From a strictly mechanical point of 5

[Keith Martineau]

From a strictly mechanical point of view (never mind the aesthetics or practicality for now)which is better to transit power, a short shaft or a long one, assuming all other factors are the same? What I'm wondering mostly is concerning strength (resisting breakage or distortion of the shaft), resisting twisting, lifespan of the material. I have in mind a situation that I can vary the length of an axle shaft (within limits, of course) and I am wondering if there is any practical difference or any advantage in choosing a longer or shorter shaft. I may need to carry a lot of power. I am also under the impression that a higher speed of the shaft is my friend (except maybe in bearings)??? I am considering a portal axle for this reason. I suspect that the length will make a difference in the way the load is carried (twist per foot of length, etc.), but my expertise is electrical, not mechanical, so any hints, insights or suggestions would be appreciated. I will choose the diameter depending on the calculations I will make and whether a short or long shaft has any real advantages.

 

[TugboatEng]

Shafts are not like wires. Torque is equal on both ends, it is not lost.

 

[IRstuff]

While torque might be fixed, inertia isn't, so a higher inertia results in lower acceleration

TTFN (ta ta for now)
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[TugboatEng]

So shafts are like capacitors and not wires. Got it.

 

[NormPeterson]

Full power would be around 70,000 horsepower per shaft at ~125-150 rpm and 4 shafts for a total of 280,000 horsepower.

That's in the ballpark, at least with respect to what the Navy publicly admits to.

https://www.naval-technology.com/projects/nimitz/

50 years ago, I was working on the Nimitz itself (shock grade equipment foundations, mostly) and I'm pretty sure I saw some SHP numbers at the time. Filed these days under "things you were sure you'd always remember until it turned out that you couldn't".


Norm

 

[EdStainless]

Thanks TBE, yes divide total by 4, not 2. Though I can't imagine turning props that size at over 120.


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[TugboatEng]

100-110 rpm is the norm on commercial ships but the speed is dictated by the direct drive slow speed diesel engine.

https://youtu.be/2pZbfd2zNvk

It seems the props on the GWB were 250x250. Inches are customary. A shaft speed of about 145 rpm would be required to achieve 30 knots.

 

[enginesrus]

"Shafts are not like wires. Torque is equal on both ends, it is not lost."

My answer was a quick hypothetical example. Shafts are not wires, agree. But just like your not going to use a 100 foot long 14 gauge wire to transfer 1000 amps, you also will not use a 100 foot long .250 inch diameter shaft to transfer 1000 lb ft of torque will you? I was just trying to explain to a person that is more into the electrical than the mechanical. Oh and the torque would be lost in my example just like the electric current would be lost as well.

 

[Keith Martineau]

Thanks for all the help guys! I do appreciate it! Also I have researched power for ocean going ships. The RPM is, in fact, mostly governed by the power plant (often a VERY slow diesel). The prop design reflects this. But some are turbines and some diesels are run at slightly higher RPM (there are several different classes of diesels according the their max shaft speeds). Some have more than 1 power source (often a slower one is used in port to maneuver more easily, and a faster one, turbines for example, at sea. This is a fascinating subject to me. I sometimes wish I had selected mechanical instead of electrical, or studied both more in depth.

 

[TugboatEng]

My other comment may have been a bit confusing. There is certainly an ideal speed range for the propeller. In the 8ft to the largest 30ft diameter propellers, the ideal speed ranges from 250-80 rpm. Slow speed engines are built around the propeller rpm, long stroke versions for the lower rpm and short stroke versions for higher. Those of us running medium and high speed engines use reduction gears. We match the ratio to our ideal propeller rpm. It really is the propeller that dictates the shaft speed.

 

[GregLocock]

Here's a general introduction to prop design

http://www.navweaps.com/index_tech/tech-034.php

And here's some idea of the tradeoffs

http://www.navweaps.com/index_tech/tech-077.php#Propeller_Design

for a particular prop,

Cheers

Greg Locock


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[SwinnyGG]

Oh and the torque would be lost in my example

That's not how torque works. If you're not going to help then don't post.

 

[Tmoose]

In some drives a long-ish skinny quill drive shaft is used to de-tune the system to de-claw torsional resonance or de-fang shock loads.

mechanically driven centrifugal superchargers from days-gone-by came to mind.
I'm thinking some more recent race engines or furious motorcycles' alternators more recently