Yeah I have bent cranks with the original factory blade and everything up under the mower. I'm guessing if you hit something at precisely the wrong moment during the power stroke that's when its most likely to happen.
Factory RPM's for lawnmower engines
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The free mower was just out of warranty and the guy who had it was done messing with it. To me it looked like the self propelled mechanism was designed to fail after a year or 2 especially in dusty environments.
Generally, mower engines are designed to operate at a constant speed.
Even on mowers where the operator has an engine speed control, it's not actually directly connected to the throttle disk in the carburetor. Opening and closing the throttle is the job of the governor-under light load it keeps the disk nearly closed and as load increases it opens the throttle more and more(at least up until the point where you've overloaded the engine, at which point it will start to slow down). If you have a speed control, it's adjusting the governor to a lower speed.
On a lot of push mowers, the governor is an air vane tucked up under the flywheel/fan, and often actually directly attached to the throttle disk. Typically there will be a spring on the air vane that will hold the throttle at the full open position. Air from the engine fan pushes on the air vane to close it, so in operation the air vane will settle into a spot where air speed counteracts the spring pressure. If the engine bogs and starts to slow down, the spring will pull the throttle more open, and if the engine starts to speed up the spring closes it. The operator's speed control generally will change the spring tension, although at least on the older carbs I'm familiar with there's also a gross adjustment for spring tension that lets you change the maximum operating speed and the range over which the speed control works.
There was a thread on this once and I think there’s an ANSI limit for blade tip speed.
Sometimes the same engine shows up on multiple decks with different sized blades, although it seems anymore most of what's out there in consumer mowers is 21-22".
In any case, ANSI standard is 19,000fpm tip velocity. This, incidentally, is not so much for concerns about the blade breaking or shattering, but rather how fast the mower can throw projectiles.
A 21" blade will stay just under 19K fpm at 3400rpms, although it seems like very few mowers actually spin that fast as set from the factory.
Incidentally, if you have a 19" mower, you can run 3800rpms without exceeding that, although for some consumer engines I'd not be surprised if that's getting a bit too close to rod-through-block speeds.
1910.243(e)(2)(ix)
The maximum tip speed of any blade shall be 19,000 feet per minute.
I Believe It’s got something to do with how fast an object gets discharged.
I haven't seen the air vane ones in a long time now. My oldest backup mower has the air vane it gets clogged up with grass and runs a little fast.
Factory RPM is like 2800-2900, due to federal blade tip speed regulations. You do not want to set the rpm to that for best results. I always turn them up to 3100-3200 for best mowing performance.
You don’t need to use a tach to set it. Just go by ear. A little faster than stock is good.
You don’t need to use a tach to set it. Just go by ear. A little faster than stock is good.
Yeah my riding mower is over that by a lot if running full speed.
What’s the RPM and the blade length?
They about 23 inch and spin upward of 4,200rpm. The Briggs 590cc has the power to do it.
Is that the engine RPM or the blade RPM? If engine RPM, is the engine to blade drive ratio 1:1? How old is it? Is that the factory governor setting?
4200rpms on a 23" blade is a tip speed a little over 25,000fpm(or 287mph if you want to calculated it that way. That's well over even the "old" ANSI 21,000fpm regulation(I think pre-1998 for that number?).
I have seen some references to there being an exception to this for commercial mowers, however ANSI B71.4-2004, which covers commercial mowers, makes this statement
Or just do some due diligence and pay attention before you run stuff over. It’s a lawnmower, not a stump grinder, wood chipper or rock crusher.
I certainly am not going to play backyard knowitall with the blades on my deck that are going 205mph. Even with the thinnest cheapest blade you can find, do you wanna know what that chunk of steel will go thru if it shatters and flies off at centrifugal speeds? Just about anything it encounters.
I have found that shi happens.
I already know. I ran my over driven riding mower blades till they broke several times now.
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It was 1:1 when I got it.
How the governor is adjusted has a big influence on the running rpm. Try playing with the spring. Most likely, something changed when the carb was swapped.
I will clean the old carb this weekend and try it on and see if the correct RPM returns again. I can tell by the sound.
You may want to play around with the spring tension on the governor first. I had a similar problem with my Honda mower after a rebuild, only the engine was running too fast. Adjusting the spring tension brought the engine rpm down to where it should be.
Every Bittogger should have one of these.
That's why there's a governor.
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