Crankshaft Misalignment Repaired

[Captain Jerry]

OK Jerry,,,,I think I understand what you getting on about.    The crank stopper on the other side of the journal bearing isn't in torsion during the "push", but in bending.....for those of you following along...look at the top of fig 6A in the story on page 1 of this thread.


Under those conditions, its impossible for the pin joint there to rotate about the axis of the pin.....I would want to be sure it wasn't bent though.


Is that what your getting at?

Dave

Not quite , Dave.  I was certainly not concerned with rotating the side crank pin on #3.  Did you know before that it was a single piece forging that included the side webs and pins of both #3 and #4 as well as the main bearing journal between them?  I didn't and all previous descriptions of the crankshaft said that it was completely assembled which lead me and maybe some others to think that both ends of all crank pins were a shrink fit.   They were not and that is what I discovered for myself and maybe some others from doping out the hidden clues contained in the Doxford Works photographs.

Jerry

[Maryak]

all previous descriptions of the crankshaft said that it was completely assembled.
Jerry

I suspect this was completely true in the original design where both pistons had an equal stroke.

Best Regards
Bob

[Anzaniste]

Here is a better link to the works photos:
http://www.shipsnostalgia.com/guides/William_Doxford_and_Sons#The_Manufacturing_Process

Dan

What a fascinating series of photographs. It really brings home how much knowledge and experience has been lost.

[IanL]

Regarding the building of the crankshaft, The earlier type was simplified in the mid 1960's whereby the main journal bearing and the adjacent webs was incorporated into a single large diameter bearing.  In the case of the 760mm bore "J" type engines the main journal was 1150mm in diameter and about 300mm wide.  It was machined by feeding in a 300mm wide tool on the lathe.  A tightly wrapped roll of steel foil was the result. Perhaps the widest parting off tool in the world!

So this section of the crankshaft was effectively the journal described above with a pin on each side of diameter 625mm sticking out by about 600mm.

The centre part of the crankshaft then designated the crankshaft as either "Semi-built" or "Fully-built".  The fully built was then formed from two huge thick plates with a 625mm hole at one end and a 640 diameter hole at the other end.  These were temporarily joined together for machining on a large vertical borer.  Then they were separated and the 640mm bore end heated with gas burners.  (Always turn off the gas supply before the air supply or just lift the burner out whilst burning correctly, otherwise the carburising flame deposited carbon on the inside of the bore which was tricky to remove and reduced the friction between the web and the pin which could lead to a slipped shrink in service).  Each heating sequence took about 6 hours to heat and a few minutes to insert the pin followed by a long cooling period so about one shrink per day was possible.  Once the two webs were together with the centre pin, then the whole crankshaft was shrunk together in modules.

The semi-built version had the centre webs and pin provided as a single forging, but the rest of the assembly was the same.  Machining the centre pin on the vertical borer was quite tricky as the tool had to reach along way between the webs.  Semi-built webs were mainly used on the smaller bore engines, i.e. 580mm and 670mm and these gave smaller polar inertia values.  I think that the 670mm bore engines could use either type and this allowed selection of torsional vibration natural frequencies to suit the application.

I hope that this explanation has helped, I was an apprentice at the time and it seems so long ago now (1973)
Ian