koofruff key keeps breaking

I'm fairly new to bike restoration so could do with a bit of help.
The bike I'm working on is a 1936 Sokol, which is a Polish built
knockoff
that is basically a 995cc Vtwin a la Indian combined with Harley front
suspension.
The problem is that the woodruff key on the kickstart keeps breaking.
The engine starts fairly easily with no appreciable kick back. The
advance/retard mechanism of the distributor has been screwed down in
the maximum retard position and will stay like that until I can
fabricate the cable for the manual mechanism.
I'm reluctant to keep on making new keys until I understand why they
keep breaking.

 

the title should, of course, say Woodruff key, excuse the type

 

Is the kickstarter shaft tapered? In that case, the tapered parts may
be worn out, allowing the rotational load to be on the key instead of
the tapered parts which are supposed to carry the load.

At one time, Husqvarna countershafts were pressed onto a tapered
countershaft...

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

B-12 schreef:

If you use genuine parts, then it could be that the key is too hard.
I once owned a russian Ural (sorry ;-) that had parts as hard as
glass (valve stems, dynamo axle'among other). Fabrication standards
simply were no good.
Those parts broke under load.

Rob.

 

The woodruff key is supposed to break to protect the engine internals in
the event of an excessive mechanical shock. If it's breaking in the event
of
a normal mechanical shock then check:

1) should be zero lubrication on the keyed shaft
2) key should not be normally taking any shear load
3) key must fit exactly, make sure the slot where the key goes is not
hogged out, or the key isn't too small, etc.

Ted

 

In response to the various comments:
1. The shaft is not tapered
2. No original parts available, the company disappeared during WW2.
3. Shaft lubrication is OK

The answer may be in my construction technique. The slot in the shaft
is 5.5mm wide while the slot on the gear is closer to 6mm (is that a
design issue or sloppy manufacturing?). My key was cut from a piece of
angle iron which was the closest I had to 5.5mm so needed the least
grinding down. Again, its iron not steel which my mentors say is OK,
the key should break before the shaft. When grinding into shape I kept
dunking it in cold water so that I could pick it up, was that a mistake?

 

I suppose that the average person cannot smell his own brain fart. I'm
not saying that
my brain has never flatulated, but this particular emission from Ted is
amusing.

The function of a locating device such as a Woodruff key is to locate
parts accurately and prevent relative motion. If the engineer had
actually intended the Woodruff key to break, he probably would have
designed the device with a shear pin instead.

A mechanical engineer *knows* what the mechanical loads on his parts
are, before he begins designing. He selects a key made of a material
which will withstand deflection and chooses a part from a table of
standard keys to obtain a key which has the physical dimensions not to
bend.

Woodruff kets are semi-circular, a Woodruff cutter is used to mill a
precision crescent shaped slot into the shaft, and a broach is used to
scrape a rectangular slot into the rotating part.

Part of the Woodruff key sticks up out of the shaft, and goes into the
slot on the rotating part.

If a Woodruff key breaks, the broken bits become a machine tool
themselves, they will act as a lathe tool and ruin the adjacent part.

 

The slot in the shaft is called a keyseat The slot in the gear is
called a keyway.

Keyseats are milled with a rotating cutter. Keyways are broached with a
device that scrapes a slot by linear motion.

Your dimensional quandary is 5.5mm versus almost 6 mm. Half a
millimeter. A silly half a millimeter. Less than 20 thousands of an
inch. Is it sloppy or is it shrewd?

It could very well be a design and manufacturing issue. I think that
the engineers might have been cutting precision keyways and keyseats in
the machined parts that they had quality control over, but they may
have realized that replacement Woodruff keys might vary widely in
dimension.

If you're going to machine precision parts to plus or minus 1/1000th of
an inch, the available Woodruff keys are all going to have to be
thinner than the keyway and the keyseat or the mechanic will have to
make them so. The tools available in the field might be crude...

I don't happen to have a Machinist's Handbook right here, but I do have
some books on technical drawing. So I did a little detective work and
reasoning with what I have available, an old book and an old mind...

The American National Standard # 808 Woodruff key is 8/32nds of an inch
wide, and the base circle of the key is 8/8ths of an inch in diameter.

IOW, it's a key that is nominally 1/4 of an inch thick, and if it was
circular instead of crescent shaped, it would nominally be one inch in
diameter. The Woodruff key would stick a nominal 1/8th of an inch out
of the keyseat.

The maximum depth of the keyseat would be .3130 inches. Now we're
getting into the tenths of thousandths of an inch. The technical
drawing book shows that the width of the milled keyseat (in the shaft)
should be 0.249 to 0.250 wide. That's a min/max range of only 1/1000th
of an inch. That's what the engineers controlled at the Sokol factory.

But that *nominal* 8/32nds of an inch thickness of the key leads back
to fractional tolerances. Fractional tolerances are often specified as
plus or minus 1/32nd or 1/64th of an inch.

So that nominal 1/4th inch thick Woodruff key could theoretically be
8/32nds *minus* 1/64th of an inch wide. The standard #808 key could,
apparently be only 0.234 inches thick. That's 16/1000ths of an inch
thinner. It's pretty close to your half millimeter.

That's a rather sloppy fit that might lead to broken keys. Maybe there
are keys made of a stronger material that you could use? It's not like
the starter shaft and gears are used continuously...

 

If you do that you will create a weakness at the transition from the 5.5mm
to the 6mm size.

The big question, though, is why is the key taking a shear load? What keeps
the kickstarter from sliding off the shaft? Is there a bolt that clamps it
on
to the shaft?

Ted

 

Lateral motion of the kickstart on the shaft is limited by a collar and
gudgeon pin.