Flywheel key question

[imdying]

Non standard crankshaft. End of crank has some damage, but not too bad.






Ok, both the crank and flywheel grooves are straight and level as you can see, no indent for a woodruff style key (the standard crankshaft does however).

The key that came with the flywheel appears be a factory one, 5.9mm high 3mm wide and 10mm long. Grooves in the crank and flywheel are about 1.5mm deep and 3mm wide.

It seems to me that I'll need to use a straight 3 by 3 key about 10mm long, either by getting some key stock from a bearing shop (easy enough) or grinding the one I have (or not).

I've never come across a straight key on a crank which either means I don't get out enough (or perhaps too much) or there's some engineering reason for it. If a straight key is suitable, how critical is the height? It can't 'slide out' either way as far as I can see, but too much might stop the flywheel somewhat imperceptibly (but enough to matter) snugging up on the taper? Any advice welcome.

At this stage I'm going to get some 3x3 stock and ensure it is snug, but not too high as to prevent the flywheel from seating correctly on the taper.

[Brian d marge]

the taper does the holding

the key is just a locater

Clean everything up and just use a mild steel ( soft ) bar ... ( I make them myself from an old bolt )

Lapp the taper and re install

Stephen

[imdying]

That seemed logical to me, thanks for your help.

[unstuck]

The keyway in the crank is tapered slightly,or is that just my eyes.Looks like it narrows slightly towards the seal? May just be optical olusion because of the damage?

[imdying]

The keyway in the crank is tapered slightly,or is that just my eyes.Looks like it narrows slightly towards the seal? May just be optical olusion because of the damage?

Yup, optical illusion. With hindsight I would have pulled the camera back from the work further

/edit: Any idea how one would actually do that sort of damage in the first place?

[Motu]

B&S use a flat key on their flywheel,and BSA used one on the clutch shaft.As mentioned the key is nothing but a locator...but even so,for some reason a woodruff always does a better job.

[tri boy]

Straight keys do way more than locate.
Otherwise 50hp electric 3phase motors driving hydraulics/pulleys/high load sprockets would only use tapers.(they use straight and taper depending)
Key steel is dirt cheap. Use it. The tolerances are more important than penny pinching. MHO.

[Brian d marge]

Straight keys do way more than locate.
Otherwise 50hp electric 3phase motors driving hydraulics/pulleys/high load sprockets would only use tapers.(they use straight and taper depending)
Key steel is dirt cheap. Use it. The tolerances are more important than penny pinching. MHO.

this will be good; last time i looked at that photo it was a taper ......

Keys , Locate , and transmit loads , and are used as a fail safe , ie designed to fail before major components. Designed for crushing , and shear ......Those crank ends are really soft , and key steel one could imagine would have a field day

As for tolerances, Even I can get a bit of Chinese steel to fit quite nicely , given a few min with a hand file .

In this case , its the taper doing the holding and the key doing the locating.

Stephen

[tri boy]

Duh.
I know the crank in the pic is tapered.
My point is there is more to key steel than locating.(you yourself just admitted it)
Me thinks you should step back from theory more often, and get your hands more accustomed to real world situations.
Engineers have always been a bit aloof, but rarely very good working in the field

[Ocean1]

Any idea how one would actually do that sort of damage in the first place?

Yes. Both the taper and the key are needed to take the torque from the sort of full compression stop that can occur, f'rinstance pre-ignition issues. The shelling damage on the shaft shows that it's a tad on the hard side. The fretting on the key shows that it's been "working" for some time.

As someone said, lap the flywheel onto the shaft until you get 95% contact shown by bearing blue on a light push / light 20 degree twist.

Place the old key into the keyway and lay a 5thou plastiguage along it's length. Place and torque the flywheel up. Remove it and check the clearance. Now that you know what the radial clearance is chuck the key away. Get a fresh bit of 4mm keysteel, (yes, there's a standard, and a reason for it) and find someone with a surface grinder.

Cut a piece of keysteel at least 100mm long, (less and the mag chuck won't hold it) and grind the piece to 3.2mm. Use that as a feeler gauge in both the shaft and the flywheel, grind a thou at a time until you can feel it drags hard on the width of the keyways without being so tight it won't seat right down.

Grind the key to the exact radial dimension you've already measured above. Cut it to length and carefully chamfer the corners, 'prox 0.2mm. Trim the key to length and radius the ends as/if required.

