Crankshaft Counterweights relation

Discussion in 'Engines' started by Brian.G, Apr 19, 2010.

  1. RobT

    RobT Forum Junkie

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    if you look in the Burton catologue, there are narrow bearing versions of the BDA/BDG series engines for precisely this reason - extracting out every last hp and sacrificing longevity
     
  2. Mikey C Forum Member

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    You can shave as much as you want off the counter weights, as long as you take off the same off all 4. The crank will still balance statically and dynamically.

    Consider the crank in two halves, without any counter weights. Each half is no where near dynamically balanced but it is statically balanced. Put them back together (still without counter weights) and the imbalance in each half is equal and opposite to the other so they cancel each other out so it is overall dynamically and statically balanced.

    The forces that go across the whole crank to balance these moment cause bending. So the counter weights are added to reduce the dynamic imbalance of each half, closer to the source so the crank overall bends less.

    Put simply, shaving equal weight across all of the counter weights will not affect the overall balance, but it may cause more bending in the crank leading to bearing damage.

    I had to dig out the text books today, if anyone wants to fully understand the subject of engine balancing, or torsional vibration for that matter - i recommend you do the same :-)
     
  3. Brian.G

    Brian.G Forum Member

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    Wrong:thumbup:
    If you want me to demo what happens if you do I will, I cant believe you think you can actually considering that the 4 counterweights on both 2+3 then weigh far less than journals 2+3 instantly causing hop in the centre of crank. Your thinking in static terms not dynamic.
    I mocked up a crank in technical lego(yes I kept it) to display exactly this. I supported crank both ends just. I weighted the 4 journals and 8 counters the same(four pairs), it spun up dead straight up to I dunno, the max of the motor with the lego. I then lightened just the counterweights by approx 15% and spun it up. The whole lot whipped originating from the centre.
    With the counters lightened the crank would be fine statically if placed on blade rollers and spun by hand, as in it wouldn't stop in the same spot each time, or a spot 180 degrees from the first place of rest.
    But dynamically its a different ball game as displayed by the good old lego.

    The fact that the lego crank was not supported at the 3 points in-between the ends is irrelevant, and possibly better in order to display the hop/whip.

    Edit>

    ''The forces that go across the whole crank to balance these moment cause bending. So the counter weights are added to reduce the dynamic imbalance of each half, closer to the source so the crank overall bends less.''

    You totally contradicted yourself there. ''Counter weights are added to reduce the dynamic imbalance of each half''
    And you then say that you can chop as much as you want off them and it will have no effect? But in doing so it may bend more? going by what you said they were there to stop bending in the first place?

    I suggest you put down the books and make a model to see what happens in real life.
     
    Last edited: Apr 23, 2010
  4. mark25 Forum Junkie

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    Have a look at this statically balanced situation:

    [​IMG]

    It is balanced, but if you sawed it down the middle at the current fulcrum, the weights would be quiet different! This demonsrtates the futility of statically weighing two halves of a crank.

    What's also interesting, is that while the structure is currently statically balanced, if you spun it around its current balance point, the dynamic balance would be quiet awful.
     
  5. alexisblades99 Forum Member

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    Although he has demonstrated that the centre of mass of each journal and it's counterweight is equal in the case of the crankshaft halves. I think the point about drilling the big end journals to ensure this remains the case after knife-edging is a valid one, borne out by the fact that cosworth have done the same thing (and eurospec too by the looks of it, if brookster's crank is anything to go by). It's really important that each section of crank between each pair of main bearings is balanced, both statically and dynamically, rather than relying on the imbalance of one section to counteract the imbalance of another. Which is the point Brian's making here.
     
  6. Mikey C Forum Member

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    Brian, your lego broke due to bending in the middle, exactly as i have described. If the lego was stiff, it would have balanced dynamically and statically without the counterweights.

    I stand by what i have written, take it or leave it, only trying to help. I design engines with theory, not with lego.
     
