Crankshaft lore

[OldTee]

I heard that if you allow a crankshaft to lay on its side for a period of time that it will warp. That crankshafts should be stored on their ends. Fact or fiction?

[camgrinder]

I now think its a myth....

[john walker's workshop]

i've heard that, and that they're supposed to be hung instead of stood on the floor. don't know if it's fact or fiction either. you would think that laying sideways, standing or hanging, would all cause some sort of stress of a finite amount. mine are laying on the shelf until i know better.

[Eagledriver]

Maybe you could straighten one out this way

-Andy

[camgrinder]

I have seen cranks stored flat, but they had supports on the end and center main journals.

[Wayne 962]

Hmm, I suppose anything is possible. However, crankshafts are very stiff, and very strong, being made out of steel. I would seriously doubt that any crankshaft would bend under it's own (minute) weight. Seems bogus to me...

-Wayne

[philippeF]

i agree

Philippe

[garibaldi]

That is definetely, true, the cranck can bend if laid flat. The proper way to store a crank is to hang it by its neck. A good crank shop will store their cranks this way. Althought they are strong, treated and made from iron or steel, they can certainly flex, if not, there woudl be no need for multiple main journals running down its length to support it during operation.

To give another example, the bed of a Bridgeport milling machine will sag if run out to either end by a significant amount since the main area of support for the bed is right under the centerline of the head in its straight ahead position. A bridgeport is a 3k lbs plus machine made from thick cast iron, and the bed is about 3.5- 4 inches thick, and you can actually run a dial indicator along its surface referencing from the column, and read the sag.

Another example, if you chuck up the nose of a crank in a lather, and let it hang out over the bed with no support on the other end, it will droop.

You can also rest a crank on its mains in vee blocks, and set up a dial indicator along its length, and using your one hand, squeeze the cheeks of the counterweights together and watch the reading move.

[Eagledriver]

The question is not will it flex. Of course it will. The question is will it flex beyond it's elastic limit under it's own weight. I doubt it.

-Andy

[rcwaldo]

It is indeed true. A buddy of mine once worked at crank straightening and balancing place that mostly straightened Ferrari cranks. Funny as a customer would bring in a crank, holding it ever so delicately like a baby and set it on the counter with best care. Employee would take it to back into the shop and wing it across the floor.

Then they used BFH's to straighten them. Rock-apes

There have been some instances of bent cranks in cars that have been in wicked accidents. Who would of thunk?

Chris

[OldTee]

Obvisouly a crank will not bend overnight. It will take some time. For those who may have one in the garage what is the drill. Turn it over, say 90 degrees, every week, month????? Set it on end???

[camgrinder]

I asked my favorite crank grinder today about this myth. He laughed and said no, he has never heard about a crank bending because it was laid flat. He said the crank has a better chance of bending if it fell over and hit the floor.
He also said most cranks he works on are fully counterweighted and would have sufficient support. and if the crank only had counterweights on each end he thinks it could bend. But he has never seen any evidence of this.

[Wayne 962]

Right - it's a question of creep from elastic to plastic deformation. This type of creep is common in synthetic composites (like fiberglass), but not common in metals like steel.

I still vote for bogus. Maybe we need a mythbusters team to figure it out...

-Wayne

[john walker's workshop]

i could see it happening to a extremely heavy and long industrial crank, like out of a ship or something, but a short, lightweight, counterweighted unit like a 911, i rather doubt it.

[NevenM]

Guys

This can happen on 'green' or untreated cranks, but as Porsche heat treats their cranks I doubts its a problem, It is the residual stresses in the metal that would cause this. This is different to the beam deflection which Christian talks about because all beams deflect to the 4th order of thier lenght

The mechanism IIRC is grain boundary creep.

Neven

[beepbeep]

I wonder if they "hang" this one:

[NevenM]

Goran

Is that a non-counterweighted T crank? :-)

Neven

[350HP930]

Technically slow molecular creep from low stresses could cause distortion to occur, but it would take hundreds of years before it would be measurable with a micrometer.

Even if they are hung or stood on end, the crank is still being distorted.

All that changes are where the tensor vectors are pointed.

If you think laying on the side is bad, imagine what a crank with repeated several thousand lbs forces applied to it hundreds of times a second is seeing?

Its not unusual for those forces to noticeably twist a crank over time.

[OldTee]

Somewhere in the dim past I either recall or dreamed that Lycoming had a Service Bulletin or Service Letter or Advisory or something warning that cranks should be supported. A aircraft crank has a very long nose bearing from the flange to the first journal. I don't have access to ATP anymore. Anybody else?