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View Full Version : Blade tip speed?


A. Kirk
09-16-2008, 03:31 PM
Does anyone know why the ANSI spec recommends a maximum blade speed of 19,000 ft/sec?

JimQ
09-16-2008, 04:07 PM
Taken from ANSI B71.1

A.22.4 Blade speed
This subclause specifies a design requirement intended to minimize the possibility of a hazard from thrown objects.

This was established long before I was part of the industry.

I'm not sure why a 19K FPM limit was established instead of 18K or 20K.

Good question. Any industry old-timers out there?

Q

Richard Martin
09-16-2008, 04:14 PM
And always remember.... It's a spec and not the law.

JimQ
09-16-2008, 04:37 PM
And always remember.... It's a spec and not the law.

Very true... BUT as an OEM, you can kiss your butt goodbye in court if you decided not to comply.

Q

TomberLawn
09-16-2008, 06:15 PM
When something is turning that fast, does it really matter exactly how fast it's going? I know for cut quality that faster is better, but as far as slinging stuff, looks like 19k fpm is just as dangerous as 20k fpm. So let's make it 20 and get a better cut.

Runner
09-16-2008, 11:08 PM
Also, thank you for clarifying that it is ft/MINUTE, not ft./sec.. lol THAT would be pretty dog gone fast! Somewhere around 12,000 mph. Almost twice the speed of sound.:laugh:

TomberLawn
09-16-2008, 11:32 PM
If my math is right, the speed of sound is somewhere around 750mph, so that would be like Mach 16 blade speed!! Grass wouldn't know what hit it, if the blades could stay together at that speed. Imagine the G forces.

FaithNoMan
09-17-2008, 03:20 AM
Also, thank you for clarifying that it is ft/MINUTE, not ft./sec.. lol THAT would be pretty dog gone fast! Somewhere around 12,000 mph. Almost twice the speed of sound.:laugh:

No, that's about 15.4 times the speed of sound. I'd imagine your machine would take off(in the air) very quickly and/or disintegrate the blades, deck, wheels, and operator also.

JimQ
09-17-2008, 10:50 AM
I was thinking about this last night.

Knowing how standards are developed, sometimes you just need to pick a number that you're "comfortable" with, and go with it.

It might be rooted in technology that existed when the standard was developed.

I'm thinking of a single blade walk behind mower.

3300RPM seems to be the standard engine RPM for 21" mowers.

A single 22" bade rotating at 3300RPM would have a blade tip speed of 19,006 FPM

A 21" blade rotating at 3300RPM would have a blade tip speed of 18,142 FPM

That's my best guess. I'm still waiting for someone who actually knows to come along and educate me.

Q

cybervision
09-17-2008, 11:08 AM
19,000 FPM = 215.90 MPH

Anything coming out of the machine at that speed sure is dangerous.

Richard Martin
09-17-2008, 07:10 PM
19,000 FPM = 215.90 MPH

Anything coming out of the machine at that speed sure is dangerous.

You mean like this stuff?

This is a 3 foot long piece of angle iron I hit about a month and a half ago.

http://i208.photobucket.com/albums/bb56/RMartin631/mower%20stuff/angleiron.jpg

This is a 2 foot long stake that was ejected from my Dixie today.

http://i208.photobucket.com/albums/bb56/RMartin631/mower%20stuff/stake.jpg

cybervision
09-18-2008, 12:42 PM
Hey, you screwed up a perfectly good length of angle there. Hopefully your blades don't look like that now.

I'm thinking you may need to mount a metal detector on the front of your machine.