Crankshaft balancing
03-14-2009, 07:33 PM
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Crankshaft balancing
There isnt much out there on crankshaft balancing. I found this on another post on another web site and found it very interesting. I would like to hear what all you know about crankshaft balancing.
I saw this on the rcu forum and from here there are two different bob weights percentages used so now I am wondering just what is the number and procedure to use to balance a cingle cylinder crank
Make a jig for a bearing to fit in that will support the end of the crank, put the bearing in jig then the crank in the bearing. Use a new bearing, take it apart spray out all of the grease and soak in break clean. That will get the bearing free enough to let the crank spin the heavy spot down. Take the piston, rod and wrispin and weight it. Use 3/4 of the overall weight say 3 grams (made up number) as a guide to take some solder enough that weights the same and wrap it around the crank pin. This is a close weight of the force on the crank from the piston. Spin the crank and see where it ends up. You allways want the counterweight of the crank to be heavier then the weight of the piston and rod on the crank.
Do the required cuts to get the crank where the counterbalence is heavier then the weight on the crank pin and your done, now put the crank back in the engine to discover nothing is different. The engine still vibrates like crazy, and there is nothing that you can do about it.
A single cylinder engine will never be balanced. Balancing the crank on the engine does nothing for it, because there is nothing to oppose the force from the piston. On any real engine there are multiple pistons whose force is used to counteract the force that any one piston will make on the engine. In a single cylinder engine there is nothing to counteract the force. A counter weight will not work because the force that the piston places on the crank from combustion and compression cycles are never the same. Point being the force on the crank is constantly changing because the speed of the piston is consantly changing as it go's through the engines stroke. In a multi-cylinder engine there are other pistons going through the same phenomenon to counteract. But in a single cylinder engine there is nothing. To have the engine truley be balanced you would need a counter weight that is constantly changing its effectiveness on the crank to match the pistons g-force on the crank. But thats impossible in these engines.
Anyone can balance the parts, but having it actually make a positive effect is just a myth. Its dosent make a difference to a point. I try to get the crank close and I allways check it, but spending any more then 5 min on it is a joke. If you cant precisely measure, or duplicate the constantly changing force on the crank pin, and you cant replicate it on the engine then all your doing is just waisting time.
Not to sound negitive but its just a lame rumor that uninformed or fly-by-night motor guys spread by saying that you can balances a crank to make an engine better.
Do the required cuts to get the crank where the counterbalence is heavier then the weight on the crank pin and your done, now put the crank back in the engine to discover nothing is different. The engine still vibrates like crazy, and there is nothing that you can do about it.
A single cylinder engine will never be balanced. Balancing the crank on the engine does nothing for it, because there is nothing to oppose the force from the piston. On any real engine there are multiple pistons whose force is used to counteract the force that any one piston will make on the engine. In a single cylinder engine there is nothing to counteract the force. A counter weight will not work because the force that the piston places on the crank from combustion and compression cycles are never the same. Point being the force on the crank is constantly changing because the speed of the piston is consantly changing as it go's through the engines stroke. In a multi-cylinder engine there are other pistons going through the same phenomenon to counteract. But in a single cylinder engine there is nothing. To have the engine truley be balanced you would need a counter weight that is constantly changing its effectiveness on the crank to match the pistons g-force on the crank. But thats impossible in these engines.
Anyone can balance the parts, but having it actually make a positive effect is just a myth. Its dosent make a difference to a point. I try to get the crank close and I allways check it, but spending any more then 5 min on it is a joke. If you cant precisely measure, or duplicate the constantly changing force on the crank pin, and you cant replicate it on the engine then all your doing is just waisting time.
Not to sound negitive but its just a lame rumor that uninformed or fly-by-night motor guys spread by saying that you can balances a crank to make an engine better.
Since I was originally looking for a severe vibration problem, I then checked the balance. The short shaft engine was WAY off. It was nowhere near in balance, which makes me wonder if it originally had an off balance flywheel that helped to balance the engine. (I was not running the magneto, I was using electronic ignition.) The long shaft engines I have balance perfectly. 1/2 of the reciprocating weight perfectly balances with the counterweight on the crank with those. Remember that there have been questions about whether the Ryobi had a zero balanced flywheel or not?? I now think some models do and others don't!!
03-14-2009, 08:01 PM
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RE: Crankshaft balancing
Piston, rings etc.. recipicating weight
Small end of the rod reciprocating weight
Big end of the rod, rotating weight.
