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View Full Version : Heavy flywheel engine for Lawnmower???



radkins
04-03-2016, 11:33 AM
I have the chance to buy locally a new still in the shipping box Briggs 5.5 hp (8.75 torque) electric start engine for $100 from a neighbor that paid over twice that amount with shipping. This thing, while being a different brand engine, is still a direct bolt on replacement for the trashed engine on my Cadet self propelled walk behind mower but the problem may be that the guy who has it bought it for a log splitter he was building. Since it was to power a hydraulic pump it has a heavy flywheel instead of the lightweight flywheel used for engines with a crankshaft mounted blade so would that heavy flywheel cause a problem if used with a blade? This would be a real bargain for me if it will work, he changed the design of the log splitter and now needs to use a horizontal shaft engine so he offered me this one for less than half what he has in it. Will the extra weight of that iron flywheel be a reason to pass on this and just get the right engine instead?


I was told already that the heavy flywheel will be highly prone to shearing the key that secures it to the crankshaft if the blade encounters even the least sudden resistance even heavy clumps of grass? I can understand that happening if the blade hits something solid but just a heavy clump of grass? Would it really be that bad? How about any other problems I might encounter with a heavy flywheel engine on a walk behind mower?

sarge41
04-03-2016, 11:52 AM
radkins: The part about shearing keys sounds reasonable, but my reservations would be because the engine is probably Chinese made. Go somewhere, like a lawnower shop and observe one that is disassembled. Mostly plastic parts, such as cam and cam followers and timing gears and carburetor to name a few.

Sarge

brian Rupnow
04-03-2016, 12:14 PM
Radkins--the engine with the heavy flywheel should work fine on a lawnmower. True, if you run into a rock or some other "immovable object" you may damage the crankshaft---but that is true of any lawnmower. The only reason that conventional lawnmowers don't have heavy flywheels is that they don't need them. No manufacturer is going to put something "not needed" on any device you buy. It adds to their cost and cuts into their profit margin. As far as concerns about the engine being made in China, it's time to face reality people!! That's where 90% of everything is being made now, and no amount of whining and bitching is going to change that.

wierdscience
04-03-2016, 12:24 PM
I have several of the HF Predator engines which are Honda knock offs.Here's the deal,I prefer them,since unlike the US made Briggs I can actually buy parts for the carburetors if needed.

I don't think you will have any problems using the Briggs engine on your mower,other than the usual Briggs problems that is.

firbikrhd1
04-03-2016, 12:26 PM
I'm with Brian, key shearing should not be an issue. Many old Briggs and Stratton engines used cast iron flywheels (heavy) on lawnmower engines. I suspect that the use of lighter aluminum flywheels is one of economy in shipping and perhaps manufacture rather than necessity. A heavy flywheel may actually be a benefit, carrying the engine through intermittent heavily loaded periods with minimal slowing of RPM. In any case, the taper on the crankshaft is really what makes the connection to the crankshaft, not the key. The key simply assures proper registration to assure correct ignition timing. The keys are made of a soft material so they won't damage the crankshaft should a suddenly impact occur at the blade end, a point at which I am again in concurrence with Brian, the crankshaft is likely to be damaged (bent, twisted or broken) upon such impact that is heavy enough to allow the flywheel to overcome the friction of the taper fit.

radkins
04-03-2016, 12:55 PM
This is about what I had suspected already and I will go ahead and call the guy to tell him I will pick it up this afternoon.


BTW, the engine that came from the factory on that Cub Cadet walk behind mower was a Chinese engine, a Lifan I have been told, and it was certainly nothing to brag about. It has been trouble from day one barely outliving the warranty, just like the rest of that junky thing, and even the Briggs would have to be considered a major upgrade. As far as Lifan engine reliability I have read some good reviews on them but if this one is indeed a Lifan I definitely was not impressed! The self propel system on that mower looks as if it was designed by Moe, Larry and Curly so it's not as if I am installing a cheapo Briggs engine on an otherwise well built mower.

Willy
04-03-2016, 12:59 PM
I too would not hesitate on using the Briggs in this application for the same reasons already mentioned.
Actually like firbkrhd1mentioned the heavy flywheel may be a feature. It's not a motocross bike engine so the instant throttle response of a light flywheel won't be missed. Not sure what your lawn looks like but I'm assuming that all of the obstacles and rocks have long since been removed.

