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Thread: Knife Edges for Crank Balancing

  1. #41
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    Dec 2015
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    Blighty
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    Quote Originally Posted by Peter Cooke View Post
    What about milling a slot in a length of ground round bar and clamping your straight edges into the slot. Then, if you wanted, you could rotate 180 degrees and use the ground round bar edge instead. You get extra rigidity and 2 sets of edges to choose from.
    Peter, that is a really good idea. The one drawback (for me) is that I don't have a functional mill at the moment although I am hoping to remedy that in the near future. If I do then I will give it some serious consideration.

    Thanks,

    John

  2. #42
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    Aug 2012
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    Victoria, Australia
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    No problem John,
    I have to admit I've not done it myself. I was reading through the thread and it wasn't until you mentioned sandwiching your straight edges between flat bar that it occurred to me. I'll be needing to balance my VL engine somewhere down the track so am interested in the topic. There are many ways to skin the cat...

  3. #43
    Join Date
    Jun 2001
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    Central Illinois, USA
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    My apparatus is flawed, Folks,...

    But it illustrates the pitfalls of attempting to clamp to planer blades, as a firmly set set screw screws them.

    (Horizontal against the blade in the attachment.)

    Ideally, nothing should need to be retained by anything but gravity.
    Round stock is quite secure in v-blocks, especially with the weight of the flywheel assembly upon it.
    The trick is getting the v-blocks up high, rock solid.

    And a steady hand placing the wheels on them!

    ...Cotten
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    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  4. #44
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    Dec 2015
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    Hi Cotten, thanks for your input again. More food for thought. I now have several ideas for my edges and a few days to ponder my plans.

    Thanks to everyone who have contributed to both this thread but also everyone on the AMCA forum for your help and words of advice over the past year.

    Seasons greetings to everyone.

    John

  5. #45
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    Dec 2015
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    Blighty
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    Seasons greetings to everyone, I hope you (for those who celebrate it) had a great Christmas. The last of my guests left earlier today and so I have less distractions although it was nice to have them, it is also nice to have some peace.

    Thanks to BoschZEV for taking me on a trip down memory lane, following Bosch's post I had a look at some options for my knife edges. The trip down memory lane was because I did a year of Structural Mechanics in my first year of University in 1996.

    Because I have a few different ideas I put it into Excel so I could compare the various amounts of sag for the different knife edges I have in my head. The first thing I noticed was that one of my options gave an angle of slope similar to the theoretical example that Bosch posted about.


    Quote Originally Posted by BoschZEV View Post

    .....plugging 10 kg (22 lbs) into the calculator along with an angle of arctan(0.002"/12") = 0.01 deg., a force of 0.0017 Newtons is required to pull that weight up the plane. The same 0.0017 N is trying to get it to slide/roll down that plane. Plugging in the acceleration of gravity, this is 0.17 grams. That is, an imbalance of only 0.17 g will cause your flywheel to roll down that slope.
    However my maths gave a result for the mass of the imbalance needed to move the 10kg flywheel to be a factor of 10 greater than the example. I double checked the on-line calculator that was posted and I think Bosch made a typo in his original example. I think it should be 1.7g rather than 0.17g. What do you think?

    Anyway, my plans for the knife edges have settled on 300mm between supports and using the beveled straight edge with a stiffening bar, probably the thicker of the two in my table below. (note the straight edges are approx max 38mm x 5mm with the bevel starting 8mm from the edge) In order to get the maximum stiffness in the edges I will need to fix the straight edges to the bars. I plan to use something like Loctite Epoxy Adhesive rather than mechanical fixings although I may mill a small "shelf" into the support bar so that the load is also physically bearing onto the bar rather than being transferred to the bar 100% by the adhesive in shear.

    I only have bathroom scales for weighing the crank assemblies but I am sure they are accurate enough for this estimation. My 20F crankshaft assembly and 2 rods weigh 12kg's and a 1953 Matchless single crankshaft and rod came in at 12.6kg's. So I based my calculations on a weight of 13kg's.

    The results (assuming I have not made some stupid error) are summarised as follows:




    If I were to go with round bar for the edges then to get the sag minimised I would need quite large diameter bars. I would then wonder if the width of the bar would cause issues when trying to fit the crank on them?

    The final decision is a couple of weeks away at least because I have some, essential, distractions to take care of. I have a small mill that I am rebuilding, it is not far away from being functional and I want to get it to that stage before I look at these knife edges.

    Also, I bought a new to me Lathe (an upgrade from my current one) which is being delivered next week. It will need some attention before I can use it because it is 3 phase so I need to wire it to retain it as 3 phase but power it through a vfd. Also, remember my workshop is at greater than 100% capacity, in parallel I need to sell my current lathe.

    John

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