Telescope extension mechanism

I am in need of a device that is tubular, and when rotated, will extend multiple orders of the original length, space constraints will require the device to extend 3-4 times its original length. I was thinking of the mechanism behind simple extensible telescopes, which when twisted, presto gonzo, extend multiple orders of the unextended length. Then I hit a problem, I am not entirely sure what is going on inside those telescopes. I figure that each section is threaded into the prior section with a different pitch thread on each section.
Does anyone know the exact mechanism or have a link to some pictures that a simple lad could understand. My google skills are not all they were and I haven’t found what I am after.

Most telescopes I have seen (admittedly the very cheap toy kind) are not threaded at all. They have tubes of increasingly smaller diameter, with a lip on the bottom of the outer tube and the top of the inner tube to keep it from pulling out.

Something like this:

|            |
|            |
|            |
| +        + |
+-|        |-+
  |        |
  |        |
  | +    + |
  +-|    |-+
    |    |
    |    |

Does it need to be rotated to extend? How much load will it support? Automatic car antennas work by using a motor to push and pull a flexible tape. Would something like that work?

Telescopes usually don’t thread – they slide, with stops on the ends.
If that’s what you’re going for, places like Small Parts, Inc. sell brass and steel tubes that nest precisely inside each other. But the sizes they sell are, as the name implies, pretty small. ASnd you’ll have to build your own end stops.

The extension mechanism needs to be more or less contained within the device, and as I have something that rotates that will screw in to either the fat or the thin end I figured it could drive the extension mechanism. I am thinking telescope like as the bore needs to have no obstructions.
Basically as the rotation twist this thing it should extend and the extending end will stabinto a larger stationary tube and seal.
Load wise, during the extension and retraction, just its own weight, which I figure would be several hundred kilos. The weight is high because once the device is extended is going to have to hold internal pressure of several thousand psi. Now if after the extension it bottoms out on some real load bearing sections and it could support several hundred tons that would be super cool, that is a problem for another day.

This is all conceptual, if any actual engineering work is done it would be by competent adults, not me.


Ok I am probably misremembering then, I was pretty sure i have seen small telescopes that twist and extend , the smaller section rotates more then the larger sections hence my thought that the pitch on the threads must change.
Does anyone else recall telescoping devices that twist and extend or is my version of reality a distant relation to actuality (a not unheard of occurrence)

Well, telephoto lenses do this.
They have a helical track cut into the inside of the barrel, and pins that slide in the track. When one section is rotated, the rotary motion is turned into linear motion by the travel of the pins in the helical groove.

Good lord, hire an engineer!

Well that would take all the fun out of it.
Seriously we were just bouncing some ideas around and the whole twisty turney extendy telescopy thing came up as a concept, hence my question as to how they worked (or even existed).

beowulff - that sounds like the idea. The telephoto lenses I have seen just have one extending element, have your (or anyone else) seen a device with multiple extending elements from a single rotating element?

cheers again

Yes, the Nikon 18-200mm VR does: Looky here

Cool, that’s the fella I need. Now does anyone have one and mind breaking it apart and telling me what the internal bits look like?

Oh, sure, it’s only a $700 lens…

You need to be talking to drilling rig people for those kinds of weights.

Another telescoping solution can be seen in an auto/truck lift at a garage. Hydraulic, pneumatic, and rotating screw lifts are available. This will get you into the 12/20 ton range. Multiple lifts would get greater extension. Some lifts already come with multiple sections. Same types of mechanisms in low-rise elevators (just a few floors).

I’ve taken apart some (smaller) lenses. It’s pretty much what you imagine. You have a helically machined barrel around the inner sleeve and either followers or a few turns of matching thread inside the outer sleeve. Or the other way around, I forget. (It probably varies anyway.) To drive multiple segments, you’re probably going to have to have something like a second, inner set of splined telescoping tubes to hold the inner drive tube stationary while the outer tube turns (or, again, vice-versa).

But if this thing is going to be containing so much pressure, why not make up a plain telescoping stack of tubes and use the inside pressure to extend it hydraulically? You would need some overlap to act as bearing between tubes, and seals, etc., but it sounds like you need all that anyway.

Smithsb, mwbrooks, those are helpful sugestions, thanks.
With respect to using the internal pressure, good thought but this is not possible, the device has to extend then lower end stabs and seals into another pipe, at which point fluids would be pumped through the device into the other pipe and it would pressure up. It also has to be retracted once the volume of fluid has been pumped through. Certainly will need overlapping seal areas which would also probably be load bearing if it came to that.
So multiple segments with splines or possible rotating screw lift with multiple stages.

Aerial refueling boom?

I did some end-of-life work on the KC-10 refueling boom. I don’t recall (*) what the extension mechanism was, but I know it had a travel of less than the non-extended length, and that compensating for the tendency for it to extend under hydraulic pressure when the fuel was flowing took some doing.

  • It was long-since in production by the time I got on board, and all the mechanicals were done by the airframer (Douglas). I worked on the boom control electronics…trouble shooting and designing out/around obsolete electronic components. For those interested, it used a Sperry 18/19b processor…NO not a microprocessor, a processor made from LSI logic components spread over three cirduit boards.

Do you remember the Yankee screwdriver? ( as example ) The mechanism is similar to Beowulff’s description, and capable of load bearing far beyond the fine pitch threads of lens barrels.
Using typical rotary/ linear translation it has the advantage of differing advance/return speeds without use of clutch, half-nuts, etc.