Blade speed........rpm's

bob3275 · Aug 28, 2019

I would like to know if the blade speed on a John Deere X530/X590 is different than the blade speed on a John Deere X740/X750. All the mowers have 54 inch decks. Thanks

IH3444 · Aug 30, 2019

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 · Aug 30, 2019

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?

KennyG · Aug 30, 2019

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 · Aug 30, 2019

KennyG said:
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.

Why is there an OSHA limit?

beenthere · Aug 30, 2019

Here is what I found..

here is a semi-engineering answer: I have been unable to get the spec on the web, but several sources name ANSI B71.1-1990 and ANSI B71.4 (commercial) as the governing safety spec "USA standard safety specification for powered lawn mowers" and list the reason for the limit as hazard for thrown objects. By the way, the limit is 19,000 fpm, 19,000 fpm equates to about 215 mph,

KennyG · Aug 30, 2019

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.

Richard001 · Aug 31, 2019

KennyG said:
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.

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 · Aug 31, 2019

Richard001 said:
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.

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 · Aug 31, 2019

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.

bruceatlam · Sep 1, 2019

Thanks for all that information.

As I see it, the answer boils down to "because we say so!"

two_bit_score · Sep 1, 2019

In doing my research it seemed that finding blade tip speed was difficult to find for most models. Yet it痴 the most telling thing about how good they cut grass. The faster the better. So called, designated pro models like most Scag mowers have higher blade tip speed.

If you could get a comparison chart of mowers by blade tip speed it would probably make deciding on a mower just too easy.

4570Man · Sep 1, 2019

KennyG said:
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.

That’s about what I had figured. Mower blades aren’t even close to that fast. I’m not sure where that miss information came from. But blades make a lot of noise at much slower speeds than that.

4570Man · Sep 1, 2019

KennyG said:
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.

I have a Howse finish mower that claimed be the manufacture to be 27,000 FPM. My calculations support the claim and the blade noise is the worst I’ve ever heard that would further support the claim.

Grumpycat · Sep 1, 2019

two_bit_score said:
In doing my research it seemed that finding blade tip speed was difficult to find for most models. Yet itç—´ the most telling thing about how good they cut grass. The faster the better. So called, designated pro models like most Scag mowers have higher blade tip speed.

If you could get a comparison chart of mowers by blade tip speed it would probably make deciding on a mower just too easy.

Country Clipper doesn't hide it. But again, 19,000 FPS is the ANSI/OPEI limit. Anyone exceeding that is inviting liability lawyers to send registered letters.

Feature Comparison for Residential & Commercial Grade Zero-Turn Mowers

buickanddeere · Sep 1, 2019

Operate the mower, snowblower etc at WOT redline like it was designed to . You are not 都aving the engine by operating at reduced rpms .

beenthere · Sep 1, 2019

I prefer not WOT. Designed for it or not, I just don't push many of my engines to WOT. Mower engines, more rpm's than others. Just do what feels good.

RickB · Sep 1, 2019

Blade noise isn’t a concern if you have a Kawasaki engine. They are so frigging loud you never hear the blades.

Topcat14 · Sep 7, 2019

Same blade tip speed on most mowers where 590 shines is the electronic governor the keeps engine rpms— up in high load conditions ( heavy grass) reacts much faster than mechanical governor used in most mowers can react some things that affect cut quality- what type of mower blades do you use- depending on what part of country a lot of different type grasses. Next thing is how often do you mow- constantly hear people complain about mulching, only to find out they mow once every 10 to 14 days in heavy growth season. If you have 6 inch high grass and try to mulch it down to 3 inches in one pass, no mulching system in world will work. If you are occasional mower, your best bet for quality cut is good blades and power flow bagger. Or take two passes of your lawn in heavy growth conditions if you want that golf course look! never cut more than 1/3 of the grass blades in single pass