It's of little concern as to where it was built, as I'm sure it will be built to a standard, country of origin is not that much of an issue these days. Of the two Honda power equipment engines I have, one is Japanese and the other Honda was built in Thailand and I cannot discern the difference.

JCHannum
04-03-2016, 03:11 PM
I doubt that the heavy flywheel will cause problems, but, it might be a good idea to pull the flywheel and make sure that it has the aluminum "shear" key installed since it apparently was not built for lawnmower use.

LKeithR
04-03-2016, 03:36 PM
...As far as concerns about the engine being made in China, it's time to face reality people!! That's where 90% of everything is being made now, and no amount of whining and bitching is going to change that.

Exactly. The Chinese are quite capable of making quality products. The only reason we get a lot of junk over here is because the buying public is demanding that everything they buy be "cheap" and affordable. You get what you pay for folks...


I have several of the HF Predator engines which are Honda knock offs.Here's the deal,I prefer them,since unlike the US made Briggs I can actually buy parts for the carburetors if needed...

In Canada Princess Auto--and others--sell the "Chonda" engines. They're as good as anything else out there and the warranties are bulletproof...

Black_Moons
04-03-2016, 03:54 PM
Go for it. It might be a pinch harder to pull start. I doubt you'll find any other issues.

Yea, it might shear the flywheel key in a collision.. but your more likely to bend the crankshaft and make the motor near damn useless before that happens.

It certainly won't shear the key hitting clumps of grass. Not unless someone removed the flywheel and did not torque it up to spec when reinstalling it. As others have said, its the taper that keeps the flywheel on, the key is only so you put it back on with proper ignition timing.

That same sort of taper that secures your MT drills, your R8 tooling, etc. (No, that little nub in your R8 spindle is NOT what keeps your tool from rotating)

RB211
04-03-2016, 04:37 PM
If your grass is anything like mine in Florida during the summer, that heavy fly wheel should help the blade keep momentum when it is mulching the heavy thick grass. My cheap push behind with its 6.5 hp motor stalls out unless I go painfully slow. Needless to say, it has been replaced with a John Deere 285 garden tractor that has a liquid cooled 18hp Kawasaki 2 cylinder engine, and the deck is a 48" 3 blade job. I can cut my entire yard in 15 minutes or less now. Bought the tractor used for 350$ which was an absolute steal.

Black_Moons
04-03-2016, 04:46 PM
If your grass is anything like mine in Florida during the summer, that heavy fly wheel should help the blade keep momentum when it is mulching the heavy thick grass. My cheap push behind with its 6.5 hp motor stalls out unless I go painfully slow. Needless to say, it has been replaced with a John Deere 285 garden tractor that has a liquid cooled 18hp Kawasaki 2 cylinder engine, and the deck is a 48" 3 blade job. I can cut my entire yard in 15 minutes or less now. Bought the tractor used for 350$ which was an absolute steal.

Yes and im sure its the bigger flywheel that is getting you through that thick grass, and not the fact you have 3x as much horsepower now. :P

A flywheel is only going to store enough energy to keep those blades turning a couple revolutions under 6HP of load. Its not going to help you push through thick grass any faster then you could before, might just keep you from stalling when you drop the mower on thick grass/clumps but.. that is about it.

sasquatch
04-03-2016, 05:38 PM
I know of a few owners of these chinese knock offs, and they have been very good engines! I had a generator with a Briggs engine, and when i pulled the recoil shround off the engine was made by Misubushi,, stamped right in the block. I bought a chinese engined generator as a back up once, (it was cheap), and used it for hundreds of hours overloading it at times to build sons house with. That was almost 6 years ago, and it's still running. For the price they're getting to be decent engines.

CarlByrns
04-03-2016, 05:46 PM
radkins: The part about shearing keys sounds reasonable, but my reservations would be because the engine is probably Chinese made. Go somewhere, like a lawnower shop and observe one that is disassembled. Mostly plastic parts, such as cam and cam followers and timing gears and carburetor to name a few.

