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Old 05-18-2008, 08:12 PM
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DIY Crank Shaft Balancing

I had posted a similar discussion to find anyone who has kinfe edge their crankshaft on their own.

I have a two cylinder crankshaft from a motorcycle that I am experimenting with. It is trashed so I dont mind messing with it and IT WILL NEVER BE INSTALLED.

I wanted to practice machining the crank to see how difficult it is. What I need to know now is:

HOw are the crankshafts balanced? Are the counterweights weight equalt to the weight of the entire connecting rod assembly?

In order to fine tune the weight balancing I have seen them drill holes into the counter weights then cap the holes off. Is this the best way?

I want to understand how the balancing process is done in order to determine how much I want to remove from my counter weights.
I want to have a well balanced crank in the end.

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Old 05-21-2008, 11:50 PM
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I see your question hasn't been answered so I'll give it a shot, first off I know nothing about the procedure but it takes special machinery and a lot of knowledge. Unless you have a crank balancing machine forget it. I've seen it done on my parts and they have big bob weights they bolt to the crankshaft rod journals that corresponds to your piston/rod weight (I think) and spin it on a lathe type machine.

Kudo's to you for wanting to do your own stuff but it's just too easy and correct to have an engine balancer tech do it right and not have to worry the first time you take it to 6500. Give Speed-O-Motive in West Covina a call, I think they still have a balancing shop in house. Maybe they could give you the REAL scoop.
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Old 05-23-2008, 06:29 PM
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Quote:
Originally Posted by 327NUT
I see your question hasn't been answered so I'll give it a shot, first off I know nothing about the procedure but it takes special machinery and a lot of knowledge. Unless you have a crank balancing machine forget it. I've seen it done on my parts and they have big bob weights they bolt to the crankshaft rod journals that corresponds to your piston/rod weight (I think) and spin it on a lathe type machine.

Kudo's to you for wanting to do your own stuff but it's just too easy and correct to have an engine balancer tech do it right and not have to worry the first time you take it to 6500. Give Speed-O-Motive in West Covina a call, I think they still have a balancing shop in house. Maybe they could give you the REAL scoop.
Thanks for your response. I appreciate you taking your time to answer my question.

I am not going to give up. If i knew exactly how a "crank balancing machine" worked I would be able to replicate it and possibly make my own.

I have seen the balance machine for a motorcycle crank, I just dont understand how it works exactly.
look what i found. Nothing special there. I just dont see what they are doing? I can tell how they are balancing the crank
http://www.youtube.com/watch?v=cxWp2...eature=related

I will search for that other video I found after the Lakers beat the Spurs!
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Old 05-23-2008, 10:50 PM
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Quote:
Originally Posted by Riot Racing
I had posted a similar discussion to find anyone who has kinfe edge their crankshaft on their own.

I have a two cylinder crankshaft from a motorcycle that I am experimenting with. It is trashed so I dont mind messing with it and IT WILL NEVER BE INSTALLED.

I wanted to practice machining the crank to see how difficult it is. What I need to know now is:

HOw are the crankshafts balanced? Are the counterweights weight equalt to the weight of the entire connecting rod assembly?

In order to fine tune the weight balancing I have seen them drill holes into the counter weights then cap the holes off. Is this the best way?

I want to understand how the balancing process is done in order to determine how much I want to remove from my counter weights.
I want to have a well balanced crank in the end.
It is necessary to know the engine configuration...ie V-style, inline or oppossed cylinder, as all of these styles could have very different methods for balancing. A V-style will require a bobweight formula and bobweights to attach to the rod journal, and then spun on a balancing machine to sense the "out of balance" of the assembly. And them either remove or add weight to the counterweights to balance it.Much like spin balancing a tire,using a strobe light to pinpoint the areas where weight needs to be added or taken away.Also many 1 & 2 cyl. engines fall into this category for balance as well.Traditinal engines such as modern inline 4 and 6 cylinder engines have 2 and 3 crank throws(respectfully), that are 180 degrees apart, therefore opposing the same rod and piston weight to each other(much like a beam style scale for balancing). You put 5lbs. on one tray and 5lbs. in the other tray,and the needle on the scale is in the middle. The inline engines crank is balanced by itself with no bobweights, and the rod and pistons are all made to weigh the same. Maybe this gives you more of a heads up on how to proceed with your engine.
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Old 05-23-2008, 11:10 PM
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This was just a wasted post. I should know better by now.

