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| Hi,
While I know many lawn mowers have the cutting blade connected direct to the engine's crankshaft, several others may not have the blade mounted direct on the shaft, but rather on a spindle or a pulley. The Toro 23267 25" commercial mower with its 6 hp Tecumseh engine may be an example of a mower whose blade isn't mounted directly on the crankshaft, because the engine turns 3,200 rpm, which coupled with the 25" blade length gives out a blade tip speed of 20,944 fpm (nearly 2,000 fpm above the government-mandated maximum that is 19,000 fpm), until you figure out some math which would have the spindle traveling at no more than around 90% of the engine's speed to bring the blade tip speed on this unit below the CPSC maximum of 19,000 fpm. So while the engine may be running 3,200 rpm, the spindle diameter and other underdeck mechanicals on this mower would be designed thus that the blade will travel at 91% or less of the engine's speed to stay within the limit. This issue also baffles me in regards to certain mowers produced in the 1960s, when the ANSI (then the USASI) had then imposed a 21,000-fpm max limit. Some engines have a 4,000 rpm governed speed, which was the case on some Allis-Chalmers mowers (among other manufacturers); this speed was OK on the WB1930 19" cut model (19,897 fpm, which back then was below the maximum; after August 1, 1968, however, the engine speed on mowers with this blade length had to be less than 3,820 rpm), but it becomes a problem on both the WB2235 and WB2235S models, where coupled with the 22" blade length, the blade tip speed is 23,038 fpm (2,000 over limit) unless these two particular mowers have spindles that are designed to travel at no more than 91% of the engine's speed. Sources: Briggs & Stratton Engines and Short Blocks for Replacement of Allis-Chalmers Original Equipment Engines, form MS-5741-48 (May 1, 1968). ~Ben |
Follow-Up Postings:
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| Don't forget: The engine is also used to move the tractor, as well as turn the blades on the deck. Notice that the tractor main drive pulley on the "stacked pulley" is smaller than the deck drive pulley below it. If you think the deck blades are revolving too fast--then go out and buy a different stacked pulley that has a smaller deck drive pulley,(Approx. $75 new) and experiment with shorter belts, until you find one that fits, and you will have lowered the blade tip speed! I am quite sure that you will be able to find the pulley you need to fit your application, as i have several here in my "used stack pulley" department! That way, you won't need to tell us "uneducated in college" folks all that stuff abot blade tip speed, and stuff like that! :o) And, who cares? As long as the blades send the cut stuff out from the underside of the deck and out the opening, thats what they are designed to do! If you are a lawyer looking for some Company to sue, well, keep looking! Please leave us out of it! We're here to help folks who might not have the mechanical "smarts" to fix their machines, not to worry about "tip speeds", nor who to sue about it! by: Rusty Jones |
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- Posted by orangedotfever (My Page) on Sun, Nov 1, 09 at 11:58
| What a government agency considers safe and what is actually safe in the real world can very well be two different things. 2000 FPM over the maximum sounds like a BIG number. Divide that by 60 and it's a whopping 33 feet per second. Kids shoot paintballs at each other at point-blank range at 300 FPS. Sure, projectiles can be, and are, dangerous. Just use some common sense and don't sweat the small stuff. |
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| Our Yardman, with a 22 inch blade, has a 6 hp engine, which looks like overkill, for the size of the mower. But, the engine is air-vane governed, & sounds like it runs at about 3000 rpm. The mower occasionally stalls in thick grass. Since I can't open the throttle, and crank up the rpms, the seemingly oversize engine is actually about right, for the application. |
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| 'seemingly oversized' engines began to be required when mulching started to get popular, then a 'horsepower race' ensued, finally (hopefully) topping out at around 7HP. Your 22" mower can legally run up to 3300 RPM, so I would tach it and set it there. |
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| rustyj14, You beat everything...What's up with the attack on a guy that's been a registered user for a year and a half. Sounds to me like he was trying to strike up a conversation about blade tip speed. Sorry you can't comprehend it. That's no reason to bring up "uneducated in college" stuff. That's so lame. Speak for yourself. I saw nothing in his post that indicates a possible lawsuit. Don't worry they wouldn't subpoena you anyway. Blade tip speed is a hot topic among ZTR owners and pro mower guys because it affects how fast they can mow. I found his points interesting and informative. By the way blade tip speed is calculated by finding the circumference of the blade tip circle in feet and multiplying that by the engine rpm. (Circumference is the diameter of the blade multiplied by pi.--Pi is figured by the average number of pieces of chocolate pie a typical guy can eat. That's 3.1415927. |
