Woodruff key questions

I managed to weld up the clutch basket / shaft for the lathe I am repairing okay
Didn't damage the threaded sections and it is still straight if we can trust my
I used the fiberglass tape suggestion in an earlier threads, wrapped tin foil
mounted bearings that I couldn't get of non destructively.
Welding, I put opposite strips weld on the fill and non fill side to keep
minimized. I took my time too.
I used the short bapps of weld on opposite sides.
One trick that helped was I set my 3W LED maglight positioned so that I could
really see
what I was doing from under the hood.
Decided the section could stand losing 0.015" on diameter, I have to bush the
anyway so I'm taking the easy route on this.
Now I'm down to putting a woodruff key in. I can't exactly tell what was in it
due to the
damage. The pulley groove is 3/16" and a fragment of key was 3/4" long but I
wouldn't bet
it wasn't 7/8" or 1" too, those see to be sizes I can get a key in.`
So I started calculating things. A 3HP motor puts out ~4.4 lb/ft of torque at
doubled if the motor is half that rpm, I can't remember what it is atm.
There is a pulley ratio of 1:2 motor:driven clutch so now I have 8.8 lb/ft at
the pulley /
shaft interface that is a ratio based on diameter of shaft of 24.34 giving 214
lb of force
on the key at the shear line.
A 3/4 x 3/16 key has .140 sq/in in shear. Using 60Ksi that would mean I'd have
generate 8437 lbs at the shear line to shear the key.
So the questions are:
Is there a standard shear value in KSI for woodruff keys? I'd think we would
want them to
give rather than the expensive parts but my assumption could be wrong.
What kind of safety margin is typically used in this kind of application?
Thanks in advance,
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want them to
Don't know the answer, but the numbers in your sums look pretty safe.
Woodruff keys were not designed for transmitting torque and should not be used for such. It causes the keyseats to self destruct.. They are for location only. If a designer wants to transmit torque they should use a proper taper key. Preferably gib headed for easy extraction.
God Save the King!
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Mark Rand
I haven't seen a gib head since I repaired Sheridan double crank die cut press(s).
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I'm curious about your comment, I've seen woodruff keys in many things. The most notible thing is the handles on a bridgeport. That may be a case of location being primary over torque since those are not stressed.
I'm tempted to just cut it for a pocketed square key and be done with it.
I *thought* there might be some sort of rational to the woodruff key that made it better than a square key.
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A square key usually is held by a setscrew. I cant think of a time that a woodruff key had a setscrew on the shaft. A woodruff key cant work out of the slot. they are used when the part has to slide along the shaft.
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They are also used on tapered shafts. Look at the flywheel on your average single cyl air cooled engine. (where they also transmit torque - starting torque as well as, in many cases, accessory drive torque.
Just blew two so-called "facts" about woodruff keys with one example. Not a sliding fit, and delivers torque.
Then go to the harmonic ballancer (front pulley) on many car engines - Woodruff key there too - definitely not a sliding application, and definitely delivers torque - runs the alternator, air conditioning, and power steering loads.
It is also used to hold the drive pulley on many alternators/generators/etc as well as keying timing gears to both camshafts and crankshafts..
A woodruff key is used where the loss of the key through lateral movement is to be avoided. It is also used where a controlled torque shear is required. They fail very predictably under high impact shear loads, providing a certain level of mechanical protection to drivelines.
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Lawn mower flywheels typically have an aluminum woodruff key so whey you hit that rock or whatever, the flywheel can slip instead of damaging the crank.
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