balancing crank - turning counterweights

R.Brown · Post by **R.Brown** » Thu Dec 25, 2008 4:12 pm

Thanks again guys, and for sure Dave K for weight info.

Happy Holidays!

rb

Post by **SchmidtMotorWorks** » Thu Dec 25, 2008 11:06 pm

Jon
Define "fan"?
Thanks

Remove material off the leading and or trailing edge of the counterweight not the outside radius.

If it isn't knife edged, it is a good improvement to make at the same time.

Generally I mill until within about 25, then use a body grinder to get it within a few then a resin roll or drum sander to finish it by rounding off edges.

Wolfplace · Post by **Wolfplace** » Fri Dec 26, 2008 1:24 am

SchmidtMotorWorks wrote:
Jon
Define "fan"?
Thanks
Remove material off the leading and or trailing edge of the counterweight not the outside radius.

If it isn't knife edged, it is a good improvement to make at the same time.

Generally I mill until within about 25, then use a body grinder to get it within a few then a resin roll or drum sander to finish it by rounding off edges.

=
Thank you Jon

Happy Holidays

andyf · Post by **andyf** » Mon Jul 06, 2009 3:34 pm

I have an additional question on this topic and rather than starting a new thread I figured I would add on here.

I have a crank here that is about 350 grams heavy on the balance machine. As I understand it, what that means is that 350 grams needs to come of each end. So in other words, the crank is actually 700 grams heavy?

So I believe I need to remove 350 grams from each end. I can turn the end counterweights down in a lathe or I could hang the crank in the mill and reduce the fan width such as was recommended. But in either case it sounds like I need to remove 350 grams from each end.

I guess I don't really understand what the balance machine is telling me. There are four bobweights so if the crank is 350 grams heavy why isn't it 350x4 heavy?? The balance machine must do an internal calculation that splits the bobweights into front and back? I suppose that would be easier for an operator to understand.

machine shop tom · Post by **machine shop tom** » Mon Jul 06, 2009 4:10 pm

andyf wrote:I have an additional question on this topic and rather than starting a new thread I figured I would add on here.

I have a crank here that is about 350 grams heavy on the balance machine. As I understand it, what that means is that 350 grams needs to come of each end. So in other words, the crank is actually 700 grams heavy?

So I believe I need to remove 350 grams from each end. I can turn the end counterweights down in a lathe or I could hang the crank in the mill and reduce the fan width such as was recommended. But in either case it sounds like I need to remove 350 grams from each end.

I guess I don't really understand what the balance machine is telling me. There are four bobweights so if the crank is 350 grams heavy why isn't it 350x4 heavy?? The balance machine must do an internal calculation that splits the bobweights into front and back? I suppose that would be easier for an operator to understand.

Most crankshaft balancers will read out in two planes -- rear counterweight and front counterweight (or some, as our Hines, can be programmed to read on other counterweights instead). So the screen or readout will specify the amount of imbalance per designated measuring point (counterweight) and the specific location of the imbalance.

tom

andyf · Post by **andyf** » Mon Jul 06, 2009 4:37 pm

Okay, that must be the answer then. The machine is indicating that we're heavy on each end and we know we're at least 300 grams heavy but we're actually off scale. That is one problem with this type of operation: You have to be in the ballpark before the machine tells you what to do.

rustbucket79 · Post by **rustbucket79** » Tue Jul 07, 2009 1:10 am

If my balancer starts giving me questionable results, or just won't give me a reading, I'll start by spinning the crank at a slower speed. (normally happens if someone convinced me to internal balance a GM 454 crank

)

In your case if the machine is asking you to remove 300+ grams per end, you could always hang an additional 200 grams on the front and rear bobweight to see if the results make sense. (IE now it only needs 100 grams removed to balance) It sounds like you're new to balancing, so take your time, prove you're going in the right direction BEFORE you start machining/drilling, and good luck! Better to take an extra hour or two proving your next move is the correct one rather than making an expensive paperweight.

What crank are you working with, and what is your bobweight?