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- Posted by orangedotfever (My Page) on Tue, Nov 3, 09 at 0:30
| The easier formula is: BTS = Engine RPM X Blade length X 0.262 Thus my 21 inch R7070 running at 3300 Rpm is 3300 x 21 x 0.262 = 18156.6 feet per minute blade tip speed. PS. I typed this really slow so some people could follow along. |
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| Every mower dealer I talk to complains that mowers are supplied by the manufacturers with the engine speed set at least 400 rpm too slow to mow properly, and a lot slower than they used to be. The blades are the same length as they were when the engines were set to run faster. So I'm guessing that safety issues are less of a problem now than cutting issues. On machines where the blades are belt driven (ride-ons) I've noticed large variations in blade rpm. Which seems to imply that certain manufacturers are leaving themselves a large margin for error. |
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- Posted by orangedotfever (My Page) on Fri, Nov 6, 09 at 1:20
| "On machines where the blades are belt driven (ride-ons) I've noticed large variations in blade rpm. Which seems to imply that certain manufacturers are leaving themselves a large margin for error." Blade RPM is meaningless unless you know the length of the blade. An OEM running 2 blades to get the same deck width compared to one using 3 blades must run at a much lower RPM to remain within safety regulations. Don't get RPM and linear speed at the blade tip confused. |
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| I was comparing like with like. I'm not so ignorant as to confuse 2 blades with 3. What I'm saying is that a lot of machines appear to be designed with blade tip speed considerably lower than is strictly necessary to comply with safety legislation. I draw this conclusion because machines with the same length of blades and a considerably higher blade rpm are sold legally. |
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| EGAD!! My mind is boggled! (But, i do remember what PI is, but it took the preceding message to jog my memory.) But, even after all of the highly educated letters about blade tip speed, I'm just going to mow with anything i have here! Tip speed bedanged! Dang the torpedoes--full speed ahead! Up (or down) with the throttles--I'm going mowing! |
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- Posted by james_garfield (My Page) on Thu, Jan 21, 10 at 23:49
| I'm no expert, just another yard guy and part time mower fix'r. As I understood it, the 'government regulations' about blade tip speed apply primarily to machines intended for the Residential or Homeowner use market. Machines marketed for Professional use have provisions to adjust blade tip speed beyond what you'd find on residental grade machines. But legality issues cannot take the place of common sense. Know your machine. Know your surroundings. |
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| Thank you, James Garfield!! You said it like i did, only differently! I truly think the whole thing about tip speed is a crock--heck, you can get yer fingers, or toes, cut off, if yer foolish enough to stick them in the underside of the deck while the blades are turning, even if they are turning at the recommended blade tip speed! I firmly believe the whole maximum tip speed thing is to help the lawyers convince a jury, consisting of little old ladies and men who have never even used a lawn mower, that the machine was very dangerous, because it exceeded guvment regulations as to tip speed! After all---lawyers have to eat, too. To quote Red Green:"Keep yer stick on the ice!" |
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| Rusty / Ray: Your both the Best Entertainment for the Buck , I have experienced in Yrs. Just at opposite ends of the Spectrum ! I mean this in the Best and Most Complimentary Fashion ... I have not learned as much so fast in any College or University Level Institution . But then back in my day Slide Rules were in Vogue lol . Ifin She Don't Find Ya Handsome She had Better Find Ya Handy..Eh ! |
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- Posted by orangedotfever (My Page) on Sat, Jan 23, 10 at 13:50
