is this possible weight of hollow soft = solid soft, if made up in same dia and with same materials,
Same Weight... Not possible.
Think about it...? Same material... Same diameter... The solid bar of the same material has an entire bar running through the center of it that weighs something.
But if you want to talk strength... Then that's another story. The tube CAN be as strong as a solid bar in torsion once the wall thickness has reached a certain point. Actually it doesn't take a very heavy wall to get 70 - 80% of the strength of solid. Tubing is an awesome material for transmitting power. It's lighter and in instances where the diameters required are large the cost is usually significantly less. To produce the bar is less expensive and therefore you pay less per pound and the tube requires more processing so it costs more per pound. But… if you drop the weight by 80% of bar and have adequate torque carrying capability then you’re gonna save some greenbacks. The larger the diameter… the more savings potential… In small diameter shafting it’s cheaper to just use bar but when weight is an issue you can always drop weight by removing the useless center portion through the length of a bar… and call it tubing.
Another way to sort of cheat the cost of your torque carrying capability requirements is to bump up the diameter. Like say you need a 1” diameter bar to carry the torque for you “thing.” If space allows it you could purchase a fairly light wall … say maybe 2” diameter tube that would carry the same load.
Yep… tubing ranks right up there with the invention of the wheel and sliced bread in my book…!!