Thread: Advice on Broken keyway for Crank

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[QUOTE]Originally posted by pat mccarthy
[B]

Originally posted by kitz
I didn't look at any pictures. I thought it was common knowledge that blowers require double keyways on the crank.

this quote right here could be bebated stock key ways 4130 steel crank fluidampr 548 7500rpm over 4 years stock keyways

Yes Pat, we understand you have done it with one key. It could also be argued that you can buzz a cast crank to 7,500 and get away with it. But it's just like everything else, you might get away with it and you might not. All I was saying is that Blower Drive Service recommends 2 keyways and that's good enough for me, 'cause they've built a few more blower motors than I have.

yes but i am no jack ass .the bore in the hub has to be very good and the hub of the damper as to be made of good stuff but how about that key way in the small block ???? do you know no one will back there crank up for a blower not even with a big block nose on it ??and what about 10/10 cut crank??? i think some of ther stuff is off so at the point the press will fail?? cut one more key way in the crank make it weaker lose full surface by cutting one more big key way in it?? and now at this point we have to use the key ways to drive off ??now we do not have the same press??? i guess i must be very lucky ?? No .at a point you may need it but not mild stuff and i would not try this with a iron hub

There is typically more than one engineering solution to meet the demands of mechanical torque transmittal.

My favorite involves the use of a heavy interference fit involving a tapered shaft and hub. The hub is expanded hydraulically using high fluid pressures. 30 ksi is typical. With this method there are no stress risers in the shaft or hub from keys or splines. Torque is transmitted purely through friction from the interference fit. But few could afford this on engines in the automotive world.

Pat I certainly don't doubt that a single key can handle the loads when properly designed and implemented.

And I for one am comforted in knowing that you are not a jack ass

Kitz

That is a very effective coupling. We use them on the ships to couple the segments of prop shaft together. You should have seen some of the early shafts with bolt together flanges and keyways. A shaft would have several 4 inch thick square keys to align the flanges aand keep the flange from spinning in unusual circumstances. All keyways had to be radiused inside and out to reduce the risk of cracking. It was still a problem. Now, most ships use a muff coupling that is similar to what was described in the previous post. The most horsepower per shaft I know of at the moment is in the realm of 110,000 hp at 80ish rpm so you can imagine the torque produced. That works out ot 6.9 million ft lbs of torque at 84 rpm. It doesn't slip.

Originally posted by kitz

Pat I certainly don't doubt that a single key can handle the loads when properly designed and implemented.
Kitz

In a properly designed joint doesn't the key not take any load? It is primarly there for alignment. When engineering calculations involving the strength of the joint are done, the key is ignored. The keyway is taken into consideration because it reduces the strength of the shaft, but they key itself is left out.

Absent a heavy interference a keyed shaft relies totally on the key for torque transmittal. Design issues are then the shear strength of the key(s), bearing stress on the shaft and hub slots that contact the key, and torsional shear stresses in the shaft and hub including the presence of the keyways.

Kitz

thankyou this is what is was trying to say .that the key way is for alignment and on the chevys so if the press is right it should get the job done . have seen the dampers not work on engines with out a good press and at that point the damper will not do the job of damping the engine harmonics the press has to be right like i said if not make it .like i out line and posted this is nowhere to do a half ass fix .and i have no PHD but i think that this is more load carrying surface with the press shaft then two key way cut in the crank now i have put stress points were i may get failure and now NO full contact on the shaft and now the hub has great spots to fail from the cut key ways?? i think you lower the bar when out add key ways the press is better . you may think i am nuts thats is ok. BUT harmonics need to be dampen and this can not be done as good with out a good press and as you can see i am not a big fan of cutting steel out of the surfaces of any press crank or the hub

Looks like your crank is toast, guess I would be inclined to go with the BDS recomendations. They do blowers for a living and probably know what works best....

Keyed or splined?

Ok we have a couple opinions here, but one is starting to get repeated a bit. I have talked to a couple machine shops in town and here's what they said.

1. this guys claims he can take out the key and weld a new ceramic key in the slot and that will take all the power I can give it. So he claims. I never heard of a ceramic key, but then I'm kind a noobie.

2. Two speed shops in town have suggesting taking the crank them repair the key then cut another key slot 180 degree.

I beleive #2 is what BDS is trying to say. Although I don't under stand the whole 10/10 thing. What is that?

curious why is it that all of Denny W's posts are deleted, is something getting "moderated here".

Thanks Guys!

Oh also, the crank pulley just a basic pully with a 6 bolt pattern front, there is NO dampener on the front.

FYI this engine was built in '78 (father in law had it built back in the day) So I don't think it was terrrible common back then to be using the 6-71 blower.

denny i do care if you are piss off at me . i am sorry BUT i have had this happen. I do build engines for a living and if i was not good i would not have any work ?and i do .if you would like to know what i am working on rigth now i would be happy to share one 632 drag engine one 572 mud drag engine one 565 drag engine one 540 drag engine. this is what i do and think i have some thing to add to this?? if not ok. i told him how to fix this rigth. it works it has been done and i have had hubs built up with hard chrome to and had shafts spray weld to. that is what needs to be done .

LIVE4SPD on behalf of CHR as one of their younger members (I'm 45) I apologize to you.

You asked a seemingly innocent and important technical question and landed in the middle of a squable between experienced experts (myself included) who all have the best solution for you. Some of us think more of our own methods than others, and all have had definite measures of their success. Well crap.

To me the facts are your key/keyway failed, the hub has definitely spun o0n the crank, and remediation of this is required.

The fact is indeed that there are options for you to pursue. I believe you need to evaluate your wallet in particular as to what road you take. And based upon your specific application the options range from repairing the crank to replacing the crank. To this end I say don't hessitate to get further options from qualified individuals and shops.

I think you do have enough info to talk details with a qualified shop that will absolutely be required to perform the repair.

Best Wishes and Good Luck, Kitz

Originally posted by LIVE4SPD
Ok we have a couple opinions here, but one is starting to get repeated a bit. I have talked to a couple machine shops in town and here's what they said.

1. this guys claims he can take out the key and weld a new ceramic key in the slot and that will take all the power I can give it. So he claims. I never heard of a ceramic key, but then I'm kind a noobie.

2. Two speed shops in town have suggesting taking the crank them repair the key then cut another key slot 180 degree.

I beleive #2 is what BDS is trying to say. Although I don't under stand the whole 10/10 thing. What is that?

curious why is it that all of Denny W's posts are deleted, is something getting "moderated here".

Thanks Guys!

The "10/10" thing is part of BDS's recommendations for the crank when running a blower. They are saying don't cut the mains or rods more than 0.010". Sorry if that was confusing, it was part of the paragraph about the 2 keyways, so I just copied the whole paragraph.

Oh yeah, BDS refers to Blower Drive Service, a company that specializes in supercharges. They've built tons of them and are generally regarded as the experts to go to in blown street engines.