[LoveLearn]

I'm reassembling a Lawn-Boy S21BSN. The hub to which the blade bolts has a slot cut into the bore side and the engine's shaft also has a matching slot. These two cuts APPEAR to have been machined for the purpose of mechanically joining with a woodruff key. If such a key was in place when I disassembled this mower, I lost it. When I went to Toro/Lawn-Boy's on-line drawings for the S21BSN, the drawing titled "Housing and Shroud Group" shows a group of parts described as "30 1 Clutch Brake Assembly." I do not see a woodruff key illustrated in that group. I stopped by a local Lawn-Boy dealer who also looked for any key there or a part number for it. He said that if it exists, it's not shown.

As a Dentyne gum TV commercial said years ago when kids left home without brushing their teeth, "WHAT'S A MOTHER TO DO?"

Does anyone know if this model is supposed to have a woodruff key locking the crankshaft to the blade-mounting disk?
John

 

[johnsonmk]

I think it's an unused keyway...
Parts diagrams show no key.

 

[hankthecrank]

I have had the blade clutch apart on several M-Series and there is a key in those and the drawing looks similiar to me.

 

[johnsonmk]

I have had the blade clutch apart on several M-Series and there is a key in those and the drawing looks similiar to me.

Post some model numbers so we can look them up.

 

[hankthecrank]

I cannot find any breakdowns that show a key either but the piece on the breadkdown ( that does not have a number)shown does have a key in it that is what keeps everything from spinning on the shaft when the blade is engaged. And the numbers are 10590 and 10591.

 

[LoveLearn]

Thanks for the feedback so far.

I examined model 10590 exploded views which show a bottom-tapered crankshaft and matching friction-locking tapered blade mounting piece. Its blade attachment configuration appears very different from that used on the S21BSN.

The S21BSN's crankshaft bottom is constant-diameter rather than tapered. It terminates with a flat end into which an Allen-head cap screw clamps a disk against the blade-mounting hub. The blade is attached to the hub by two short bolts. The hub slides onto the constant-diameter crankshaft.

Interestingly, the hub is attached to the crank through a sealed anti-friction bearing. Perhaps crankshaft torque drives into the outside diameter of the clamping disk, but if the blade hits an obstruction which instantly stops its rotation, the friction bond between the disk's outer clamping surface and/or inner clamping surface momentarily behaves like a clutch, allowing the hub-mounting bearing to rotate. That could prevent destructive blade-strike-torque loads from being transmitted into the crank.

This photo shows the
constant-diameter crank with flat end and keyway,
clamping disk,
Allen-head cap screw,
blade-mounting hub with anti-friction bearing and keyway.

No matching woodruff key is shown in this model's exploded view so dealers have no part number for any such key. Maybe they should list it as a phantom key and offer them for $0.00, limited to one per customer as a promotional item. Hope you enjoy this kind of "mind candy" more than cross-word puzzles.
John

 

[hankthecrank]

The key needs to be there, there should have one in there, if not someone has had it apart and not replaced it.

 

[LoveLearn]

I combined parts from two F-series engine powered S-model mowers to restore this mower's operation. I took the parts machine's flywheel woodruff key and found that it fit snugly into these blade-driving bottom keyways. It mows fine with that key installed.

Pulling the starter cord slowly against first compression cycles is a time-honored way to roughly reveal compression leaks and peak compression. That test after rebuilding, which included hand-lapping the reed valve surface plane within the lower case, revealed noticeably higher compression in this engine than it developed in recent years. My friend who ran the mower said that it's exhaust note running frequency no longer dips when he cuts into heavier growth. These mowers are too small to connect to my old dynamometer. But I think that if we were to connect mower engines to a suitable DC motor, we could drive the electric motor as a DC generator. We could plot the DC output curve over mower engine rpm ranges to generate comparable curves. That would be very instructive. We already know direct-drive mower rpm curve legal limits. They are determined by blade lengths and the federal blade tip speed 19,000 feet per minute limit. Has anyone ever tried plotting Lawn-Boy torque curves? I'm sure in-house Outboard Marine Corporation designers did that frequently during their engine development work, but sadly, I've found no published curves. Converting torque curves into horsepower curves is simply mathematical conversion, so if you have one you have all you need to generate the other.

I've read diverse conflicting opinions, both favoring and disputing Lawn-Boy 2-cycle engine torque curves as compared to 4-cycle mower engines. It would be fun to settle those conflicting speculations by publishing some perfectly-comparable horsepower, torque and BSFC (brake specific fuel consumption) curves. Somebody's got to be wrong in their strongly-held opinions about these comparisons. I for one am more impressed by this 4-horsepower 2-cycle Lawn-Boy engine's torque response behavior than a nearly-new 5-horsepower rated B&S 4-cycle engine powered mower I recently tried. But maybe that comparison just indicated that Lawn-Boy's governor system response was better. Maybe the Lawn-Boy's blade and enclosure configuration is more efficient so it better uses available engine power. There are so many possible explanations that can cause behavior which can appear similar from only one observation view, that we need to make observations from different views. That makes real-world puzzle solving more challenging and fun than cross-word puzzles.
John