Re: [PSUBS-MAILIST] Precision prop shaft

[Antoine Delafargue <antoine.delafargue@gmail.com>]

Hi Alec,

another example of calculation using this time a bit more realistic figures and I still get .0002" reduction of hole radius.

here is what I used:

-the stress at the center of a 3/8" thick 30" diameter semielliptical cap  is around sea pressure * diameter / thickness / 2

so here around 20000psi

(In case you are looking at through hulls in a cylindrical shell, the stress in a cylindrical hull is given by seapressure*diameter/thickness/4 in the direction of the axis of the cylinder, and seapressure*diameter/thickness/2 in the direction perpendicular to it that is in the place of the circular cross sections of the cylinder)

-and then ri= .5", re=2" and the fraction of the hull stress applied to the through hull as 1/3 (2/3 was probably way conservative, 1/3 probably still is. By comparison, with a plain circular hole in a hull the stress builds up by a factor 2 around the hole)
 

You could underream the hole section in the area of the hull to prevent it from squeezing on the shaft. That would help with the strain potentially building up with the welding too. At least your critical seal is protected from parasiting strains from the hull as it is not located in the alignement of the hull.

 

But I was wondering whether the bearings could have a self aligning capability sufficient to centralize the shaft within 0.0001 to 0.0005" accuracy. because if it is not the case I imagine that even slight bending non axi-symmetric forces on the shaft (reduction gear) or the tunnel (welding/ exostructure) would be enough to bring the two in contact.

 

If you put two seals and oil in the tunnel I guess the friction would be dramatically increased in the tunnel with the current clearance.  

 

This whole problem of locating: the seal, potentially back up seal, bearings, spider couplings to isolate the seal from dangerous bending forces is really interesting. I have yet to figure it all out.

I initially thought a coupling on the shaft would be nice on the outside of the hull to protect the seal in the event that a collision or entanglement damages the propeller or fairings. and also that could limit the shaft back and forth movement when going in reverse mode again to protect the seal. but on the other side that would mean adding more bearings and friction from the coupling to the propeller (and I guess there is more friction in bearings underwater than inside the hull). (In the case I am looking at, the propeller stands 50cm  behind the hull).

Then when Vance said there were spider couplings in the inside of the subs he knew, I realized that the set up could be more simple with one bearing inside the hull and one outside (or two inside is the prop is close to the hull). the spider coupling inside would protect the seal from vibrations (esp with a diesel) and non symmetric forces with gear or belt).

regards

antoine