After reading a lot about crankshaft design, and their counterweights I decided to put all the myths around the counterweights to bed. A lot of very high up people told me that some of the reciprocating weight of the rod and piston is also taken into account on these engines when picking the counterweights weight. That is that the counterweight is heavier than the journal its trying to balance in order to counter act the weight of the piston and rod also. Well, its not. Its the very same weight as the journal opposite it that its there to balance. The weight of the the reciprocating parts does not come into it at all, not even the slightest bit. Ill let the pictures below do the talking. I stuck my name on them because this took me a hour to do and and don't want these pictures showing up somewhere else with someone else claiming they did it as happened me before. I cut the crank right down the centre, its a 2.0L cast crank. After I checked a few things after with the dial gauge I realised my cut was not exactly in the centre which is why the measurements aren't exactly the same, journal, and counterweights. There's 100grams between the weights on 2+3 Which is exactly how much the volume of metal adds up to that my cut was off. Keep in mind 50grams is the same weight as a bic cigarette lighter, so even at that 100grams is nothing compared to the combined weight of one piston, rod, assembly and its bits which adds to over a kilo! Anyway the Pictures, there pretty easy understand I hope> Showing cut centreline accuracy> Cut into the counter balancing pairs> The weights> Close-up of the all important journal radius at ends of journal> Interesting weights aren't they.. Another thing which also came to me when doing this after weighting is people that knife edge these cranks. Im not going to put up a picture of a vw knife edged crank because I don't want to cause war with the 'tuner guys', you'll find loads on google. They always profile the counterweights to a knife edge, following the angle opposite it either side of the big end journal. If you look at the above figures again this can only do one thing, throw the crank miles out of balance per crank web as both halves(Journal side, Weight side) are exactly the same weight to start with. The only way to recoup this weight lost and to bring the crank back into balance is to bore out the big end journal, which I have never seen done on any of them, 'yet'. So.......'my opinion' on knife edging a Vw crank is that, ok, it may reduce the rotating mass AND cut through the oil mist better, but after doing the above, Im concerned about the material removal on just one side causing internal stresses in the crank, and main journals. If you look below you will see Cosworth bore the big end journals, mainly for weight over all Id guess, but I bet both halves would weigh the same taking into account that it is also knife edged. I know the above may not appeal to everyone, but it does to me as I now know for sure about the above journal/counterweight relationship, this solid info is not to be had from anyone that I have spoke with. Brian.G
Another excellent piece of work there Brian. This shows how accurately designed the parts are from the outset. For the weights of each dissected section to be so close to it's corresponding section is quite remarkable. The combined weight of the 2 piston / con-rod assemblies going up are balanced by the other 2 going down, so they will be the same overall weight and effectively balancing themselves against each other. So does the cylinder which is on it's power stroke have any affect on balance, or does it only cause a harmonic issue with it's attempt to twist the crankshaft at it's own big end journal? Presumably the overall mass of all of the components is calculated to try to dampen these resonant frequencies down to acceptable levels.
Cheers Brian, well that's fantastic information, settling any debate. You've got some metal saw - it's had 2 engine blocks and done a crank lengthways! Mega! Plenty more before that, I'm sure! Whilst I appreciate you want to sit outside the debate on knife edging, I'm going to point back to the harmonic thread bump of a few days ago for those who wish to consider or comment further on that score: http://www.clubgti.com/forum/showthread.php?t=207823&page=4
Your welcome Guys, corrected the figures in the pictures too. I must read that thread now Chris, Ive done a lot of reading on them the last while.
This is an interesting reply without much explanation. I would not have thought a load of stationary components (effectively an inanimate object) would need to be balanced at all. So what changes when it starts? It must need to be balanced to a certain extent so it would be of interest to hear what point is being missed?
I haven't missed any point I have been told for yrs upon years that the counterweights per bank hold some of the weight to counteract the piston and rod as well as journal weight. I know all the maths end of things once its started, but above, Im simply stating that the counterweights are the same weight as the opposing journals when its left on a desk, which was not was I was told by the 'pros'. They stated that the counterweights contain some of the reciprocating weight also(when left on a desk), and another man high up explained that counter weight picking was ''a black art'' to which he later told me that nobody actually knows how it should be calculated, or how its arrived at I believe this guy, as he was designing engines for GM for I think 40yrs! Edit, Im starting to realise why its hard calculate, the reciprocating weights will all be different anyway depending on rpm and throttling opening. Lets not forget all 4 piston and rod combos ''weigh'' different anyway as they are all on different cycles/strokes, with different pressures above them.(ontop of them trying to shoot out through head on the exhaust stroke at elevated rpm while at the same time another is getting fully compressed!)
