T

 

[evelrod]

Dan, not all 'shrink fit' ring gears are compatible with really light wt Al flywheels. In my case it was a Tilton/Borg-Beck unit for a Lotus twincam. I also had a similar unit for an MGB at some point. On super light flywheels, it was common to either dowel the gear in place or, as my example, use the pressure plate mounting screws to lock in the ring gear. I still have the flywheel and, save my one moment of stupidity, it has never given any problems...It's just a little old fashioned by comparison to our latest duel disc AP/Tilton setup.

Your second link is to a stock Mini flywheel. Even that is old fashioned compared to the SIX Lb. units we race with today. Certainly requires a definite "attitude adjustment" when it comes to making one of these little dears work properly.

Rod

 

[gruntguru]

Grunt, I was directing my query to "Now, that is for an engine built from a balanced crank and a balanced pulley and a balanced flywheel, and we still ended up balancing the flywheel plane for EVERY engine *after assembly.

My comment was assuming the situation under discussion where a well-balanced motor has the flywheel removed, machined and refitted. It should remain well-balanced if the machining was accurate.