Terminology problem - meaning of neutrally balanced - PeachParts Mercedes-Benz Forum

Stretch · #1 01-22-2012, 11:15 AM

G'day All,

I'm suffering from a terminology problem - I see it bandied about quite a bit on the internet as well as here on the forum.

What do people mean when they say that a flywheel (for example) is neutrally balanced?

Are they talking about it being what I call statically balanced?

My understanding is that a rotating something is statically balanced when it favours no particular point in its rotation.

For example you hold the rotating onject in one position - let it go and it won't move to another more favourable position. Hold it in another position and let then let it go and it won't move back to that favourable position.

So you could also spin it - evil friction slows it - and it stops at one point. Spin it again - evil friction slows it - and it stops at another different point...

...statistically speaking if you were to mark your rotating object with 360 equal degrees the chances of it falling between two points should be 1/360 => it shouldn't favour a particular point like a dodgy roulette wheel.

Or has it got something to do with how the rotating parts have been balanced? I've seen several references on 'internet that talk about internal and external balancing where external balancing is adding weight on the outside of something (like balancing a tyre - you add weights to correct for the any uneven thickness in the rubber or the valve stem weight) and internal is material removal somewhere inside. These terms seem to get included with the term "neutrally balanced"...

See for example

Pro-race

Surely it doesn't have anything to do with dynamic balancing???

I haven't found a source of information that defines this term. Can someone help?

bmwpowere36m3 · #2 01-22-2012, 11:45 AM

Static balance is single plane (or a correction at a single point), for example: a wheel put on a bearing assembly and let to freely orient itself. Thus the heaviest part will naturally rotate to the bottom. This is typically checked a few times to insure the correct heavy point. To correct, weight is added 180 degrees opposite the heavy spot.

Dynamic balance is dual plane (or a correction at two points), for example: a spinning wheel on a dynamic balancer. Typically their will be two heavy points (one on each side of the wheel. Weights are added opposite of each heavy point. Dynamic balance also accounts for static balance.

So neutral balance is just a generic term for balancing something close to zero (or whatever the tolerance is).

Stretch · #3 01-22-2012, 12:00 PM

Quote:

Originally Posted by bmwpowere36m3

...

So neutral balance is just a generic term for balancing something close to zero (or whatever the tolerance is).

Sorry I don't get you zero what?

Which balancing? Static or dynamic?

t walgamuth · #4 01-22-2012, 12:08 PM

What is commonly talked about here as neutrally balanced matches your definition of statically balanced, I believe. (...and I am not disagreeing with BMWpower on this...)

The term internally or externally balanced is another term entirely and I only know of it as applied to engines.

An engine has only so much room inside the block to add mass to the crankshaft so if the crank is too far from being neutrally (or statically) balanced you may run out of room inside the block to balance it. In this case the crank needs the assistance of the flywheel in making a balanced whole.

Most of the 123 engines I have run across had neutral cranks and neutral fw.

My 84 euro 280e though had a fw that was balanced with the crank. If you were to look at the tightness of the inside of the block where the crank lives you would not be surprised that sometimes they could not balance the crank for them.

Another way to solve the problem is to drill holes in the crank weights and add plugs of heavier material. They did this on my 300SD replacement crank when they found it to be a bit out.

Stretch · #5 01-22-2012, 12:10 PM

Thanks Tom!

47dodge · #6 01-22-2012, 07:14 PM

the crank is normally checked for balance when spinning with "bob" wieghts to simulate a portion of the weight of the rods and pistons, and a strobe light. My crank balanced to within one gram on my 240d.

The flywheel is normally balanced statically as per fsm instructions. This is between two shafts with presision ball bearings. When the flywheel does not roll to any one point it is balanced. This all assumes what Tom has said and that the crank can be balanced without needing additional weight

97 SL320 · #7 01-22-2012, 08:10 PM

There are bits and pieces of information floating about, but no single post puts them all together where they fit.

The original poster asked about a flywheel that is neutral balanced. The term "neutral balanced" pertains to the state of balance and _not_ the type of balancing. A neutral balance flywheel ( or front pulley/hub/damper for that matter ) has zero _imbalance_, it can be spun on it's own and not vibrate.

Some flywheels have a imbalance by design. ( counterweighted flywheel ) This is done when a crankshafts counterweights can't be made large enough and still fit in the block. So, spin one of these cranks by itself* and it will vibrate, same goes for a counter weight flywheel. Bolt the two together and they will spin just fine. ( * this assumes the pistons/rods are in place or machine shop bob weights are being used for testing )

Now onto the two types of balance, static and dynamic.

Static is used for items that have a thin section width or systems that only need rough balancing. As its name implies, this type of balancing is done with the object stationary and relies on gravity.

Dynamic is used for items with thick / long section widths or high speeds. Also as its name implies, balancing is done with the object spinning. This requires displacement sensors to measure the force of imbalance and some sort of indicator to show where the imbalance is occurring. A strobe light that flashes when imbalance is detected is one method, another is keeping track of rotational position then after running a test, manually slowly rotating the object until the meter shows the imbalance is at some predetermined point.