Later, mikey
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Old 05-24-2008, 06:04 PM
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Quote:
Originally Posted by CNC-Dude
It is necessary to know the engine configuration...ie V-style, inline or oppossed cylinder, as all of these styles could have very different methods for balancing. A V-style will require a bobweight formula and bobweights to attach to the rod journal, and then spun on a balancing machine to sense the "out of balance" of the assembly. And them either remove or add weight to the counterweights to balance it.Much like spin balancing a tire,using a strobe light to pinpoint the areas where weight needs to be added or taken away.Also many 1 & 2 cyl. engines fall into this category for balance as well.Traditinal engines such as modern inline 4 and 6 cylinder engines have 2 and 3 crank throws(respectfully), that are 180 degrees apart, therefore opposing the same rod and piston weight to each other(much like a beam style scale for balancing). You put 5lbs. on one tray and 5lbs. in the other tray,and the needle on the scale is in the middle. The inline engines crank is balanced by itself with no bobweights, and the rod and pistons are all made to weigh the same. Maybe this gives you more of a heads up on how to proceed with your engine.
Well I am working with a 2 cylinder engine BUT, I am practicing to hopefully in the future do 4 and 6 cylinder engines for real. These would be inline 4 and inline 6 cylinder engines.

So the first engine that I would like to do for real is: a 4 cylinder yamaha 600c engine. the year would be 2003.
I have several of these crankshafts from the numerous motors I have plan from my formula car.

One thing that you may have answered that I over looked, or maybe you didn't answer.

SO do the counterweights which are on the crankshaft weigh the same as the connecting rod, piston and piston assembly (wrist pin, bearings, bolts, e.t.c)?

Again, it doesnt seem so because the crank is so damn heavy, but I could be wrong.
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Old 05-25-2008, 08:57 AM
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The balancing of a crankshaft will require that "bob weights" are installed on the crank rod journal areas and then the crankshaft can be balanced. In some automotive engines the flywheel and the harmonic dampener are also installed. Weight will be either added or removed from the crankshaft counterweights to achieve this.

There are several different formulas (different manufacturers have different ideas as to how to determine the correct weight for the bob weights) to get the correct weight to be used for the bob weights. The bob weights are to simulate all of the rotating weight and half of the reciprocating weight (consisting of the rod, piston, pin, keepers, rings). How the components are weighed is very important.

Balancing the crankshaft with no bob weights or the incorrect bob weights will yield a very "out of balance" assembly.

Some engine designs can not be 100% balanced. The Harley Davidson V-Twin is one example. Some Japanese Motorcycle manufacturers designed a separate counter shaft that is driven off the main crankshaft to enable a better "balancing" of there V-design engine.
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Old 05-25-2008, 09:15 AM
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Maybe you'll get lucky and find a shop that's selling an old balance machine, or a shop that's closing down and selling off all their current gear.
I've always been curious about how exactly they balance the crank, too. I know that if you cut down and knife edge most of the counterbalance weights, the guy will have to drill holes and use heavy mallory metal to bring it back into balance.
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Old 05-25-2008, 07:05 PM
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Quote:
Originally Posted by Frisco
The balancing of a crankshaft will require that "bob weights" are installed on the crank rod journal areas and then the crankshaft can be balanced. In some automotive engines the flywheel and the harmonic dampener are also installed. Weight will be either added or removed from the crankshaft counterweights to achieve this.

There are several different formulas (different manufacturers have different ideas as to how to determine the correct weight for the bob weights) to get the correct weight to be used for the bob weights. The bob weights are to simulate all of the rotating weight and half of the reciprocating weight (consisting of the rod, piston, pin, keepers, rings). How the components are weighed is very important.

