Balance

[sharpe 1]

Why is balanceing an engine important?

[brainsboy]

All engines need to be balanced when you do a rebuild. This is necessary to extend the life of your engine, it allows smoother operation, increase horse power, and improve gas mileage. Most machine shops will balance within a few grams, when I build all my motors I balance with in .5 grams. This reduces the centripetal force inside the motor. Centripetal force not to be confused with centrifical force causes bearing failure and wear. I bought a digital gram scale off ebay for about 70bucks, since then I check everything the machine shop does. I write down on paper all the piston, rod, bearing, and bolt weights before and after the machine shop does the work.

Ben

[auto10x]

Ben,
Please help me understand how centripetal force is involved and causes bearing failure and wear.
Thanks,
auto10x
Bill

[brainsboy]

TAKEN FROM A WEBSITE NOT MY ACTUAL WORDS!!!!!<---

The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation (which is expressed in units of grams, ounces or ounce-inches). A crankshaft with only two ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs.! Double the speed again to 8,000 rpm and the force becomes 227.2 lbs.

This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back and forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.

[Tommy]

Let me try a slightly less technical explanation. When you spin an object, it will always try to spin about its center of mass. For example, if you spin a childs toy top, it will spin about a line passing through its center of mass. If you spin a crankshaft assembly, it naturally wants to spin about a line through its center of mass. If the assembly is not balanced so the center of mass aligns with an imaginary line through the centers of the bearings that hold the crankshaft, the shaft will constantly be pushing against the bearings. As an unbalanced crankshaft rotates, the magnitude of the push on the bearings increases as the speed of rotation increases. So, balancing the rotating assembly should increase the life of the main bearings and the crankshaft, especially in engines run at high speeds.

[ERA600hp]

You Must Have Balance!

Grass Hopper

The Royal Rhubarb

[Excaliber]

In the past engines were seldom balanced beyond what the manufacturer had all ready done. A typical re-build using the the same parts (crank, con rods, pistons etc) would be cleaned up brought to within specs and bolted back in.

Parts were marked so as to go back in exactly the same place.

Building a new engine, a block here, con rods from over there, custom pistons etc will require balancing.

Ernie

[Ron61]

Tommy has stated it really well without all of the technical jargon. If the top he used as an example were out of balance or has just a little more weight on one place than another you would get wobble. If it is balanced it spins smoothly. An out of balance crankshaft will tent to try and wobble if that is the correct term and that causes uneven stress on the whole shaft.

Besides that it makes it easier to carry the engine with the weight even.

Ron

[Bluedog]

What is the difference between internally balancing the crankshaft and externally? How is it done? Is one method better than another?
Regards,
Bluedog

[Excaliber]

Internally balancing the rotating parts would be more accurate than "just" the crankshaft.

As I understand an engine internally balanced will accept virtually any flywheel that will bolt up. An externally balanced rotating assembly requires a balanced flywheel. External balance is not as common as internal balance.

Ernie

[Bruce Edwards]

Most Fords are externally balanced. That extra bit of steel on the flywheel and the front balancer are what you see on an external balance. An internal has all of the weight on the inside. So the flywheel and balancer would be with out any weight.
In this case the crank is drilled and heavy metal is added to balance the rotating parts.
The best way is to balance it from the inside. The cheap way is the outside. The factory can spin the motor and drill the balancer till the engine is in balance.
The best way is to balance the rotating mass is to spin the crank with weights that match the weight of the rods pistons and rings after these items have been all balanced to the same weight with the rods being balance for the same weight on both ends.
The 427 NASCAR crank has to be balanced with the weight of the oil figured in as it has a hollow crank pin.
Just want to throw that in.

[Lowell W]

OK...

How important is the distance from the crank centerline to the imbalance? In other words: If a crankshaft is heavy at a distance of, say, 2 inches from the centerline and the flywheel is what, maybe 14 or 15 inches in diameter with the eccentric weight 180 degrees off but out near the edge, does this affect how smoothly the assembly spins, even though the entire assembly is "in balance"?

How about the same question if a crankshaft is "light" on one side at the front and "heavy" on the opposite side at the rear?

Is this measurable? No cause for concern?

Anybody care to address "percentage of balance" or is this only an issue with one or two cylinder engines?

Thanks.

[brainsboy]

Lowell,


Balancing takes the weight of the pistons and makes them all the same weight. Then they make the rods all the same weight. Rods are actualy done at both the big and small ends. Weight for the bearings, rod bolts, oil, all is taken to account for. This weight is then calculated for whats called a BOB weight. The crank is then spun on a electronic machine which will tell the machinist how many grams your crank is off based on the BOB weight given. So having a crank heavier on the front or rear is not important. The weight the crank produces while turning is whats important. A steel crank is heavier then a cast crank, but both can be balanced for the same piston & rods. Lightweight race cranks will often need heavy metal added to the counter weights when balancing.

[CobraV8]

On a daily driven vehicle with a stock engine with cast pistons etc. that is not worth the money for having the complete rotating assembly balanced would it increase engine durability if I matched all the piston's weights, polished the rod beams and equalized the small and big rod end's weights without having the crankshaft balanced at a shop (sort of cheap do-it-yourself balancing)?
Or would this only disturb the coarse factory balancing and make things even worse?

Simon

[Mr.Fixit]

Must balance rotating assembly!!!!!
balance piston weights to one another
balance rod weights to one another
balance crankshaft counterweights to the piston/rod bobweight

whgen your whole vehicle vibrates like a soil compactor, it's not just the engine that will have a decreased lifespan.

[Bruce Edwards]

Simon anything you do to change the weight of the rotating mass will require you to balance the engine.

[brainsboy]

About 15 years ago I built a 302 and skipped on the balancing, I had custom crank, rods, and pistons. When I started it, my car had what I call parkinsons disease, it shook so bad I thought the doors were gonna fall off.