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Thread: Knife Edges for Crank Balancing

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  1. #1
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    Default Knife Edges for Crank Balancing

    Hello folks, I have decided to finally get around to making some knife edges for static crank balancing. Its something that I have wanted to get around to for a while but have been putting it off because of higher priorities.

    Their intended use will be for vee twins and singles. Parallel twins, triples, fours and mores will be farmed out to be dynamically balanced.

    I am pondering on the best way to construct them. Obviously only one corner can be at a static height and the other three will need to be adjustable. However, for them to be of use on a wide variety of engines I was wondering on the benefits of trying to make the width between the knife edges adjustable. There must a wide variety of physical dimensions of crank shafts so it would be good to try to design the apparatus to be of use on as wide a range of cranks as possible.

    So I was wondering if those people who have this type of thing in their own workshop could share their opinions and experiences. I am looking for dimensions such as width (between knives), height and length. Plus how have you designed the adjustment mechanism to get them level? And finally are yours adjustable for width or are they fixed?

    Also, what did you use for the actual knives?

    I have done a fair bit of googling and have read a few books but I am keen to hear the experience of the folks on here.

    I did find a you tube video showing the late George Yarocki balancing an indian crank and his rig looks nice and simple but it is fixed width which I guess is a reflection of his specialising in Indian Scouts.

    Thanks,

    John

  2. #2
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    I have a set of Crown balancing wheels of sensitivity 1 gram-cm I use for this, but maybe some of my experience might be helpful. I set these wheels on an Al holder I made that raises the centers above the table by ~14.5".

    As you already know, they have to be adjustable for height and width. Having balancing wheels rather than knife edges makes leveling a bit less critical, but I set them on a surface plate whose surface is level so it isn't a problem anyway.

    I recently balanced a 1928 Ariel crankshaft, having a 1" shaft on one side and 7/8" on the other. Instead of adjusting for the difference in height I was lucky in that inexpensive 2" OD bearings are made with both of those IDs. Slipping those on the shafts gave me identical heights for the balancing wheels.

    Actually, even if tried to adjust the heights I would have had the problem that the bases of my balancing wheels are 1" wide so the centers are inboard by 1/2" and a spline on one of the shafts extends closer to the flywheel than that. Because of that spline, using the wheels directly would have been impossible so I would have had to machine an adapter for one of the shafts had those bearings not been available. No matter what, I had to machine a new spacer for the Al holder that moved the centers closer together than the other two spacers I have for it (if I were to make a new bracket I would design it differently to allow variable spacing).

    For what it's worth, on that Ariel crank a difference of 8 grams hanging from the connecting rod makes a 1% difference in the balance factor. That is, if you want to achieve this, ahem, level of accuracy you will have to be able to level your knife edges well enough that you can tell whether the crank is "perfectly" balanced to within ~8 g (and, ideally, better than that). Others with direct experience with knife edges should speak up about this, but I would think that, at a minimum, a machinist's level like a Starrett 98 (good to 0.005"/ft.) would be required. That, plus gage blocks to use along with the level when the edges have to be at different heights.

  3. #3
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    Default

    John!

    I have conjured a few knife-edge contraptions, but if I were to do it all over again, I would use 3/4" or larger ground round stock instead.

    Somewhere in "Essentials of Speed" or maybe it was "Uncle Frank's" there is a picture. Its too simple to put into words.

    ....Cotten
    Last edited by T. Cotten; 12-17-2017 at 02:42 PM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  4. #4
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    Hi Bosch, thanks for the detailed reply, I had wondered as to what rig you are using for the Ariel (I have a alter ego over on another site where you are describing your Ariel build) . I have seen quite a few pictures of the type of rig that you describe and if I could find the wheels only it would be a simple job to build the rest of the apparatus. I will see what I can find.

    Tom, thanks for confirming about the ground bar. I think I may have seen the picture that you are referring to. (also you have reminded me that I must get a copy of Uncle Franks Q&A book)

    Going with Bosch's KISS principle then ground bar on a simple base would be the easiest to build and also (I think) the most sensitive as long as they were both straight and level.

    Referring back to Bosch's post, I had thought that a machinists level would be the best tool to level a knife edge. I have heard to expect it to take much longer to level the apparatus than actually use it.

    If I do go with a knife edge/ground bar type of arrangement then how long would you go for? One revolution of a 1 inch shaft is obviously 3.142 inches of travel along the edges and I guess that you only really need room two or three revolutions which equates to a little under 10 inches. I was thinking 18 to 24 inches, Tom what length would you go for?

    I am off work now for a couple of weeks and tomorrow is errand day. I will ponder what approach I am going to take with this tonight and tomorrow I can feel a trip to the steel stockist coming on. I will let you know how I get on.

    John

  5. #5
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    Here's mine.
    The knife edges are levelled by eccentrics.
    It mounts on my mill table overhanging the side to give con-rod clearance.
    Straight edges are just over 600mm long and mounted on an angle to achieve knife edges.
    Knife edges are 165mm apart and the bottom cross pieces are 150mm long
    Has stops at either end to prevent the crank rolling off
    The bottom cross piece is mounted so you get feet at each corner.
    I thought that I might need to widen it at some time so the cross pieces are bolted in so wider ones can be made and fitted easily.
    The only pain is having to make ground rings to fit those assemblies that have mainshafts of differing sizes.
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    Peter Thomson, a.k.a. Tommo
    A.M.C.A. # 2777
    Palmerston North, New Zealand.

  6. #6
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    I use my homemade truing stand,flipped 90 deg.and clamped over the bench edge.Seems to work pretty good.
    Tom

  7. #7
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    Quote Originally Posted by tfburke3 View Post
    I use my homemade truing stand,flipped 90 deg.and clamped over the bench edge.Seems to work pretty good.
    Tom
    Thanks Tom, I have a truing stand but its not the greatest and wouldnt want to use it so whatever I use will be a "new build".

    John

  8. #8
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    Yes, John!

    That drawing sums it up, although I don't remember the arbor confusion.
    (And I would drill vertically through the ends of the rods, so then they could be leveled with threaded rod and nuts.)

    So keeping it as simple as possible, a "truing stand" and "knife-edges" are different animals.

    You can true wheels between any machine centers with enough throw.

    Knife-edges are pretty much for balancing only.

    (But no doubt, a reasonable balancing could be achieved on centers, if great care is taken.)

    ....Cotten
    Last edited by T. Cotten; 12-17-2017 at 05:57 PM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  9. #9
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    Tommo, many thanks for posting the pictures of your setup, that gives me more food for thought. I have two HSS plane blades about 600mm long that I acquired years ago thinking I could use the HSS to make tooling from because the sharp side of them has a few chunks missing although I have not actually done anything with them yet. Maybe I can use the other side of them if they are tilted at an angle? I will put that thought into the mixing pot with the other ideas and see what comes out.

    Bosch, I found this on google, I think it is is like your jig?



    Tom, I remembered about the picture of the ground bar. It was actually Tommo who provided it to me when I first asked about balancing some time ago. Here is the picture, I assume its the same one that you were referring to?




    I found this picture on an aussie site here:




    Similar to Bosch's setup but it looks homemade. I guess the key is concentric and accurately machined disks plus sensitive bearings. It would have a smaller footprint but would take longer to make and, maybe, not as sensitive as knife edges/bars.

    All good food for thought, I will sleep on it and decide how to attack this tomorrow.

    Thanks to you all once again,

    John

  10. #10
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    Quote Originally Posted by TechNoir View Post
    Bosch, I found this on google, I think it is is like your jig?

    I guess the key is concentric and accurately machined disks plus sensitive bearings.
    Yes, that Crown picture you found is one just like mine, except mine only say "Crown" without the "20 in." The stand I machined has flats for them to sit on with vertical tabs through which I use bolts to secure them using the holes otherwise used for the rods in the photo.

    The homemade version shown in another photo shows no evidence the wheels in it had been balanced. Each of the Crown wheels has several shallow balancing holes drilled in it. When I went to the garage to check how they compared with the other photo I gave one of the wheels a flick with my finger and it spun for 35 sec. I've never measured their runout but given everything else about them I'd bet it's pretty small. Just as you wrote, the keys are good disks and sensitive bearings.

    Having recently been fiddling with my Ariel's crank makes me appreciate the properties of these wheels all the more. The fact the crank stays in one location, rather than simultaneously rolling and turning, is a definite advantage both when the imbalance is large as well as when getting down to the final few grams. Not having to spend the time to precisely level it is another definite advantage of wheels over knife edges.

    I was lucky in finding my set on eBay some years ago and having to pay relatively little to get it (I don't remember how much). Sometime later I came across the retail price; again, I don't remember what it was, but I do remember it was a pretty significant amount.
    Last edited by BoschZEV; 12-17-2017 at 07:13 PM.

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