Balancing the crankshaft with no bob weights or the incorrect bob weights will yield a very "out of balance" assembly.

Some engine designs can not be 100% balanced. The Harley Davidson V-Twin is one example. Some Japanese Motorcycle manufacturers designed a separate counter shaft that is driven off the main crankshaft to enable a better "balancing" of there V-design engine.
You are partially correct as to your statement of balancing a crankshaft without bobweights! Crankshaft designs that are of the 180 degree design(most inlines are this style), are not balanced with bobweights at all! In fact, the rod and piston weights have no affect on the balance, as long as they(the rods and pistons themself)are balanced and matched to each other. A 180 degree design crank, has the same rotating mass 180 degrees apart(in a four cylinder),2 crank throws in the opposite direction to counteract the balancing. In over 20 years of professionally building Pro Stock, Comp Eliminator, Winston Cup and other engine types that spend much of their life in extreme RPM situations, I have seen a trend of actually "overbalancing" the rotating assembly. This seems to help smooth out harmonics that are created at these higher RPM levels. Overbalancing is balancing the crankshaft with a bobweight that is heavier than the actual calculated bobweight(usually by 1-1/2 to 2 percent). Back to the original question, as to whether the counter weights reflect the same weight as the rods and pistons. For his application, they do not. The crank is balanced independent of the rods and pistons,rings,wrist pins,bearings, pin locks.....In a V-style engine, and others that require bobweights to be attached to the crank for balancing, the counterweight mass is affected by rod and piston weight and vise-versa! So for knife edging the counterweights on his scooter crank, the same amount of weight will need to removed from the crank pin area of crank to compensate for the knife edging....

Last edited by CNC-Dude; 05-25-2008 at 07:13 PM.
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Old 05-25-2008, 08:12 PM
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http://en.wikipedia.org/wiki/Engine_balance

http://www.diynetwork.com/diy/ar_rep...272193,00.html

http://www.universal-balancing.com/c...FQ3_sgod9VjHCw





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Old 05-25-2008, 08:17 PM
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Quote:
Originally Posted by powerrodsmike
This was just a wasted post. I should know better by now.

Later, mikey
Dude mikey dont be so sensitive. It was nothing personal. I thought your post was very helpful. I wish you didn't delete it. Whats up with that?
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Old 05-25-2008, 09:16 PM
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I figured you had me on your ignore list.

http://69.20.125.73/temp/instruction/sheet/51-1037.pdf

I posted that link to the S&S instructions because you said you were doing a 2 cylinder motorcycle engine as a practice . The S&S instructions show how to do a 2 cylinder motorcycle crank, which may or may not be the type that you are playing with. Still, the step by step process that they show is probably very similar to balancing any other IC motor, you'd just add counterweights..

Also, you said you'd like to make up the machine yourself. The balancing fixture they show could be made up easily by a resourceful and innovative guy like yourself.



I always thought that a set of shaper blades, honed flat on a surface plate and mounted in a steel frame would make a good home balancing stand.

Another type of static balance fixture, with hardened discs and very sensitive bearings..



Very much like the machine shown in RobKellers 3rd link...


All that being said, I do know that dynamic balancing is best for high speed, high power shafts of any type, but by starting at the basic end of things a guy gets a better understanding of the sophisticated methods.

Any small indication that you had even looked at my first post would have been enough for me to not behave like a girl.


Later, mikey.
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Last edited by powerrodsmike; 05-25-2008 at 09:23 PM.
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Old 05-25-2008, 11:10 PM
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Quote:
Originally Posted by powerrodsmike
I figured you had me on your ignore list.

http://69.20.125.73/temp/instruction/sheet/51-1037.pdf

I posted that link to the S&S instructions because you said you were doing a 2 cylinder motorcycle engine as a practice . The S&S instructions show how to do a 2 cylinder motorcycle crank, which may or may not be the type that you are playing with. Still, the step by step process that they show is probably very similar to balancing any other IC motor, you'd just add counterweights..

