Half VW Balancing

Discussion in 'Half VW' started by 12notes, Oct 14, 2018.

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  1. Oct 14, 2018 #1

    12notes

    12notes

    12notes

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    I have a 35hp Global engine, I have ran it briefly with the prop it came with it along with a random assortment of bolts and washers holding it on. It shook badly, but it had no hope of being in balance with 3 bolts of correct length and 3 bolts each of different lengths with different numbers of washers. However, it did not come with an external counterweight to go on the prop side, as I've seen other 1/2 VWs have. I know some Globals were balanced and some were not. Is there an easy way to tell if the crankshaft is balanced? Is there a way to balance it along with the prop to correct any out of balance conditions, or do I need to have the engine balanced initially?

    I know I can ship it to Scott Cassler at Hummel Engines in Arizona for balancing, but shipping will be fairly expensive to do so from Louisville, KY. Is there anyone East of the Mississippi who can balance a half VW?
     
  2. Oct 14, 2018 #2

    Hot Wings

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    If it was balanced with the external prop hub weight the aft counterweight will also be heavier than the forward internal counterweight. 1/2 VWs come in both flavors of balance. The other thing to note is prop indexing with the crank. The 1/2 VW shakes around the vertical axis due to piston offset. The external weight helps to reduce this. The prop should be such that it gets twisted around pitch axis during this motion rather than trying to flap the blades.

    A visual I was given by an Onan/Quickie guy: Hold a hard stick out in front of you vertical and rotate it around the vertical axis. Now twist your hand 90 degrees so the stick is horizontal and try to rotate it about the vertical axis.

    Edit: Incomplete example. Add gyroscopic precision to the mix.
     
    Last edited: Oct 14, 2018
  3. Oct 14, 2018 #3

    TFF

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    There has to be someone in Louisville that has balance equipment. Nashville does. I'm east of the Mississippi by 200 yards but 5 hours from you. I would copy someone's weight first. The problem you have is adding weight requires fabrication not just bolting some washers to a flywheel. You will need the plate to get it in the ballpark. Grinding on it or making a bigger one is probably how the weight needs to be adjusted and that takes time. It may require a couple of weekends to do it. Some RV builders buy the cheaper balancers. Have to be some of those guys around. Must have for helicopters. If you really can't find someone, start looking in Nashville. I have a friend there who may know of someone.
     
  4. Oct 15, 2018 #4

    N8053H

    N8053H

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    This weight you install on the front of the prop is mounted in a certain spot. You put the cylinders at TDC then the weight goes at the 9 o:clock position standing in front and looking at the prop. Call Scott and he can send you one of these counter weights. You will need to know what size 1/2 vw you have. A 38 hp 1/2 vw uses a different counter weight then a 40 hp 1/2 vw.
     
  5. Oct 15, 2018 #5

    blane.c

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    Bob weight. For a V-8 the total weight of the big end of the rod plus the rod bearing plus 1/2 the weight of the small end of the rod, piston, piston pin, and rings. Make a weight equal to that and put it on each crank throw. Put the crank in a cradle and spin it, have a switch mounted to a strobe light so the pressure of the imbalanced crank blinks the strobe each rotation, this will light up one spot on the crank and you can find it in relation to a mark you place on the crank. Add or subtract weight on the crank counter weights until the light quits blinking.

    The Bob weight for a horizontally opposed is probably different than that for a V-8 but you get the idea.
     
  6. Oct 15, 2018 #6

    N8053H

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    I spoke with Scott Casler in length about this subject. There is a reason why this weight is not added to the crank. You would have to ask him about this reason. But if I remember correctly, there was a reason he did not do this.
     
  7. Oct 15, 2018 #7

    blane.c

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    Bob weight is temporary to account for the rotational and reciprocating mass of the pistons and rods assembly in a blueprinted unassembled engine. If you were going to use say a propeller instead of counterweights on the crank you could mount a triggered strobe to the running engine and add and subtract longer/shorter bolts and or more or less washers, it is going to have to have some counterweights on the crank to work even if it is a boxer certainly on a horizontally opposed … well at least I can't imagine it working well without.
     
  8. Oct 15, 2018 #8

    Hot Wings

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    The bob weights Blane.c is talking about are a temporary weight used only during the actual balancing operation. The reason the prop weight is not added to the crank is that it is there to help offset the natural rocking of a twin boxer. When a 2 cylinder boxer crank spins, even if it is perfectly statically balanced along it's axis, the mass from the offset rods will tend to make the crank wants to spin about an axis that is not the same as the cranks. The added bob weights simulate the rotating part of the mass from the rods - which add to this off plane rotating mass. The half circle internal counterweights offset the out of plane mass, but don't quite get the job done on a 2 cylinder crank.

