Torsion shaft special required please 3

[AlphaH2Omega]

Hi all, first time here in search of some advice please.
I am building a machine that is required to transfer torque to a generator and would like some opinion on the drive shaft material and sizes required.
Given the shaft is submerged and low rotation speed 10rpm, I was thinking maybe nickel plated 4340 bar with Vesconite sleeves pressed onto the bearing surface and then housed within a vesconite bearing to prevent wear to the shaft (if vesconite on vesconite is not advisable and you think I would be better off having the shaft bearing directly to the vesconite please let me know)
The shaft is 3000mm long with a 250mm long vesconite bearing within a captive housing at each end with 1800diameter sprockets locked to the shaft with a sufficient boss using either a spline or multiple keyways (I would prefer keyways as a fail safe to prevent over loading)
The torsion on the shaft will be up to a continuous 500,000Nm applied through the sprockets(38mm thick) fixed to the shaft through the bosses (200mm wide) with the outboard edge of the sprocket 12mm from the most inboard edge of the vesconite bearings to accommodate a thrust bearing to prevent shaft end play.
The load is dynamic and will be applied to a generator stator through a gearbox on the outside of the machine, 500kNm being the torsion load that would accumulate if the stator was locked from rotating, the driveline and stator is to be designed in conjunction with eachother and the main output further down the track, however the force origin has an output of 500,000Nm of torque at 10rpm. The distance from the outboard face of the sprockets to the driven joint on the gearbox outside the tank, having gone through a sufficient shaft seal, that will transfere load to the stator is 400mm.
At this stage I do not know the resistance that the stator will offer, so intend to work the initial design as if the resistance to tha torque is 100%, thus having stalleed the gearbox kinetics - see that as a safety factor ensuring than any initial failure point is the loint between the outpt shaft and the gearbox.(Please let me know if you have a better idea)
What I really need is design advice on material and dimension requirements for the strength of such a drive shaft, and a description of recommended arrangements of keyways that would be suitable for the task.
Based on the 12mm offset between the innermost of the bearing surface and the 400mm from where the torque is applied to its transfer point, I consider a reverse bending moment through the length of the shaft between the bearings requiring a straightness, and fatigue calculation; and a torsion through the shaft between the sprocket and the output joint across 400mm of shaft length to be the critical factors.
Please offer what design advice you see fit.

Thankyou
Kyle.

 

[dvd]

To be clear, is this 500,000 N-m of torque as in five-hundred-thousand, or do you use a comma as the decimal point to indicate five-hundred N-m?

If this is 500,000 N-m of torque being transmitted through a chain drive, where are you getting this chain? It seems like it would be massive. What are the particulars of the sprocket and chain - i.e., dimensions, and capacity?

Can you provide a better description of the equipment, maybe indicate the industry? The sketch is hard to understand? Is there a CAD model, or assembly drawing of some sort.

Is this an invention that you wish to make, or an actual machine?

 

[EdStainless]

A plated steel shaft will have a limited life in water, even clean water.
You really need to think about an alloy.
You can get cold finished Nitronic 40 or 50 shafting, it is used as prop shafts in boats.

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P.E. Metallurgy, consulting work welcomed

 

[AlphaH2Omega]

3" link chain UTS 123000lb,56T
Duel sprockets, one each end of the drive shaft, 2500mm space between the sprockets.
Sprockets are 38mm 6061-t6 alloy with laser cut and machine milled jigsaw style couplings around the circumference to which will be pressed sections of toothed gears relative to the chain specification c/w sandwich plates.
Yes it is rather large.
Mining industry.
An actual machine that is an invention.

 

[AlphaH2Omega]

Elevation view

 

[dvd]

Are the sprockets heat-treated to T6 after laser cut, or is this -T6 plate? You may have locally annealed the material and lost your T6 strength in the laser HAZ during the laser cutting. Waterjet may be an alternative that retains temper. Why did you choose aluminum? Aluminum does not have infinite fatigue life. Granted a 56 tooth sprocket has to have a lot of revolutions to fatigue the teeth.

56 tooth, 3" pitch sprocket has a pitch diameter of 53.504", at a torque of 500,000Nm (=368,780 lb-ft or 4,425,375lb-in) the tightside tension will be more than 4,425,375 lb-in / (53.504/2in) = 165,000 lb, before adding in any tensioning forces. You would probably want a 10:1 chain ultimate to chain working strength ratio, if not more for a severe duty mining application. I think your chain is overloaded.

To me, the moment would be from middle of sprocket to middle of plain bearing so 38/2 mm + 12 mm + 250/2 mm

I would advocate for a Ringfeder-type shaft locking device in lieu of keyways or splines - thoroughly discuss the application with Ringfeder.