After measuring the crank weights in respect to their journals they are counterbalancing I found them to be the same. I dont want anyone saying that changes when the engine is started, all I have expressed by saying the above is that both halves of crank(journals, weights) weigh the same. Meaning, you could spin the crank on a lathe to 20,000rpm and it would be fine. Fine both statically and dynamically. Nowwwwwww, if you knife edge the crank it still comes out fine if you were to check it statically for balance because there are the same amount of new knifed edges above and below the crank centre line. BUT, if you now check it dynamically its miles off as the weights no longer weigh the same as their journals opposite. This can only be bad for internal stresses and main bearing wear. The only way to do this correctly and keep the crank dynamically balanced is to bore the big end journals, either in a bit, or all the way through(mind the oil way) You then calculate the weight of the material you have removed and this is what you can take off the counterweights in order to knife edge(using the best profile you can in respect to what you have removed from the big end journal centre.) This in my opinion is the best way and only way to knife a crank correctly. AND If you look at extremely expensive cranks in high end cars they do just this, they dont bother with the ''shave your face cool as hell look'' , they simply round the leading edge and knife the trailing edge. This is the best profile. I can also confirm it to be because Ive made a lot of tillers for sailing boats and the profile of them are exactly the same for reduced drag. My god that was a long rant, does it make any sense at all? Edit, for those worried about oil mist and losses, do the above profile, and polish the bejasus out of the crank to cut down on the irregular surface area that the oil can cling to. Ive another rant on all I know about torsional dampers coming later, god help us!!
You have neglected to consider the distance of the mass from the centre, just weighing each half means nothing about the balance
How so then? Anything that spins need to be balanced to a certain extent. eg washing machine drum, wheel and tyre on a car. There will be a tolerance of acceptable "unbalance" depending on application. This forum is generally a helpful place where people share their views and knowledge, so it would be polite to engage a little with your insight rather than this somewhat negative post.
Balance is to do with moments, force x distance from the centre Out of balance tolerance is generally quoted as g.cm or similar, so something balanced to 10g.cm is something perfectly balanced with 1gram mounted 10 cm from the centre, or 10grams 1 cm from the centre.... etc Just to be helpful, for the crank you would need to find out how far the centre of mass of each half is from the centre, multiply this by the mass and compare
Taking a piece of cast steel 'rod', 30 centimetres in diameter, solid. Whirl that up to 20,000 rpm, and it should be perfect? No harmonics? What does that give us? Perfect moments. So what's being missed here? I get the point about counterweights: the whole lot needs to be equal on both sides, on a force x distance basis. Easy to see. Clearly the design suggests this is v. hard to do. Is that (plus rods thumping down, twisting things) what leads to the actual source of our harmonics?
Cheers mikeyc, thanks for that. I understand what you are saying there about the weights and distances from the centre. Similar to a longer lever needing less force than a shorter one would require to tighten a bolt to the same torque. The cranks must be designed and manufactured to have close to equal moments though and the drillings one sees must be for dynamically balancing them. Which is pretty much the only way to check for equal moments? Maybe these last few posts should go to the other thread.
So the harmonics come into play due to the power strokes twisting the crank in localised areas, and the frequencies that this twisting / untwisting (oscillations) creates needs controlled or the constructive and destructive energy at certain rpm and loads this creates gets out of control?
Indeed, I hear you and understand that, I do have to start somewhere though, I do find the weights very interesting though, Ignoring all the moments for a minute() Lets just say that both halves are very near weight-wise no matter where there mass centres are. The counterweights pretty much mimic the journals until the outer edge(both are the same on the same outer radius too) Its on the outer 20mm that the most mass lies on the counter-weight, similar to the journal. This is vague I know as yet, but keep in mind I was told/read everywhere that the counter-weight holds 25-35% of the reciprocating weight also, this would be approx 260g. I dont think that could be in there also no matter where the mass centre lies in the weight, but I could be wrong. I do intend finding out though as this info does not exist, even if I have to cut it into microns, weigh every one of them, stick them to a steel rule and then spin them about. Brian.
danster, sounds about right, click first two links, first one is fairly clear and helpful - even has a bit on race engines Brian, you would be better to cad the bits up, or:
