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Oh my, wait until Bloomerbumb finds out about this.

2K views 33 replies 22 participants last post by  buckhorn_cortez 
#1 ·
#2 ·
Dealt with an like issue the other day at a lawyers office.

A city riding mower with multiple blades was mowing near some train tracks and it caught a discarded rail road spike/huge nail. It went across the field, past a parking lot that has stalls on each side with room for two way traffic between them, across two lane street and side walks, into a lawyers tempered window that it damaged rather well.

It was just luck that it cleared a bunch of vehicles, traffic and people and only caused a broken window. The spike was on the sidewalk a few feet away from the lawyers window and it had a large fresh gash on it from a mower blade, along with some fresh grass marks. He thought his office was being shot. I thought a person might had thrown it at first, but I once had an officer shoot through a tempered window like it and the rounds barely went a few feet inside and another officer hit one with a baton a few times and did not break it, along with the fresh nick and grass, I had to look around and observed way over yonder was the only grass and there was a team mowing.

It could have killed a person and/or caused a heck of wreck and/or damaged a few vehicles, and the mower had safety guards around all the cutting areas. The mower operator could only tell me he hits things all the time, but the blades can take it and the guards do their job most of the time...

Mount your lawn mower upside down and running (if it would run that way ;) ) with a bucket of nails on a string above it, you should be able to take out an entry team without a firearm.
 
#5 · (Edited)
A projectile launched by a mower cannot exceed the velocity of the blade. I calculate the tip velocity of a 24" blade to be 376 fps at 3600 rpm. To equal the speed of a .357 Magnum (1400 fps), the motor would have to turn 13,400 rpm. Three times that would rival a dentist's drill. That would certainly attract attention :)

Actually, the report says kinetic energy, not velocity. A railroad spike weighs about 2 pounds. the energy = mv^2/2/32 , or about 4400 ft-lbs. The blade is going to slow down a lot hitting something that heavy, so this figure is too high. We would have to know the mass and center of momentum for the blade (about 1/3rd the distance from the hub). In an elastic collision (i.e., nothing bent), momentum is conserved, not energy. A lighter object, like a nail, will not slow the blade much, but the energy would be proportional to its mass.

Sorry to spoil this party, but it was fun while it lasted.

Pouring nails into an inverted mower - which way would the nails go?
 
#6 ·
A projectile launched by a mower cannot exceed the velocity of the blade. I calculate the tip velocity of a 24" blade to be 376 fps at 3600 rpm. To equal the speed of a .357 Magnum (1400 fps), the motor would have to turn 13,400 rpm. Three times that would rival a dentist's drill. That would certainly attract attention :)

Actually, the report says kinetic energy, not velocity. A railroad spike weighs about 2 pounds. the energy = mv^2/2/32 , or about 4400 ft-lbs. The blade is going to slow down a lot hitting something that heavy, so this figure is too high. We would have to know the mass and center of momentum for the blade (about 1/3rd the distance from the hub). In an elastic collision (i.e., nothing bent), momentum is conserved, not energy. A lighter object, like a nail, will not slow the blade much, but the energy would be proportional to its mass. They seemed to be mowing at fast rate/speed.

Sorry to spoil this party, but it was fun while it lasted.

Pouring nails into an inverted mower - which way would the nails go?

The one I dealt with was a commercial mower that had like 1800MM slasher main blade and other smaller side blades that could cut what looked like a 12 feet wide area in one pass and as much horse power as motorcycle if not more (who knows the TQ). The spike was less than a pound, like 12oz.

So say a 0.75 lbs spike and a 70" blade that I don't think would slow much if at all.
 
#7 ·
I suck at math but found this, but I am going to guess the RPM's at 3000, however I am sure the blades being gear driven might have been going faster than 3000 RPM's. I get about the same as a subsonic .45 only a much higher grain... ;

Propeller and Fan Tip Speed Calculator

Determine Propeller RPM Redline

Would you like to know how fast your propeller tips are traveling? Enter some different combinations of prop diameter and RPM setting below to find out. Note: Ambient temperature is needed to determine the speed of sound for a particular day. The speed of sound varies according to air temperature.


Compensation for outside temp in Celsius. Speed in Mph.

* = REQUIRED INPUTS



80 * Prop Diameter. In inches.

3000/3600 * Engine Rpm. ( Max )

1 * Engine Gear. Example. 2,58:1 set 2.58, with no gear use 1.

714/857 = Prop Tip Speed in Mph.

20 * Outside Temp. in Celsius ( Normal is 20 Celsius. )

767 = The Speed of sound in Mph. Calculated using outside temp.

0.930899608/1.117340286 = Prop Tip Speed in Mach.
 
#8 ·
You're right I did miss quote the article, it was not intentional. The danger is the point of the comparison.
 
#9 · (Edited)
If a mower blade propelled stone or object could equal .357 ballistics, the flimsy sheet metal or plastic blade guard would be full of "bullet holes".

Mythbusters did "test" the lethality of mowers and concluded that the momentum of objects thrown by mowers could match a .357. Of course, they conflated energy and momentum and lethality.

Sure, you could throw a punch with more momentum than a .357, but that does not make it more lethal than a bullet. If that punch is delivered via a knife or hammer, then the energy or momentum involved hardly matters, other than to say it does not take much effort to cause lots of damage when it is delivered to a small focused area.

Mythbuster killer lawnmower videos:
https://www.youtube.com/watch?v=CtRT0hxXwlA

https://www.youtube.com/watch?v=AOugpYXnWFk

https://www.youtube.com/watch?v=nOqdQ84Zn0s
 
#10 ·
When I was a kid we had an electric mower with a fiberglass shroud. My brother got hit in the chin with a rock that blasted through the thick fiberglass. He had a good amount of swelling and a gash that required stitches. Not having learned the lesson, the next year the exact same thing happened with a rock going through the same hole. If he was hit a few inches up on either occasion it would have likely been ugly.

My kid when he was smaller liked to run near the mower. A few stern warnings has ensured he's nowhere near it when running.
 
#11 ·
Mythbusters already did that one. Not sure what they proved but they sure tore up a bunch of blades
 
#15 ·
Rocks thrown by older gasoline mowers would take out a tempered car window.

Managed to do it once on a Buick station wagon.

CPSC lowered blade tip speeds years ago on homeowner mowers.

Around the same time they mandated the 'dead man' handles (the ones everyone bent up clip from coat-hanger wire to disable).
 
#18 ·
Gravel n' stuff hits the shroud on my mower all the time. No holes. The ones that have hit my feet or the sides of my cars have done little damage.

The ".357" comparison is clearly horsecrap. Even stone that causes bleeding & stitches is nowhere near comparable to a bear-killing headshot with a .357!

Now as for "total energy" sure. That means nothing. An RV rolling at a walking pace has more "energy" than an elephant rifle. So what? It's not that dangerous.

As a kid I worked at a range that hosted handgun silhouette shoots. Guys would hit, but fail to knock over, targets that I could kick over with my foot.

Later, when I heard the term "knockdown power" I laughed, wondering what the hell they were talking about.
 
#20 · (Edited)
Chicago heard about it and banned lawnmowers, now thugs are going around with them cutting grass. But Chicago is paving over all grass and putting up city run parking lots. Illinois is now doing background check and issuing M.O.I.D.(Mower Owners Identification) Cards at a $250 fee. An illegal market of goats and sheep are cropping up, but peta people are protesting farmers.
 
#24 ·
Airplane prop tip speeds are kept under 700 mph. & RPMs under 3000 rpm.

& four stroke mowers run at pretty low RPM's. Unless they are geared-up with insanely tall transmissions (& they aren't) there's just not going to be much speed there.

I have no tach on my Briggs mower but if it spun faster than 3000 rpm when mowing I'd be shocked.
 
#25 · (Edited)
A projectile launched by a mower cannot exceed the velocity of the blade. I calculate the tip velocity of a 24" blade to be 376 fps at 3600 rpm. To equal the speed of a .357 Magnum (1400 fps), the motor would have to turn 13,400 rpm. Three times that would rival a dentist's drill. That would certainly attract attention
Power mower blade speed is generally given as the blade tip speed in FEET PER MINUTE.

The mowing industry standard "safe blade tip speed" is 19,000 feet per minute (FPM). It is defined in ANSI/OPEI B71.4-2004 Commercial Turf Care Equipment - Safety Specifications.

The highest speed commercial, rotary mower is the King Kutter rear discharge with a blade speed of 18,463 FPM.

The reason for wanting a high blade tip speed on a rotary mower is to lift the grass so it is cut cleanly, and at the same height.

The blade is going to slow down a lot hitting something that heavy, so this figure is too high
.

I doubt that the blade will be slowed much, if at all. Commercial zero turn mowers will have 25+ HP motors. If the mower was a rotary cutter on tractor then the PTO horsepower running the mower could be anything from 30 HP to well over 150 HP (larger HP tractors would be impractical for usage outside of farm work).

Having used a variety of large mowers, I have hit: basketballs, metal sprinkler heads, cement blocks, pop bottles, rocks, branches, hay bales, and logs.

The mowers never slowed down. In fact, the day I was using a 60-inch rotary cutter on a 33 PTO HP tractor, I hit two hay bales hidden in a large 4-wing salt bush - the tractor automatically slowed its forward speed as it transferred additional hydraulic flow to the PTO output - and PTO speed (2500 rpm) never moved.

In order to help with thrown objects near buildings, I often use a flail mower for rough cutting, as the mowing blades (hammers) are attached by a short chain to a central rotary drum and fold up if they hit something. I've hit a cement block with a flail mower and it just left a pile of dust with no damage to the mower and nothing thrown out of the mower.
 
#27 ·
Woah! Did you guys catch that? FPM vs. FPS.

That's where the crazy comes from.

FPS (feet per second) is sixty times faster than FPM (feet per minute) That explains the whole misunderstanding going on here.

Also, Yes, a spark plug insulator will pop a car window with little effort. But have you tried a ball peen hammer? The flat side will bounce off a car window, But the round side will explode it.
 
#28 ·
I wonder the effect if the mower hit the holy hand grenade.:scratch:

The one time I took a major hit from a mower as a kid, I was cutting the parents' lawn by a sweet gum tree and one of the gum balls shot out the back of the mower (pre skirt days) and hit me right in the family jewels. :eek: That really taught me respect for checking the area before mowing.
 
#29 ·
The mowing industry standard "safe blade tip speed" is 19,000 feet per minute (FPM).

That's 316 fps. I guessed high.

The blade will slow - it's a matter of physics and conservation of momentum. If it didn't slow or deform, the force exerted on the shaft would be infinite. Somewhere there is a clutch or slip joint that allows it, otherwise the blade and stub of a shaft would accompany the railroad spike or sprinkler head.

You feel it when a bat hits a ball because the bat decelerates. You feel it less when it hits the sweet spot, otherwise known as the center of inertia, because the bat doesn't twist in your hands. Then there's something called "recoil" which is occasionally mentioned in this forum. Newton's Laws rule.
 
#34 ·
The blade will slow - it's a matter of physics and conservation of momentum. If it didn't slow or deform, the force exerted on the shaft would be infinite.
No it would not be "infinite" unless what you've hit is larger than what the drive system is designed to deal with; OR whatever you've hit is permanently anchored and stronger than the drive system.

The amount of energy required is simply the moment of inertia - and that's not infinite for a railroad spike.

What you seem to want to ignore is the strength of larger mowers - and the fact the spike is simply a loose object, not something that is permanently anchored.

As an example, if you hit a metal sprinkler riser with a 24-inch rotary push mower - it will stall the mower and probably bend the shaft.

If you hit the same sprinkler riser with a rotary cutter with a 100HP rated gear box, and mower itself rated to cut up to 3-inch diameter material - it will cut right through the metal riser pipe just like it was a small tree. The mower will never slow down.

The only way you'll know you've hit the riser is the noise of metal hitting metal. It will leave a slight divot in the front of the cutter blade, but it won't impede you from cutting for the remainder of the day.

Commercial mowers have slip clutches that are part of the drive system to protect the mower is you do hit something that is immovable or beyond the rating of the mower. Rotary cutters will have either a slip clutch or shear bolt as part of the PTO drive line.

All that's being talked about is a railroad spike. Yes, if you hit the spike with a 24-inch push mower it will slow the mower and possibly even stall it.

Hit the same spike with a commercial ZTR or a commercial rotary cutter and the machine will make a "clank" as the blade hits the spike and that's about all you'll get out of it as the weight of that object is well within the rating of the machine's drive system. The slip clutch won't be activated and neither would you break a shear bolt.

It's not like the spike is anchored solidly into something - it's no different than hitting a rock or large branch. The blades have a great deal of inertia, and the drive system is designed to absorb that kind of momentary shock.
 
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