Experience With "Balanced Cams?"

[headman]

Any rotating part whose center of mass is not collinear with its center of rotation should be balanced.

Nope, not always, it depends on the source of imbalance and how you plan to correct it.

If the object being balanced is flexible (like a cam is) and you are balancing it with modifications to the ends of the cam you have no reason to believe that you are reducing bearing loads and plenty of reason to believe that you are increasing them where you have added or removed metal to compensate for imbalance in a region of the body that will not be effected by change in balance to a timing gear.

I think the part of this that everyone is missing is that dynamic balancing of the entire body doesn't mean anything to the local balance of sections of the body. This is of critical importance when the body is flexible (as a cam is). The cam already bends more from loads from the lifter than it ever will from out of balance loads.

If you really wanted to do this right, you would have to cut the cam into pieces, balance those pieces then duplicate the changes on an intact cam.

I've done this on crankshafts until I figured out a way to determine the internal imbalance with other measurements.

This does not refute balancing.
Simply, consideration should always be made, (with any rotor), to minimize the distance along the axis of rotation between the off centered mass, and the respective correction.

The example shown in Jon's image should be manfactured with the shaft on either side of the encentric, moved opposite the center of rotation from the "green" cam

Obviously, the red counterbalance is the worst possible senario,
yet imagine the shaft both with and also without the added "red" counterbalance spinning, in a free suspension without bearings to contain ampitude. The large encentric will cause movement of the axis of rotation even prior to sufficient rpm for flex to be an issue.

[SchmidtMotorWorks]

Jon's graphics shows insight. ................but with a shaft that is balanced you have eliminated one source of imbalance/ harmonics?

Nope, think tire tread harmonics on a rocky road, insignificant.

[SStrokerAce]

I think to do this right (cam balancing) you would need to have the setup in a spintron and try it that way.

I think Schmit is right about the dynamic loads here. The lifters with spring pressure and the valve events load the cam and bearings more than the out of balance does but probably not at a equal force during the rotation.

Bret

[SchmidtMotorWorks]

but probably not at a equal force during the rotation.

Yep, that's right and unless you know what those forces are and where they are and how the shaft is distorting you have no way of knowing if balancing the cam will improve bearing loads or harmonics an any given part of the cam.

Any external balancing in the gear is more likley to do harm than good.

[Dick Phillips]

Gentlemen,
I just stumbled across this chat site and noticed your discussions on camshaft balancing. My comments are founded on personal experience and testing of the subject.
Two and a half years ago, we pondered this subject because of today's valve train breakage tha a lot of race engines experience. Upon investigation, we noted that most cams were out on the average of 40 grams per inch/ounce at the front and 25 grams at the rear checking this at a radius dimention of .800". This compairing it to a 4.25" crankshaft radius would exceed 120 grams. Everyone can agree that an engine with that much imbalance will fail in short time.
When the camshaft vibrates it (a): transmits that vibration throughout the valve train, and (b): the camshaft bends amplifying the imbalance. CWT has the software necessary to locate the imbalance and also location the "vectoring positions" of both counter-balance plains to counter-act the bending tendacies while in motion.
Testing showed reducton and/or elimination of lifter axle failure, spring fatique, and enabled higher rpm's of the engine. This process has been run on spintrons and only show minor power improvements, however, the friction testing also showed little gains, but were encourageing. This has also been track tested showing improved reliability and smoother high rpm operation.
Hope this helps as a testimonial.

Dick Phillips

[SchmidtMotorWorks]

Upon investigation, we noted that most cams were out on the average of 40 grams per inch/ounce at the front and 25 grams at the rear checking this at a radius dimention of .800". This compairing it to a 4.25" crankshaft radius would exceed 120 grams. Everyone can agree that an engine with that much imbalance will fail in short time.

25 grams at .8" doesn't equal 120 grams at 4"

You are simply wrong about a crankshaft failing if it were out of balance by 120 grams. 99% of V8 crankshafts are more than 200 grams out of balance internally. Long stroke racing cranks could easily be out 400 grams.

Doubt it? Cut a crankshaft into three pieces, cut off the first two CWs and the last two so that you have only the middle portion of the crank with the 2 & 4th main. Put some bobweights on it and spin it up. I think you will see dynamic balancing in a new light.

[headman]

You are simply wrong about a crankshaft failing if it were out of balance by 120 grams. 99% of V8 crankshafts are more than 200 grams out of balance internally. Long stroke racing cranks could easily be out 400 grams.

Doubt it? Cut a crankshaft into three pieces, cut off the first two CWs and the last two so that you have only the middle portion of the crank with the 2 & 4th main. Put some bobweights on it and spin it up. I think you will see dynamic balancing in a new light.

Crankshafts that are out of balance roughly 100 grams at a 3.5" radius will noticably "shake" the engine at certain rpm's.
That's roughly 12 oz/in.


Much less 200 or 400 grams. That would shake the whole car.
Remember, the crankshaft can and does "successfully" pass loads through the mains and rod journals. These loads can cancel each other out collectively. Typically, the final minor corrections are accomplished at the ends.


Sure, the idea situation would be for the crankshaft (or camshaft) to be a collection of independent neutrally balanced subassemblies.
The next best alternative, keep the bobweights and crankpins to a minimum reducing the need for excessive counterweighting.

[headman]

Back to the cam...

25grams at .800" is just over 1 oz/in, not but so bad, but I would want to reduce the imbalance.
camshafts with encentrics for mechanical fuel pumps are much further out of balance on that respective end.
Especially offensive are large heavy bolt on encentrics that need correction regardless of the actual camshaft.