Sarge

Sarge- go to a go-kart track. All the stock-block series engines are (by rule) the Honda knockoff Harbor Freight sells. They're unkillable in racing and built to tighter tolerances than Briggs.

firbikrhd1
04-03-2016, 08:09 PM
When it comes to Chinese engines I believe it's luck of the draw and what factory made it. I have a friend that bought a Chinese water pump with Chinese engine a few years ago and it quit running. It turned out to only be that the valve adjuster had gotten loose and wasn't opening the intake valve. When I pulled the valve cover the rocker arms and screw adjusters were pretty cheesy looking. Then again, the Chinese make a diesel engine that is (or was, things change) nearly un-killable. I bought a new commercial mower (Encore) several years ago and opted for the Briggs engine (made in Mexico) because it wasn't going to be used in a commercial application (only once a week in a rather large yard in FL). Like RB211 I needed more power. I mowed one time with it and a friend came over to see it, considering the purchase of one. I started it up in the driveway and it threw a rod while idling. The dealer replaced the engine no charge and I got one with 1/2 HP more, but as I originally stated, to some extent it's luck of the draw.
Globalization may have made things cheaper but it certainly hasn't made quality any better.

Tim Clarke
04-03-2016, 10:32 PM
Back when I was young, I fixed lawnmowers by the score. Jacobsen's had a Briggs with a dished shaped rotor, and 4 little triangle shaped cutters. Also some flaps for airflow. These were as good as it got in the 60's. At the time, vertical Briggs had an Aluminum flywheel, too light for naked operation. They depended on the added weight of the blade to make up for the too light flywheel. These old jakes with the heavy rotor/blade assy. were as smooth as it got. I tried the iron flywheel from a Horizontal shaft on a couple rotary mowers, they ran so smooth I couldn't believe it.

At that time Briggs was careful to use an aluminum key for the flywheel. Prevented crank damage most of the time. I don't have a clue what they do now. I have a John Deere mower with a Briggs on it now, it's 20 years old and it never has missed a lick. Maybe I'll need to pull it down sometime, and then I'll know. Get this engine on your mower, you'll love it.

Tundra Twin Track
04-03-2016, 10:49 PM
I ran a 3.5hp Tecumseh mower engine with no blade,just a 2.5" pulley on my self propelled ladder.You really had to pull quickly on the rope to start it.This past fall changed it out with similar version of that engine,but added a 6" pulley for some rotating mass ,starts easier now.

RB211
04-03-2016, 10:58 PM
My favorite small gas engine of all time is the Kohler K series. Restoring an old John Deere 140 with one... Anyhow, back to the discussion on Briggs, if you want a really good Briggs, you need to buy one of their Vanguard series engines. These are popular for repowering old garden tractors. They are up there with the Honda's. I've only heard good things about the Predator engines from Harbor Freight, mainly from the Go cart crowd. I do know of some people taking the twin cylinder Predator and repowering older tractors with. I have no idea how long they last. I know a K series can go 30 years before needing to be rebuilt.

J Tiers
04-04-2016, 12:18 AM
Actually, there IS some added risk with a heavy flywheel, depending somewhat on the key system.

The flywheel is normally on top, while the blade is on the bottom. In between is the crank, which transmits torque well to either top or bottom, but less well THROUGH itself from top to bottom.

The blade is normally the dominant "flywheel" in the system, with the light aluminum unit functioning to operate the magneto, and act as a fan. it has far less effective rotating mass than the blade.

If the blade is stopped abruptly with the light "magneto disc" flywheel in place, probably not much bad happens. May partly shear the key, I've seen that to the point that the timing is messed up, but you do not really see a key problem visually. With a heavy flywheel, a sharp stop will certainly shear the key, if it is an aluminum shearable key, and the torque on the crankshaft as the flywheel slows down due to friction may distort the crankshaft and/or score the flywheel seat. If the key is not a designed-to-shear key, there may be serious crankshaft distortion, depending on the flywheel mass, and the actual rate of stopping.