Add piston and upper rod weight, take 50-60% of that weight and add to the weight of the big end of the rod.
That is balance weight. Start with 60% so if its wrong you can remove more later.
You can add weight to crank balance OR remove weight from crankpin side.
All this is with the understand that you can NOT balance a single cylinder engine, but you can make it smoother in the rpm band where you run it.
Small end of the rod reciprocating weight
Big end of the rod, rotating weight.
Add piston and upper rod weight, take 50-60% of that weight and add to the weight of the big end of the rod.
That is balance weight. Start with 60% so if its wrong you can remove more later.
You can add weight to crank balance OR remove weight from crankpin side.
All this is with the understand that you can NOT balance a single cylinder engine, but you can make it smoother in the rpm band where you run it.
03-14-2009, 08:44 PM
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RE: Crankshaft balancing
I am not sure what you mean by the rod end reciprocating weight and the rotating weight and how this weight is found?
So let me get this straight i take the weight of all the reciprocating parts and the wieght of the reciprocating end of the rod and add them together? then i take 60% of this weight and add it to the rotating weight of the rod end? this is esentially what is considered the bob weight? I then need to figure a way to add this weight to the crank journal to do the balance?
How would you know the 60% weight is wrong if your going to modify the crank to balance with this weight?
It is my understanding you want to balance a crank much like in something for a prop where there is no resistance to the rotation of the crank and you want the crank just slightly heavier than the rod end?
This is so darn confusing? a video of this procedure would be great but im not sure if one exsists
So let me get this straight i take the weight of all the reciprocating parts and the wieght of the reciprocating end of the rod and add them together? then i take 60% of this weight and add it to the rotating weight of the rod end? this is esentially what is considered the bob weight? I then need to figure a way to add this weight to the crank journal to do the balance?
How would you know the 60% weight is wrong if your going to modify the crank to balance with this weight?
It is my understanding you want to balance a crank much like in something for a prop where there is no resistance to the rotation of the crank and you want the crank just slightly heavier than the rod end?
This is so darn confusing? a video of this procedure would be great but im not sure if one exsists
03-14-2009, 09:40 PM
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RE: Crankshaft balancing
Balancing is black magic, as you change the weight of the counter shaft then you change the "balance RPM". Remember that there is no way to do a perfect balance job on a single.
You hope to find a smooth rpm balance point that matches your desired RPM.
You will need 2 GOOD scales, weighing the piston is easy you just weigh it. Then place one end of the rod on each scale and then you know what the big and small ends of the rod weigh. Just to make sure your scales are on the up and up weigh the rod on one scale. The weights of both ends of the rod should equal the rod weight.
Then do the math and this is your BOB weight. Weigh out the correct BOB weight in solider and wrap it around the crank pin.
The 60% is wrong if it vibrates worse at your desired RPM
Like I said black magic
I've always been told you want the counter weight slightly heavy.
You hope to find a smooth rpm balance point that matches your desired RPM.
You will need 2 GOOD scales, weighing the piston is easy you just weigh it. Then place one end of the rod on each scale and then you know what the big and small ends of the rod weigh. Just to make sure your scales are on the up and up weigh the rod on one scale. The weights of both ends of the rod should equal the rod weight.
Then do the math and this is your BOB weight. Weigh out the correct BOB weight in solider and wrap it around the crank pin.
The 60% is wrong if it vibrates worse at your desired RPM
Like I said black magicI've always been told you want the counter weight slightly heavy.
03-15-2009, 12:15 AM
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RE: Crankshaft balancing
Scott sent me an artical that showed how to balance an assembly using a prop balancer.
I set up an Echo engine I was working on but is was perfect, well as good as it gets any way.
No matter where you placed the the crank and piston assembly it stayed there.
It used the same principle as a spoked wheel. The heavy end whent to the bottom.
I came across it the other day. It was printed, so I'll have to scan it to post it.
Ralph
I set up an Echo engine I was working on but is was perfect, well as good as it gets any way.
No matter where you placed the the crank and piston assembly it stayed there.
It used the same principle as a spoked wheel. The heavy end whent to the bottom.
I came across it the other day. It was printed, so I'll have to scan it to post it.
Ralph
03-15-2009, 09:10 AM
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RE: Crankshaft balancing
TKG could you go over the math again? im kind of fuzzy on that. From what you describe I take 60% off all the reciprocating weight and add it to the rotational weight to get bob wieght right? I am unsure if the rotational weight end of the rod is at 60% also, Or if you use that whole weight of the rotat. rod end to add together with the 60% reciprocating weight to get the bob weight?