[SchmidtMotorWorks]

Remember, the crankshaft can and does "successfully" pass loads through the mains and rod journals. These loads can cancel each other out collectively.

Are you sure about that? How do you know the crank isn't held straight by the main journals transmitting force on the bearings? Measure the wear on the main journals and you will see that is exactly what does happen.

If you still think a crank has to be balanced to prevent bearing failure, think about a one cylinder engine. There is no perfect bobweight for a one cylinder engine, you can only bias the direction it will vibrate. the range of optional bobweights for a one cylinder the size of a V8 would be easily 200 grams.

A 45 degree V twin like a Harley is another good example, there is no bob weight that will make them vibration free, you can only adjust the bobweight to control what direction the engine will vibrate. I haven't balanced a Harley but I have seen how much they vibrate, I would guess they are equal to 100 grams out (or more).

[headman]

There is no perfect bobweight for a one cylinder engine, you can only bias the direction it will vibrate. the range of optional bobweights for a one cylinder the size of a V8 would be easily 200 grams.

A 45 degree V twin like a Harley is another good example, there is no bob weight that will make them vibration free, you can only adjust the bobweight to control what direction the engine will vibrate. I haven't balanced a Harley but I have seen how much they vibrate, I would guess they are equal to 100 grams out (or more).

These are examples that unfortunately came oem with a significant amount of underbalance. There is not enough counterweight mass, so the crankshaft is being displaced severely by the attached rotating and reciprocating weight. These engines can be balanced, but not everyone may be willing to make the required effort.

The Harley engine has a reputation for its unique firing event's "pulse" (which should not be confused with the actual rotating components poor dynamic balance).
These unusual engines spin smoothly like a completely different style engine once balanced. (you still keep the trademark sound) But it requires a lot of correction with heavy metal.
Also the fact that the crank must be separated makes the job more involved.

I also have worked on marine outboard engines that have similar issues due to the space constraints.

[SchmidtMotorWorks]

These are examples that unfortunately came oem with a significant amount of underbalance. There is not enough counterweight mass, so the crankshaft is being displaced severely by the attached rotating and reciprocating weight. These engines can be balanced, but not everyone may be willing to make the required effort.

Nope, there is no bob weight to balance a single cylinder crank assembly to be vibration free. This is because the frequency of the piston vibration is twice that of the crank vibration and on different plane/axis.

The same reasoning applies to a 45 degree Vtwin except it is more complex. The V Rod has balance shafts to hide the crank vibration.

A 90 degree Vtwin can be balanced to have very small vibration because the vibrations of the pistonS are coincident to those of a crank with the proper ammount of imbalance.

[headman]

These are examples that unfortunately came oem with a significant amount of underbalance. There is not enough counterweight mass, so the crankshaft is being displaced severely by the attached rotating and reciprocating weight. These engines can be balanced, but not everyone may be willing to make the required effort.

Nope, there is no bob weight to balance a single cylinder crank assembly to be vibration free. This is because the frequency of the piston vibration is twice that of the crank vibration and on different plane/axis.

The same reasoning applies to a 45 degree Vtwin except it is more complex. The V Rod has balance shafts to hide the crank vibration.

A 90 degree Vtwin can be balanced to have very small vibration because the vibrations of the pistonS are coincident to those of a crank with the proper ammount of imbalance.

I'm not disputing the concept, but this holds true for any engine. Because the pistons are never exactly opposed to each other. And the reciprocating load will change with the rpm. The 90º engines cannot be balanced perfectly.
If I take your liberty, I can say the single cylinder can be balanced to have very small vibration, If the counterweight is increased sufficiently and the reciprocating weight is reduced.

Out of balance shafts are an awful way to hide reciprocation induced vibration

[SchmidtMotorWorks]

I'm not disputing the concept, but this holds true for any engine. Because the pistons are never exactly opposed to each other. And the reciprocating load will change with the rpm. The 90º engines cannot be balanced perfectly.
If I take your liberty, I can say the single cylinder can be balanced to have very small vibration, If the counterweight is increased sufficiently and the reciprocating weight is reduced.

Nope, a single cylinder engine will always be out of balance by 50 percent of the reciprocatiing weight. There are many books that explain why this is in detail for every cylinder configuration.


A 90 degree V can be perfect except for the cylinder offset and difference in piston acceleration at the top and bottom of the stroke. This is a small fraction (less than 2%) of the imbalance in a single cylinder engine.

[putztastics]

If engines were balanced the same way steam turbine/generators are balanced they would be balanced between each pair of main bearings. As SchmidtMotorWorks says this is not the case.

The only reason engines can be balanced the way they are is because the crankshafts and engine blocks are massive enough to "average" the balance between all the main bearings.

This is a very crude way to balance in my opinion and requires parts much stronger than they would have to be if balanced between each pair of main bearings.

[headman]

a single cylinder engine will always be out of balance by 50 percent of the reciprocatiing weight.

Sure, and like I said, if you reduce the Piston Mass, it becomes even less of an issue.
The bigger issue is that these engines come stock considerably underbalanced regardless.
They are not counterbalanced even close to a minimal 50% bobweight, much less the 70% or greater bobweight required for high rpm's.

A 90 degree V can be perfect except for the cylinder offset and difference in piston acceleration at the top and bottom of the stroke. This is a small fraction (less than 2%) of the imbalance in a single cylinder engine.

At which bobweight configuration would you consider the 90º V twin perfect? 50%, 52%, 57% ?? It's still a function of rpm.