  7. Mikey C Forum Member

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    I dont think you've understood what makes something dynamically unbalanced. not a particularly good analagy

    sorry if i just sound like i'm disagreeing with everyone, just trying to help
     
    Last edited: Apr 23, 2010
  8. A.N. Other Banned after significant club disruption Dec 5th 2

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    Yep, please (all) keep this as a good discussion.

    It's a good topic and all we're enjoying trying to understand it & it isn't easy :thumbup:
     
  9. Brian.G

    Brian.G Forum Member

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    If you think bending in the middle or ANY stress that is present or amplified by lightening the weights is ok then thats ok with me:thumbup: The crank is under enough stress by adding to it even more.
    I dont on the other hand, and its looks as thought Cosworth, Ferrari, or Eurospec dont think its ok either.
    I did all the above to demonstrate that what I had been told was wrong, that is, that ''the counterweights do not weight the same as there journals opposite and also hold some mass for the piston and rod too''.

    Dont dis the Lego, its a very valuable tool for real life testing.:thumbup:
     
  10. Brian.G

    Brian.G Forum Member

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    Exactly, bending! The crank is indeed supported in real life, but that does not mean these stresses go away. Ive also noticed something from trawling the net, a lot of engines that spit rods have these style knife edged cranks;) All failures go unexplained.
     
  11. Mikey C Forum Member

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    Yeah, i never said it was ok, just that it wont affect the balance. The example you mention does back up that it is not a good idea - due to bearing damage perhaps from bending the crank.

    With regards to the counterweights balancing some of the rod and piston mass, the engine is split into reciprocating and rotating masses. The primary reciprocating forces are balanced by having two going up, while two go down. This isn't exactly equal but you would need balancers rotating at 2x engine speed to balance the secondary forces. The rotating forces are balanced by symmetry across the engine, but the bending forces are reduced by having counterweights (as described above).

    Interestingly (or not) the reciprocating mass is considered to be the piston etc and some of the rod, and the rotating is the crank and the rest of the rod. How much of the rod depends how you look at it, some sources say 2/3rds are rotating, some that its upto the centre of mass.

    So.... about 2/3rds of the rod mass (xdistance) should be in the counter weight. I don't know if the accuracy of your method covers this, or if VW didn't do this for whatever reason. I suspect the size of the counter weights may be limited by what will fit in the space, vs bending forces on the crank.

    I always preferred technic or meccano to be honest, my sisters all had lego :p
     
  12. mark25 Forum Junkie

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    In this thread you're trying to prove that counter-weights have no effect on reciprocating components, but when it suits you, you use that very arument, but in opposite, to prove your myth-busting point, see above.
     
  13. Brian.G

    Brian.G Forum Member

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    Your sisters? Your really cool so, I dunno where my meccano went actually..
    Ive found out what I needed to anyway as its been on my mind for oh...I guess 10yrs, Since I replied last I ve found another 7 cases where engines have exploded(not that big a hp) where these factory cranks have been knife edged. For that and the above findings etc, Im now putting knife edging out of my mind forever:thumbup::lol: Unless I can bore out the journals. I was looking this morn and I reckon I can get an 17mm hole in there, that should allow me to remove a nice bit off the counters in order to round the leading and knife the trailing sides:thumbup:
    Now all I need to do is figure out how to build a machine to hold and bore it. Boys oh boys..:o

    On a side note, my two engine sims count the big end cap only not 2/3rds of the rods mass.
     
    Last edited: Apr 23, 2010
  14. Mikey C Forum Member

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    and how to drill the oil passages....
     
  15. Brian.G

    Brian.G Forum Member

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    17mm will miss them all, or you could go to 25 and re-sleeve the drilling's across the hole with some press fit tube.
     
  16. A.N. Other Banned after significant club disruption Dec 5th 2

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    Mark - you are clearly sat on knowledge. Please put that into the thread :thumbup:

    As before folks, this is an interesting thread, but it requires us to get proper engineering points down and get (at least) to the absolute crux of where you're going to agree to disagree on - in order to have ongoing value or a springboard for others to come in. Please don't leave implied insults in your statements - it will be detrimental. "My understanding is... & this is because" is better than "gospel says".
     