Also, you said you'd like to make up the machine yourself. The balancing fixture they show could be made up easily by a resourceful and innovative guy like yourself.



I always thought that a set of shaper blades, honed flat on a surface plate and mounted in a steel frame would make a good home balancing stand.

Another type of static balance fixture, with hardened discs and very sensitive bearings..



Very much like the machine shown in RobKellers 3rd link...


All that being said, I do know that dynamic balancing is best for high speed, high power shafts of any type, but by starting at the basic end of things a guy gets a better understanding of the sophisticated methods.

Any small indication that you had even looked at my first post would have been enough for me to not behave like a girl.


Later, mikey.
I didn't respond to your post right away because there was a lot of reading involved and at that moment I didn't have time.

Now, that I have read throught he instructions. I see how helpful that is. Thank you. Thats interesting that something that seems to be valuable and possibly a trade secret would be so easily available.

Something that I am having difficultly finding answers to is:
Balancing the crank after the kinfe edging has taken place.

If i know the counter weights are all equal mass, I could do my machine work equally so that they would stay relatively equal. But, I feel like this isn't the safe method of practice.
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Old 05-26-2008, 10:08 AM
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Quote:
Originally Posted by Riot Racing
SO do the counterweights which are on the crankshaft weigh the same as the connecting rod, piston and piston assembly (wrist pin, bearings, bolts, e.t.c)?

Again, it doesnt seem so because the crank is so damn heavy, but I could be wrong.
I think I may have answered this portion of your question in a previous comment. Also, the balancing fixture shown in several of the other replies, is a very good economical method to get your hands dirty and see the concept of balancing. Your actual crankshaft to be used in your formula car will be balanced independently and seperately from the rods and pistons. You still will need to balance them,however. Most racing shops balance them(rods and pistons) to within a 1 gram +/- of each other. Also, the rods will need to be balanced on the small end as well as total weight. If you use a fixture similar to the one shown in other posts, i'll use the following illustration to give you a visual on how to proceed with the mods you have in mind! In a perfect world, when you set the crank on the fixture, it shouldn't try to rotate or move in either direction. Indicating, that it is neutral, and no area on the crank is heavier or lighter than any other spot on it. Now compare that to a seesaw that is completely level. When you remove material from both the leading and trailing edges of the counterweights(im assuming you will do both),the opposite end of the crank will become heavier and rotate to the bottom of the fixture. You will have to drill, mill, or grind material off of the rod pins boss area of the crank to get the crank back in balance. Now, picture the seesaw again. Put a 50 lb. bag of sand on one side, and a 75 lb. bag on the other. You obviously will have to scoop out sand from the heavier bag to make the seesaw level again. That is the same concept. After you do one of your cranks, take it to a machine shop and get them to check it and see how close it really is in balance. This will give you a heads up as to how accurate that fixture method really is....
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Old 05-27-2008, 08:42 PM
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I decided to take some crappy pictures with the camera on my phone. The lighting wasn't too great so they came out bad.

The holes you see drilled in the counterweights came like this when i purchased the crank. The crank is damaged which can't be seen. There is scoring where the rod journals would be. I may repair this in the future, again as practice. I think a grinder is needed for the rod journals.

View#2
Looking at the counterweight which has the drilled holes pointing up in the air. This is the edge I was planning on knife edging. Does that make sense? This seems like the edge which has the most mass. AND in the case that the crank hits the oil, this seems to be the likely side to hit the oil.

View #4 & #5
Are to demonstrate the practice machine work I have done. This was done on a manual lathe. The material is extremely hard. If I were to conintue to machine this area we are talking .010" cuts at a time, maybe .015" cuts.

ALSO, in these views you can see a metal ball which was likely pressed in. What the heck is this for?

View #1


View #2


View #3


View #4


View #5


View #6
[img]
http://img2.putfile.com/thumb/5/14722411032.jpg[/img]


What do you guys think?

Last edited by Riot Racing; 05-27-2008 at 09:00 PM.
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