Hey Guys,
I'm in the middle of fixing an MTD yard bug - not a highly rated lawnmower, but the price was right (free) and it has electric start (request of Wife, who might, someday, mow the lawn =)
I have the thing apart, and I noticed this - its like every other lawnmower blade I've ever seen - the "sharpened" part of the blade only extends from each tip to about 5 or 6 inches in from each tip.
There is about 2 feet of unsharpened blade - It wouldnt take me and Mr. Dremel more than 15 or 20 minutes to sharpen the rest of the blade - My question is - Why didn't the factory do this from the beginning ?

-John

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2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)

i believe it has to do with cutting,as towards the inside the blade speed wouldn't be fast enough to cut much grass,and if it did it would more then likely just beat it rather then cut it clean.

The blade is spinning so fast that it's always the outside part that is doing the cutting. You'd have to be pushing the lawnmower very fast to get a piece of grass past the outer portion of blade before it is cut.
How do they figure out how much of the blade to sharpen? Would it work with just 2 inches sharpened?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Kerry covered that one dead on.

Now that the question has been answered. How about a highjack to some trivia that came to mind along this same issue.

Did you know that in forward flight (fast). There is a length of a helicopters rotor blades, where the air is actually flowing backwards across the blade surface?

Things that make ya go: Hmmmmm.

Or maybe they just make ya go: Who gives a ...?

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1980 300D - Veggie Burner !

Do the blades on helecopters get ice on them? If so how would the pilot ever know?

It would depend on how high above ground they were flying when the ice formed. High altitude, pilot knows for a long time that something is amiss; lower the altitude, the less he has time to know.

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Never a dull moment at Berry Hill Farm.

Did you know that some helicopters have hollow blades that are pressurized with nitrogen gas? A sensor detects the presuure of the gas and when it drops activates a light on the instrument panel. The thought being that if the pressurized nitrogen escapes, there is a crack in the blade, and it's time to land and check it out.

The amount of energy required to rotate a blade is going to be related to the moment of the blade (mass times distance^2, approx) and the resistance to the blade as it moves in rotation. If the grass is a consistent density along the length of the blade in rotation, and the blade is honed to a constant sharpness, then the greatest transference of energy is at the blade tip and decreases as the square of the moment arm.

In other words, a blade does more work at the tip than anywhere else. Thus, at length r, the work done will be proportional to torque at r^2 X rotational velocity. Since torque is related to the moment arm length, every unit of decrease in length (n) will see a decrease in work related to (r-n)^2. As n approaches r, work goes to zero.

Thus, the metal at or near the axis of rotation is nearly incapable of doing work, regardless of how sharp. In fact, it is more effective to have the blade sort of hump-up near the axis of rotation so that the metal wont be a frictional drag as the mower goes through rectilinear translation.

Finally, if one were to sharpen the blade from tip to axle, and run fast enough to make cutting at the axle possible, it is more likely that you would stop the rotation than cut the grass. This is why shaft locks are put on the shaft, not on the moment arm.

Bot

I like the way you think.

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1980 300D - Veggie Burner !

Yes, they do.
The ship starts to fly like *****.

(Things get out of balance and vibrate. Requires more power to maintain same rotor speed. )
(You know it's gotten pretty bad when you fall out of the sky like an anvil chained to a manhole cover.)

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1980 300D - Veggie Burner !

(or anyone else that may know)

Two identical airplanes at the same speed, altitude, and heading the same direction. The only difference between the two is that one is heavier than the other. Both are within weight and CG limits.
They are both flying along and loose all power at the same time. Assume both pilots think alike.

Question(s): Which one glides the farthest and why?

Here’s a hint:
My wife’s Grob G104 sailplane has a ballast tank capable of carrying 300+ pounds of water!!

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1980 300D - Veggie Burner !

the one that weighs the most

Nope....Guess again.

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1980 300D - Veggie Burner !

well i had changed my original answer after seeing your hint..

it would have to be the lighter one because ... im basising this on the fact that less weight=less pull from gravity or something... gravity was months ago in physics

When you lose the water(ballast) you gain altitude, the higher you are the further you can slide?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).