  17. Brian.G

    Brian.G Forum Member

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    Really? Im basing all my findings on facts that I have found to be, albeit it doing it the hard way but still...
    I have been told many many times that the counters hold a percentage of the mass of the reciprocating weight per bank. As I have found, this is not true. Im not trying to make this complex. I do know all about what happens a crank inside the engine one its fired up, but Im not talking about that. My first book was hillier's fundamentals of motor vehicle technology and I read that in 1989 and far more like it ever since, Im NOT an engine designer nor am I a qualified Mech engineer, but I have done a vast amount of research on the subject and It all suggests that the mass should be kept the same either side of crank centre, that is in the journal and the pair of weights.(Nothing to do with whats bolted to it) Failure to do this can result in internal stresses I feel. And I also feel that the big guys that win races Know about this and therefore drill the journals to offset the shaved weights.
    I did get in touch with one guy in China this morn that has worked with a large company over there solely designing cranks for inline fours. He told me that messing with counterweight mass on a factory crank is to be avoided at all costs otherwise premature main bearing failure will occur due to the crank having to now handle and dissipate stresses that it was never designed for.

    He also went onto say that when a crank is being balanced it should be done so on its own. The flywheel and torsional damper should be done on there own also. He went onto say that they are either balanced or they or not so it then does not matter which way they are bolted together(he called it indexed).

    If you want to look further on this browse the net, look at all the engines that have failed using this method to knife a crank. Then look at all the modded engines that are running the same rpm, but the running factory crank that have not failed. Failures display spat rods, or shot shells for one reason or another. Perhaps the piston has been skewed in the bore causing it to lock. I dont know.

    And In respect to what Chris said, Keep in mind guys to whoever I may have come across as being a smartass, Im really not and it is not my intention. Im just doing research for my own engine with which I intend pulling out ll the stops for. If a modded part gives me 5hp extra I want to include it. But if its going to cause fatal reliability issues they I dont want it. I drive a car hard. I do believe that a modded engine can be reliable if you do your homework correctly. :thumbup:

    Peace out.

    Brian.
     
  18. mark25 Forum Junkie

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    A 4 in-line crank counter weight is designed to cancel 100% of the rotating forces F1, and 50% of the reciprocating forces F2. Where the big end 1/2 of the con rod is considered as a rotating mass.

    This is a compromise because cancelling 100% of the F2 forces in an in-line 4 would result in excessive block loading in the axis of the crank, due to the compromise involved in cancelling an F2 force with an F1 mass. Cancelling F2 forces is better achieved at F2, with balancer shafts running at 2x crank rpm, which is the only solution when F2 becomes too large a proportion of F1, ie when capacity reaches >2.3 litres.

    Most modern high-torque long-stroke engines are under counter-weighted, eg K-series and Duratec, because there isn't enough room to get enough mass in the right place within the block to cancel F2 forces. This not a problem for Honda with their low-torque short-stroke engines, becuase F2 is a lower proportion of F1. F2 is much better managed in a in-line 6, with a similar result in smoothness.
     
  19. Mikey C Forum Member

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    Mark, i think you have misunderstood something you have read, some of what you have written applies single cylinder engines. On these the counter weight is designed to balance 50% of the reciprocating masses. This is because on in balancing the up and down forces of the piston with a rotating mass, you create a forward and backward force. So 50% is chosen as it reduces the up and down forces by half and the forward and backward forces are no more severe.

    Since there are the same number of pistons going up, as going down in a 4 pot, the primary reciprocating forces are balanced without counterweights.
     
  20. infinity

    infinity Forum Member

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    Good Thread Guy's,

    I was always of the belief that the counterweights balanced out the rod's and pistons so interesting info there!!:thumbup:

    Still trying to get my head round all this, but don't manufacturers use balance shafts to balance things out too?? i believe the TFSI has them??
     

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