This demonstration works much better with a shaft and bearings but the theory still holds true with lesser props.

Slide a CD onto a pencil then hold the pencil horizontally on each end with both hands. Imagine the CD spinning, there will be zero vibration because the mass is evenly distributed through the CD.

Now, take a few coins, make a stack and tape it to one side of the CD, rotate the CD so the coins are near the top then let go. The coins will fall ( gravity ) and cause the CD rotate then stop. This is a static imbalance / counter weighted flywheel. If we spun the CD it would vibrate _up and down_.

Generally to bring a flywheel back into balance, we will have material removed by drilling. We could remove the coins to balance but they are needed for the second part of the demonstration.

Take a second stack of coins that matches the same mass as the first set, tape this to the _opposite _ side and 180* from the first stack of coins. We now have corrected static imbalance and no longer have up / down vibration however we now caused a dynamic imbalance. Dynamic imbalance is where the vibration forces act _left to right_. To correct this, weight must be removed from both sides of the CD.

Dieselkraut23 · #8 01-22-2012, 10:46 PM

So not to jack the thread, but the 617/616 manual flywheels are suppose to be static balanced so why would some have vibration problems swapping the two and others not?

Sorry just I have been having the same problem as i thought static/neutral were the same.

t walgamuth · #9 01-22-2012, 11:05 PM

Apparently some of the cranks could not be balanced by themselves so they had to use the fw to get the assembly balanced.

Hence the FSM calls for a switch of flywheels to be match balanced.

As a practical matter when you disassemble a crank from the motor you need to mark its location. then get the fw checked for neutral balance. If it is neutral you need to make the new fw neutral. If it is not they must be match balanced.

In about five motors I have had apart we found one to have a fw that was used to balance the crank assembly.

bricktron · #10 01-22-2012, 11:29 PM

hey tom - thank you for explaining this so many times!

Brian Carlton · #11 01-23-2012, 01:16 AM

Quote:

Originally Posted by 97 SL320

Now onto the two types of balance, static and dynamic.

Static is used for items that have a thin section width or systems that only need rough balancing. As its name implies, this type of balancing is done with the object stationary and relies on gravity.

Dynamic is used for items with thick / long section widths or high speeds. Also as its name implies, balancing is done with the object spinning. This requires displacement sensors to measure the force of imbalance and some sort of indicator to show where the imbalance is occurring. A strobe light that flashes when imbalance is detected is one method, another is keeping track of rotational position then after running a test, manually slowly rotating the object until the meter shows the imbalance is at some predetermined point.

This demonstration works much better with a shaft and bearings but the theory still holds true with lesser props.

A few points need to be corrected above.

There are two types of unbalance. "Static" and "Couple". Static unbalance is the unbalance generated at the CG of a component and is corrected with a single correction weight, preferably placed at the CG. The machine utilized to measure static unbalance can be a simple bubble balancer that does not rotate, or it can be a very expensive rotating piece of machinery with a precision spindle. The type of machine used for the correction does not affect the terminology.

When couple unbalance correction is desired, in addition to static unbalance, this is the generalized state of unbalance and it is now referred to as "dynamic" unbalance. To fully correct dynamic unbalance requires the addition of weights in two separate (as far as possible) correction planes.

The more costly machines for this purpose do not utilize any stroboscopic pickup for angle reference and rely on a probe or a photocell to determine zero degrees. The balancing machine instrument then measures the difference in angle between the peak force and the zero degree position of the rotor.

BTW, there is no term "imbalance". The proper term is "unbalance".

Stretch · #12 01-23-2012, 02:47 AM

Thanks for all the responses.

Is everyone in agreement that neutrally balanced is statically balanced as I described in post #1?

Brian Carlton · #13 01-23-2012, 09:31 AM

Quote:

Originally Posted by Army

Thanks for all the responses.

Is everyone in agreement that neutrally balanced is statically balanced as I described in post #1?

I am not in agreement.

"Neutrally" balanced and "statically" balanced are not synonymous.

The component can be neutrally balanced and statically balanced. It can also be neutrally balanced and dynamically balanced. It can be statically balanced while attached to the crankshaft (not neutrally balanced). It can also be dynamically balanced while attached to the crankshaft (not neutrally balanced).

bmwpowere36m3 · #14 01-23-2012, 10:35 AM

Like I said, neutral balance is a generic term for balancing an item down to a certain tolerance (as nothing is perfect, or zero imbalance). The FSM shows a method to match balance one FW to another. That procedure is static balancing, but generally when you bring a FW to a machine shop they will dynamically balance it due to its high rotational speeds (more so in gasser engines).

tangofox007 · #15 01-23-2012, 11:55 AM

Quote:

Originally Posted by bmwpowere36m3

Like I said, neutral balance is a generic term for balancing an item down to a certain tolerance (as nothing is perfect, or zero imbalance).

You can keep saying it, but that won't make it so. The concept of "neutral balance" has nothing whatsoever to do with a particular tolerance. Nor does it qualify as a "generic term."