what size/type steel?

[472viper]

Hello, 1st time poster here. I'm new, really new. Just unboxed my 7X12 Cummins 5278 mini lathe this afternoon. As I'm typing this I think I have a little cosmoline on the keyboard. Although my 1st project may sound a little technical for such a newbie, it really is just basic turning. I have a motorcycle drive shaft that I need to shorten. It is .82 in dia (solid) at the middle of the shaft with splined ends on each end. This shaft is 18 inches long. My plan is to cut the splined ends off each end of the shaft (spline and stub about 3.5 in long). I will insert each spline/stub into a piece of steel tube 9 inches long, and weld the stub/spline ends to the tube. I will bore the I.D. of the tube to 0.75 in. a depth of 1 inch. I will turn the stub of the spline to 0.75 dia and lap to fit. This will produce a shoulder on the spline stub and the shaft tube will set against that and be welded at that point. My question, (finaly) is how do I know what wall thickness of steel tube is sufficiant to maintain driveline integrety, (resist twisting and breakage) and what kind of material do I look for? I don't want to turn the dia of the stub less than .75. The O.D. of the tube cannot be more than 1.18 to accomadate a grease seal that must slide over it. I plan on building about two dozen of these shortend driveshafts and will be ordering material from a place like "metal depot". I would like to use 1.125 steel tube (the grease seal would pass over it no problem. That would leave a wall thickness of .375 nominal. How do I know that is enough and what kind of steel tubing do I look for. Like I said I'm new to this but I am going to do ALOT of reading and ask ALOT of questions before I ever turn lathe on. Thank you for any help any of you can provide.
Viper

[Ken_Shea]

Viper,
DOM (drawn Over Mandrel) or 4130 tubing should be a good choice leaning toward a thicker wall as permitted.

Ken