| "I truly think the whole thing about tip speed is a crock--heck, you can get yer fingers, or toes, cut off, if yer foolish enough to stick them in the underside of the deck while the blades are turning, even if they are turning at the recommended blade tip speed!" Maximum blade tip speed has nothing to do with sticking your foot under the mower. It's about things that you hit being turned into projectiles and flying out from under the deck at high speeds. Limiting the BTS limits the projectile velocity. I have no idea how they come up with their figures for this though or if it makes that much real world difference. |
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| Orange: I'am with you , never heard of these Regulation's , but then never heard of Lawn Mower Racing either till recently lol . Ray: That Pie eating Dude must be one Heavy Dude ifin his Calorific Intake Directly correlates to Pi (repeating) lol :) P.S. Your Guys are Killin Me...Great Posts ! |
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| The whole thing boils down to one thing: KEEP BYSTANDERS, KIDS, DOGS, WIVES, CARS, AND ANYTHING YOU DON'T WANT DAMAGED BY THROWN OBJECTS, AND THE PROBLEM WILL NOT REAR ITS UGLY HEAD! i'M GONNA GO PUT ON THE FEED-BAG! HAVE A NICE DAY! RUSTY j. |
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| Have a Great Day Rusty ! Keep Yer Stick on the Ice..Eh ! :) |
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- Posted by orangedotfever (My Page) on Sun, Jan 24, 10 at 13:21
| Yep, if there's one thing I've learned over the years, it's that not matter how hard the government tries to foolproof things, modern society will come up with bigger and better fools. All that ends up happening is that things get heavier, more complicated and more expensive. Or, the good stuff just gets outright banned. |
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| Well, after reading all of the messages from the "learned heads" here, i have finally figured out how to make lawn mowers as safe as possible. I don't know why you folks haven't found the secret to having safe lawn mowers. Go buy a goat, or several sheep, tie them out in the yard, and let them go at it!;0) As for the manufacturers--simple--sell mowers without blades! And, don't sell blades to fit their products. Just think how safe lawn mowers will then be--no more chopped off toes, feet, fingers, hands, or any broken windows, and no more kids chopped up by mowers! Why, a man can buy a safe mower, take it out of the box, and push it around, looking like he owns the world, smiling at the neighbors, and knowing he is perfectly safe, in his yard, and knowing he can never injure anybody by using his perfectly safe lawn mower! Or, he could ride around on his lawn tractor, happy as a lark, and full of the knowlege that he won't be sued by anybody who might have been injured by thrown objects, like his old tractor or push mower used to do. He could even let his young kids ride safely on the tractor, while he simulates mowing his lawn! OOOHH! Just think of the possibilities. And, no more worries about blade tip speeds! By: Rusty Jones, The Mower Man! PS: I knew what PI was/ is--learned it in high school! And, Grand-ma knew what it was back then--she made the best tasting ones! A thought just occured to me---why doesn't some egg-head invent grass that doesn't grow any higher than 3-4 inches? Problems solved. |
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- Posted by orangedotfever (My Page) on Mon, Feb 1, 10 at 9:19
| Sorry Rusty. You can't do the goat/sheep thing. They create and emit methane gas and therefore contribute to global warming. You would have to purchase a number of carbon credit offsets per animal to be allowed to have them. Don't ya just love the government? |
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| Rusty Jones sez: :0) ;0) |
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| Gotta put catalytic converters on them. Who is going to volunteer to help catch all the squirrels & chipmunks and retro-fit them? Walt Conner |
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- Posted by oldlawnmowerman81 Milwaukie (Oak Grove (My Page) on Mon, Aug 2, 10 at 19:44
| If both the WB2235 and WB2235S mowers have their engines set to run at 4000 rpm, then doing the math properly, we get: = engine speed x blade length x 0.262 But if the engines (model 92902-0409 for WB2235 and 92982-0385 for WB2235S) on these same two mowers runs at 3500 rpm, we would then arrive at: = 3500 x 22 x 0.262 But, as I said before, after August 1, 1968, the engine speed would now have to be limited to less than 3299 rpm for a 22" blade, but we'd also suggest rounding up (or down) these maximum safe speed figures to their respective nearest hundred, and then subtracting 200 rpm from these rounded figures. The 200 rpm subtraction mentioned is to compensate for governor lag time. = 3100 x 22 x 0.262 So, for all those of you who own one of these or other 22" cut deck mowers, how fast do you run them? I suggest up to 3600 rpm on the 1968 and older examples, but from 1969 on, up to 3200 rpm is suggested. ~Ben |
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