Crank tapers and key ways

Chaps,

The Alton thread raised the topic of the woodruff key on the crank taper, and some posters questioned the role of the woodruff key in actually taking any load, stating instead that its purpose was for parts location.

Gotta be honest here, this is always something that has confused me...

IF... the keyway is only to help locate parts, then why is it there on the crank to sprocket taper? There is no specific location issue there. Triumphs have a taper for the clutch to mainshaft, that too has a key way, and that too has no specific location requirement.

AND... IMHO, I agree that the role of the taper is to hold the torque.

SO... what is he purpose of woodruff keys when fitted to tapers where there is no specific location requirement?

It's what the engineers believed was needed. We since know otherwise. My 8,000 rpm 150 h.p. snowmobile engine (ALL snowmobiles ) has the drive clutch on the crank with just a taper.

SO... what is he purpose of woodruff keys when fitted to tapers where there is no specific location requirement?

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the woodruff key locates the rotor in proper position so you can read the timing mark with a strobe light

1up3down said:

SO... what is he purpose of woodruff keys when fitted to tapers where there is no specific location requirement?

Click to expand...

the woodruff key locates the rotor in proper position so you can read the timing mark with a strobe light

Click to expand...

Yes but in that instance there is a specific location requirement (and the rotor also fits on a parallel section not on a taper with a different woodruff key) but the question was 'where there is no specific requirement' such as the crankshaft sprocket.

I have also wondered why they used a Woodruff key. It is possibly to prevent creep, as it is a rather heavy duty size. A key used only for location could be much smaller.

I have sheared a crank sprocket woodruff key......and lost drive.....suggesting that the key had a role in holding the sprocket in place.... but....

I suspected that the crank nut had come loose first! Which possibly allowed the sprocket to come loose on the taper then sheer the key under drive forces, rather than the key sheering being the cause of the problem.

No significant damage to crank simply replaced the key, fitted new shakeproof washer, never happened again.

As one who has sheared a number of keys in various equipment I'm in the "it transfers torque camp and indexes if necessary". A quick google of engineering texts would seem to back that up however, I've seen various machine tools that get by on the taper alone. It's certainly necessary on a non-tapered shaft. One of the advantages of a woodruff key in particular is that the keyway can be easily cut in a tapered shaft whereas a straight keyway cannot be as conveniently cut.

Briggs and Stratton use a straight aluminum key in a tapered shaft on the flywheel. The purpose is for sudden stoppage. If, for instance a lawn mower blade hits an immovable object the inertia of the flywheel will cause it to slip on the tapered shaft and the aluminum key will shear thus preventing damage to the crankshaft. Of course the crank can be damaged on the other end but it won't be twisted by the heavy flywheel.

In my time in engineering i have learned that if the taper of the two parts has a large enough area an slow enough taper (less degrees of included angle) like the morse tapers used on drills and lathe etc. it will hold together against tremendous force (try using a 4 inch diameter drill you see what i mean). But if the length of taper is short and steep as i would IMHO say the drive sprocket is and the tapers of both parts due to manufacturing tolerance an worn production machines are not perfect (in a toolroom you can take all day about it your not on piece work) then a key is needed. Its just a sensible precaution an will stop warranty claims. Snow mobiles made on modern equipment with different engine torque characteristics, taper angles and gearing requirements due to traction available on snow not tarmac may well not need keys however.

toppy said:

In my time in engineering i have learned that if the taper of the two parts has a large enough area an slow enough taper (less degrees of included angle) like the morse tapers used on drills and lathe etc. it will hold together against tremendous force (try using a 4 inch diameter drill you see what i mean). But if the length of taper is short and steep as i would IMHO say the drive sprocket is and the tapers of both parts due to manufacturing tolerance an worn production machines are not perfect (in a toolroom you can take all day about it your not on piece work) then a key is needed. Its just a sensible precaution an will stop warranty claims. Snow mobiles made on modern equipment with different engine torque characteristics, taper angles and gearing requirements due to traction available on snow not tarmac may well not need keys however.

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1 in 30 is common snowmobile engine taper. What is the Commando?

:mrgreen: :roll:



The acceleration will nearly rip your arms out of socket....

Don't know what taper is never bothered to measure it. Keys on older stuff is norm in my experience if you want it to stay put an are not going to change it every five minutes like a drill bit (even old versions of larger drills have a taper plus a slot through for a driving drift wedge to be hammered through drill spindle and drill to lock it in place).

I don't see any snow but can see some wheels so does that really count as a snow mobile?

Larger drills are driven by the taper alone. The tang on the end is used to break the taper by using a drift through the slot.
I was wondering if the snowmobile engines being 2 stroke might have smaller but more frequent pulses allowing the taper to hold better rather than the wider spaced bangs on a 4 stroke?

triumph2 said:

Larger drills are driven by the taper alone. The tang on the end is used to break the taper by using a drift through the slot.

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Take a look at any older machine tool that has a Morris taper larger than number 5 (Google image of Asquith Radial Arm Drills for example). There are normally two slots to be seen in the spindle the upper most slot is for the ejection drift the lower slot near the end face of the spindle is for the drive wedge. Large drill say over 3 inches had drive wedge slots as did reamers over 1 11/2 inches (these often have more than 10 cutting lands an use less the 50 rpm but feed rates of over 30 thou per revolution. To let each edge cut properly and achieve a smooth surface finish).

The tang loosely locates in a slot on many machines which will try to transmit drive should the taper slip. The taper of old worn machines and tooling is often worn an so fails to grip effectively. This can often mean to much load go on to tang and it becomes twisted or shears off completely. Most who have spent time in older machine shops have seen plenty of such damage or when lifting the large heavy tool into the machine had it not grip the taper or tang foul it's groove an the tool fall back out.

Enough of this though as its not what Eddie asked an i don't care if you don't believe me as i know am right about the machines. After 30 years i still have all me fingers toes arms legs and eyes but even these days people lose them in my line of work or worse. I have seen this to with my own eyes including fatal injuries (that as only few years back in a modern stafety paranoid firm not some old dangerous place).

My Goldstar gave me fits for 30 years until I went to a crank triggered Boyer ignition. The Taper on the magdyno shaft would only hold for a short time before skipping. Nothing I did improved it. Now, it just runs the Alton generator . I don't care if it skips.