Lathes and dynamic balancing

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Hackasaw
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Lathes and dynamic balancing

Postby Hackasaw » Sun May 18, 2003 5:19 pm

thought about buying a stroboscopic wheel balancer and using it to attempt balancing crank shafts.

I've used them to actaully touch up balance problem wheels that came off a computer wheel balancer so I do know how to use one but crankshafts do not readily lend themselves to clip on weights and such.

Anyone experienced with dynamic spin balancing heavy metal objects?

Any thoughts?
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gamachinist
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Re: Lathes and dynamic balancing

Postby gamachinist » Sun May 18, 2003 7:49 pm

Hello Hackasaw.Balancing crankshafts is done on V blocks or twin rollers.The pick ups are seperated and the shaft is balanced on each end.Either weight is removed or added in the form of tungnsten rods inserted into holes drilled in the counterweight.As tung is heavier than steel or cast iron you gain weight to correct imbalance.Weight can be removed by drilling(most common) or grinding.Grinding can be used to profile the counterweight for a more aerodynamic effect in the crankcase.I grind when I'm almost finnished to keep from removing too much(A real PITA).
A tire balancer might could be modified for M C shafts like a Harley or single cylinder two stroke.You'd have to remove the arbor and replace it with a chuck or something to hold the shaft.
Robert.

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Hackasaw
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Re: Lathes and dynamic balancing

Postby Hackasaw » Sun May 18, 2003 8:10 pm

not exactly what I am thinking........ I am aware of how it's done with vblocks or rollers but I am thinking beyond static balancing and the strobe can be used for both planes by simply moving the pick up where you want.........

I was thinking just put the crank pin in the chuck and spin away but I suppose a thrust surface would have to be present for the dynamic part of the operation

I'm not talking about the conventional spin balancers either but the old school stobe type that have a magnetic pick up

I've been known to think and speak out of the side of my neck and this time could be one of them
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mbensema
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Re: Lathes and dynamic balancing

Postby mbensema » Sun May 18, 2003 8:28 pm

I do vibration analysis and balancing of process equipment, but crankshaft balancing is a special area all to itself. There is much more to it then a wheel balancer or a machine with round components. This article gives some basics about what is involved. http://www.diynet.com/DIY/article/0,2058,4937,00.html

Since the crank has many planes, the calculations involved would be fairly complex to avoid a lot of trial and error. You might be able to drill and tap the bottom of each crank counterweight to mount your trial weights and see if you can get it close enough.

Mike

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gamachinist
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Re: Lathes and dynamic balancing

Postby gamachinist » Sun May 18, 2003 8:29 pm

Actually,I was refering to "spin" balancing on an automotive or industrial balancer.If I can get the scanner to work I'll post some more info if you'd like.
My balancer has an aux pickup and a magnetic block to attach to the part.I never have to use it but your idea will work as far as I can see.My machine uses a strobe to indicate where to remove metal.(It shows where to ''index'' the shaft,then weight is added or removed.Robert.

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Hackasaw
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Re: Lathes and dynamic balancing

Postby Hackasaw » Sun May 18, 2003 8:37 pm

Well........... I was thinking since I know for certain I can static balance the assemblies I do with surgical precision........... the dynamic would sorta invovle a bit of trial and error to sweeten a good one just that much more

I'll chase the links and look forward to seeing the scans for sure


It is a shame you can't just undo a zipper and some velcro to stuff cranks in and out of their cases

thanks
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gamachinist
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Re: Lathes and dynamic balancing

Postby gamachinist » Sun May 18, 2003 8:51 pm

Thanks for the link mbensema.That is the same machine I have.I may send my costomers to that site as I'm sure they can explain it better than I can.
Drilling and tapping the shaft for trial purposes would create a problem when they were removed.One gram can make a big difference(a 5/16 hole 5/16 deep =4.4 grams in steel or brass).The bobweights have tapped holes and studs that I use for this purpose.On a external balanced engine(these have a counterweight on the flywheel and/or balancer)you can use the flywheel or balancer for trials.