I've seen the problem with built-up crankshafts, like Gravely, where the pin is held by friction, no splines. That starts the back of the crankshaft wobbling as you look at it, due to the pin shifting in the two discs. The problem seems to be the electric start gear on the back side, which is pretty big and has a fair amount of rotational inertia.

radkins
04-04-2016, 09:47 AM
A hundred bucks later and now it's mine! :cool:


I really like the electric start feature of this engine due to my disabling neck condition that makes pull rope starting not only difficult for me but in fact quite risky. From what most are saying here I think it might be a good idea to remove the flywheel before I install this thing to make certain the key is shearable aluminum, also whenever I have a flywheel off of one of these engines I like to coat the shaft end with antiseize compound to make it easier to disassemble later if the need ever arises. I have done this many times in the past and the compound has never caused a problem, in this case I am thinking it might even prevent damage if the key shears and the flywheel turns on the shaft, well maybe anyway. That's interesting about the taper securing the flywheel in place and although I had never thought of it that way it makes perfect sense, I guess that because I had never thought about it one way or the other I had just always "assumed" the key did that job also along with properly aligning the flywheel for timing.

In any case I should have my mower going by this afternoon.

firbikrhd1
04-04-2016, 10:35 AM
Actually, there IS some added risk with a heavy flywheel, depending somewhat on the key system.

The flywheel is normally on top, while the blade is on the bottom. In between is the crank, which transmits torque well to either top or bottom, but less well THROUGH itself from top to bottom.

The blade is normally the dominant "flywheel" in the system, with the light aluminum unit functioning to operate the magneto, and act as a fan. it has far less effective rotating mass than the blade.

If the blade is stopped abruptly with the light "magneto disc" flywheel in place, probably not much bad happens. May partly shear the key, I've seen that to the point that the timing is messed up, but you do not really see a key problem visually. With a heavy flywheel, a sharp stop will certainly shear the key, if it is an aluminum shearable key, and the torque on the crankshaft as the flywheel slows down due to friction may distort the crankshaft and/or score the flywheel seat. If the key is not a designed-to-shear key, there may be serious crankshaft distortion, depending on the flywheel mass, and the actual rate of stopping.

I've seen the problem with built-up crankshafts, like Gravely, where the pin is held by friction, no splines. That starts the back of the crankshaft wobbling as you look at it, due to the pin shifting in the two discs. The problem seems to be the electric start gear on the back side, which is pretty big and has a fair amount of rotational inertia.

In the case of the Briggs engine the key is a designed to shear key made of aluminum. They will shear if the flywheel isn't tightened properly or if the blade end comes to a sudden stop causing the inertia of the flywheel to shear the key. That is not to say that then crankshaft will never be damaged. Even with lightweight flywheels on Briggs engines abuse such as sudden stops can twist or even break the crankshaft, at times even at the blade end or at the counterweights.
On older Briggs engines, flywheels were heavy regardless of intended use. That changed over time probably more due to cost effectiveness than any mechanical reason.

J Tiers
04-04-2016, 12:24 PM
...
On older Briggs engines, flywheels were heavy regardless of intended use. That changed over time probably more due to cost effectiveness than any mechanical reason.

yes they did....easy die casting, far cheaper than cast iron. But the shear key is certainly, as you point out, not a guarantee. Much less so in the case of the heavier flywheel, as I mentioned.

The heavy flywheel increases the risk for devices subject to sudden dead stops like a direct-drive mower with no intervening belt.

Briggs was not interested in what happened with a sudden stop. Not their concern, but that of the OEM customer. Mower companies could have put in a slip clutch to prevent that damage, but chose not to, again for cost reasons. If their consumer customer was blame fool enough to jam the blade, that's outside of the "proper usage" and not warranted.

aribert
04-04-2016, 01:11 PM
I may be off base, but I would tend to think that the lighter flywheel on a mower engine is part of the quick engine stop on consumer grade mowers. I remember mowing my parents yard 40+ years ago and when I ran the mower too fast into a thick clump of grass, and the mower was about to stall, just pushing down on the handle to lift the mower deck up (and clear out the excess grass) the mower would speed right up to normal RPM. Its rare that I am able to do that today w/o the mower stalling anyway. I like the old unsafe way (without the quick engine stopping) much better. I bought a pressure washer (expecting it to have a bad pump) to use the engine as a push mower engine. Turns out the $75 pressure washer works just fine so I never got around to swapping the motor onto a spare push mower deck.