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How would you make this crankshaft?

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  • How would you make this crankshaft?

    I took this photo earlier in the year while visiting a small museum at Port Chalmers, near Dunedin, here in NZ.

    This is an old air pump, one of the hand cranked variety used to supply air to a diver.

    The crankshaft is very interesting, it appeared to be one piece, finely finished all over and notice how the crank 'webs' are tapered - it is a nice piece of work.
    To give some idea of scale, the crankshaft is roughly 600mm or 2ft between the main bearings.

    [This message has been edited by Peter S (edited 11-03-2003).]

  • #2

    That's an interesting crank! I would have thought that this crank was possibly a tube fabrication, the tapered big end 'webs' being developed & rolled to give the taper form. Is there any evidence of welding at the 'mirror' bends (the 45* joints), and is there any evidence of welding along a seam for the tapered webs?

    Is the crank hollow? I know most museum displays have a prominent notice saying "Please do not touch", but in the interests of engineering research, a 'tap' with a finger knuckle would have been helpful!

    Why would the crank be hollow? Is this air pump set a portable unit?? i.e is it weight sensitive? But, a bit of inertia in the crank would seem the ideal way of getting 'over compression' but then again maybe the old diving suits didn't need 'compressed air' just an air mover? I seem to remember from my past experiences with SCUBA diving that 32ft of water exerted 1 Atmosphere of air pressure on the human body, so the other side of the argument is that for every 32ft of depth you would need <15psi gauge just to breath 'normally'.

    RR (confused!)


    • #3
      Those "corners" seem to suggest that the pieces were machined and then maybe hard-soldered together. Otherwise that "3phase" crankshaft would be difficult to machine like that.

      It would seem pointless and very difficult to finish it like that in the solid, but both easy and sensible if the pieces were assembled.


      • #4
        Sure looks assembled to me. Doweled maybe, then silver soldered? Doesn't look like you could get any hard out of it with out it folding up on ya.
        Smitty.... Ride Hard, Die Fast


        • #5
          Seems like it could have been cast or perhaps forged and then the bearing areas machined. If it was a slow speed (hand cranked) pump then the bearing areas need not be that accurate. Perhaps just ground to fit with abrasive.

          Paul A.
          SE Texas

          Make it fit.
          You can't win and there IS a penalty for trying!


          • #6
            I think it must have been intended to be used with a rigid diving suit. They were first devleoped in 1873 and only required air at one atmosphere pressure. In that case then the pump is just a "air mover" rather than a compressor and would not require a very strong crank.
            Free software for calculating bolt circles and similar: Click Here


            • #7
              Is there anything to indicate that this really is the original crank, as opposed to a newly-fabbed restoration? Seems to me, that a machine that old would've rusted to bits after a life on the high seas. That crank looks brand-new and TIG-welded to me.

              Regardless, that really is nice looking. Love the taper. Inspirational, even.


              • #8
                Unfortunately, the pump is now about 1000kms away from where I live, so I can't go back for another look! However, I was particularly struck by this crankshaft, and looked closely at it.

                I wish I had tapped it to see if it was hollow, however, there were no seams or welds visible, nor the slightest indications of any joins or part lines, and it looked to be an old, original part.

                I have to admit, at first glance a built-up construction seems most likely, however considering that it is a vital part in keeping a man alive under water, my guess is it would have been built strong and safe as # 1 priority, lightness not as important.
                I really need to get some more information from the museum, even a date to find out what kind of welding options were available at time of manufacture.

                A local museum here in Auckland (MOTAT) has a similar vintage set-up with which they did demonstrations (some years ago), they had a large tank of water, one guy pumps, the other gets suited up and climbs into the tank. The suit used is massive with one of those bolt-on type helmets, lead weighted boots etc. From memory that suit and pump was Siebbe Gorman (sp) pioneers in diving.


                • #9
                  That sort of pump could also have been used to supply air to a diving bell. It certainly isn't something where you would skimp on quality. A lot of the early diving equipment inventors found out the hard way that thier inventions had fatal flaws.
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                  • #10
                    If it's not built up, I'd guess it was forged, then the surfaces cleaned up by hand.

                    But as Oso says, the corners seem more likely for a built-up job, less likely for a forging.
                    Try to make a living, not a killing. -- Utah Phillips
                    Don't believe everything you know. -- Bumper sticker
                    Everybody is ignorant, only on different subjects. -- Will Rogers
                    There are lots of people who mistake their imagination for their memory. - Josh Billings
                    Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                    Don't own anything you have to feed or paint. - Hood River Blackie


                    • #11
                      Are those allen cap screws on the mains?

                      Maybe a forged crank,kinda reminds me of a T-model crank.
                      I just need one more tool,just one!


                      • #12
                        I just can't hold back anymore . . .

                        Anyone who's been through (military) officer training knows that the crankshaft results from ordering "Seargeant, get someone going on that crankshaft!"

                        Seriously, it's a beautiful piece of work that I'd guess started life as a forging, but I wouldn't rule out a plaster-mold casting.



                        • #13
                          "Are those allen cap screws on the mains?"

                          GOOD eyesight! I didn't even notice! They sure do look like allen cap screws. I though those where developed around the 1930's or 1940's.
                          NRA Life Member, and loving every minute of it!


                          • #14
                            I would guess the socket head cap screws are bogus for this unit. Steel would be out of place among all what appears to be bronze hardware. The rod screws look correct. If the crank is legit it may be a lost wax cast marine bronze piece. JMHO

                            Neil Peters
                            Neil Peters

                            When on the hunt, a broken part is better than no part at all.


                            • #15
                              I get the impression that it's cast, and the original 'crankshaft' made from wax, or other material, and pieced together, accounting for the 'homemade' look. The lack of welds or other methods of joinery support this theory. I wonder about the material, sort of looks like stainless, or maybe some brass alloy. It must be chosen for it's non-corrosive quality, or it would be a forging out of cast iron or steel, since that was very common then. Weight didn't seem to be much of a factor in those days, so I don't think it would be hollow. If it's brazed, it sure is a nice job of it. There doesn't seem to be any significant ammount of machining done to it to clean it up. It's cast, using a special alloy, using a cheaply made master, that's MHO.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-