That is, they allow you to install a weight-bearing structure between two walls, or between the floor and ceiling, without leaving any holes behind.
If you have a significant amount of weight you want to support, it's more structurally sound to install two tension rods vertically, then connect a cross piece to the two vertical posts. A vertical member is going to be able to support much greater loads than a horizontal one whose vertical forces are supported only by friction. (true, lateral loads would still have to be supported by friction, but hopefully those aren't too great)
If you wanted to build a tension rod, these are the specific things you would need:
Commonly available tubes in the US that fit inside one another:
| material | nominal size | inner tube OD | outer tubeID | gap | ||
|---|---|---|---|---|---|---|
| inner tube | outer tube | inner tube | outer tube | |||
| PVC Sched 40 | PVC Sched 40 | ½" | 1" | 0.840" | ~1.029" | 0.19" |
| " | " | 1" | 1 ¼" | 1.315" | ~1.360" | 0.05" |
| " | " | 1" | 1 ½" | 1.315" | ~1.590" | 0.28" |
| " | " | 1 ½" | 2" | 1.900" | ~2.047" | 0.15" |
| " | " | 2" | 2 ½" | 2.375" | ~2.445" | 0.07" |
| thinwall Class 200 PVC | thinwall PVC | ¾" | 1" | 1.050" | ~1.185” | 0.14" |
| " | " | 1" | 1 ½" | 1.315" | ~1.750” | 0.44" |
† assume round tube unless stated otherwise
Other pages about this topic: [1] [2]
On the end of the smaller pipe, install something to block the very end. If you have the correct hole saw or an adjustable circle cutter, you can cut a wooden cylinder and epoxy it inside the end. (To really secure it, maybe put some headless nails through both the PVC and the wood?)
At several intervals down the larger pipe, drill a hole through both walls. Then, to select a particular length, run a bolt through one of the holes.