A reasonable shaft calculation for material handling applications would be found in ASME B106.1M -1985

 

[TugboatEng]

Let's take 10 steps back here. This is absolutely the worst nightmare of a materials selection I have ever seen described. Can OP please provide some reasoning behind their choice of materials so we can help understand the situation better? How long does this machine need to last? Aluminum sprockets aren't great under dry conditions and they will be terrible when wet.

Ed, I am curious, Aquamet is the premier global supplier of SS boat shafts. I see their materials are nitrogenated 304 and 316 stainless steels. Are these comparable to Nitronic alloys? I have no experience with stainless shafting, everything above 10 inches is clad carbon steel shafting.

 

[AlphaH2Omega]

6061-t6 alloy with laser cut and machine milled jigsaw style couplings around the circumference to which will be pressed sections of toothed gears relative to the chain specification c/w sandwich plates.

I thought it was obvious where I said "toothed gears relative to the chain specification" that thr gears would likely be heat treated high carbon steel "press fitted - c/w sandwich plates" onto the 38mm 6061t6 disc.

As far as the "worst combination of materials" what are you saying? That 4340 is not a good shaft material?
That vesconite is not a good bearing material for submerged applications at low rpm?

 

[TugboatEng]

Plastic bearings do present manageable issues in wet applications. Plastics do absorb moisture which causes them to swell. I'm not familiar with Vesconite but Google leads me to believe their product is parallel to Orkot which is common in the marine industry. There are guidelines to accommodate swelling. Generally, bore diameters will be oversized during machining.

Your bad material selection has a few issues:

Plastic bearings are sensitive to abrasive wear so you must not have corrosion products in your bearing areas. Stainless steels or ceramics are mandatory.

You mention aluminum sprockets which are going to have galvanic corrosion issues with both your steel shaft and steel chain. Aluminum also galls against itself and most other materials so that will present wear issues, especially in wet environments. I won't even run aluminum sprockets on my motorcycles anymore.

Can you please provide any reasoning behind your material selection? It will help us understand your goals or misunderstanding.

 

[AlphaH2Omega]

I probably know more about stainless steels than you dude. I was a aeronautical fabricator for a significant part of my career and got to know a fair amount about a wide variety of high alloy stainless steels that there is a high likelihood that you have not even heard of.
Especially considering that instead of assisting with my actual question regarding torsion and the drive shaft arrangement, it seems you are rather desiring to wank on about the gear material that I did not ask any questions about and then direct me to some boat place thinking they specialise in loads of 500,000Nm.

Is there someone here please who, instead of trying to project their ego at me in at attempt to critique and subsequently invalidate my design in search of amplifying their already disproportionate ego at the behest of my self worth - a vain attempt albeit; but rather prioritise the answering of the question I asked regarding a suitable shaft material and diameter, taking the potential for shaft flex into account- which is likely not neccessary, providing the shaft meets the specific requirement that it remain straight in operation to prevent fatigue, with only a 12mm offset from the bearing - providing the shaft does indeed not flex and thus the bearing IS in FULL effect across the width that has a safe working capacity of 500,000Nm of torque transfered through a length of the shaft 400mm long.

If not, I can go somewhere else and ask someone.
Sheesh, what a reception.
What drives you?

 

[TugboatEng]

I will ignore your personal attack. You seem infatuated with your torque number. How does that correlate to power in your generator?

As a marine operator I have learned to despise stainless alloys, including the ones you think I haven't heard of.

 

[3DDave]

Materials with very low deflection still fatigue. Trying to force parts to be straight under bending load can create a stress concentration that reduces fatigue life. The shaft will flex.

The drawing looks like a bucket lift. Is there a free body diagram?

 

[MintJulep]

I'd avoid nickel just due to the environmental and environmental liability risks.

 

[dvd]

The shaft is probably in the 18 inch to 25 inch diameter range, depending on materials and geometry.

 

[AlphaH2Omega]

Thankyou for all the offered answers.
I have come up with an applicable equation from which to derive a selection based on an equivalency model.

