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.

 

[3DDave]

What factors do you think apply?

 

[BrianPetersen]

If you need to transmit the power a long distance, a short shaft won't do it. If you need to fit in a small space, a long shaft won't fit.



Define "better". Define what "all other factors" you really mean.

There are some technical limitations to how long a shaft can be versus how stiff its cross-section is versus how fast it can spin. Look up "critical speed". Is that what you are concerned with?

 

[IRstuff]

Did you include inertia?

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[TugboatEng]

Carbon fiber shafting being used in the marine industry is a game changer. Long runs, no line shaft bearings. A 20ft long shaft rated for 2500 hp at 1800 rpm can be carried by two people. It's 12ish inches in diameter which makes it very torsionally rigid so it does require elastomer couplings.

 

[Keith Martineau]

Great response Tugboateng! BTW I was a missile tech on Trident subs. 18" diameter sand-filled hollow shaft EXTREMELY long (think like part of a football field). Under certain conditions the shaft twisted more than 18 times from end to end without damage. I will try to research carbon fiber shafting. I was hoping for a diameter that can be used as an axle shaft but that is something to look into for sure.

 

[Keith Martineau]

Thanks to you also IRstuff. I will check that out too.

 

[CapriRacer]

From a strictly mechanical point of view, a short shaft is better. Less prone to harmonics and buckling, However, usually the choice isn't made so much as the shaft length is dictated by the design. If you are saying you have a choice as to where to mount the power unit, then mount it close.

 

[NormPeterson]

Suddenly-applied loading? U-joint angularity?


Norm

 

[TugboatEng]

Quill shafts are often employed in turbine gearboxes. They make the length of the system shorter by making the shaft longer.

 

[EdStainless]

Short, stiff, and very well aligned is one option.
But if there are alignment and motion issues I would rather have a long shaft and not force the issue.
We used to supply some high strength 4" OD thin wall SS tube (4" x 0.090" with 150ksi UTS) for drive shafts. We made them 30' long (though they were used no more than 12') and they would handle a lot of torque.

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[BrianPetersen]

Don't forget the mid-1960s Pontiac Tempest, engine in front and trans in back, with the "rope drive" from the engine to the transmission! It was intentionally not straight, because the intentional misalignment stopped the shaft from "whipping" due to exceeding what otherwise would be its critical speed.

 

[BrianE22]

A long shaft will store more torsional energy - sometimes a dangerous condition. Short shafts will usually deliver torque to the output quicker.

 

[TugboatEng]

Keith, I've heard those stories from other nuclear guys. I doubt the deflection exceeded 18 degrees. Also, those 18 inch shafts are solid unless they are controllable pitch. Boring long shafts is... challenging.

I sailed on an ex-Navy sonar ship. While I never saw the shafting come apart I know it was solid. It was joined using the SKF OK coupling system which needs quite a bit of rigidity to generate the friction forces for keyless power transmission. A hollow shaft could not support that.

I do know that hull stanchions get filled with sand to combat vibration.

 

[Keith Martineau]

Thanks for all the great replies guys! Those are the things I was looking for. Short and stiff seems better for my application, I think, but good to know about long shafts storing more energy, sudden loading, and all the other ideas. This helps tremendously! I am planning a LOT of extra strength due to exactly this kind of thing that is hard to measure or calculate. TugboatEng, I suspect (but don't really know) that the shafts were cast hollow and then machined on the outside only. They are filled with sand to deaden sound (silent service, you know LOL),and probably to balance the shaft too. I did not see it with my own eyes (I was busy elsewhere) but I really believe (or at least think it plausible) about the twisting. It is a VERY long shaft, and being hollow too, not to mention I would guess is is made of very flexible material. They really used to tape rags to the shaft so that they could keep track of the twisting during emergency drills, so this makes the story more plausible too. It is an interesting story any way!

 

[Keith Martineau]

I don't know the exact length of the shafts, but I would estimate a minimum of 30 yards; probably a lot longer. Imagine 18,400 tons of submarine coming to a VERY sudden stop as the prop is suddenly and dramatically reversed using full power. Anyway, thanks again everyone!

 

[TugboatEng]

When a shaft fails due to torsion the outside surface is seeing the highest strain. Solid or hollow, the deflection at failure will be the same.

 

[EdStainless]

US aircraft carriers have hollow shafts, as I recall something like 26" OD x 18" ID x nearly 200' long, in 3 or 4 sections connected by flanges with a buttload of bolts in them.
And with 150,000HP at 40-60rpm you will get twist in the range of 1/2 to 1.5 rev over the full lenght.
Yes the stored energy is significant.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Full power would be around 70,000 horsepower per shaft at ~125-150 rpm and 4 shafts for a total of 280,000 horsepower. None of this is official but I've been able to figure some things out by comparing to other ships and a few pictures that may have slipped through the censors (a picture of the George Bush's props left the pitch and diameter numbers in). Of course the Bush is markedly slower than previous generations of carrier. The SS United States had an aircraft carrier plant in it. 70,000 horsepower is the largest geared turbine I have heard of.

 

[enginesrus]

Short shaft of course. Who needs a torsional spring added to the mix. You say you do electrical and not mechanical. So what is better to transmit electrical current? A long wire or a short one? Same with RF long distance or short?- inverse square law right.