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gamachinist
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Re: Lathes and dynamic balancing

Postby gamachinist » Sun May 18, 2003 9:28 pm

Sorry guys,the scanner I was given is for Windows 2000 but I'm on XP.I don't want to take a chance on fouling up the computer as I'm not so good at fixing that.I'll try to pick up a scanner this week and put that stuff up here for you.Robert.

stephen thomas

Re: Lathes and dynamic balancing

Postby stephen thomas » Sun May 18, 2003 10:07 pm

Can you go over how to figure what percent of the rod-piston-pin-rings weight to use for the balance weights? The DIY article glosses over this. For instance, on a single or 360º parallel twin, I believe the percent runs between about 65% and about 85% depending on desired rpm range. (because the radius at which the center of mass acts is constantly changing with the rod and piston installed, instead of the fixed radius with the weights on the balance machine. Also the change is more rapid at TDC than at BDC) On an opposed ("boxer") twin or four, the opposing rod & piston sets cancel, so it might seem the crank can simply be balanced without any weights? Or? Between these extremes, there would seem to be a range of options, say for various V configurations depending on angle and arrangement of the crank throws?

Have any relatively common bike or auto engines used dynamic counterweights? (NOT balancer shafts, I know those are used in bikes these days). Some recip (piston) aircraft engines use dynamic counterweights and I would be interested to hear the advantages/disadvantages.

Thanks! smt

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Bill_Cook
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Here's what I wonder about...

Postby Bill_Cook » Sun May 18, 2003 10:26 pm

In ballancing a crank, how do you guys compensate for the weight of the crank end of the con rod that's wingin' 'round on that journal?
Is it a percentage thing?
Doesn't matter?
Nobody'll notice?

bc
BC

If there was only one way to do each machining job, the smell of sulphurized cutting oil smoke would have fewer fond memories.

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gamachinist
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Re: Lathes and dynamic balancing

Postby gamachinist » Sun May 18, 2003 11:16 pm

Some of this is dated from 1980 or earlier and may be out of date:
Rotating weight is the rod bearing if used.and the large end of the rod and an estimate of the oil in the throw and on the bearing surface(usually 1 gram).
Reciprocating weight is the piston,wrist pin,locks(if used),rings,and small end of the rod.The rods are weighed with one end on the scale and the other supported on a ball bearing fixture.
So:
v-8:Rotating=100%,Recip=50%
v-6 with split crankpins:
3.8L Ford:Rotating100%,recip39.2%
3.8L G M:Rotating!00%,recip36%
Single cylinder engines are inherently imbalanced.They cannot be brought into perfect balance but:use 100% rotating and
55-58% for 7500-8000RPMs
60% for 8000-10,000 RPMs
65% for 10,000-12,000 RPMs
Now onto Motorcycles etc.
This says for single cylinder engines use 61% recip and 100% rotating
Verticle twins:60-70% recip and 100% Rotating
BSA use 72-80% recip and 100% Rotating
3 Cyl use 42% recip and 100% Rotating
V twins use 52% recip and 100%Rotating
Inline 4 and 6 cylinder engines are balanced without bobweights except rare circumstances where the rods can't be matched and different weight bobweights are used.
As you noted "Flat" engines don't use bob weights either including the Corvair 6 cyl.
I hope this helps,Robert.

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Hackasaw
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Re: Lathes and dynamic balancing

Postby Hackasaw » Mon May 19, 2003 1:04 am

Good stuff for sure.......... there are a whole lot of people that don't understand the percentage stuff

recently I saw a advertisement or post from a gent that claims his 90 degree crank for brit twins make them "SMOOTH AS A BMW"......... a bmw twin running on one cylinder maybe [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

I've played with offset cranks of varying degrees for vertical twins and there simply is no way you can make a vertical twin have both perfect primary and secondary balance short of running a balance shaft......... heck........ you can't even make a 90 degree v-twin as smooth as a bmw boxer (opposed twin) but for sure somewhere between 70-74 degrees would come much closer than 90

another incredible exception to physics and science is the claim that the 490cc Norton single Flat Track engine is "vibrationless at all speeds" had me rolling in the floor almost as much as the 90 degree vertical twin jive.........

I guess some people really would have you believe you can balance a brick from idle to 10,000 rpm........crazy

the dynamic balance machine in the link......... how fast does it spin the crank assemblies?
I see a lot of people that have really dumb signatures they add to their posts on many forums. Why?


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