Monday, May 30, 2011

Rigger or Rigor ?

Snack or a meal ? It depends on how much you want to learn. I found that out as I began to explore and assemble for the Gin Pole and winch set ups that will be used to raise each assembled lattice bridge truss to vertical. Earlier in the project using what we learned about rough sawn lumber and calculating Board Footage we determined the weight of each truss as being 5,700 pounds. From Milton's book we saw how Gin Poles and winches were rigged and used to handle this kind of weight and raise a bridge truss to vertical by hand.  Alex and I tested this out using the 3/8 scale model shown in the pictures below.

There is some rigging at the top of the Gin Pole too but for now I will just be talking about the winch at the bottom.
After considerable comparison winch shopping I decided on a nice 2000 pound capacity geared winch with 25 feet of 3/16" wire rope (wire rope - sounds like an oxymoron to me). The wire rope was packaged separate and needed to be attached to the drum of the winch. Simple enough, right? Right off the bat I could see there were two small holes and one larger hole in the flange of the drum. Humm? I thought and then for all the "right reasons" picked a small hole and passed the cable through it to the clamp on the side (see picture below). This required considerable maneuvering to make the bend and hold the clamp and screw so the nut could be tightened clamping the wire rope securely. After several tries I got it!

Then I looked at the routing of wire rope and how it would be wound on the drum (see picture above). They say there is nothing scarier than the shadow of a doubt. The more I looked the more I wondered. Did I have it wrong? There is another saying too, "when all else fails look at the instructions." Good idea. Sure enough, ten pages and a pretty good picture showing the wire rope threaded in the larger hole and then there was the reference to winding under or over the drum? OK, time to get out the ol' Machinery's Handbook and get more input on the subject. Page 465, "Installing Wire Rope" First it talked about avoiding kinking and how to remove wire rope from a reel vs a coil. Oh, this was good! Then they went into right lay and left lay rope (the helix of the twist just like screw threads). Overwound or underwound on the drum and which drum flange to start winding against! I started out thinking this would be an little educational snack but I soon discovered there was enough new learning here for a seven course meal! I dug in and got more answers to my questions (I also got more questions to my answers!). Even the capacity of the drum. I needed to know if the drum could hold all the wire rope used in the block and tackle set-up to to raise the truss.
ABOVE - With the wire rope routed correctly ( no kinks, right lay, overwound and started against the right flange of the drum) the winding could begin.

The red tape marks the five "dead wraps" that must remain on the spool for anchoring by friction against the drum. The first layer held 7 feet of 3/16" wire rope. The Machinery's Handbook had a nice formula for calculating the capacity of the drum and introduced several new technical terms. Finally I went to the computer and did a Google search on this. Ingersoll-Rand has a great wire rope drum capacity calculator and plenty of info too. Looks like our winch will do the job just fine, with a 1/2" freeboard it will hold 50' of 3/16" wire rope.

Puzzler - What is the Recommended Working Capacity of this winch?

Tech Vocab - Total Drum Storage Capacity, Total Working Drum Capacity, Recommended Working Capacity, Dead Wraps, Freeboard, Barrel diameter, Flange diameter, Drum length, Cable diameter, Fleet Angle,    

No comments:

Post a Comment