Start with identifiers for the sections, call them base, 1st, 2nd, 3rd (dead) sections, from biggest to smallest.
Telescope 1st section about 3'
Telescope 2nd and 3'rd (they are pinned together to start) to full extension, at 0 degrees, or negative, so you can reach the boom from the ground.
There's a little screw jack , accessed through a hole in 1st section (that's why it got telescoped 3'- to expose that hole and jack). You insert a little handle with a socket welded to it, and crank that jack up, locking the 3rd (dead) section to the 1st section. Then walk out to the end of the boom, and pull the 3" pin that's holding 2nd and 3rd sections together. It comes up from the bottom side of the boom, held in by a keeper pin.
Now you have 1 and 3 locked together with the little jack, and 2 is disconnected from 3 because you pulled the big pin on the end.
Retract #2 all the way. Reinstall the 3" pin, from the bottom of the boom, locking 2 and 3 together, only now 3 is stuck all the way out. Now that 2 and 3 are attached together, you use the little handle and socket to retract the jack pin, unlocking section 1 and 3 from each other.
Now you can boom up, 3 is stuck all the way out and pinned there. 1 and 2 will extend normally - each with their own cylinder.
Uninstall dead section is the reverse.
Most guys get hung up when installing or removing dead section, when they try to crank the little jack with boom telescope cylinder "wedging" the pin locked. You have to bump the boom section just a bit back out after retracting all the way when installing the dead section. That releases the tension on the pin, and you can crank the little jack easily. The same when removing it, you have to telescope all the way out, then just bump the lever enough back (retract) to release the tension on the jack.
You can hear the boom "bump" itself free off the pin from the cab, if you're totally out of the throttle and just bumping the lever. If you do it all revved up, you'll go past the sweet spot. Once you've done it a couple hundred times, you get the hang of it. Actually the most important thing is having someone that's done one before, show you how to do it, that's the fastest learning curve.
If you really want a mess, have someone retract or telescope without remembering to uncrank the little jack, and shearing something off inside, when you have the dead stick out. Then its torch time, if its wedged, because there's no way to get at it.
In this picture I've got the boom laid down to pull dead section back in, before swinging my jib back around, after my job.
1 is out the 3', you can see the little hole in the side of the boom, it's right by the passenger mirror of the crane in that picture.
2 is out all the way, pinned to 3. I've retracted the jack pin , and I'm ready to climb back in the cab, boom up to 30-40 deg or so, and retract both sections all the way so I can uninstall the jib.
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