Assemble perfectly clean and dry. I cheat, I warm the flywheel a tad. That might make it ever so slightly more difficult to remove... up to you, the keyways have already been compromised, I'd take every advantage I could.

There's other ways of doing it. They don't work.

B&S use a flat key on their flywheel,and BSA used one on the clutch shaft.As mentioned the key is nothing but a locator...but even so,for some reason a woodruff always does a better job.

They do eh? theory says they shouldn't, but there's probably more load-induced distortion going on there than theory says there orta be. I've never seen a shaft sheared through the deeper keyway either, usually further up, at the business end of the taper.

[Wannabiker]

All sound advice.

Note: B&S use a flat aluminium key...the theory being that if the blade hits a solid object (rock, concrete kerb, old clothse line pole) the inertia of the flywheel shears the key, rather than breaking the crank.

Always a good thing to check if your B&S mower won't start after hitting rocks. It does not take the flywheel to move much and it wont spark. (Replace with standard item!!)

[Motu]

The B&S is a good example that the key is more than a locator.There are a lot of keyless tapers used - in motorcycles all the British magnetos used a plain taper,and even the later camshaft driven advance units were a plain taper too,although Triumph used a small locator tab.Never saw an ignition drive move.The Mini flywheel was keyless too,the whole car drove through a taper just located by a keyed washer.Anyone who has tried to remove one knows the strength of the taper.In modern times Ford uses keyless tapers on cam drives - the belt tension is set with them loose,then they are tightened onto the shaft....don't see them moving either.

I don't think we've proved anything...but if a key is fitted,use it.

[Ocean1]

There are a lot of keyless tapers used - in motorcycles all the British magnetos used a plain taper,and even the later camshaft driven advance units were a plain taper too,although Triumph used a small locator tab.Never saw an ignition drive move.The Mini flywheel was keyless too,the whole car drove through a taper just located by a keyed washer.Anyone who has tried to remove one knows the strength of the taper.In modern times Ford uses keyless tapers on cam drives - the belt tension is set with them loose,then they are tightened onto the shaft....don't see them moving either.

Self-locking tapers. Most common is the Morse taper on taper shank drills, 5/8"/Ft. It's reasonably closely defined, a tad more included angle and the torque able to be applied is much reduced. Much narrower and you get actual fusion.

[Brian d marge]

Duh.
I know the crank in the pic is tapered.
My point is there is more to key steel than locating.(you yourself just admitted it)
Me thinks you should step back from theory more often, and get your hands more accustomed to real world situations.
Engineers have always been a bit aloof, but rarely very good working in the field

I have spent my life ,,fixing stuff that people who "think " they know .... use key steel by any means , I dont care

Stephen

[Brian d marge]

Yes. Both the taper and the key are needed to take the torque from the sort of full compression stop that can occur, f'rinstance pre-ignition issues. The shelling damage on the shaft shows that it's a tad on the hard side. The fretting on the key shows that it's been "working" for some time.

As someone said, lap the flywheel onto the shaft until you get 95% contact shown by bearing blue on a light push / light 20 degree twist.

Place the old key into the keyway and lay a 5thou plastiguage along it's length. Place and torque the flywheel up. Remove it and check the clearance. Now that you know what the radial clearance is chuck the key away. Get a fresh bit of 4mm keysteel, (yes, there's a standard, and a reason for it) and find someone with a surface grinder.

Cut a piece of keysteel at least 100mm long, (less and the mag chuck won't hold it) and grind the piece to 3.2mm. Use that as a feeler gauge in both the shaft and the flywheel, grind a thou at a time until you can feel it drags hard on the width of the keyways without being so tight it won't seat right down.

Grind the key to the exact radial dimension you've already measured above. Cut it to length and carefully chamfer the corners, 'prox 0.2mm. Trim the key to length and radius the ends as/if required.

Assemble perfectly clean and dry. I cheat, I warm the flywheel a tad. That might make it ever so slightly more difficult to remove... up to you, the keyways have already been compromised, I'd take every advantage I could.

There's other ways of doing it. They don't work.



They do eh? theory says they shouldn't, but there's probably more load-induced distortion going on there than theory says there orta be. I've never seen a shaft sheared through the deeper keyway either, usually further up, at the business end of the taper.

I held back from saying why , I couldn't see any evidence on the shaft,( Hetzian? but wasn't sure) We are talking a motorcycle ( possibly inline 4 ) those fluctuations ain't as big as me Enfields flywheel. .you working for Rolls Royce??

Stephen