The the ratio of the offset between the inner edge of the bearing and the outer edge of the sprocket (relevant considering an implied perfect rigidity) and the center of the shaft length is 12.5mm:1250mm (1%)
Assuming a 550000 N load acting on the two chains, divided - is 275000N per chain in tandem with a spacing between the sprockets of 2500mm, therefore is an equivalent load at the center of the shaft implying the shaft center of length to be a separation point that offers zero rigidity is 1% of 275000N =2750N =250kg at an offset of 1.25m in a static load setting.
So, I can calculate the fatigue based on the applied deflection such a shaft at 1250mm long would experience being mounted rigid and having a 250kg load applied to the opposite end from which in is mounted, which for an 8" 4340shaft is close to negligible as a physical displacement.
It is safe to assume that given there would physical displacement at the shaft center as a static demonstration that there is little to no stress accumulation occurring in the location of the bearing originating from the bending moment force due to the low offset and any stress in this area due to a bending moment would result in bearing wear prioritised over fatigue hands down. - remember this machine is only doing 10rpm.
In this application however, the shaft either side from length center are not independent of eachother and as such the bar crnter is also offering resistance to the bending moment.
In any case of deflection therefor; the stress concentration as the result of any deflection, likely to be less that 0.004" or (phi-1)×(the physical deflection caused by 2500N at an offset of 1250mm) would be happening through the entire length of the shaft between the sprockets having a tendency to accumulate 0.618 of the stress in the center 0.382 portion of the shaft length and the remaining 0.382 portion of the stress distributed evenly to the remaining 0.628 of the shaft toward each end.
A phi-coefficient distribution.
Having isolated bending moment and fatigue as an issue in this shaft that is doing 10rpm with a consistent and continuous load I diagnose the torsion capacity of the shaft to be a paramount consideration provided it is selected as to be congruent with negligible deflection in a static load of 1250N at 1250mm.
Such a shsft material is easy to find.

 

[geesaman.d]

At those specific speeds within the bearing, you have an ideal grinding machine. "Only" 10RPM is convenient for kinematic considerations, but for fluid-lubricated plain bearings with any amount of abrasives, it's brutal. Orkot/Vesconite/etc will take that continuous load and collect any abrasive grit present to convert themselves into lapping tools that will eat into the rotating mating surface faster than you would expect. Just a single dose of abrasives will permanently lodge into a plain bearing and begin the accelerated wear pattern. So for me, unless this is fully filtered water, the main design question is how long the bearings need to last before the end user will accept replacing them. And how will they replace them. At a minimum, consider a plasma-sprayed ceramic coating on a replacement shaft sleeve.

If endless bearing life is very important, call ChampionX. Their bearings are almost impossible to wear out, even in the presence of nasty abrasives, as long as they're kept cool and wet. A larger bearing from them runs into 5 figures cost, but maintenance outage costs can easily justify it.

Fatigue calcs are only valid if the material is not corroding. Nickel plated 4340 has lovely fatigue strength on paper but it's highly notch sensitive and corrosion will accelerate that process. 300 series stainless is not fun to machine and doesn't show off on paper but it's far more tolerant of real-world factors. For large, turned/ground/polished stainless bar, you can consider the Aquamet option especially if you need longer than 20ft lengths. They seem to be one of the only stockists to keep really big stainless bar around.

Marine propeller bearings are a useful reference but without adequate flow and sliding speed they can also pick up abrasives and perform as lapping tools.

Shaft deflection is always important. First run the fatigue calcs (ASME B101.6 if you want specifics) and then analyze the deflection. If using plain bearings, also analyze the situation where the bearing surfaces have worn and possibly determine a wear limit on those bearings to prevent secondary damage to the machine that comes from the misalignment.

I've seen too many 3-bearing shafts employed to control deflection without accurately considering bearing loads, shaft stresses, and proper alignment. They fail unless that loading scheme is built into the design from the start.

I can't comment on chain drives as I just don't do them. But aluminum only has corrosion resistance from its aluminum oxide outer layer, and in continuous rolling contact I would tend to think that protection is voided?

Regarding gearbox stall torque - is there any shock? Stall torque is an easy calculation but how it gets to a stall is far more crucial - gearbox service factor will need to be boosted if it must withstand an abrupt stall condition.

 

[EdStainless]

TBE, They market it as Aquamet 22 (and 22HS) for Nitronic 50 shafting.

https://www.aquamet.com/aquamet-22

Once upon a time I knew someone who was actually making this stuff.
The small sizes were cold drawn and larger ones were rotary forged.
Don't know about today.
I agree, the mix of materials is a disaster waiting to happen.


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

 

[dvd]

Another point to make about aluminum is that the coefficient of thermal expansion is about twice that of carbon steel and 1.5 times that of stainless steel. If there is any significant temperature swing, the sprocket hub diameter will increase more than the shaft diameter.

I don't know whose money is involved in this project, but it is going to be expensive, and it would certainly pay to hire an engineer.

 

[AlphaH2Omega]

The fluid that is in the machine that the moving parts are inside of is clean fresh potable drinking water with the chosen corrosionprevention additives and soluble lubricants neccessary.
I should have mentioned that from the start

There is likely to be a static electrical charge build up in the tank but it will he earthed and I can install galvanic balancing plates if neccessary to protect the AlO² from degradation.

Thankyou all for your input.