Crankshaft balancing question
Discussion
I have just got back my crankshaft from balancing, and to check the new cluch, I asked another company to put it on the balancer. Unfortunatly they have found 20gr runout in the shaft. Before getting it corrected, I would liket o know if it is a nasty misstake or the cause of using counterweights - or the worst, the first company balanced the crankshaft with the flywheel on...
camelotr said:
or the worst, the first company balanced the crankshaft with the flywheel on...
Depends if the crank was designed to be externally balanced, in which case it must be balanced with the flywheel (and I think) the front pulley/damper in place.Ideally you need to speak to the original balancing company.
I was puzzled by the term "counterweights" but I'm just going to say what needs to be done regardless of what the actual question meant.
Straight six cranks don't require anything bolted to the big end journals like cross plane V8s or to have the flywheel fitted like some V6s which use a flywheel offset weight to counterbalance the crank. The crank itself should be in perfect balance on its own and the flywheel and front pulley are also symmetrical on this engine and therefore in balance either on their own or when fitted.
If the crank itself is not in balance then either the first company has cocked it up or the second one is wrong but it needs to be fixed before anything bolted to the crank can be balanced because you don't want a crank imbalance being compensated for by another imbalance in something else.
Personally I would have left the crank well alone because factory balancing will be perfectly adequate and in fact probably not easy to improve on. I never have cranks rebalanced unless they've been messed about with in some way. You have to remember that at the factory they would have been using really good balancing machines tailored for each application and working to specified limits set by the designer. Chances are that the average engine reconditioner has a balancer so old and knackered it has an inherent error and they might even be making perfectly good cranks worse. You can read the horror story I got involved in about so called "professional" flywheel balancing actually making things about 10 times worse than OE spec on my website.
http://www.pumaracing.co.uk/flywheelbalance.htm
Sounds like your crank has gone the same way. No great surprise given the awful jobs everyone you use out there seems to do for you. You need to get a better grip on what it's worth doing on a stock or near stock engine rebuild and what's it not worth doing. Crank balancing on this engine was just a complete waste of money and would have achieved nothing detectable in the running of the finished engine anyway. To waste that money and also then end up with a problem that didn't exist to start with doubles the indignity though.
I can teach you how to make your own flywheel balancer if you want but you really need your own lathe first or the use of one. In fact if you stopped paying incompetent engineers to cock up every job they do for you you'd be able to afford one by now. Something like my Colchester Student will do just about any job you'll ever need it to. You can get by with a little Myford or similar for some stuff but not lightening flywheels which the gap bed Student excels at. Always buy a gap bed lathe if you can. It doesn't have to be modern or even in very good condition to do good work. A good machinist can manage perfectly well on a lathe with wear in it. My clapped out old thing has about 2mm play in the cross feed screw but it doesn't stop me working to under a thou precision on it. You just need to know how to work around the backlash in the system.
Straight six cranks don't require anything bolted to the big end journals like cross plane V8s or to have the flywheel fitted like some V6s which use a flywheel offset weight to counterbalance the crank. The crank itself should be in perfect balance on its own and the flywheel and front pulley are also symmetrical on this engine and therefore in balance either on their own or when fitted.
If the crank itself is not in balance then either the first company has cocked it up or the second one is wrong but it needs to be fixed before anything bolted to the crank can be balanced because you don't want a crank imbalance being compensated for by another imbalance in something else.
Personally I would have left the crank well alone because factory balancing will be perfectly adequate and in fact probably not easy to improve on. I never have cranks rebalanced unless they've been messed about with in some way. You have to remember that at the factory they would have been using really good balancing machines tailored for each application and working to specified limits set by the designer. Chances are that the average engine reconditioner has a balancer so old and knackered it has an inherent error and they might even be making perfectly good cranks worse. You can read the horror story I got involved in about so called "professional" flywheel balancing actually making things about 10 times worse than OE spec on my website.
http://www.pumaracing.co.uk/flywheelbalance.htm
Sounds like your crank has gone the same way. No great surprise given the awful jobs everyone you use out there seems to do for you. You need to get a better grip on what it's worth doing on a stock or near stock engine rebuild and what's it not worth doing. Crank balancing on this engine was just a complete waste of money and would have achieved nothing detectable in the running of the finished engine anyway. To waste that money and also then end up with a problem that didn't exist to start with doubles the indignity though.
I can teach you how to make your own flywheel balancer if you want but you really need your own lathe first or the use of one. In fact if you stopped paying incompetent engineers to cock up every job they do for you you'd be able to afford one by now. Something like my Colchester Student will do just about any job you'll ever need it to. You can get by with a little Myford or similar for some stuff but not lightening flywheels which the gap bed Student excels at. Always buy a gap bed lathe if you can. It doesn't have to be modern or even in very good condition to do good work. A good machinist can manage perfectly well on a lathe with wear in it. My clapped out old thing has about 2mm play in the cross feed screw but it doesn't stop me working to under a thou precision on it. You just need to know how to work around the backlash in the system.
The first firm to do the job is one of the best reputed companies in my country, building very competitive race cars.
I have ordered a full balancing, soo the crankshaft first, the flywheel second etc.
Unfortunalty they have mounted the cluch 180 degrees wrong, as while they lightened the flywheel they deleted the marking - amateur misstake. For this reason they could not balance the cluch assembly and gave it back, as an item they thought to be damaged.