Post by **SchmidtMotorWorks** » Tue Jul 07, 2009 3:42 pm

So I believe I need to remove 350 grams from each end. I can turn the end counterweights down in a lathe or I could hang the crank in the mill and reduce the fan width such as was recommended. But in either case it sounds like I need to remove 350 grams from each end.

Since the crank is basically rigid, the limitation of balancing is that it can only tell you the out of balance of the entire crank.

You have to use some estimation to know where the crank is heavy. The first clue is the thickness of the counterweights. Since there isn't enough room in the block to make all the counterweights the needed thickness, the end counterweights are made extra thick and offset to compensate for the thin counterweights.

I have posted this before in detail; but the goal of balancing a V8 crank is to end-up with the end counterweights symmetrical across the plane of the first and last rod pins. You can't always accomplish that though, it is just a goal. But never reduce the thin counterweights while the end counterweights are doing the work of the thin counterweights.

In almost all V8 cranks you can't go wrong by balance by reducing the fan of the end counterweights to bring it into balance.

Similarly, you will almost always increase bearing loads by turning down the thin counterweights.

andyf · Post by **andyf** » Tue Jul 07, 2009 5:04 pm

Thanks for the info Jon. It is a lot more difficult for me to set up the mill to reduce the fan size of the end counterweights than it is to use the lathe so I'm going to go with the turning opertion.

I think I'll just turn down the diameter of the two end counterweights though rather than turn all of the counterweights down. It is easy to set up to do that and it seems like a better way to balance the crank.

By my calculations I need to take 0.250 off the radius of each end counterweight to take 350 grams off of each end. One complication is the counterweights are currently cam ground so I'll have to just take a guess at when I hit the .250 average cut depth.

Post by **Dave Koehler** » Tue Jul 07, 2009 8:05 pm

Andy,
True up the front and rear CW and respin before going further. The math works out fairly well if you have a true curve and not cam. You then should be able to hit it close as you dare with the lathe work.

fastvette · Post by **fastvette** » Tue Jul 07, 2009 8:55 pm

I have talked to Callies about one of their cranks that need over 300 grams removed from each end. I was told to lathe down all the counter weights. But after reading this it may not be the best way to do it.

Randy

andyf · Post by **andyf** » Tue Jul 07, 2009 9:12 pm

Dave Koehler wrote:Andy,
True up the front and rear CW and respin before going further. The math works out fairly well if you have a true curve and not cam. You then should be able to hit it close as you dare with the lathe work.

That is exactly what I'd do if the lathe was next to the balance machine. But, the engine builder doesn't have a lathe big enough to turn a crank so I have to run back and forth between shops that are 20 miles apart.

My original mistake was to not have the crank rough balanced by the vendor when I purchased it. I've built enough of these engines to know where the bobweight is going to be that I should've just told the crank supplier to leave it 50 grams high so the engine builder could finish it off with a dimple or two. My mistake and now I have to pay for it by running all over town with my hair on fire.

Post by **SchmidtMotorWorks** » Tue Jul 07, 2009 11:22 pm

350 is a lot to take off if the cam cut is deep. Keep an eye on how much is being cut on each side of the cam cut or the you may get a big surprise.

250 on the radius sounds like a lot to do in one cut but 20 miles is long way to drive. It's one of those Clint Eastwood moments,,,"do you feel lucky"

Wolfplace · Post by **Wolfplace** » Tue Jul 07, 2009 11:34 pm

Andy
I know making two or three trips may be a pain in the ass but having to install Tungsten is even less fun
I would take no more than about a third, maybe half of what you feel necessary the first time to see what you have

When you cut the counterweights in the lathe you are most likely going to rotate the point of imbalance & you do not want to rotate it right off the counterweight ,,,, it is a real bitch to remove weight where there is no material,,,

andyf · Post by **andyf** » Wed Jul 08, 2009 7:34 pm

I took .250 off the radius on each of the end counterweights and then took it back to the engine shop. It still needed another 100 grams or so off of each end but they did that with a couple of drilled holes. The holes wanted to be drilled near the center of the counterweights so the balance location stayed favorable. Two more passes on the lathe and I would've been perfect, three more and I would've been too far. So my guesstimate turned out okay.