Not sure if others have done this before, but while thinking about making an extendable boom, I came up with this:

This is just a proof of concept, of course.

In case you don't want to watch the video (or if it's not clear), the concept is based on the fact that for the LAs to work in the normal way, the end of it must be held in some way, otherwise when driven, the end just spins. So one of the LAs (A) is driven, and the other (B) has its input axle fixed (not clear in the photo). When A is driven, there's no resistance (hopefully) so it turns its end, which in turns the end of B. Holding the input and turning the end is the same as holding the end and driving the input, so B extends/retracts. When this is complete and B resists, it prevents the end of A from turning and it works as normal.

It seems to work fairly well, except that if the LA with the motor directly attached extends first (as in the first attempt in the video) the clutch often kicks in once it's done, preventing it from turning and extending the other one. Switching back & forth a little bit seems to fix it when this happens.

But it does seem as though this would work better under conditions where the greater load was during retraction rather than extension.

Has anyone ever used this concept in a MOC before?

I haven't used any like this, the theory of how it works is not hard but haven't had use for it (the boom is required).

I would really like to see LA with ~twice the length and pneumatic cylinders with 2-2.5 the length of current version.

Been there, done that:

That is a good design Zblj.

Does the whole thing spin apart from the far away end or does the end closest to the motor spin on it's own?

All spins :)

I would really like to see LA with ~twice the length and pneumatic cylinders with 2-2.5 the length of current version.

Firgelli makes 50mm (2") and 100mm (4") Lego-compatible Linear Actuators: http://store.firgelli.com/SearchResults.asp?Cat=1822 .

Firgelli makes 50mm (2") and 100mm (4") Lego-compatible Linear Actuators: http://store.firgelli.com/SearchResults.asp?Cat=1822 .

50$? Maybe made with gold?

50$? Maybe made with gold?

IIRC, they make some other NXT stuff as well. They're probably very strong and may even have a sensor port for measuring extension length.

Been there, done that:

Fair enough; nothing new under the sun, I guess.

Does your setup have the problem with the clutch engaging when only one of them is extended?

I have to say though, if that issue can be avoided, I like my setup better. There's less of it spinning around, and it should be much simpler to mount the driven end, just driving it like you drive a normal one.

Edit: I take that back. I had a go, and mounting them with the big ends together prevents the clutch problem entirely.

Edited August 9, 201212 yr by Hopey

Fair enough; nothing new under the sun, I guess.

Does your setup have the problem with the clutch engaging when only one of them is extended?

I have to say though, if that issue can be avoided, I like my setup better. There's less of it spinning around, and it should be much simpler to mount the driven end, just driving it like you drive a normal one.

Edit: I take that back. I had a go, and mounting them with the big ends together prevents the clutch problem entirely.

Heh, we all learn...