# How to balance a engine ?



## Tom T (Nov 7, 2007)

Would you guy please explane to me how to balance a engine . Flywheel, cranckshaft, rod, Thanks to all who reply.  Tom T


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## 1Kenny (Nov 7, 2007)

Hi Tom,

First level the rod and put the big end of the rod on a scale to see what that end weighs. Write that number down to remember it. Then make a weight that looks something like a "C" clip that weighs the same as the big end of the rod. Clip the weight to the rod pin on the crank. Level two smooth round bars and set the crankshaft between the rods. Let the crankshaft roll on the main shafts and see where it stops. The heavy side of the crankshaft will be on the bottom. Put a thin felt ink mark on the top of the flywheel and recheck it two or three times to make sure the mark stops in the same place every time. Use a small drill bit and lightly drill the side of the flywheel flange 180* from your mark. Roll the flywheel on the bars again. You may have to do this several times. When the crankshaft is balanced the mark you made will stop in random locations. You should be able to move the crankshaft to any location and not have it try and roll on its own to a different spot. When you have it so it stays in place you can reinstall the rod to the crankshaft. If this is a one cylinder engine the small end of the rod, wrist pin and piston is not balanced because with one piston there is not another piston to balance it to.

Kenny


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## Tom T (Nov 8, 2007)

Thank you Kenny thats what I was looking for.  Tom


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## 1Kenny (Nov 9, 2007)

That is a very nice fixture Don.

There is a misunderstanding about balancing an engine. The big end of the rod or rods is balanced with the flywheel. If the rod and piston is used to balance the flywheel, the flywheel will be out of balance when rotating. This is due to the rod and piston is going up and down not round and round. There for it is not part of the rotating mass. A single piston engine has no other piston to match its weight to so what ever its weight is is fine.

_I shall use a tractor engine with two pistons to explain._

First match the pistons and mix their related parts to get them close to the same weight prior to drilling the heavy piston to get the weights the same to the gram.







Then you weigh the little ends of the rods and smoothly grind the heavest little end to match the other rod little end.






You then weigh the big ends of the rods and write down the two numbers and add them together. If there is one rod that is the only number you need. This number determines the amount of weight to attach to the rod pin on the flywheel for balancing.






Once the heavy side is found drill the necessary metal from the flange in a fashionable pattern to achive balance.






Recheck for balance and assemble the the rods.






This is the best I can explain balancing. Hope it helps.

Kenny


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## 1Kenny (Nov 9, 2007)

It was late last night and I failed to explain a couple of other ways to get the balance.

Lets say there is two rods and each one rides on its own crankshaft pin and the pins are 180* apart. The big end of the rods are then weighed and matched. Then attach weights that match the big end to each crank pin. If two rods share the same crank pin then add the weights together for the required weight needed on the crank pin.

Kenny


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## 1Kenny (Nov 9, 2007)

When balancing a flywheel with the rod and/or piston assembly installed it is increasing centripetal force (not to be confused with centrifical force) when the flywheel is rotating. This will disturbe the center of gravity of the flywheel. If the flywheel is rotating dead center and the center of rotation coincide the engine will spin smooth. By install only the weight of the big end of the rod on the pin center line it will maintain the center of gravity. The added weight of the rod and piston will cause the the center of gravity and axis of rotation to become misaligned resulting in an imbalance and vibration.

As explained by Mr. Newton. 

http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#strmas

Kenny


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## Tom T (Nov 9, 2007)

Guys would you look at my post in pictures [ Atkinson cycle engine} and let me know if you have any more thoughts on balancing an engine of this type. I am building this engine in a large scale and would like to balance it the best I can .  Tom  T


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## 1Kenny (Nov 11, 2007)

Looking at the Atkinson I noticed a design flaw. There is _*2*_ cranks on the engine. The upper one with the flywheel is a crank with a shaft and is balanced seperate to it self as explained.

Looking at the lower one is the flaw. Where is the counter balance for the rotation for that arc? It isn't there. You have the combined end weights of three rod ends with no counter weight where the rod is attached to the column. The rod end attached to the column needs to be treated as a center pivet. Extend that rod and add the necessary weight to counter balance the weight of the three rod ends on the other side.

This is the way it is in the design.

             R R R---------------O

This is what it should be.

             R R R---------O--------- X X X

Or you can shorten the length of the counter balance arm this way.

             R R R---------O-- X X X X X

The R is the rods, X is the weight and the O is the pivet on the column.

http://www.keveney.com/Atkinson.html


Kenny


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## Tom T (Nov 11, 2007)

Kenny this imfo came in the nick of time. It saved me from having to remake parts. I believe with this imfo. Ican  make my engine run nice and smooth. Thank you for your help  Tom


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## 1Kenny (Nov 11, 2007)

You are very welcome Tom.

Kenny


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