Monday, August 6, 2012

Jib Crane Part 2

The neighbor I mentioned in an earlier post that has me fix all his broken antiques has a nice tall rolling ladder I borrowed for the next phase of the Jib crane installation. All this steel is heavy and I was getting pretty nervous torquing bolts to 80 ft/lbs off a aluminum extension ladder. This platform ladder saved my ass. The right tool at the right time. Nothing beats that.
Before I did the work up high I test fitted the brackets and fittings close to the floor. This had the added benefit of making the sequence of assembly operations obvious. This exercise told me I needed a couple of spacer blocks and more nuts to keep the studs from turning.
In this picture you can see the threaded studs that provide the clamping force to sandwich the concrete column. Just for scale sake, the white column is eighteen inches square. The pivot plates are through bolted and welded to the channel. The studs I chose are high strength  4140 steel with a tensile strength of 125kpsi. Each one when properly preloaded provides over 10,000 lbs of clamping force.Getting them torqued evenly and to full preload took a little time especially up a ladder.
In this shot I have the brackets up on the column. They had to be set a specific distance apart to make the assembly go together and to agree with the design calculations. Yes I wimped out a little and added a couple of half inch anchors to each bracket. We call these chicken bolts in engineering speak. Its for the things you didn't anticipate or just plain conservative insurance. So the next step here is to get the main beam and tension rod up into the brackets. It weighs around 400 lbs so don't think I going to whip that up the ladder on my shoulder.
I ended up adding a couple of lifting lugs in the ceiling. Pretty handy to have a concrete ceiling that you can do that with. No forklift to help me out just an Egyptian come along and a bunch of trips up and down the ladder to get it rigged and hanging right. The tension rod has a turnbuckle in the middle so I can level the I beam and adjust the length for any bracket placement errors.
Looking pretty good here. Its almost time for a test drive. More like miller time. My sweaty head is covered in concrete drilling dust at this point.
First load test. Four hundred fifty pounds. I ran the jib through its full arc and area with the load. I'm listening for any creaks or pops or just out of the ordinary. It may not look like it but I'm ready to run in a split second if anything goes south. Nothing happened just like it was supposed to so I'm pretty happy. Now for the 125% load test.
All is well. The installation passed the 125% test with honors. You are pretty confident during the design phase but when you actually test it and verify it gives the important peace of mind you want with something like this. Motto for the day is, "Credere sed verificare"  "Trust, but verify"

1 comment:

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