Tuesday, June 26, 2018

Big Wheel Keeps On Turning

I mentioned my redesign of the flywheel support bracket for the flame eater engine.  I turned a solid, one piece bracket with quarter inch thick pieces into a three piece assembly.  I did this mostly to add ball bearings for the flywheel shaft, but also to avoid a long operation of turning about 2/3 of the metal into chips.  The new design and the old side by side look like this.  The colors were chosen to highlight the three separate pieces.  (The blue block is a duplicate of the middle piece copied over to the right so I could get dimensions).

In real life, it came out looking like this, posed holding the flywheel as it's supposed to. If you look closely around the shaft, you can see the ball bearings; 1/4" shaft and 3/8" OD. 

When I was a larval engineer, an old graybeard told me that "engineering is the art of compromise".   There are rarely ever perfect choices that everyone agrees are the best way to do anything and this design is like that.  It does what I wanted.  All the parts worked out just as modeled.  The drawback is that it's three parts and they need to be aligned properly during assembly.  When I first put it together, the flywheel spun, bit also made the characteristic scuffing sound of something rubbing.  The parts aren't perfectly aligned, but I improved that by loosening the four screws on the bottom and putting a shim between the side bracket and the flywheel hub (I used one of my old business cards - still useful after retirement!).  When I retightened the screws and pulled out the shims, the scuffing was gone.  The flywheel still wobbles which argues that the two sides aren't exactly the same height.

These facts together make me think I need an alignment fixture when I build it.

I took a video of the flywheel running, but let me be honest: I built every part in here and I can't stand to watch it at real time speeds for a minute and a half.  So I switched the recording for YouTube to run at 2x real life speed.  The only thing you can see in the video that you can't see in the picture, besides motion, is I spin the setup 180 degrees so you can see both sides of it.

I need to come up with a fixture to hold everything in the right positions for assembly, or else I go back to the author's design.  I have a piece of 1" thick aluminum that I could turn into the original design.


  1. If you add a couple .1250 reamed dowel pin holes next to your bolt holes you can pin it together, tighten the bolts, then line ream both holes through the ears. That will give you perfect alignment for the shaft.

    1. Unfortunately, there's no room for a dowel pin. The space between the screws is a tapped hole, vertical into the bottom of the brackets to mount them to base plate. Those are kinda visible in the right hand side of the top picture.

      I'm thinking a milled out pocket in a piece of scrap that I can set the pieces in and holds them in the right position, but I don't see how to put the screws in that. Slots lined up with the screw holes?

  2. I was going to suggest what Irish just did. One thing I've learned from building engines is that you want the "Line Bore" for the crank to be dead-nuts correct. Same with the camshaft, but people rarely check it.

    I can hardly wait to see this little gem run!

  3. McMaster to the rescue!

    Self-Aligning Flanged Ball Bearings

    No need to worry about precisely aligning these bearings—they swivel to compensate for up to 5° of shaft misalignment. The flange ensures proper positioning inside a tube or housing. Shields keep out dust and contaminants.

    For technical drawings and 3-D models, click on a part number.For technical drawings and 3-D models, click on a part number.


    Radial Load
    Cap., lbs.

