balancing crank - turning counterweights

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Post by ptuomov »

SchmidtMotorWorks wrote: Thu Dec 25, 2008 11:14 am Generally I would rather reduce the fan of the first and last CW rather than turning because the first and last CW are overweight in 99% of cranks.

Depending on the design of vise and jaws you have, you might be able put the vise sideways on the table, and then clamp it on the counterweight that you are milling. I have balanced hundreds of billet cranks this way and never drilled a single hole.

Since the CW is probably not symmetrical across the first pin, be aware that the amount you remove form each end (leading / trailing) will be different.
Searching on the topic because I have crank that will get a lot of material removed due to much lighter pistons. This is the usual sort of six counterweight V8 crankshaft with thicker end counterweights.

Am I interpreting these instructions correctly if I cut along one of those lines depending on how much weight needs to come off?

...
6AEF82EC-11DD-4D50-BB3D-D2339BC5B75D.jpeg
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Re: Re:

Post by SchmidtMotorWorks »

ptuomov wrote: Wed Jan 22, 2020 6:07 am
SchmidtMotorWorks wrote: Thu Dec 25, 2008 11:14 am Generally I would rather reduce the fan of the first and last CW rather than turning because the first and last CW are overweight in 99% of cranks.

Depending on the design of vise and jaws you have, you might be able put the vise sideways on the table, and then clamp it on the counterweight that you are milling. I have balanced hundreds of billet cranks this way and never drilled a single hole.

Since the CW is probably not symmetrical across the first pin, be aware that the amount you remove form each end (leading / trailing) will be different.
Searching on the topic because I have crank that will get a lot of material removed due to much lighter pistons. This is the usual sort of six counterweight V8 crankshaft with thicker end counterweights.

Am I interpreting these instructions correctly if I cut along one of those lines depending on how much weight needs to come off?

...

6AEF82EC-11DD-4D50-BB3D-D2339BC5B75D.jpeg
Do the lines on just one side of the cw indicate that the imbalance is that far off of the center-line?
If so, we need do think the situation through a bit more carefully.

Can you take a few more pictures of the crank?
And explain the situation in detail?
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Re: balancing crank - turning counterweights

Post by ptuomov »

The bobweight will go down by more than 250g with 50% balance factor. The crank doesn’t have any balance or main bearing wear issues to start with, but the redline will go up from 6700 to 8000 rpm.

The crank looks like this:
960EFF89-AACE-40B7-A683-46955FDE0EBD.jpeg
45D69A56-A95E-4137-B507-DB9C7CA62723.jpeg
B466DECB-DF15-461E-9E99-B228435ADC51.jpeg
.,.

The only way to get the counterweights more behind the rod throws is to cut the leading edge of the thick end counterweight #1 and the trailing edge of the thin cw #3 (and symmetrically the trailing edge of cw #6 and leading edge of cw #8).

If I understand the issue correctly, large section of cws #3 and #6 would match a very small section of cws #1 and #8, both because of the thickness of the cws and because cws 3/6 are closer to the center main than cws 1/8. But since cws #3 and #6 are small to start with compared to bobweights on rod journals 2/6 and 3/7, I don’t want to take off much from those cws #3 and #6, only the section that is close to 90 degree angle to the corresponding rod journal or, equivalently, on the outside edges of the 180 degree fan angle relative to the corresponding rod journal.

To reiterate, the reason I’m asking is because I’m not sure I understand the issues fully.
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Re: balancing crank - turning counterweights

Post by ptuomov »

Some better photos by others:
crank-a.jpg
crank-c.jpg
DSC_0165lg.JPG
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Re: balancing crank - turning counterweights

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ptuomov, you have mail.
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Re: balancing crank - turning counterweights

Post by pdq67 »

I probably bought the old Lunati Co's last welded GM 454 crank that they stroked a 1/4" for my 496".

Well, I didn't pay any attention when they told me to use 1/4" longer rods so I used my good thumb rods since my engines rpm is only going to be less than 6,000 rpm.

Long story short, my piston bottoms hit the counterweights so I called Lunati and they said they could cam-cut my counterweights as well as finger the flywheel flange after adding the needed heavy-metal!

I called again and made an appointment to drive everything to them and then I came back a week later and hauled my stuff back home.

The cost of the crank rework figured out about what a new set of decent 1/4" longer BBC rods would have cost so I was HAPPY! Now this has been almost 20 years ago..

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Re: balancing crank - turning counterweights

Post by ptuomov »

I wish there would be a high school physics or geometry book that would really work out all the balancing math from the first principles!
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Re: balancing crank - turning counterweights

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Like for example I think there are 3*2*1=6 alternative crossplane V8 crankshafts but I think in practice we only see two kinds in production. Why?
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Re: balancing crank - turning counterweights

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Here’s another one. 100% balance factor balances the primary vibration in a V-twin. So I’d assume that under but close to 100% balance factor for each V8 throw would also minimize bearing loads if mains journals were flexible. But then how does this setup balance the whole V8 crankshaft that has the rocking motion? Options include: Perfectly, not at all unfixably, or well with added weight externally? And is this 100% balance V2 (not the rocket but 90-degree V-twin) thinking something that doesn’t show up on a V8 balancer as it’s only measuring the relatively rigid coupled system of four V-twins?
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Re: balancing crank - turning counterweights

Post by MadBill »

Would it help to cut a scrap crank into 4 pieces and mess with them separately, then reproduce the resultant CW sizes and locations on the real deal?
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Re: balancing crank - turning counterweights

Post by SchmidtMotorWorks »

Sorry for the delay in replying.

