Engine balancing question
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From: Phoenix, AZ
Car: 1986 Z28 / 2012 CLS550
Engine: F-1R -> Aluminum block 540
Transmission: T56 Magnum + GForce gears
Axle/Gears: Ford 9" 3.00's + Eaton Truetrac
Engine balancing question
Assume that the crankshaft is perfectly balanced internally. This is possible because for every crank throw there is an opposing one to balance it. Now assume that all rods (big end weight) are exactly the same weight. Not likely, but theoretically assume it is true. Now when assembled, the result should also be balanced, correct? Even if a rod is too heavy for its associated counterweight, it will be balanced by the rod on the opposing crank throw.
Any truth to this logic? Not trying to dodge the cost of balancing, just wondering if you had perfect components, would it be necessary?
Thanks
Any truth to this logic? Not trying to dodge the cost of balancing, just wondering if you had perfect components, would it be necessary?
Thanks
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From: Loveland, OH, US
Car: 4
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Transmission: 5
Yes and no.
It will be statically balanced but not dynamically balanced. i.e. if you suspended it in space through its axis, it would not rotate.
On the other hand, imagine a crankshaft with one giant weight on one end, and one giant weight at the other end, 180° opposite the first weight. By the above definition, it would be statically balanced. Now imagine what happens when you spin it. One end will try to fly off one direction, and the other will try to fly off the opposite way; in other words, it will vibrate like a mother, even though if it was sitting still, it would be appear to be "balanced".
Now imagine what would happen if you took the one weight and put it at the center; and took the 2nd weight, split it in half, and put the halves on either side of the 1st weight. Now, it will run smooth, at any speed, including sitting still or spinning at 10,000 RPM.
THat's what you try to do with a crank. Put the counterweights as close as possible to being directly opposite the rod journals, with equal amounts of weight on either side of the journal (since it can't be directly across).
It will be statically balanced but not dynamically balanced. i.e. if you suspended it in space through its axis, it would not rotate.
On the other hand, imagine a crankshaft with one giant weight on one end, and one giant weight at the other end, 180° opposite the first weight. By the above definition, it would be statically balanced. Now imagine what happens when you spin it. One end will try to fly off one direction, and the other will try to fly off the opposite way; in other words, it will vibrate like a mother, even though if it was sitting still, it would be appear to be "balanced".
Now imagine what would happen if you took the one weight and put it at the center; and took the 2nd weight, split it in half, and put the halves on either side of the 1st weight. Now, it will run smooth, at any speed, including sitting still or spinning at 10,000 RPM.
THat's what you try to do with a crank. Put the counterweights as close as possible to being directly opposite the rod journals, with equal amounts of weight on either side of the journal (since it can't be directly across).
Last edited by RB83L69; May 23, 2003 at 02:34 PM.
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I'm glad I don't build piston engines for use in space! Forget about balance - We'd either need one huge ***** of a tank of air, or some hydrazine. Then the EPA would probably be all over us for "contaminating" space.
The theory is to have all the pisto/rod assemblies balanced to one another, AND to have the same mass (at the big end of the rod) balanced to the counterweight on a given crank throw.
One of these will get you started on the first part:
The theory is to have all the pisto/rod assemblies balanced to one another, AND to have the same mass (at the big end of the rod) balanced to the counterweight on a given crank throw.
One of these will get you started on the first part:
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