Thats why I had to find another company to refit the cluch and check it. While there, I have asked them to check the other components.
There we found the problem with the crankshaft.
At the end of the day, my suspect proved to be true: to save time, they have balanced the crankshaft with the flywheel on.
A would never find it out if they could balance the clutch.
Now the whole assembly is set right - I have been there and followed the work, which was carried out by a balancer specialist rather than an engine reconditioner. It was done to very tight tolerances.
BTW it was well worth balancing the crankshaft, as it was a way off-balance. I dont know how they did balancing in the late 50s, but I suspect their machines limited their possibilities.
Why I run into theese problems? Donno. I never go for cheap solutions. Maybe, I am not quite agressive type, and people think they handle me easier. Maybe. Or maybe I am just unfortunate.
Nevermind. Mission completed, engine assembly going on!
This time I check every tolerances according to Your guidence - and pay extra care on the damn honing pattern also. Up to this point all clearances were acceptable, well within factory data.
I have ordered a full balancing, soo the crankshaft first, the flywheel second etc.
Unfortunalty they have mounted the cluch 180 degrees wrong, as while they lightened the flywheel they deleted the marking - amateur misstake. For this reason they could not balance the cluch assembly and gave it back, as an item they thought to be damaged.
Thats why I had to find another company to refit the cluch and check it. While there, I have asked them to check the other components.
There we found the problem with the crankshaft.
At the end of the day, my suspect proved to be true: to save time, they have balanced the crankshaft with the flywheel on.
A would never find it out if they could balance the clutch.
Now the whole assembly is set right - I have been there and followed the work, which was carried out by a balancer specialist rather than an engine reconditioner. It was done to very tight tolerances.
BTW it was well worth balancing the crankshaft, as it was a way off-balance. I dont know how they did balancing in the late 50s, but I suspect their machines limited their possibilities.
Why I run into theese problems? Donno. I never go for cheap solutions. Maybe, I am not quite agressive type, and people think they handle me easier. Maybe. Or maybe I am just unfortunate.
Nevermind. Mission completed, engine assembly going on!
This time I check every tolerances according to Your guidence - and pay extra care on the damn honing pattern also. Up to this point all clearances were acceptable, well within factory data.
Edited by camelotr on Thursday 14th April 21:53
camelotr said:
BTW it was well worth balancing the crankshaft, as it was a way off-balance. I dont know how they did balancing in the late 50s, but I suspect their machines limited their possibilities.
How do you know if the crankshaft was really originally out of balance if the first company messed things up? I seriously doubt if it was and they could balance crankshafts perfectly well in the 50s and indeed long before that.camelotr said:
Your the specialist, soo I belive what You say.
Than it must have been the first company to mess up things.
You've pretty much already said they did. You said they lightened the flywheel which would have put that out of balance if the material was not removed evenly then they balanced the crankshaft with the unbalanced flywheel on it as well as cocking the clutch up - assuming I have understood you correctly. These are race engine specialists? They sound like complete morons.Than it must have been the first company to mess up things.
Anyway, to balance flywheels is very easy. You just need two ordinary ball bearings, I use 1/2" I/D ones, and an accurately turned or ground 1/2" shaft about 6" long to join them. A bit of silver steel will do nicely. Actually I made a 3/4" diameter shaft with a 1/2" diameter machined at each end so it was sturdier but if so you need to turn this very accurately and concentrically. Or just use 3/4" ball bearings and 3/4" shaft.
Then for each type of flywheel you need to make on the lathe an adaptor with a 1/2" hole through the middle (3/4" in my case) and an O/D to fit at a light push into the flywheel central hole. This adaptor must be machined exactly concentric so I bore the hole in it rather than just drill and ream. Push the adaptor into the flywheel, push the shaft through the adaptor and put a bearing on each end of the shaft. Rest the complete assembly on two level rails or blocks with the flywheel hanging between them. I use a couple of old cylinder heads resting on the bed of my mill with two old main bearing caps upside down for the ball bearings to go into so the whole lot doesn't roll off and end up on the floor!
The flywheel is now suspended true between the ball bearings and will just turn until the heaviest spot is facing down. If the bearings are clean and dry there will be very little friction and you can easily balance to 1 gram.
There will always be a little friction so the flywheel will tend to stop just short of the true position so you find this roughly on the first try then you turn the flywheel back a little one way, let it come to rest again and mark this spot, turn it back a little the other way, let it come to rest and mark this spot also. The true heavy point should be midway between those marks.
Then you get some plasticene and stick a ball of this on the opposite side of the flywheel and keep adding plasticene until the flywheel is balanced and will stay still wherever you set it. This gives you an idea of how much weight you need to drill out. I use a 10mm drill usually. You need to make very sure before you drill that there isn't a bolt hole nearby or on the other side of the flywheel. Sometimes you need to drill in two places either side of the heavy point. A little practice and it becomes very easy.
Drill a bit of metal out, test the flywheel again, repeat. I can usually get it balanced in two or at most three tries. With a 10mm drill every 1mm you drill out weighs about 0.6 grams. Very quickly you get the feel for how deep the hole needs to be in each case. If you take too much off then then drill a little hole on the opposite side.
The level of accuracy you can get with this simple device is very high if the ball bearings are clean. I keep mine sealed in a plastic bag so they don't get dusty, give them a blow out with the air line before use and check they spin freely and perhaps a squirt of WD40 every now and then to stop them rusting. No oil though or it will create friction.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff