Composite pipe to resist torsional loading

[Marine10]

Hi,

I am looking to replace an existing steel pipe with a composite pipe for the steering shaft for an 86ft Steel motor barge.

I need to do this as this piece needs to be lightweight and removable.

Any suggestions as to suitable materials would be greatly appreciated.

Our main concern is related to the torsional loading of such a pipe.
Thanks

 

[desertfox]

Hi Marine 10

What is the torsional loading and what size is your steel pipe at present?


desertfox

 

[Marine10]

DesertFox thanks for the reply.

The torsional loading is provided by the water resistance on the rudder, probably 1m2 which can be turned into the oncoming water flow to about 30degrees. The vessel speed is approximately 8mph. The max loading occurs when the vessel is in reverse.

Unfortunately I cannot give you any numrical data for the scale of the loading at this stage.

The shaft connects the rudder to the chain attached to the steering wheel and is about 4m in length and 20cm in diameter. Thickness curently unknown.

Thanks

 

[desertfox]

Hi Marine10

We really need the pipe size as well as the loading, however if you can tell us the pipe thickness we could give you an equivalent polar moment of inertia in various materials to that which you have with the steel one at present.

desertfox

 

[Marine10]

ok that could work. I would estimate a pipe thickness of about 5mm.

cheers

 

[desertfox]

hi

I'll run some numbers and post later, in the mean time you might like to look at this:-

http://www.eurekamagazine.co.uk/article/22442/Industry-wakes-up-to-carbon-fibre-driveshafts.aspx

desertfox

 

[gwolf2]

I think you may have problems with water ingress into the carbon fibre if it is continually submerged in water. The end fittings will need to be carefully designed.

If the shaft is impacted by debris that will break any surface protection and again let the water in.

Once the water gets in it will delaminate and fall to bits.

Metal is probably best.

 

[Marine10]

gwolf2,

Thanks for the reply, the existing shaft runs from the engine room to the steering gear room where it connects with the rudder post and so the entire shaft is inside the vessel and does not come into contact with water (if it does we have sunk!)

Cheers,

Marine10

 

[gwolf2]

In that case a composite one would work well I think - carbon fibre torque tubes work very well but large custom ones will be very expensive because of the tooling involved to make the shaft.

gwolf.

 

[RPstress]

A typical boat composite might be E-glass woven or knit in something like Derakane 411 vinyl ester. Depending on your steel, this might have similar allowable strengths to steel, permitting, as a very rough first estimate, a size for size replacement.

Layup should be with fibers oriented mostly at 45° to the tube axis. However, while that should exceed the steel tube's torsional capacity, it will give a deficit for the bending properties.

A tube of 200 mm x 5 mm wall in E-glass should have a reasonably well factored allowable torsion of, say, 75 kNm.

If cost is less a factor than weight then a carbon tube made of something like Panex 33 fiber/epoxy should be about half the weight.

Close attention is needed for design of attachment fittings.

 

[Compositepro]

If weight and stiffness are important then carbon is the only way to go. Carbon is three times stiffer than glass while the strength is about the same. Also carbon/epoxy generally has no problem with water.

 

[MikeHalloran]

If cost is important, an amorphous iron- carbon composite may be indicated, despite the cost and complexity of necessary anticorrosive surface coatings.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

Mike,
I thought that was what he was trying to replace,
God knows why.
B.E.

 

[George77577]

Why not consider combining carbon and E-glass fibers or Basalt fibers which are inexpensive and stronger than E-glass. Derakane 411 or 8084 which has highest elongation might fit the application.

Thinking out of the box here but might a cored laminate reduce the weight and provide the necessary stiffness?

 

[RPstress]

George77577 has a point about basalt. It seems to come out with mechanical properties intermediate between S-glass and E-glass with a cost claimed to be closer to E-glass. Never used it myself. A cost- or weight-benefit is claimed for basalt vs. E-glass. However, it doesn't have that many suppliers (all Russian as far as I'm aware; Basaltex is a Belgian company but I have a strong suspicion they import from Russia). It won't come close to carbon for weight.

I personally don't see any benefit in going to a sandwich.

Compositepro is right about glass's stiffness being low. (Basalt is a bit better but not that much.) The only way to match a steel part's stiffness with composite is to use carbon. However, for a tube that's not in bending, stiffness is unlikely to be a major factor. A glass tube with a lot of 45° plies is likely to have a stiffness in torsion about 1/7[sup]th[/sup] of a steel tube's, maybe less depending on exact layup.

However, for a shaft dominated by torsion you should still be able to take a lot of weight out with glass, probably about 40% to 50%, and with carbon maybe 60% to 70%.

Some info on the steel used for the original part might help refine these sorts of very, very rough (gu)estimates.

 

[George77577]

Basalt comes from Russia because they have the basalt mines. You can get the material locally. They carry both epoxy and vinyl ester compatible.



Aliene Elkins
Advanced Materials Group
The ET Horn Company

http://www.ethorn.com