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Removed Balancing 'Counter Weights' in Crankshaft?

05/04/2011 3:30 AM

I recently picked up a bit of info that has left me wondering "How do they do that?" Reduced the rotating mass in a reciprocating engine by doing away with the counterweights built into crankshafts just for that purpose.

Adding weight in the webs (the members between the bearing surfaces themselves)? Though I doubt that this would reduce the overall mass of the crankshaft, just altering the design so that it is more 'compact' with no 'balancing members' persay sticking out so far from the centerline. If done this way, it seems to me that the crank would actually be heavier because the mass would be closer to the centerline of the crank so they would have to use even more mass to balance it?

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#1

Re: Removed balancing 'counter weights' in crankshaft?

05/04/2011 5:01 AM

Dunno...I agree with your observations...
Del

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#2

Re: Removed balancing 'counter weights' in crankshaft?

05/04/2011 5:35 AM

I'm not quite sure what you are trying to say. But adding weight anywhere will not reduce overall mass. The location of the weight will effect it's rotational inertia (resistance to rotational acceleration). If one were to remove the crankshaft counter weight, then it's rotational inertia would be reduced......but at the expense of an unbalanced shaft, which could have disastrous effects.

Automotive engineers have been refining engine parameters for decades so I would venture to say that automobile engines have the lowest mass and least inertia that is safely possible for their designed purpose.

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#3

Re: Removed balancing 'counter weights' in crankshaft?

05/04/2011 7:51 AM

The idea is to move the rotating mass closer to center of the axis. This will allow the power that would normally be used to spin up the mass to be used for acceleration. Because the crank shaft changes speed frequently the storage of energy in it's mass is a waste. They are not trying to reducing the mass per say. But that in the change they may.

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#4

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/04/2011 8:37 AM

This is a Scat lightweight crank that we use in Sprint Cars.

By reducing the mass that is opposite of the counterweights it is possible to reduce not only the overall weight but also the inertia weight.

This crank has also been lightened in every way possible.

We also use lightweight pistons and rods which allows us to reduce the needed counter weight while balancing the rotating assembly.

Unlike V6s and V8s a Four cylinders balance is not affected by the overall weight of the pistons and rods. People sometimes cut the counter weights completely off of four cylinder cranks.

This is a bad practice. The overall crank will still be balanced but each individual rod journal will not have a counterweight opposite of it so at high RPMs it will put stress on the crank and main bearings that eventually leads to failures.

I hope this helps!

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#7
In reply to #4

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/05/2011 4:12 PM

You are right. The overall balance is not so important it is the balance at each journal that matters, otherwise the loads cause the crankshft to bend. A bend in the shaft causes the main bearings to be exposed to additional loads which can lead to their failure. The oscillating loads of an out-of-balnce crackshaft could also result in broken crankshaft or cracked crankcase.

Lightening must be done cautiously as removing weight is also removing strength, both in bending and torsion. Changing the torsional strength of a crankshaft can result in a torsional failure especially if the crankshaft is long (eg. in-line six) and many engines have torsional vibration dampers to control this. You could take this off to save weight but it may come back to bite you. If you change the torsional stiffness of the crankshaft you should really look at changing the torsional damper to suit.

Nice to see bored out journals, the centre of a round bar does not do much for the torsional or bending strength.

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#5

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/04/2011 3:28 PM

Get rid of the counterweights, remove material in non-structural areas, i.e., the counterweight mass. This would have the same effect in balancing the shaft. We can't do this on tires, the next obvious comparison unless you want to start removing rim material each time you put on new rubber.

Cams can't be as easily balanced, but their rotating mass is significantly lower and much closer the the center of gravity and pressure making the overall effort less technical, especially if the production process is more precise.

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#6

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/05/2011 6:01 AM

It's not the weight of the crankshaft that is important but its moment of inertia. The moment of inertia determines how much torque is required to increase its RPM. Take an example: a 1 lb mass 1 ft from the axis of rotation balanced by a 1 lb mass on the opposite side. The moment of inertia is proportional to the mass times the square of the distance. (2x12=2). If the 1 lb mass were counterbalanced by a 2 lb mass 1/2 ft from the axis of rotation (still balanced), the moment of inertia would be 1x12 + 2x(1/2)2=1.25.

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#8

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/07/2011 6:35 AM

Though it's not by any means technically efficient or cost effective, by making main bearings and crankshaft strong enough you can 'ignore' the per-cylinder imbalance if your most important goal is the lowest possible moment of inertia. But this isn't a modification option on an existing motor. It should be designed from scratch. You'll know when you hear something like that trying to idle at 3500+ rpm. S.M.

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#9

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/07/2011 8:52 AM

Hmmm, not sure what's the point of a low moment of inertia when a huge flywheel is bolted onto the end of it anyway
Del

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#10
In reply to #9

Re: Removed Balancing 'Counter Weights' in Crankshaft?

05/07/2011 9:34 AM

Also minimized to special clutch size Del. Not aplicable to CatMobile. S.M.

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#11
In reply to #9

Re: Removed Balancing 'Counter Weights' in Crankshaft?

06/21/2011 6:24 PM

Yep... I was going to forward this question to my friends at "Scat" mentioned above by another poster but I'm afraid they would send me a letter bomb........

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