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Ideal Blade Tip Speed?

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Well-known member
Jun 20, 2006
Ryan D Wilke
Mornin' Fellas,

The other day there was some discussion on the main board about the cutting quality of various mower decks. It was suggested by some that the Crapsman deck cut very well. So what makes a mowing deck better than another - is it cutting blade design, blade tip speed or what?

I pondering this (as it continues to rain) with my coffee this morning & I decided to try to determine the cutting blade tip speed on my 42" IHC mowing deck. Now those of you way smarter than me please review my (likely) mathematical errors, and if so, PLEASE point it out to me.

Ok, here goes --
Things I assumed:
My 12HP Kohler will be run at the recommended full throttle crankshaft speed of 3600 rpms.

The PTO is directly connected to the crank. The mechanical PTO pulley (driver) is 4 3/8” (4.375") in diameter. So it is also turning at 3600 rpms.

The (driven) pulley on the top, center of the deck is 6 ¼” (6.25”) in diameter.
So: 4.375/ 6.25 = 0.70
3600 x 0.70 = 2520 rpms

Now, the sheave below the top center deck pulley which is being driven is 7-1/8” (7.125”) in diameter. It is turning at the same rpm as the sheave above it, 2520 rpms.

The center pulley drives the two outer blade pulleys, which are only 3” in diameter.
So, the driver pulley is 7.125” @ 2520 rpms.
7.125 / 3.0 = 2.375
2520 x 2.375 = 5980 rpms

Assuming my above math is correct, it's interesting to note that the two outer blades turn at nearly twice the speed of the center blade......

Blade tip speeds:
The center spindle is driven 2520 rpms. The center blade is 22” long, so this is equivalent to a pulley 22” pulley. The circumference is the distance around the imaginary 22" pulley. So, Circumference (C) = Diameter (D) x Pi (3.1416)
C = 22" x 3.1416
C = 69.11”

So, the center 22” blade tip is making a 69.11” trip with every rpm =
69.11" x 2520 RPM = 174,157 inches per minute.
174,157 / 12"/ft =
14,513 feet per minute (FPM).

The two outer blades are 10.75” long or equivalent to a 10.75” pulley.
10.75” x 3.1416 = 33.77” circumference
33.77” x 5980 rpms = 201,944 inches per minute
201,944 / 12 =
16,829 feet per minute (FPM).

So, again, assuming my above math is correct, then we've determined that my 42" IHC deck cuts grass with spindle speeds of 2600 to 6000 rpms and produce blade tip speeds of 14K to 17K FPM.

First off, is my math correct?

Secondly, I KNOW modern mowing decks have a higher blade tip speed. Does anyone know just how fast do they go?

Does anyone know what the blade tip speed of a Crapsman deck is? If not, if someone would provide some engine rpm and sheave diameter measurements here, we'd be able to calculate them for discussions sake.....

Thanks for any and all input, opinions, & discussion!
Ryan Wilke
Ryan - You really need to move to a dryer climate

I wish I'd found out the speed of our disc mowers at the farm because they cut so well but it never crossed my mind. The NH mower had very small disc heads compared to what someone else had on a different brand. NH was like 16" diameter to the other one at maybe 30-36". I never seen the other one cut so I couldn't compare speeds.

The 42" and 38" deck's biggest down fall were/are the wrong blade designs.

A friend down the road has a Huskee that cuts smooth, I'll see if he's able to measure it or I may have to go down there. (he's half way through getting his neck brace off from surgery)

Steve B - What was that speed stuff you posted years ago ? I don't remember if I saved it or not. Something about not breaking the sound barrier ??
RYAN - I didn't recalculate all your numbers but the math is correct in theory, so I'm sure the numbers are correct.

I haven't done an internet search but seem to remember something about a maximum tip speed of 19,000 ft/min. Think it was mentioned in one of my CC history books written by Hank Will or Ken Updike. Think the story was Purdue University helped IH rework mower decks to reduce noise and it turned out the decks wouldn't cut grass when they were done, and the engines made more noise then so the Q/L side panels were created.

I should figure up how fast the outer blades on my 38" deck turn. Those outer blades are only 9 or 9-1/2" long, they have to turn mighty fast.
Thanks for your input!

So you feel as though the 38" & 42" decks had a poor blade design..are you referring to the tapered-back cutting edge?
I can't seem to understand why that would make a difference...

As the blade turns, doesn't the sharp edge of the blade come into contact with the grass regardless of the taper of the cutting edge?

If you schedule permits, it would be interesting to see just what your neighbor's Huskee deck has for pulley sizes, engine rpm, length of blades, etc. so we could plug those values into the math and determine what, if anything, is different with them in that department.

Also, if possible, notice what style blade is on that Huskee - a straight edge or a tapered-back cutting edge?

Dennis F.,
Thanks for your input as well!

So you seem to recall a 19K ft/min blade tip speed was determined to be ideal...interesting.

