Angle of Twist- Carden shaft

Write details about torque, cross section geometry, shaft lengh, material and general load conditions. I'd be glad to review the calculation and take a look at this.

Hi bhrescobar, and good morning. When I was working as a marine engineer and some ships had midship placed engines the length of the propeller shaft could be up to 30m, and when starting up the twist of the propeller shaft could be anything up to 15 degrees. Most propeller shafts were solid but some were hollow, the hollow ones were laminated, that is tube in tube up to 5 tubes. In all the years I sailed I never once experienced a broken shaft.

Hi Scapolie.

I did my trade training at sea on steam powered destroyers.

When going from full ahead to full astern watching the shaft was frightening. There was a section of shaft that was painted with diagonal stripes that we had a mirror setup so that it could be seen from the throttle position. This was to allow the throttle watch keeper to monitor the shaft movement while the engine was being shut down or required to be at "Stop Main Engines". Being a steam turbine it must be kept just moving so the shaft doesn't "HOG" or get a bend. About 4 RPM was considered optimum for no drive but no bend.

This was a hollow shaft of about 16 inch diameter. I always thought that it was just an incredible forging that allowed it to take that kind of torque and distortion. Anyway that was one of the things that convinced me that engineering was where I needed to be so I just kept studying. "Now I are one."

Hi BlueAussieBoy. Yes, I agree with you, it could be very frightening to watch the shaft when going from full ahead to full astern. One of the last ships I shiped out on was a large 220,000 ton crude oil tanker which was powered by a large 12 cylinder Sulzer diesel engine of 40,000 bhp at 120 rpm. The top speed was 17 knots and to turn such a ship around took a radius of 11 nautical miles when at full ahead. When going from full ahead to full astern, the procedure was quite simple but it took some time. First the engine had to be slowed down or we could have broken the crankshaft, easily done if care is not exercised. Then after full stop, the engine being a 2 stroke was simply reversed. At full stop the propeler shaft would twist one way, then when the engine was reversed the prop shaft would twist in the opposite direction. The prop shaft was only 14 meters long and 21 ins dia, but the huge wieght of the prop combined with its momentum caused the shaft to twist a full 9 degrees during this operation. The shaft itself was constucted of 5 tubes, each tube a shrink fit on the previous, this was done in the event of shaft failure, as only the outer tube and or the next one would rupture, thus enabling the ship to poceed, albeit slower, to the nearest port for repairs. The engine crankshaft was of the built up type, ie, the shanks were shrunk onto the pinions, which were then pinned together. Myself I am a pomey bastard! but don't let that put you off as I have fond memories of Australia.

As I recall, the four key variables that determine the angle of torque are load, composition (molecular, temper/treatment, etc.), uninterrupted length, and diameter. Also, at what operating point(s) are you getting 8 degrees?

You may be experiencing the difference between the lab model and in-situ model because you did not take into account the effects the load of the liquid (eg frictional coefficient, speed, diameter, etc.) can have with regard to resistance on the propeller, including its various sizes, cavitational effects at speed, etc., that would be felt back along the shaft.

Mark

I am assuming that the lab tests take most other constrictions along the shaft, like water seals, pillow bearings, etc., into account.

Mark

Some interesting histories from our parties. Nice to read this here...

Ultimately, we still wait for details on the application. And, most important, is there any problem if the shaft is turning 8 degrees instead of 3? What could be the implications? Is there any torque measurement involved, or fatigue related issue?