The first thing to look at when evaluating a crank for balancing is the symmetry of the first and last (end) counterweights across the plane of the rod pin.
A perfect counterweight layout for bearing loads will have end CWs that are perfectly symmetrical to the adjacent rod pin.

If when you spin the crank up in the balancing machine and it indicates a heavy spot on the end CWs that could be reduced and centered by narrowing the fan on the widest side, that should be the first thing to do.

If the balancer indicates a need for change that will make the end CWs less symmetrical, look at the 2nd, 3rd and center CWs for places to change.
If that requires adding heavy metal to the 2nd or 3rd CW, if you can afford to do it, do that.

If you can make a change to any end or center CW that makes it more centered and opposite of any adjacent rod pin, that will reduce bearing loads on the adjacent journal. The 2nd and 3rd CWs should be mirrored on the split of the first and second rod pins as they are both working together to reduce loads on the 2nd main journal. If you see a big deviation from that , understand it was done to make the crank inexpensive to balance, not reduce bearing loads.

The idea behind non-symmetrical end and center CWs that were prevalent in before the mid 1980's was to reduce cost of manufacture. In some design, the shapes were easier to forge and easier to balance.

In cases where the bob-weight is heavy, the ideal balance job places the heavy metal in the end CWs directly opposite of the rod pin.
On the 2nd and 3rd CWs, the heavy metal should be placed on the 45 degree split opposite and between the two rod pins.

This is how high-end cranks are balanced and may not be economical for everyday hardware. It is worthwhile to understand these concepts and apply any of them that you can.
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Re: balancing crank - turning counterweights

Post by SchmidtMotorWorks »

MadBill wrote: Fri Jan 24, 2020 2:30 pm Would it help to cut a scrap crank into 4 pieces and mess with them separately, then reproduce the resultant CW sizes and locations on the real deal?
It will give you understanding but if you make a layout that would be for 4 pairs of v-twins, the crank will be unnecessarily heavy. Better to merge the work of the 2nd and 3rd, CWs.
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Re: balancing crank - turning counterweights

Post by ptuomov »

Thank you for all the feedback. I'm responding without quoting because I find the resulting quote windows annoying.

I think that a 90-degree V-Twin with a single crank pin and long rods should be balanced with 100% rotating and 100% reciprocating weight in the piston. I am guessing that secondary vibrations for short rod engines make the balance factor (the % of reciprocating weight) somewhat less than 100%. In any case, if we model the V8 crankshaft as four V-Twins with flexible plug in the mains, we'll get very big counterweights. I am not sure, however, what happens to the rocking motion if each V8 crank throw is fully counterweighted with a 100% balance factor and then put into the regular V8 balancing maching with 50% balance factor bobweights. Would it show perfect balance, or would more weight still have to be added to the end counterweights (or externally to flywheel and damper)?

In terms of my crankshafts, the front end counterweight is not exactly behind the 1/5 crank throw. It has maybe 1/3 of the weight on the trailing side and 2/3 on the leading side, relative to the plane that goes thru both the 1/5 crank journal centerline and the main journal centerline.

If you look at the crankshaft from the front such that your line of sight is parallel to the line going thru the mains, you can see the 1/5 and 2/6 crankpins forming a 90-degree angle (duh, obvious). If you bisect that angle with a line and continue that line to the counterweight, the counterweight number #3 is about centered relative to the line. In other words, the counterweight #3 seems to be placed half way between balancing 1/5 and 2/6 crank throws. If you think this thru with the mental model that assumes flexible mains, I am thinking I would like to have the cw #3 aligned more behind 2/6 crank throw and less behind 1/5 crank throw. Now, the mains are pretty rigid and other parts of the shaft flex too, so maybe that four-V-Twins doesn't capture everything.

Given all this, I'm considering adding heavy metal to counterweights #3 and #6 (4 and 5 are missing since this is a six counterweight crankshaft) on the side that lines them up behind 2/6 and 3/7 crank throws better. Then, offsetting this added weight by cutting the end counterweights from the side that is currently less aligned with the adjacent crank pin. Maybe about half the added mass, eyeballing the lengths? If the math is done right, this should result in a balanced rotating crankshaft with the stock bobweights. Then, considering the lighter pistons and rods and resulting lower bobweights, I'd further cut the fan angle of the end counterweights from both sides to get the crankshaft to balance.

Does this make sense or have I misunderstood something?

Circles are locations yo which one might be able to physically install heavy metal:
E47B3058-FA5F-4538-9637-85667996D30D.jpeg
64502141-3FD8-463D-9954-FA31741F4629.jpeg
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Re: balancing crank - turning counterweights

Post by SchmidtMotorWorks »

Your crank balancing plan sounds good.

With regard to the v-twin Bob weight, the 100 + 100 will have a lot of vibration in one axis and very little in the cross plane.
A 100 + 0 will have the same result with the axis rotated 90 degrees.
A 100 + 50 divides the vibration between the two axis.
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Re: balancing crank - turning counterweights

Post by ptuomov »

SchmidtMotorWorks wrote: Mon Jan 27, 2020 4:29 pm With regard to the v-twin Bob weight, the 100 + 100 will have a lot of vibration in one axis and very little in the cross plane.
A 100 + 0 will have the same result with the axis rotated 90 degrees.
A 100 + 50 divides the vibration between the two axis.
Doesn’t that depend on the rod length? I was under the impression that with an infinitely long rod, the 100+100 would balance a 90-degree V-twin perfectly. Perhaps my understanding is incorrect. And with rod ratio of about 2, the secondary vibrations are something like 10-15% of primary vibration amplitudes. But this again with the caveat that I may be mistaken.
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