Well, we all also know that the ability to cut grass smoothly is also a matter of the ground speed of the tractor and the type/brand of grass, how tall the grass is and how short do you want to cut it. Of course, full engine rpm is required to not only to sufficiently cool the engine, but also to keep the deck operating at its designed speed.

I'm thinking your smaller, outer 9" blades would have a slower blade tip speed, but I'm assuming your 38" deck has the same pulley sizes as my 42" deck.

9” x 3.1416 = 28.27” circumference
28.27” x 5980 rpms = 169,081 inches per minute
201,944 / 12 = 14,090 feet per minute (FPM).

Ryan Wilke
Ryan - IH was the only idiots to put a worn edge on a new blade ... the Huskee wont be like that !

A straight edge right angle corner will out cut any angled leading edge ... common sense ... when blades get sharpened wrong several times and end up with an edge like the (new) 38-42 deck blades you throw the blades away.
Well, it might be common sense to you - I guess I'm just not too swift then because I don't see how the taper or straight cutting edge would effect their cutting performance.

If what you're saying is true, then I guess I need to start looking for a blade manufacturer who makes/sells replacement blades for a 42" IHC deck that has a straight cutting edge.
Ryan Wilke
RYAN - I think the 19,000 ft/min is the Maximum tip speed according to the ANSI regs, not sure what the optimum would be.

Like Ken, I seem to remember some comment about tip speed not exceeding the sound barrier. Never really heard a reason why.

Like all things mechanical there have to be compromises, higher tip speed requires either longer blades or faster spindle speeds. And faster spindle speeds require more HP due to friction, seal drag, etc. And the longer blades are more prone to bending, and due to the longer lever arm would require more torque to turn against the grass.

Like Ken mentioned, on large bush hog type mowers, the longest blade I know of is around 6-7 feet. Anything wider gets two, three, or four blades. The disk mowers he mentions are pretty neat, old sickle mowers used to operate best at 5-6 mph, maybe just a bit faster in good conditions. These disc mowers can run 12-15 mph, as long as you have the HP to pull them and can stay in the tractor seat running that fast.
Ryan - In a simple way to say it for someone that didn't spend countless hours cutting hay or bush hogging ... an angled back edge more or less pushes the grass back , a straight edge will cut like a hand food chopper.

If you know how the old style sickle mower was made , tapered knives were used so the grass could enter BUT there was a guard on both sides of those knives that trapped the grass so it could be cut.
Now IF the angled back 38-42 blades had a "guard" to trap the grass it might cut better. Doubt it but maybe.

Take a wire and try to cut it with a pair of tin snips without holding the wire in the other hand. It trys to just slide off the blade , down toward the end and out. Same principal.

Dennis - When I was using a 15 foot bat wing I'd get into some swampy places and cut bull rush in standing water. I found that the disc mower would cut the bull rush easier and faster in 2 feet of standing water than the bat wing ever could.
That MF 2675 with the cab sure was nice !
Ryan - The ONLY blades I've seen offered for the 38-42 decks that were not OEM are made by Oregon. I've not seen the blades but I have the 32 page catalog pdf file if you want it. It's 2.06meg.
Somebody sent it to me and they were going to order a set but I never heard if they did or not.
This was some years ago when I was up in arms for the poor cut of my 42" CI deck and I threatened to buy high carbon steel and make my own.

Dennis - Remember those days when I use to speak my mind ??? Boy I'm glad I've calmed down since then ...
Dennis F.,
I suspected that the 19K figure for an "ideal" blade tip speed would take into consideration generated noise levels and mechanical longevity/durability.

Your example and the example Ken provided reminds me of my elderly neighbor who tried to cut 12+" grass with his (JD) deck all the way down and traveling WAY too fast...then he wondered why it would eat and/or toss the belt and not fully cut all the grass....go figure.

Thanks for the layman's cutting blade function description! I now understand the difference.

Oregon is the company I was going to contact. Thanks for the offer, but no need to send their catalog. I'll just give them a call and see what they say.
I'll report back with what, if anything, I can locate.

Ryan Wilke
I got in a hurry and put a stamp on it and the Yorkie ran it out to the mail box
I have a chraftsman 42" deck here and the top drive pully is 5" acrost and the blades are 21" long and if you need any more info just ask. Is that good Ken
And If you want to know the blade cutting speed I can use the digital tack and tell you that also ?
Ryan can do the math , I don't have as many toes as he does ...
DON - NO !

Do Not Run the deck and check blade speed ... to DANGEROUS since you'd have to aim at the blade tip !!!!!!
to late for that safety tip lol. I put tape on one blade and lifted the plastic exhaust shute and took a reading straight down and got 3189 at mowing speed. The front pulley is a 5.5" that is on the bottom of the 15 HP brigs. Is there something else you need to know??? Don T
Yeah ... is that your bloody finger laying there on the ground ?
21771.288 FPM ???

recheck me Ryan - I'm watching news and counting toes ...