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How does a mower blade collect grass

Discussion in 'General Industry Discussions' started by Daithio, Oct 13, 2011.

  1. Daithio

    Daithio LawnSite Member
    Messages: 4

    Hi Folks,
    Does anyone have any information on how the airflow created by a lawn mower blade works to gather grass into a grass box. I am specifically interested in the aerodynamics of various shaped blades. This is for an engineering project at college and your help would be appreciated. Diagrams, photos, videos or links would be really appreciated. Dave
  2. FoghornLeghorn

    FoghornLeghorn LawnSite Senior Member
    from Texas
    Messages: 753

    The outside tip of the blade is curved upwardly to create an upward airflow. Imagine an airplane propellor, but the size of a lawnmower blade
  3. MR-G

    MR-G LawnSite Senior Member
    Messages: 479

    And the shapes under the deck channel that air flow towards the discharge...
  4. Daithio

    Daithio LawnSite Member
    Messages: 4

    Thanks Joseph, hopefully someone might have some more info on the impact different shaped blades have on the airflow. I am specifically interested in the fluid dynamics of different shaped blades :) Sad I know !
  5. txgrassguy

    txgrassguy LawnSite Gold Member
    Messages: 3,083

    Daithio, a direct application of Bernoulli's Principle specifically related towards incompressible gases will get you started in the right direction.
    All a mower blade, paired to the minimum, really is - is a dihedral airfoil hence the application of Bernoulli especially towards low Mach gases.
    In order for your calculations to achieve specificity you require:
    blade dimension consisting of:
    length, width, thickness with the airfoil or fence surface expressed as a separate calculation.
    Blade rotational speed at the arbor, mid point then finally tip. Factor in the mean deviation and you should be with-in acceptable variations regarding accuracy.
    The real variable is in the different machines made - simply expressed that not all machines reciprocate similar mass blades at similar velocity. So, pick a machine and check that specification sheet regarding FPM on the blades then apply Bernoulli and you'll get close enough.
  6. Daithio

    Daithio LawnSite Member
    Messages: 4

    Just spent an hour reading about the Bernoulli Principle, must get around to the maths later.

    Thanks, Dave

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