Round Tube/Pipe with Tension Cable Core

[DFLewis]

The design challenge:

There is a fabric roll that needs to span a large distance unsupported (about 80 feet). The client has asked for the roll weight to be minimized since the quantity and weight of the fabric is very small in comparison (roll size < 5" dia; preferably carbon fiber tube).

The idea:

The roll has ends caps to which the internal cable would attach. Tension would be applied to the cable inside the tube to support the roll and reduce deflection. Containing the tension within the roll eliminates the need for robust horizontal supports. Multiple cables could be an option.

This concept is new to us and we wanted to see if anyone else has experience doing this or know of instances where something similar has worked.

What design challenges do you foresee? Is it feasible?

Thanks,


DFLewis

http://www.phoenix-engineer.com

 

[handleman]

Any external loading expected other than the weight of the fabric?

What is the permissible deflection at the center?

Does the roll have to rotate? If not, the cable should be at the bottom side if this has any chance of working. An 80' span seems awfully long for a 5" diameter cylinder. I think putting a tension cable inside would do more to contribute to Euler buckling than straightening the thing out.

-handleman, CSWP (The new, easy test)

 

[DFLewis]

- Just the weight of the fabric

- 12" in the center is a lot but probably on the edge of what we might find acceptable.

- Yes, the roll needs to rotate and be driven... approximately 1000 in.lbf. of torque applied.

I agree with the buckling concerns.

DFLewis

http://www.phoenix-engineer.com

 

[handleman]

I don't think a cable at the center is going to be any good at all then. If you're already expecting deflection over the diameter of the tube then a tension cable inside the tube with tension supported by the tube will only contribute to buckling. A high-tension wire run through the tube but supported externally would sort of work, but you would need some sort of bearing inside the tube to prevent the tube from being abraded by the cable.

Also, with that kind of driving torque and such a long "shaft" you are going to need to consider twisting of the tube pretty carefully.

-handleman, CSWP (The new, easy test)

 

[rb1957]

what about a fabric membrane supported by inflatable tubes ?

the inflation may be able to deploy the cover ??

 

[griffengm]

What if you incorporate several internal caps and run the cable through them?
Or could you run several tensioned smaller cables from end to end inside the tube along the id?
Griffy

 

[chicopee]

To minimize deflection, you need to increase the moment of inertia which can easily be done by increasing the roll diameter and at the same time decrease the thickness of the roll in order to minimize weight. There will be no simple solution, it will be a compromise.
An internal prestressed cable will only make deflection worse as Handleman pointed.

 

[pauljohn]

This sounds similar to the in-mast roller furling you see on sailboats. Consists of a central wire or rod (rod works better - less deflection at centre) suspended between one or two turnbuckles for tension inside the mast. A series of aluminum foils cover the whole length of the outside of the wire and have a track grooved in them which the sail slides up in. The sail is hoisted one time then a swivel unit top and bottom riding on the wire allows the whole unit - foils and leading edge of the sail to rotate around the central wire and roll the sail in and out through a slot in the mast.

Only difference in principle I see is that the sail is inside the mast which is the object in compression. I think it would sag quite a bit in the middle if suspended horizontally. Companies that make them are Reckmann, Profurl, Seafurl, Harken. Not exactly what you were looking for but might give you some ideas.

 

[MikeHalloran]

If you took griffengm's idea of multiple cables and internal diaphragms, and extended it a bit by separating the cables toward midspan and running them through radially displaced holes in the diaphragms, you'd have an internally stiffened analogue of the long pipes stiffened only with spreaders and stays that you sometimes see overhead.

E.g. say:
-end diaphrams with cable anchors clustered near the center
- a midspan diaphragm with only notches around its periphery
- quarter- span diaphragms with holes at, say, 5/8 radius
- and so on,
so that the cables are each tensioned over a series of edges approximating a half-sine.

Ok, maybe that's what (s)he meant to say anyway.

Just as in sailboat rigging, rods would probably work better than cables, because cables (or their terminations) stretch more.



Mike Halloran
Pembroke Pines, FL, USA