    Dia. For
    ID Wd. OD Thick. Ring
    Material Dynamic Static Max.
    rpm Lubrication Misalignment
    Capability Temp.
    °F ABEC
    Rating Each
    3/16" 0.645" 9/32" 0.69" 0.03" Steel Not Rated 60 40,000 Lubricated 5° -30° to 250° ABEC-3 1293N1 $16.57
    1/4" 0.645" 9/32" 0.69" 0.03" Steel Not Rated 60 40,000 Lubricated 5° -30° to 250° ABEC-3 1293N2 16.57
    5/16" 0.645" 9/32" 0.69" 0.03" Steel Not Rated 60 40,000 Lubricated 5° -30° to 250° ABEC-3 1293N3 16.57
    3/8" 0.947" 3/8" 1" 0.03" Steel Not Rated 180 36,000 Lubricated 5° -30° to 250° ABEC-3 1293N7 19.70
    5mm 16.4mm 7.2mm 17.5mm 0.75mm Steel Not Rated 60 40,000 Lubricated 5° -30° to 250° ABEC-3 1293N4 16.57
    6mm 16.4mm 7.2mm 17.5mm 0.75mm Steel Not Rated 60 40,000 Lubricated 5° -30° to 250° ABEC-3 1293N5 16.57
    8mm 16.4mm 7.2mm 17.5mm 0.75mm Steel Not Rated 60 40,000 Lubricated 5° -30° to 250° ABEC-3 1293N6 16.57
    10mm 24mm 10mm 25.4mm 0.75mm Steel Not Rated 180 36,000 Lubricated 5° -30° to 250° ABEC-3 1293N8 19.70

    1. Interesting idea - I've never seen those.

      I would mean starting over because the OD is wider than the bracket upright and the bearings are thicker than the quarter inch sheet they mount in. If I have to start over, though, it's worth a second thought.

  4. Earlier picture shows you carving the endplates out of plate by CNC, which I would think would get them similar sized enough. The screwed connection isn't a precision one. How about for alignment, clamp the assembly minus screws sideways in your milling vice? Jaw rails will set height against bottom of endplates, square edges of plates of equal width will set side to side offset against bottom of jaws. When the pieces are lightly hammer tapped together yielding no motion in your vice, then put the screws in and tighten them. The screw heads will be placed on the right spot on the side plates.

    1. I think the answer is either this approach or something like this.

      The drill size I used to pass the screw through the side plate has to be bigger than the screw's major diameter to pass the screw. That's going to allow some movement of the side plate. I need a way to get the sides precisely aligned with the bore straight and hold the sides in place while I tighten the screws.

  5. Or make a shallow keyway on the side pieces that aligns with a corresponding key extending between the screws that you machine in the ends of the (new) base piece.
    Or machine a vertical half-circle (well, slightly deeper than a half-circle) “keyway” in both the end of the base and the bottom of the side part that you can insert a dowel in from below, then tighten. The dowel will align the items vertically, and being slightly deeper than 1/2 the dowel means the faces will align it in the other (splay) direction

  6. The problem appears to be that you are trying to get alignment to be done by the screws. Not optimum for this setup. You may be able to rework the existing parts for better accuracy of shaft alignment.

    Take the uprights/towers, use a pin or sleeve that tightly fits the bores, clamp the two parts together, and mill the bottoms to just clear the flywheel height. Drill and tap vertical holding screw holes.

    Make a matching base plate. The critical design parameters are to get the bearing bores to be the same height. Changing it to get the parts to stack solid, instead of relying on the screw threads for height control is the focus.

    The change in fixturing/alignment is potential rotation of the bearing towers. The addition of a shoulder on the base plate for the towers to snug up against can add that back to the design. Just mill the mounting areas maybe .050" to .100", and leave enough slop in the screw holes in the base plate to allow the towers to stop on the shoulders instead of being constrained by the screw body or head.

    1. The problem appears to be that you are trying to get alignment to be done by the screws. Not exactly. At least, not deliberately. The idea for alignment was the vertical portion on the sides of the uprights against the vertical walls of the base. The centers of the bores are designed to be the same distance above the bottom of the side bracket, 2.000". This was done by getting the drawing of the first one right and then copying the design and pasting it with 0.4" between the widest parts of the bases. The parts are supposed to be exactly the same height, so when all of the pieces are set on a flat plate, the shaft should be level.

      I've been making the next parts (I won't need this assembly for quite a while), but the approach Anon 6/27 at 0322 suggested is what I'm leaning toward. I'll put a uniform thickness plate in the vise, loosen the screws out, and then put a sensitive indicator and sweep the flywheel shaft to ensure it's level and not canted front to back.