Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

With a lawnmower...

okay so everyone knows your not supposed to hit stuff with their lawnmower (treeroots,survey markers,rocks,small cars ect)

but did you ever wonder just what happens when you run over something .

we had an ariens tractor come in friday with a complaint of a terrible noise in the deck.

so we remove the deck and flip it and check it out. it had 2 blades and they are timed,they run from a cog belt and are designed not to pass each other .i guess you could say they overlap .

anyway the operator ran over somthing and 1 blade skipped over its cogs on the belt.

now most of you know that ariens is pretty good gear and they have a very heavy duty set of blades on this particular mower .they are almost a 1/4 inch thick,has to be more than 3/16 at least.

anyway the blades met each other and as a result one blade ended up badley bent and the other lost at least 2 inches off each end...thats a lot of smash and crash with some heavy blades.

so here is why it happens.

there is a regulation  of blade tip speed,comes from the guys that put the stickers on the mowers. i beleive its ANSI in the US.

they have determined that 18000 feet per minute is the maximum safe speed a blade can travel.

this workes out to about 3200 rpm on a 21 inch walk behind mower.

a tractor is set up the same way,blade speed is regulated the same.

the number used to be higher ,but was slowed a few years ago not sure of exact date.

so you say whats that 18000 mean to me ,its kinda hard to relate to.

lets put it in perspective.

we know that a mile has 5280 feet 

we know the blade travels at 18000 feet per minute ...

so the blade tip actually travels at  3.4 miles a minute. here is where it gets interesting.

3.4 miles a minute gets us over 200 miles an hour.

no wonder blades get bent and cranks get twisted and lawnmowers get junked.

hmm makes me wonder if i should start wearing some extra safety gear when working on them.

later chris 

That's fascinating!   

I found that very informative

Thanks for sharing!

I never realized exactly how fast they went, but do know that they will send an aluminum can a loooooong ways.

Interesting tidbit of info there.

Hey snow, good job on the Math. During my school days we did similar math on large rotating objects, like Power generators.

Imagine this. A large generator Turbine, used on a steam propeled generator, is aproaching supersonic speeds at the tips. They cant make them any larger or spin any faster, or they would disintegrate.

bontaiJoe wrote:

Golf $#%* too! 

Kevin

This interesting post prompted me to look up my blade tip speed for my exmark, as well as the ansi standard. here is what I found.

exmark states the blade tip speed on my exmark is 18,500 feet per second.

This is the "law" ;

In section 8.1.4 - Blade tip speed - of the ANSI/OPEI B71.4-1999 Safety Specifications for Commercial Turf Care Equipment, it states that
"the maximum tip speed of any blade shall be 19 000 ft/min."

Exmark states it keeps their product just under the limit due to people "tweaking" the engines, aftermarket belts, etc.

Very interesting.  Thanks Chris!!

Frank D.

Nice info. SS... Thanks !

Maybe I'll take the 21 incher to the drag strip !  

Zoommmmm..... !!!!!!!

Dave...

Cool.

Reminds me of when I was a kid mowing lawns to earn a buck during the summers.
I used the mowers of the owner's of the lawns I cut.
In general it worked out well..
Till one owner sharpened his blade, never tighten it after putting it back on, and it came out the back and almost took off my toes.
It burried itself in the ground just infront of my toes. It was a bit scary.
Mowers back then didn't have the extra protection plate hanging from the rear axles like they do now.


-Bob
LI NY

Gosh Dang