Crankshaft straightening question.

[badbirdcage]

The way you straighten the mains for finish is you grind them straight. That way they are stress free. When you push a crankshaft around in a press to attempt to get it straight you induce stress into the shaft. And yes, the flywheel flange should be machined perpendicular to the mains. You do that with a lathe, aligning on the main bearing surfaces after the shaft is finish ground.

We always used to stand crankshafts on the flywheel end or hang them in a rack from the flywheel end.

Rich:beer:

But, it had to be straight when it was ground new. Would you not be just taking the sag out and returning it to the "as new" posture?

If I understand the OP's question that was the purpose. To straighten the main bearing surfaces back out.

Have you ever taken a new GM tufftrided crankshaft out of the box and put it in a set of V blocks to check straightness and found it to be two and a half thousandths out of alignment from the front and rear on the center main bearing surface? I have; more than once.

On an aside note: Has anyone considered the energy generated and sent through the crankshaft while it is in operation? Things get out of square really quickly in a dynamic operation.

We used to have a couple of customers that wanted us to grind crankshafts in advance of factory index on the rod journals so that the piston would be in proper position in the cylinder when detonation of the fuel took place regardless of the twisting of the crankshaft.

Rich:beer:

 

[68/70Vette]

Crankshafts have been in the forefront of machining technology for more than 100 years. I have a billet forged steel BB crank. During it's creation, it must have suffered some tremendous stresses. I would assume it experienced warping. As I hand rotate it in the block, it certainly appears to be very straight. I think crank manufacturers have processes to make them very straight. After 100 years, I assume that making a straight crank is a long ago solved problem.

I've heard that forged steel cranks are a lot more difficult to straighten than cast cranks. Don't know the details.

 

[MotorHead]

Crankshafts have been in the forefront of machining technology for more than 100 years. I have a billet forged steel BB crank. During it's creation, it must have suffered some tremendous stresses. I would assume it experienced warping. As I hand rotate it in the block, it certainly appears to be very straight. I think crank manufacturers have processes to make them very straight. After 100 years, I assume that making a straight crank is a long ago solved problem.

I've heard that forged steel cranks are a lot more difficult to straighten than cast cranks. Don't know the details.

I sent a brand new Eagle 4340 crank back because it was .001" out when I rotated it in the middle journal. I got a Callies crank and it was perfectly straight. What is point of setting your main bearing tolerances to .0005" when the crank is out .001" ?

My buddy has all his cranks standing up in the middle of the shop nothing stopping anyone from knocking most of them over except no one has in 30 years, says it takes up less space like that :amused:

 

[big2bird]

I have a billet forged steel BB crank.

I have heard of cast, forged, and billet. What is a forged billet crank?:huh:

 

[JAKE]

Didn't Smokey say to straighten them with a hammer, not a press, to prevent cracking, but should only be done by a "professional hammer swinger". Seems that comment sticks in my mind from reading his book years ago.

yea, hanging 'em is the way.

Jake