Blade speed........rpm's
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IH3444
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The mower decks are designed to be run at WOT (wide open throttle). JD has taken the blade speed into account for engine speed, pulley diameters for the highest blade speed for each. I'm sure the blade tip speed is just under the speed of sound. Both decks are either exactly running the same blade tip speed at WOT, or are so close that you most likely cannot measure. I'm also certain that JD didn't retool for the model number change. I'm sure the difference in both machine is in a number designation only, with some slight improvements to frame stiffness, etc. I have a JD X750 with the 60HC deck.
CobyRupert
Super Member
If ratio of tractor's PTO pulley diameter to deck pulley diameter is the same on both tractors and decks, then blade speed (rpm) will be the same when both engine rpms are the same.
Blade (tip) speed will be the same if both deck's blade lengths are the same length and blades are at same rpm.
Why do you want to know?
Blade (tip) speed will be the same if both deck's blade lengths are the same length and blades are at same rpm.
Why do you want to know?
KennyG
Elite Member
There is a national standard and, I think, an OSHA limit on blade tip speed. It has to be less than 19,000 fpm. I think most mowers try to stay close to this.
bruceatlam
Platinum Member
Why is there an OSHA limit?
beenthere
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Here is what I found..
KennyG
Elite Member
The reason is that the lower the tip speed, the lower the potential for damage from objects thrown from the blade. Regulators regulate, that's their job. If they could limit the tip speed to 100 fpm, they would. They don't really care if the device works or not. However, there are practical limits to how far you can regulate things before you regulate them out of existence. The limit used to be 23,000 fpm and then was reduced to 19,000.
My theory on this is a follows. Push mowers need to be allowed and they directly drive the blade from the engine. If you do the math with 19,000 fpm, starting with a 3600 rpm engine, you find that the largest blade you can use is just over 20 inches. I know there are 21" and 22" push mowers, but I would guess the engines are governed to run a little slower than 3600 rpm. If they bring the limit below 19,000, it would become a real problem for mower design.
My theory on this is a follows. Push mowers need to be allowed and they directly drive the blade from the engine. If you do the math with 19,000 fpm, starting with a 3600 rpm engine, you find that the largest blade you can use is just over 20 inches. I know there are 21" and 22" push mowers, but I would guess the engines are governed to run a little slower than 3600 rpm. If they bring the limit below 19,000, it would become a real problem for mower design.
Richard001
Veteran Member
Ah Ha, so now I know why there are no large cut width walk behind mowers. And thanks for doing the math for me.
Also important, but not mentioned so far, is that with blade tip speed just at or above the speed of sound, you get a lot of blade noise.
KennyG
Elite Member
By my calculations, Mach 1 would be about 67,000 fpm, so I think scrapnel would be more of a concern than noise at that speed.
Grumpycat
Veteran Member
The governing documents are listed at http://www.opei.org/services/product-safety-standards/product-standards-listing/
Sadly, ANSI is very proud of their documents. The PDF costs $180.
Multiple sources says ANSI/OPEI B71.1-2017 specifies 19,000 FPM as the maximum tip speed for consumer mowers. That leaves the question open as to whether a riding mower can have faster tip speeds? Can push commercial mowers have faster tip speeds than box store mowers? What about bush hog-style mowers? Isn't worth $180 to me to find out.
IIRC Country Clipper and Exmark ZTRs have 17,000 FPM tip speeds.
Sadly, ANSI is very proud of their documents. The PDF costs $180.
Multiple sources says ANSI/OPEI B71.1-2017 specifies 19,000 FPM as the maximum tip speed for consumer mowers. That leaves the question open as to whether a riding mower can have faster tip speeds? Can push commercial mowers have faster tip speeds than box store mowers? What about bush hog-style mowers? Isn't worth $180 to me to find out.
IIRC Country Clipper and Exmark ZTRs have 17,000 FPM tip speeds.
