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Thread: More Mule then Rover?

  1. #231

    Re: More Mule then Rover?

    Btw, the CC/CV lead/acid cycle that works for me is:

    - Set constant voltage to 14.5 V
    - Set constant current to something less than the Ah rating of your battery, but in A (i e, <= 1C rate)
    - Wait until the battery is drawing < 0.1 of the set A limit
    - lower the CV to 13.6V for floating.

  2. #232

    Re: More Mule then Rover?

    Can't win them all.

    the motor I planed to use for the tilting of the bucket use an inline 250Watt.
    Click image for larger version. 

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    and with it being only 250W I made it's brackets out of aluminum,
    but, the first motor I used I got from ebay for $36.00 and it failed
    after one month of daily use. I found a direct replacement motor on
    ebay for $125.00 but, it started smoking after just a few times tilting
    the bucket.(crap)

    I didn't continue testing the replacement motor for fear of it causing
    the sabertooth 50 to fail. (that would also suck)

    So I changed the motor to a 1000Watt (same as used for the lifting)
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    I'm not happy with how high it sticks up so I'll look
    around for a 1000W inline motor, I'v seen some four
    pole inline motors use in hoverround's.

    I'l also have to remake the motor bracket in steel.

  3. #233

    Re: More Mule then Rover?

    It's the torque, more than the power, that matters for your bracket. Think of it this way: A motor that is stalled develops zero power (at the shaft) but develops a lot of force at the mount!
    You can calculate the torque of the mounts, and compare to yield strength of the material at the given thickness/area, and if you're > 10x away, you've got plenty of margin.

    Very simplistic rule-of-thumb: torque divided by distance from center of axle to center of hole equals force.
    Divide by number of holes equals force per hole.
    Calculate projected cross section of each hole wall -- bolt diameter times hole thickness.
    Get yield strength in PSI and multiply by cross section of hole wall (in square inches) and compare to force calculated per hole.

    So, let's say you have torque of 100 foot-pounds. (That's a lot, btw.)
    Your shaft-to-hole distance could be about 2.5 inches. 100 footpounds divided by 2.5 inches equals about 500 pounds of force.
    500 pounds of force on four holes is 125 pounds per hole.
    Cross section of a 5/8 bolt in a 1/2" plate is 0.3 square inches.
    0.3 square inches times 6061 yield strength of 16000 psi times 0.3 square inches means the aluminum should stand up to 4800 pounds per hole.
    Your ratio is 38:1 ! (I totally guessed on all the values, though -- I may be totally off.)

    Now, this is not a cerfied-engineer calculation by the book, and it's not a FEM analysis in a CAD package, so take it with a shaker of salt, but I've found this gets me to the right ballpark. It seems to me that if your aluminum bracket is anything like this calculation, you're fine.
    Last edited by jwatte; 02-23-2019 at 07:50 PM.

  4. #234
    Join Date
    Sep 2010
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    Re: More Mule then Rover?

    That's actually the rough process for determining tear-out limits for fasteners in holes, but it says nothing about the rest of the bracket surviving. Also, non-ferrous materials have finite lifespans and low temperatures reduce further ductility/durability. If you have a bit of steel, you can design the part such that the cyclic loading is small enough to allow the part to have effectively 'infinite' lifespan, but non-ferrous materials do not display that property.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    gives free advice only on public threads

  5. #235

    Re: More Mule then Rover?

    Sure, aluminum will fatigue and break, but, you know, they make airplanes out of it. With enough safety margin, you'll be fine :-)

    Anyway, if he wants to make another bracket because he enjoys making, go for it! If he just wants to plow snow, that seems like maybe not a 100% necessary re-work, of course dependent on the specifics.

    I agree that if there are narrower sections or pinch points in other places in the bracket that are more exposed, that would be a problem, too.

  6. #236
    Join Date
    Sep 2010
    Rep Power

    Re: More Mule then Rover?

    Yet airplanes still break apart more often than you would hope because stress fractures can be difficult to impossible to detect visually. They are detected/tracked during scheduled maintenance/inspections, but if they show up too rapidly/severely then the airframe possibly gets grounded then maybe scrapped well ahead of expected lifetime.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    gives free advice only on public threads

  7. #237

    Re: More Mule then Rover?

    Only one way to settle this: Build TWO snow plowing mules, one with an aluminum bracket, and one with steel, and measure what the relevant bracket lifetimes are under equivalent workloads!

    @Tommy_T, you heard @tician: Get building! :-D

  8. #238

    Re: More Mule then Rover?

    My concern is not some much that the bracket
    could fail, but that it could flex enough to let
    the chain jump a tooth.

    With a steel bracket the motor would stall before it jump a tooth.

  9. #239

    Re: More Mule then Rover?

    I would like to try to make the lifting frame out of aluminum and 10" more travel

    but all the brackets out of steel

    breaking up ice on the streets is very hard work on the mules

    also it does't give me a warm&fuzzy feeling watching the tilt motor bounce around
    on the camera as it's doing it's work
    Last edited by Tommy_T; 02-26-2019 at 08:36 AM.

  10. #240

    Re: More Mule then Rover?


    How do you figure out where the main flex is coming from? Do you have video from the sides while it's doing it?

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