Engine balancing

[xb-60]

Originally Posted by Jerry Clayton

...notice the 2 pieces of heavy metal to internal balance this crank

On this pic of a rear counter weight you can see two pieces of mallory heavy metal installed into the crank and then a drill point drilling to remove what was then excessive weight...

Thanks Jerry. Yes it sounds simpler than it really is. Is it similar in priciple to balancing tyres/wheels in that a computerized balancing machine tells you the location along the crank and the angle at which the material is added/removed?
Cheers,
Glen

[xb-60]

Originally Posted by Luce

Let me take a stab at this....

A crank is sort of like the crystal wine glass the fat lady sings to and makes it break. If you tap the rim, it rings. If you sing the exact same frequency to it, in the right location, the pressure waves will reinforce the vibrations causing the ring, and the vibration will increase until the forces are stronger than the glass, and it breaks. If you keep a finger lightly touching the rim, the vibrations are dampened and absorbed, and can never reach a dangerous level.

The soft mounted weigh of the harmonic dampener is that lightly touching finger. It dampens high frequency vibration that the pistons and rods can induce. A good crank will ring too if smacked, but not anywhere near as long if the harmonic balancer is installed.

Thanks Luce. That's making sense.

[xb-60]

Originally Posted by AL427SBF

... or just say you need to dampen the engine's resonant frequency so you don't end up like that bridge caught in the wind oscillating at its' resonant frequency until it disintegrated.
----

You're talking the Tacoma Narrows Bridge? I was just thinking about that one, after Luce's reply

[blykins]

[quote=xb-60;1165098]Reworded questions...

For example, what does balanced to 28oz mean when describing an engine?
A: It means that the engine uses a 28oz imbalance (externally balanced). This typically means that the harmonic balancer and flywheel/flexplate are 28oz items.
Re-Q: But what do you mean, that the h-b and flywheel/flexplate are 28oz items? And the engine uses a 28oz imbalance? You know me Brent, I'm slow on the uptake.

The pieces don't weight 28oz; they have that much imbalance in them. They have a weight either bolted or welded to them, clocked at a particular spot that makes them out of balance.

Q:How does this affect clutch choice?
A: It doesn't. However, some guys want to have the pressure plate balanced with their rotating assembly as an option.
Re-Q: I can't find the email, but I thought you asked me awhile ago what engine imbalance for a particular flywheel or clutch selection?

Flywheels are categorized by imbalance...0, 28, 50, etc. Flywheels matter, but pressure plates are not rated that way.

Q: Also what is external balancing?
A: An externally balanced engine uses a weighted harmonic balancer and/or flywheel to keep the engine in balance.
Re-Q: I take it this is the quick,cheap version of engine balancing. Is this static or dynamic?

It's often easier because you usually don't have to add metal to the crankshaft. Always dynamic. Always.

Q: How does a harmonic balancer work?
A: It uses different materials, such as rubber, fluid, polymers, etc. to help eliminate the vibration harmonics out of an engine.
Re-Q: Does an engine that is internally balanced have/require a h-b as well?

Yes.


Q: What items are subjected to internal balancing? And is this balancing static or dynamic?
A: Everytime you balance a rotating assembly, it's dynamic. Go to YouTube and look for some rotating assembly balancing videos. The crankshaft is placed in a balancer and spun up to rpm. There are bobweights bolted to each rod journal which simulate the weights of the pistons/rings/rod bearings/rods/parasitic oil, etc.
Re-Q: Excuse my ignorance, but is the rotating assembly just the crank, or is it the crank/rods/pistons assembly?

Everything that rotates or reciprocates. This includes the crankshaft, rods, pistons, rod bearings, piston rings, piston locks, etc.

Q: Is material typically added (weights) or removed?
A: It depends on which setup you have. You can do it either way, although the crankshaft manufacturer will design the counterweights on the crankshaft so that a target bobweight will be used.
Re-Q: Does the crank manufacturer (say, Scat?) balance his cranks, or does he leave that for the engine man?

He has no way of knowing which rods, pistons, etc. that are going to be used. They are designed for a target "bobweight." For instance, a crankshaft is designed to be internally balanced with an 1800 gram bobweight. If the weight of your pistons/rods/rings/bearings,etc. weigh more than that, then you will need to add metal to the crankshaft to keep it balanced. If your bobweight is lighter, then you can likely get away with removing metal from the crankshaft.

Q: Do tailshafts normally need balancing?
A: If you're talking about a driveshaft, then yes.....always.
RE-Q: Halfshafts too?

I would, yes.

Q: At what rpm point does extensive/comprehensive balancing become essential?
A: There's really not an "extensive" balancing. It's either balanced or it's not balanced.
Re-Q: OK...is there a a rpm level at which it is advisable to have the crank and rods and pistons all dynamically balanced?

Read Keith's explanation above....he did a good job of explaining this part.

Before you ask any more questions, you need to watch a video or two on YouTube about it. It will clear up a lot of things you're having trouble with. You need to forget the static/dynamic questions as it's all dynamic. You don't balance a rotating assembly by just letting it sit there. It's all spun at rpm.