03-15-2009, 04:05 PM
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RE: Crankshaft balancing
Combatpilot,first,a great info on the ryobi thread.
Dont know if this would help or not and didnt check to see if it was posted.If ya look at the pics below youll see the flywheel is oblong,not by much.Take one of your engines and look at the gap when the magnets are under coil.
Now rotate 180 degrees and now youll see theres a 1/8th of an inch gap.2 reasons maybe.
1]-to make sure flywheel doesnt hit the coil all the way around when running.
2]to counter balance the effect of the piston and reciprocating weight.When the oblong part of the flywheel [longest part]is at TDC,under the coil.
It is opposite the counter balance journals on the crank wich are at BDC.Now as stated,if one needs to have a counter balance moving as the rpm increases or decreases it would be easy to do just as they did and make the flywheel out of balance.
The other thing im going to try on one of my engines with a battery CDI is to put a prop with a heavy blade on it so the heavy blade is at TDC along with the piston.
The pics show a 1/8th difference from center.
Dont know if this would help or not and didnt check to see if it was posted.If ya look at the pics below youll see the flywheel is oblong,not by much.Take one of your engines and look at the gap when the magnets are under coil.
Now rotate 180 degrees and now youll see theres a 1/8th of an inch gap.2 reasons maybe.
1]-to make sure flywheel doesnt hit the coil all the way around when running.
2]to counter balance the effect of the piston and reciprocating weight.When the oblong part of the flywheel [longest part]is at TDC,under the coil.
It is opposite the counter balance journals on the crank wich are at BDC.Now as stated,if one needs to have a counter balance moving as the rpm increases or decreases it would be easy to do just as they did and make the flywheel out of balance.
The other thing im going to try on one of my engines with a battery CDI is to put a prop with a heavy blade on it so the heavy blade is at TDC along with the piston.
The pics show a 1/8th difference from center.
03-15-2009, 08:09 PM
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RE: Crankshaft balancing
That is interesting and I have no idea why they would do that. I am trying to understand and figure out the balancing of this engine for further experimentation. one thing I can tell you for sure as I started to cut down my flywheel to do the electronic conversion is that there is a steel slug in the cast aluminum part opposite the magnet. I was quite suprised to fing this and now see that with this steel slug it is verry possible to have many different options on balance per application. I need to figure out if the flywheel is balanced independantly or if it balance with the crankshaft. in a way I would have to think that the flywheel is balanced independant of the crank as the flywheel is removed when the electronic conversion is done and I have heard no complaints of vibration after this conversion.
03-15-2009, 09:35 PM
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RE: Crankshaft balancing
If you have the "big key" flywheel this is very common. The taper is cast in, not machined in. There may be core drift when they cast them.
Who cares on a $15 engine in a weedwacker.
Who cares on a $15 engine in a weedwacker.
04-21-2009, 11:23 AM
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RE: Crankshaft balancing
We could just go out and buy a Zenoa, Fuji or a DA but what would be the fun in that.
I have limited experience in conversions but from what I have found in converting older Poulan wackers at this early stage is the cranks on the Poulans appear to have the same crank stroke, crank counter weight and con rod. I say appear because I haven't actually been able to take all the measurements. Poulan seems to change the bore for the different displacements. so what I am assuming (you know were that can get us) is the difference may be in the flywheel or the different displacements, will have different amounts of tolerable vibration. Tolerable for a wacker but, maybe not tolerable for an aircraft.
I have limited experience in conversions but from what I have found in converting older Poulan wackers at this early stage is the cranks on the Poulans appear to have the same crank stroke, crank counter weight and con rod. I say appear because I haven't actually been able to take all the measurements. Poulan seems to change the bore for the different displacements. so what I am assuming (you know were that can get us) is the difference may be in the flywheel or the different displacements, will have different amounts of tolerable vibration. Tolerable for a wacker but, maybe not tolerable for an aircraft.
04-21-2009, 01:41 PM
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RE: Crankshaft balancing
Balance....get a Twin cylinder engine...or build a Twin using most of the parts you can from 2 single cylinder engines. Good luck, Capt,n
04-22-2009, 09:47 PM
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RE: Crankshaft balancing
Balance....get a Twin cylinder engine...or build a Twin using most of the parts you can from 2 single cylinder engines. Good luck, Capt,n
Im calling you out capt. Do you even actually own an airplane. If you had and airplane in one hand and a cookie jar in the other could you actually tell the difference cause you sure dont seem to have any original thoughts or ideas or even a vauge idea of what your talking about.
