Broken crank

Art Bjornestad

Art Bjornestad

Senior Member
A buddy just got back from the MAFCA meet in Canada and a few miles from home the engine really started to clatter. He flat bedded it home and today pulled the pan.

The rod journal on #1 is broken right at the face of the flange. I looked at the crank and saw that there was no radius on the journal, it was cut flush right to the counterweight face, an completely straight 90 degree cut.

I advised him that this was the cause of the failure but he is not convinced. Is the fact that crank throws need to have a radius written up anywhere?
 
You are exactly right, Art. I hope you can read the attached from Taylor's book: The Internal Combustion Engine in Theory and Practice. It's a mid-1980s edition and there are probably computer models that are sleeker and fancier, but you can take his stuff to the bank. Page 493 explains the fillet importance, and the next page (not shown) says that with overall dimensions and fillet radius the same, the crank is weaker than the stepped shaft with stress concentrations at the cheek/pin area shown in Figure 11-35a. Figure 11-35b shows the better stress distribution betweeen pin and journal with lower stress concentration at the fillets. Hope this helps some. Sorry for the crappy scan. :(
 

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Art Bjornestad said:
A buddy just got back from the MAFCA meet in Canada and a few miles from home the engine really started to clatter. He flat bedded it home and today pulled the pan.

The rod journal on #1 is broken right at the face of the flange. I looked at the crank and saw that there was no radius on the journal, it was cut flush right to the counterweight face, an completely straight 90 degree cut.

I advised him that this was the cause of the failure but he is not convinced. Is the fact that crank throws need to have a radius written up anywhere?
Click to expand...

Well then, get him to install another crank without the fillet ground in the the cheek and see if it does it again. After the 3rd or 4th time it breaks, he may form the opinion that you are correct and he is mistaken!!:D

..
 
The Ford print calls out 5/64" to 1/8" of a radius for all journals.

My brother has a crank with no radius's and it happened to have a partial crack when he cut it apart to make a tool.

The other thing to watch out for is the correct radius. Lots of shops do not want to change their grinding wheel to the correct A radius as they loose some diameter on the wheel converting back to another radius.
 
Add to the radius problem there is no harmonic balancer at the front. The result is a nasty twist force concentrated at the unradiused corners. Did somebody say "Snap"?
 
Also look at the Service Bulletins for October, 1928 where Ford talks about the modification to use the new "service drive". It mentions the importance of the radius, so the shaft won't break. I bought a starter at Hershey 5 years ago that has a broken armature shaft because it was modified without the radius. I bought it for a show and tell piece.
 
Good information!

Question: Is the radius needed on brake drums also when they are turned?

Thanks
 
On a related thought...I suppose that's something to look at when shopping for a high strength bolt.
It seems you would want a smooth gusset where it merges with the bolt head, as opposed to a perfect sharp 90 which'll shear clean off. :eek:
 
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Below are some photos of a crank which I made counterweights for. It went out afterwards for a grind/micropolish & balance.
It wasn't a cheap machine shop...they do marine racing engines and their work is superb.
I'm thinking this journal has a pretty good radius?
......any opinions if it looks OK?:confused:
(I haven't fired the motor yet)
 

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For those of us still learning (i.e. me) let me see if I understand this. The purpose of the 'radius' or the slight curve upward in the corner is the same as a curve on an airplane window. In that it does not let a crack start and eventially snap the crank?

Mike
 
1930artdeco said:
For those of us still learning (i.e. me) let me see if I understand this. The purpose of the 'radius' or the slight curve upward in the corner is the same as a curve on an airplane window. In that it does not let a crack start and eventially snap the crank?

Mike
Click to expand...

Yes, sharp corners in the filet area allows stress cracks to start. sorta like scoring glass before you break it.
As to a crank breaking because of the stresses set up by twisting they usually break at # 4. At least that has been my experience.
 
just plain bill said:
Yes, sharp corners in the filet area allows stress cracks to start. sorta like scoring glass before you break it.
As to a crank breaking because of the stresses set up by twisting they usually break at # 4. At least that has been my experience.
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Yes number 4 takes the load from all the cylinders. My experance is the same as yours, number 4 goes first. Makes you wonder if his front main was out of line, along with a sharp corner.
We used to make cylinders for Ford they always had a Radius on the prints at stress areas. It is very important.
 
RADIUS ? Yes, bigger the better ! On any part that has a chance of failing.

Air plane windows? Just ask the aircraft company from England. Spot on!

Dudley
 
In a similar vein......

The operating rod on a M1 Garand rifle (WW2) was originally made with a 90 degree angle around mid section.

Rods would break from stress cracks at the 90 Degree location. New ones were made with a slight radius added, while field units had a radius cut in when returned for depo repair.

Marc
 
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