How does a rotary mower blade cut?

Discussion in 'Lawn Mowing' started by nt1, Jun 19, 2008.

  1. nt1

    nt1 LawnSite Senior Member
    Messages: 295

    So the tip of the blade does the cutting but why are all blades of grass cut during one pass of the mower?

    If say you have a 21" blade and only 4" 's are sharp on each end, why are all the grass blades cut in one pass with the mower?

    I realize the mower creates a vacuum pulling the grass up and on a mulching blade/deck the grass is recut many times. Other than that I have no idea how the grass is cut.
  2. JimQ

    JimQ LawnSite Bronze Member
    Messages: 1,139

    Not quite sure now to answer this one...

    Assume the following - A mower with a 61" Deck (21" blade) mowing grass at 5 mph.

    Blade tip speed of approx 18,500FPM (3300 Blade RPM)

    5 MPH = 88 in/sec
    3300 RPM = 55 Rev/sec

    88/55= 1.6

    That means the mower travels forward 1.6" for every 1 blade revolution.

    Remember, there are 2 blade tips on a blade so divide that number in half.

    1.6"/2 = .8"

    So, the mower moves forward less than 1" for each cutting pass of the blade.

    If you're looking for a more basic answer, imagine a very fast machete.

  3. k911lowe

    k911lowe LawnSite Senior Member
    Messages: 526

    a good answer,why the question?are you having trouble with your cut?
  4. Happy Frog

    Happy Frog LawnSite Bronze Member
    Messages: 1,224

    You shouldn't divide the 1.6" by 2. The mower stills travels 1.6" forward per each blade revolution but the grass is cut two times.
  5. JimQ

    JimQ LawnSite Bronze Member
    Messages: 1,139

    ;) I think we're both saying the same thing here...

    The mower has moved forward .8" each time the grass gets cut.

  6. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    Sound like it's time to split some hairs here.

    The .8" is only the first opportunity for the blade to cut grass. The number of times that a blade can potentially cut grass varies depending on where in the arc of the blade rotation the grass is and how far the blade is sharpened down it's length. This is also why some mowers tend to miss blades of grass between the mower blades since you have the least opportunites to cut the grass at that spot in the arc.
  7. nt1

    nt1 LawnSite Senior Member
    Messages: 295

    Ok, I get what you are saying about speed of travel but my question relates more to what Richard Martin is saying.

    How about another question. Why are blades not sharpened the entire length of the blade?

    As far as the fast "machete" comparison, a machete is still sharpened the length of its blade or at least the part that cuts.
  8. Grits

    Grits LawnSite Silver Member
    from Florida
    Messages: 2,994

    The first few inches of the blade is the main portion that does the cutting. I would think on a mulching blade that more of the blade cuts.
  9. windsory

    windsory LawnSite Member
    Messages: 223

    The tip of the blade is traveling the fastest because it is spinning around a larger radius than the parts that are closer to the spindle so it will give the best cut, thats why the entire blade is not sharpened. The blade is constantly spinning, that's why an entire pass is cut while only 4 inches of the blades are actually sharpened. Picture a cup with wet paint on the brim. If you put the cup brim down on a piece of paper and slide it across, you will get a line of paint the width of the diameter of the cup. The brim represents the outer sharpened edges of the blade while the unsharpened edges of the blade are represented by the empty middle of the cup.

    Hopefully this all makes sense. :waving:
  10. gorknoids

    gorknoids LawnSite Senior Member
    Messages: 316

    They're not sharpened their whole length because the mid-blade speed isn't high enough to produce a cut. Just like I can't throw a golf ball 300 yards.

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