Im sure there gona delete this but hopefully somebody will se it before they do.
04-22-2009, 10:04 PM
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RE: Crankshaft balancing
Twin in answer to your original question i dont know and i dont think anyone else knows. maybee there is someone that does as there is lots of smart guys on here but we havent heard from him yet on this one. I think in the end were just gona have to get a gram scale and do so testing and comparison to find this answer.
04-22-2009, 10:14 PM
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RE: Crankshaft balancing
I weighed the flywheels that I have an old 26cc and a 32cc. The 32 was the lighter of the two but in it self that kinda means nothing, I think. It's the weight of what ever counter weights inside the flywheel we need to know. Like Combat said, It will take someone with more knowledge to answer that question.
That, is it not the purpose of this forum. Those with knowledge of a subject, selflessly sharing it with others so they may learn and accomplish their goal.
That, is it not the purpose of this forum. Those with knowledge of a subject, selflessly sharing it with others so they may learn and accomplish their goal.
04-22-2009, 10:18 PM
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RE: Crankshaft balancing
boy you would think the 32 cc would have the hevier flywheel to. The difference would have to be in the cranks weight but without a way to check the balance of it i dont think we are going to see it by eye alone.
04-22-2009, 10:29 PM
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RE: Crankshaft balancing
Could the lighter flywheel be a result of a heavier piston and ring, although the weight difference from a 22cc piston to a 32cc piston wouldn't be that much I admit. Just throwing out ideas here. I'm never at a loss for ideas, no mater how weird.
05-05-2009, 11:22 PM
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RE: Crankshaft balancing
Balancing just takes common sense, grain scales and formulas are not needed. Modify your engine then run it. Always be ready to shut it down quickly. Evaluate the vibration level, then make an educated guess [based on your modifications] as to whether heavy [mallory] metal needs to be added to the counterweight, or if you need to remove metal from the counterweight. I've modifyed engines [lighter piston and rod] that did not need any re-balancing, they ran smoother after the mods.
It's only brain surgery if you want it to be.
BTW, I believe that internal balancing is superior to external.
It's only brain surgery if you want it to be.
BTW, I believe that internal balancing is superior to external.
05-05-2009, 11:44 PM
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RE: Crankshaft balancing
One technique of balancing I've been contemplated is to deliberately un-balance a wood prop by coating one blade with progressive coats of varnish and try it in different positions evaluating static vibration.
The engine I am working on now vibrates more than I like. The prop I'm using is slightly un-balanced. When I placed the heaver blade opposite the counter weight of the crank the vibration seemed to improve slightly. I have yet to pursue this technique further but it seemed to have promise.
The engine I am working on now vibrates more than I like. The prop I'm using is slightly un-balanced. When I placed the heaver blade opposite the counter weight of the crank the vibration seemed to improve slightly. I have yet to pursue this technique further but it seemed to have promise.
05-06-2009, 08:23 AM
#23
RE: Crankshaft balancing
ORIGINAL: Joe Butch
Here ya go, and it really works. http://www.prestwich.ndirect.co.uk/t..._balancing.htm
Here ya go, and it really works. http://www.prestwich.ndirect.co.uk/t..._balancing.htm
05-06-2009, 08:44 AM
#24
RE: Crankshaft balancing
ORIGINAL: combatpilot
Man now theres a good answer to twin stars question that makes sense. lets see if i get this straight. take a problem and throw money at it or toss it out and spend lots of money to replace it with another lesser problem. is that it? did i get that right or did it go way over my head?
Im calling you out capt. Do you even actually own an airplane. If you had and airplane in one hand and a cookie jar in the other could you actually tell the difference cause you sure dont seem to have any original thoughts or ideas or even a vauge idea of what your talking about.
Im sure there gona delete this but hopefully somebody will se it before they do.
Balance....get a Twin cylinder engine...or build a Twin using most of the parts you can from 2 single cylinder engines. Good luck, Capt,n
Im calling you out capt. Do you even actually own an airplane. If you had and airplane in one hand and a cookie jar in the other could you actually tell the difference cause you sure dont seem to have any original thoughts or ideas or even a vauge idea of what your talking about.
Im sure there gona delete this but hopefully somebody will se it before they do.
05-06-2009, 09:11 AM
#25
RE: Crankshaft balancing
Combat...here are just a couple of photos to give you a idea I do work on my own and other peoples engines. Capt,n P.S. The ribbins in top of photo are for showing my engines.