[Jerry Clayton]

There are two types of engines which require two different types of cranks--the inline engines and the "V" engines---With the inline engines they don't require the bobweights to be on them for spinning during balancing--
The V types require that bobweights be attached to the rod throws for spinning during the balancing process--The bobweights are made up of 100% of the rotational weight and 50% of the recipricating weight of 2 cylinders ( V8)--since the front two rod throws(4 cylinders) are at 90* to each other as are the back 2 rod throws also 90* while the two center rod throws( OK, so I'm up to 6 rod throws ?) are 180* to each other, the V8 cranks will have counter weights to the front and back but generally not at the center throws---( unless it has been CENTER COUNTER WEIGHTED for strength )

SOOOOOOOO---the addition/subtraction of weight for balancing is generally done on the first and last counterweight---

The 28 and 50 OZ numbers relate to inch/ounces--thats why the flywheel on the external jobs have less correction than the front dampners---differance between something 6-8 inch dia and 12-14 inches flywheel

The pics that I posted that show the imbalance(when I took the pictures) relate to grams. at the time the crank was at 2 grams on a 3 inch radius---

[Mike Huddart]

[quote=Jerry Clayton;1165114]There are two types of engines which require two different types of cranks--the inline engines and the "V" engines---With the inline engines they don't require the bobweights to be on them for spinning during balancing--

Not strictly true, an inline 5 cylinder requires bobweights.

Mike

[Jerry Clayton]

So what does that add to this conversation??? no one has a 5 cylinder inline in their Cobra and I was using the inline to v type engine as a means to identify how there are some differances and why the counter weights are on the ends of the crank and not usually the middle.

[xb-60]

Originally Posted by Jerry Clayton

There are two types of engines which require two different types of cranks--the inline engines and the "V" engines---With the inline engines they don't require the bobweights to be on them for spinning during balancing--
The V types require that bobweights be attached to the rod throws for spinning during the balancing process--The bobweights are made up of 100% of the rotational weight and 50% of the recipricating weight of 2 cylinders ( V8)--since the front two rod throws(4 cylinders) are at 90* to each other as are the back 2 rod throws also 90* while the two center rod throws( OK, so I'm up to 6 rod throws ?) are 180* to each other, the V8 cranks will have counter weights to the front and back but generally not at the center throws---( unless it has been CENTER COUNTER WEIGHTED for strength )

SOOOOOOOO---the addition/subtraction of weight for balancing is generally done on the first and last counterweight---

The 28 and 50 OZ numbers relate to inch/ounces--thats why the flywheel on the external jobs have less correction than the front dampners---differance between something 6-8 inch dia and 12-14 inches flywheel

The pics that I posted that show the imbalance(when I took the pictures) relate to grams. at the time the crank was at 2 grams on a 3 inch radius---

Thanks Jerry. My experience up until recently has been with inline four and six cylinder engines. I couldn’t work out why you would add 28oz (or getting close to a kilogram) to the front and rear ends of the crank – in order to balance it!!
Cheers,
Glen

[xb-60]

Originally Posted by Keithc8

Balancing an enigne is not exact sience .... We need to write a book on this stuff.

Good luck, Keith Craft

Thanks Keith. What's becoming clear is that the guy at the local machine shop down the road may not be the best bet for skimming your flywheel, let alone to build an engine for you.
What spelling errors? You should do the book.
Cheers,
Glen

[xb-60]

Originally Posted by blykins

....Before you ask any more questions, you need to watch a video or two on YouTube about it. It will clear up a lot of things you're having trouble with....

Thanks Brent. It's a steep learning curve, and I appreciate all the replies.
I've seen some videos, but they have been the results of out-of-balance, not "how-to", and "why?" I'll look some more.
Cheers,
Glen

[blykins]

Step by step.....

The Balancing Process from Coast High Performance - YouTube

[Jerry Clayton]

http://www.scatcrankshafts.com/scatt...k_the_Nose.pdf

After you have watched that video, check out number 5 of this page---in the video look at all the holes he drilled into the counter weights to try to balance the assy with external weighed flywheel and dampner--In Scat's tech section in very plain language talk about how much to drill and this guy evidently didn't read it--

[Mike Huddart]

Hey Ho, another scrap crank, and soon to be followed by a scrap block after # 2 and 4 main caps have been shaken to bits and the main bolt registers have cracked from top to bottom.
Personally I liked the bit where he DOESN'T check the alignment of the bob weights.
I may not be able to post something on U- Tube ( whatever that is ) but I can balance a crank properly.

Mike

[blykins]

Haha....the wolves are out tonight....

Remember guys, this is a very fundamental thread on balancing. We're not teaching him to be a machinist.

[undy]

Brent, you're having about as much luck here as that 347 vs 331 thead... heh, heh...

No good deed EVER goes unpunished

[blykins]

You win some, you lose some....

This thread should be looking at the forest though....not the trees...

[Jerry Clayton]

I'm surprised that Mark let that tape out

[Mike Huddart]

Originally Posted by blykins

Haha....the wolves are out tonight....

Remember guys, this is a very fundamental thread on balancing. We're not teaching him to be a machinist.

Sorry, point taken, but when you see a post that you know is plainly wrong the reaction is to correct it...........probably just leave it there !

Mike

[blykins]

True, and admittedly I didn't even watch the whole video. But just wanted to show what a crank balancer looked like, bobweights, etc.

[CHANMADD]

Most engines have an imbalance at 1400_2000 rpm. This is harmonic imbalance.The harmonic balancer helps to absorb that frequency. Most harmonic balancer have arubber ring sandwiched berween two parts. This type of balancer should be balanced with the crank. A fluid filled type balancer cannot be spun balanced because the fluid moves around. The 28 oz thing is like balancing a wheel and on one side there is a big hole drilled , so that before you stick it on the machine you know that you drilled out 28oz, so that exactly opposite at the same circumference you attach a 28oz weight to get the balance close before spin balancing on the machine.
A single cylinder motor would have all the components meticulously polished ,shaped etc.
A blueprint is making sure that all the components match exactly the designers specs.
A multicylinder blueprint is making sure that all the cylinders and all related components are exactly the same, down to 0.002 of a gram. When they are all same the harmonics will be minimized and then spin balancing will require less addition or removal of metal. This is what makes motors live.
Look at the pulley on a good timiing chain set...it should have some balancing drill marks.
So....yes balance everything if you can..it can only make things smoother
John