Flywheel key

Gentlemen, On my Briggs & Stratton engine (yes it did come from a lawnmower), there is an aluminium key locating the flywheel to the crankshaft. It clearly says on the flywheel that only an aluminium key should be fitted. Why is this? I have wondered about this for many years. No doubt someone can put me out of my misery. Oh yes, before anyone pokes too much fun at my little engine, consider this. The engine was originally fitted to a lawn mower owned by a church and used to cut the grass in the grave yard. I'm told that God was very fond of the little engine and may not like anyone who takes the Michael. So take the risk if you wish but if you incur the wrath of the Lord, don't come running to me. As I only normally read this NG Mon to Fri, I missed the development of the O/T thread. Here's my vote. I can sympathise with John's views but I must agree with all the others who supported the present posting variety. I have enjoyed the year that I have been reading the group, both on and off topic. Long may it continue.

John

Reply to
John Manders
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My first thought would be that it is intended to acts as a shear pin if something jams.

Reply to
Nick Highfield

I wondered about that too but the flywheel is on a taper fit to the crankshaft. There is a magnet in the flywheel which makes the ignition. I was wondering if a steel key could interfere with that in some way?

John

Reply to
John Manders

That's that one out then, a well fitted taper should transmit far more torque than any key, ally or not.

Wouldn't have thought so as I assume the key is only a small piece of metal compared to the crankshaft itself.

Got me stumped.

Reply to
Nick Highfield

The B&S N type I'm working on currently has a flat, round ended key on the taper fit flywheel. Now you say, it did have cast in marks on its flats although it didn't dawn upon me it was ali. What a strange thing.

I reckon it there so if the nut comes loose and the flywheel starts to chatter on the shaft, the ali key is intended to sheer through quickly so as not to damage the groove in flywheel or mainshaft. I'm sure we've all come across that kind of damage which can occur quite quickly, especially if the revs are high & these are rated at 3,000 rpm..

Mine was a bit whoosie in the sparks dept, but the spark definitely improved when I undid the screws that hold the HT coil to the back plate and dropped the "armature" as B&S call it as close as was practical to the passing magnets. Within the limits of manufacture, as they say.

Can someone enlighten me on how the vane type governor works?

Regards,

Kim Siddorn

Reply to
J K Siddorn

With an Al key, if you hit something with the mower the key sheared, ignition timing went away, engine quits. In high school I saw the same thing happen but with a steel key, crank broke at flywheel, flywheel tore through the shroud and went walkabout flinging broken air fins everywhere. Rob Provins

Reply to
rprovins10

The flywheel key is only a safety device. It can (and does) shear if anything goes wrong, thus defeating the ignition.

The air vane functions like any other governor. It tries to close the throttle. At design speed the air flow from the flywheel is routed against the vane, pushing the vane away from the tension spring. the link holds the throttle in position. With load, the flywheel slows, reducing the force on the vane, and the spring pulls the throttle open.

When stopped, the spring holds the throttle full open for starting. If the spring fails, the engine falls to idle speed. In practice the extreme simplicity of the Briggs air vane governor produces adequate speed regulation but is not intended for precise control, as in AC generator applications, etc. Brent Wegher

Reply to
bw

It tries to close the

Well, for what it's worth, I think it is a cheap and elegant solution to keeping an engine at a steady (ish) pace. As you say, not good enough for a generator perhaps, but perfectly effective for most purposes. Any ideas where I might get a spring and the link rod - all that is missing from mine.

Regards,

Kim Siddorn

Reply to
J K Siddorn

Original parts are no longer made, but NOS probably available with some effort. I have a local contact with a hidden stash of obsolete Briggs parts if you are a perfectionist.

You could also make your own, just as I have done, but I have originals as a reference. the parts diagrams and operating manuals don't have dimensional specifications. I don't have a scanner either.

With the vane "arm" exactly horizontal, and the throttle "arm" exactly horizontal, the holes in each part will be about 1/2 inch apart, vertically. The link just holds the outermost throttle hole to the air vane arm hole. To make the link, use 1/16 inch diameter welding rod, a piece about 1 inch long. With the rod vertical, make a 90 degree bend about 1/4 inch long at each end. (Leaving a 1/2 inch in the middle) the bends should be in opposite directions, not on the same side. Now bend each 1/4 inch end in the middle (1/8 inch) back into the vertical direction. This leaves a "kink" at the top and bottom, with the short horizontal portion becoming the bearing surfaces for each hole. To get the link installed, you have to have either part removed from the engine. for example the air vane, insert the link into the hole of the vane arm, then insert the other end of the link into the outer hole of the throttle, then bolt the vane into position. the parts should move without binding, but the link should fit closely into each hole with little slop.

The spring just needs to be any tension spring with about a 1 pound pull over a distance of 1/2 inch. The originals are about 1 inch long relaxed, with 1/4 inch loops, with maybe wire about 0.020 ?? diameter. Old manual typewriters were full of this kind of spring. Looks a little like a larger version of the springs inside retractable ink pens. The top of the spring has a free loop which goes into the throttle arm hole next to the one with the link. The bottom connects to the metal slide on the back of the backplate. the little knurled screw allows the spring tension to be controlled. There were variations to how the spring tension is controlled, and how it was attached to the slide. I've done this enough to know that the adjustments to the air vane can take time to get perfect. The air vane rotates about 10 degrees to the left and right of vertical, and should stop before it touches any part of the coil. The throttle idle stop screw controls movement in the other extreme.

Just a single diagram would probably be superior to all this text. Actually, I find governor control of the old engines one of the more interesting parts of the hobby. Trying to get the design speed and idle exactly adjusted, etc. Let me know how it turns out. Brent Wegher

Reply to
bw

Brent W. said > You could also make your own, just as I have done, but I have originals as a reference.

Brent, I'm very much obliged to you for your efforts. I'm sure I'll be able to get on with it now.

Just what I needed!

Regards,

Kim Siddorn

Reply to
J K Siddorn

the soft key is the mechanical 'fuse'. when the mower blade hits something unyielding, the flywheel wants to continue to rotate. sometimes it also ruins the crank/keyway if a hard key is used. a new soft key and all is forgiven. sammm

Reply to
sammmm

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