OutdoorKing Repair Forum Forums General Discussion Forum Off Topics Something interesting.

In the USA the mowers all have solid blades, because they all have thin cast alloy
flywheels and they must have the blade as the counterweight or they cannot operate properly, as a matter of fact you cannot start an American lawnmower without the blade or the pull handle will rip out of your hand and can do serious damage to both your hand and the mower...

So that brings me to the question do all Australian mowers have cast iron flywheels? or do they just have enough weight with the disc?

Food for thought

Kori

You will find the disc is heavier than the bar blades on most American mowers.

It is usual to have just enough rotating inertia to prevent kickback during starting. How much that is, depends on the application. So, on mowers Victa uses aluminium flywheels and relies on the inertia of the blade plate. Briggs does the same. In special applications there may not be a blade plate: Victa adds a steel band around the flywheel of engines fitted to edgers, for example. The one that at first glance seems a bit odd is Honda: they use massive cast iron flywheels together with either a heavy bar blade or a similarly heavy blade disk, then use a decompressor as well. The reason for all those precautions against kickback is that they use a static ignition timing of 26 degrees BTDC, with an 8 to one compression ratio, which would kick very severely if not for the rotating mass and the decompressor.

The answer is that there are several ways of dealing with the issue - the important thing is not to take an engine specified for one purpose and fit it to a machine that operates differently.

I dunno know about that Kori (American mowers), the Tecumseh TVM 140 has a bloody heavy cast iron flywheel mate.....then again that is on a ride on, still manual start but...

It is different on ride on's....

Well, and I have learned a thing or too.

First off, the only blade disc I'd seen was a fairly flimsy contraption, and weighed almost nothing.... the mower it was on had a cast iron flywheel and overly large kickwheel and bandbreak... also a decomp valve (odd on a briggs) and it was one of the hardest staring, most poor running machines I have ever seen.. (not that that had anything to do with the blade disc)

Also Joe, I seriously doubt that, the blades on most American mowers are awful heavy(up to 15 lbs or 6.5kg), And the slasher blades are 1/4 inch thick and around 20lbs...


Kori

So how often do you see severe damage to a blade bar, boss, crank or crank tip due to the blades striking a solid object at speed?

And how much does it cost to replace your average set of blades?

The blade discs on some mowers here definatly outweigh your average bar blade (you must remember, we also get our fair share of direc american models here, complete with solid bar blades)

The disc setup is designed to provide a balanced counterwieght, while also allowing the blades to fold back if they come in contact with something potentially damaging to the engine.

You cna run into the top of an embedded boulder, and the most you will need is a new set of blades, which you can get for about $15, or a little more with the bolt set.

Remember, it is the moment of inertia of the blade or disk that matters, rather than its weight alone. The MoI depends on the square of the length (bar blade) or diameter (blade plate), so a bar blade has quite a lot of inertia effect. It seems to me that the blade disks on most Australian mowers I've seen have considerably less mass and MoI than a Honda bar blade, but genuine Honda blade plates are remarkably heavy too, like their bar blades.

I believe on mowers the weight of the blade disk is made sufficient to prevent kickback. Some manufacturers probably have larger safety margins on this than others, but generally, when the mower leaves the factory it is unlikely to kick unless the operator uses some odd starting technique.

Ty, I'm told that people manage to bend crankshafts by hitting things, whether they have a bar blade or a blade disk. It seems that some of them don't give up easily after they hit the rock, they try to machine it away to a flat surface, with the mower.

With 2-stroke Victa's, and a few others, there is the added protection that the blade disk is held on by pressure, rather than locking, however the downside to this is a slight increase in difficulty getting the setup right, and as a result a few home improvers can end up with a sore arm from kickback.

On the other end, rust, rope, grass, muck and age will do quite well to seize the blade disk bolt, and the blades to the extent where they do not provide adequate protection, when the mower is not maintained well over it's life.

But yes, even swingback systems working perfectly are not fool proof, there are always those out there who think there mower can grind rocks.

i just upgraded mowers a while ago to a toro naturally being an american mower it has bar blade i always had mowers with swing back blades and the first thing i noticed when i went to sharpen the bar on it was how much it weighed but it definately does have much worse kick back than a victa.

Very often Ty,

On American mowers the blade boss is small and when the blade hits something it is more likely to bend the crank than the blade...

one blade costs anywhere from $5 to $25 it all depends on the mower and blade(s) that need replacing...

Infact, IMO The blade disc seems like a better idea, after dirty carby's bent cranks are the most common reason for mowers being thrown out...


Kori

Well, Although we do get the odd bent carank over here, even on the disc mowers (as grumpy pointed out, you can't cover everyones 'bright ideas') it is really quite a rarity in my experience.

I can only recall having had 2 mowers come to me with bent cranks and one of them was a blade bar.