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Mechanism for height adjustment of rotating body

  1. May 23, 2016 #1
    Hi,

    I am trying to build (or at least conceptually) design a diy autonomous lawnmower.

    I want to add an automated grass cutting height mechanism.
    I have never worked with lawnmowers but through basic googling know that most lawnmowers have simple mechanisms in wheels to change cutting height.

    4 wheels = 4 mechanism sounds difficult.

    Other option is to have axial movement of the blade to adjust cutting height.
    One way is to raise complete drive ( cutting blade, central cutting motor, shaft connecting them etc), using servos, etc

    Other way is to have special couplings between cutting motor and cutting blade. These special couplings will "couple" rotational motion but allow axial movement. (examples spline coupling, polygon coupling etc)

    Spline couplings are more suited for shafts in horizontal plane rather than vertical. I am not sure of polygon couplings. Problem with both is I will need to add a bearings (flange type) and holding mechanism to the hold the bearing so that the blade does not just fall off.

    All this seems to complicated.

    Are there any other mechanism which will allow axial movement but couple angular motion without adding bearings etc?

    Hope my query easy to understand.

    Thanks in advance
     
  2. jcsd
  3. May 23, 2016 #2

    Nidum

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    Wheels on a sub chassis . All the rest on main chassis . Lifting screws at 3 or 4 points . Commoned drive using timing belt .
     
  4. May 23, 2016 #3

    Mech_Engineer

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    Typically this is done on current lawnmowers by adjusting the height of the wheels. I think your best bet is to follow the same route.
     
  5. May 24, 2016 #4

    billy_joule

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    I'd agree, that'd need just one actuator, maybe a geared motor rotating the axis the adjustment handle does.

    Autonomous lawnmowers were considered as a product for development at my work, we bought 4 or 5 different models for testing and reverse engineering, the project was canned as the unit cost for a decent one would've been too high (all the bought ones were rubbish). if they aren't packed away on too high a shelve, I'll check their height adjustment systems tomorrow.
     
  6. May 27, 2016 #5
    Thanks,

    I am also thinking of a similar version. Instead of raising the whole sub frame (say including cutting motor, blade, batteries and electronics), how about the sub frame would carry just the cutting motor, blade only. Rest would remain stationary on the main frame attached to wheels.
     
  7. May 27, 2016 #6
    Wow, that would be great.

    Thanks a lot
     
  8. May 27, 2016 #7

    Nidum

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    It is not immediately obvious which is the better arrangement . Time to do some drawings . Have you got a good 3D CAD program ??
     
  9. May 27, 2016 #8

    Mech_Engineer

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    I'm not sure any of the complexity is needed, why not just have the wheels with drive motors mounted on the exterior of the frame, and they move up and down on arms? Sub-frames and belts for height adjustment seems like overkill to me, looking at current mower designs shows a bounty of simple designs.

    hrr-mower-wheel-height-adjustments.jpg

    scotts_mow_height.jpg

    fiskars_adjustment.jpg
     
  10. May 27, 2016 #9

    billy_joule

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    Sorry, they'd all been pack away. I asked the engineer who tested them and none had automated height adjustment, it's just set and forget, which I think is how most people set any lawn mower.
    I think automated height adjustment is the least of your worries. From our testing the main problems are - inability to cut long grass, getting stuck, poor battery life, poor/non existent path planning, poor/difficult boundary recognition or definition (most come with a cable you lay at the grass boundary), poor control interface, unreliable docking, poor/non existent dual area functionality (eg front & back lawn connected by paved driveway). Automated height adjustment isn't even on the list of things that need fixing.
     
  11. May 27, 2016 #10

    jack action

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    You can use a «single-touch» height adjustment on the 4 wheels as with this lawn mower:

    CMM1200_WW_1.gif

    You pull the lever 55 which rotates the rear axle 65. The rear wheels are offset from the axle, so they lift the rear of the lawn mower as the axle rotates.

    The front wheels are offset from the front axle 63 in a similar manner as with the rear axle. The rod 58 connects the front and rear axles such that they rotate together.
     
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