# How does a rotary mower blade cut?

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

1. ### nt1LawnSite Senior Memberfrom Dallas, TexasMessages: 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. ### JimQLawnSite Bronze Memberfrom Beatrice, NEMessages: 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.

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.

Q

4. ### Happy FrogLawnSite Bronze Memberfrom Navarre, FLMessages: 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. ### JimQLawnSite Bronze Memberfrom Beatrice, NEMessages: 1,139

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

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

Q

6. ### Richard MartinLawnSite Fanaticfrom Greenville, NCMessages: 14,700

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. ### nt1LawnSite Senior Memberfrom Dallas, TexasMessages: 295

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

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. ### GritsLawnSite Silver Memberfrom FloridaMessages: 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. ### windsoryLawnSite Memberfrom Howell, NJMessages: 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.

10. ### gorknoidsLawnSite Senior Memberfrom Virginia Beach, VAMessages: 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.