    The external weight can be though of as a kind of simulation of the cut off portion of the 4 cylinder crank and helps keep the rocking down. It is likely it is this weight that Scott is telling you is not added to the crank - and the reason why.
    ===========
    @ 12notes

    Without disassembly and balancing via the traditional methods you are probably left with using something like a dynamic prop balancing tool and adding weight in a trial and error method to smooth things out?
     
  9. Oct 16, 2018 #9

    N8053H

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    What you state sounds a lot like what Scott was telling me. As to dynamic balancing. There is a man not far from me that does this. I was wondering if this could be done and not use that counter weight on the front of a 1/2 vw. From what I understand this type of balance is done with the engine running. He will have you shut the engine off and he adds weight. A friend flew his airplane with an 0-200 into this man and had him balance it. My buddy was telling me about this. He loved the outcome from this. I must admit, when we flew his airplane, that engine was smooth. I know a 1/2 vw will never be this smooth, but would this type of a balance be better then that counter weight or yield better results?


    Hot Wings Thanks for taking the time to explain this.

    Tony
     
  10. Oct 16, 2018 #10

    pictsidhe

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    Someone who does dynamic balancing will know how to balance an engine...
     
  11. Oct 16, 2018 #11

    Dana

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    You'd still need the weight. All the dynamic balancing does is tell you, more accurately, how much weight is needed and where.

    Now one of those "Balance Masters" with the rolling balls might be interesting to try...
     
  12. Oct 16, 2018 #12

    N8053H

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    I had one of those at one time. When my reduction unit failed in flight I lost it along with the prop. I spent a complete day looking for it. Never did find it. Someday someone will come upon this, I bet they start looking for the airplane after they find it.
     
  13. Oct 16, 2018 #13

    blane.c

    blane.c

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    I thought the Balance Masters had Mercury in them for weights.
     
  14. Oct 16, 2018 #14

    Dana

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    Maybe that's what it is.

    Tony, how did it work for you before you lost it?
     
  15. Oct 16, 2018 #15

    TFF

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    Statically the engine is balanced. Pistons, rods, stroke same; this stuff you have to assume are in spec. Chopping off the other two cylinders makes a firing pulse every 360 deg out of the 720 a 4 stroke makes, but one every 360 rotation. What is not in balance is where the power stroke is. One cylinder the power stoke is pushing the crank over the top; the other cylinder is pushing the crank around the bottom. As close to equal as they seem, they are not the same. The chunk of weight added to the crank fills in the difference of loosing the other two power pulses which smooth out the engine. Why is it not put on the crank on the inside? No room. The weight is in a different clock angle from where traditional mallory heavy weight of adding to the counterbalance is. You would either need another crank made to spec or add a crankcase to the manufacturing detail. I would not trust a crank with a welded weight on it. Because 1/2 vw is about cheap, a 1/4 steel plate about 4 inches square bolted to the flange fits the mission.
     
  16. Oct 16, 2018 #16

    Hot Wings

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    Picture time!

    VW crank.jpg

    The blue ovals represent the bob weights added to simulate the rotating mass of the rods. The red line shows where you can't add weight internally in a VW without hitting the case*. The green rectangle represents the external weight added to make up for the lost red to maintain static balance.

    Notice the orientation of the Ix axis. That is the axis about which the crank wants to spin. In the picture it's pointed the way it is because there are no virtual bob weights attached. If we could add just the right amount of bob weight the Ix axis would be the same as the cranks natural axis. Ideal!

    But in the real world even if the engine is assembled with perfectly matched rods, bearings, bolts and the crank is perfectly statically balanced - the engine could still shake. Ix would still needs to be adjusted by changing the weights of the internal counter balances. Take weight off the bottom of the rods - we need to take weight of the counterweights as well.

    This is related to why Pete has decided to have the rod throws on his new crank use drilled rather than cast lightning holes. Those holes not only reduce the weight of the crank directly, they also reduce the amount of weight needing to be added to the counterweights. It's a double win as far as weight. Drilling gives him more control over the process rather than having to worry about core shift during casting.




    *This crank was going into a case with out the same restriction.
     
  17. Oct 17, 2018 #17

    N8053H

    N8053H

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    Dana I really can not give an educated answer to this question. It was a brand new airplane and engine. I had put 4 hours on the A/E before it came apart.
     
  18. Oct 17, 2018 #18

    blane.c

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  19. Oct 18, 2018 #19

    pictsidhe

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    Drilled crank pins also strengthen the crank. It's all win, other than the practicalities of having to drill the crank and deal with oil.
     

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