Wedges, Zinc, and Epoxies – Oh My
by Dennis J. O’Rourke, CSP
Most Sockets today are wedge, poured, and the swaged socket. Poured sockets create equal tension on all wires, producing 100% efficiency. The wedge socket, however, reduces efficiency, about 20%. The wedge places unequal forces on individual wires that cause premature tensile failures; a generous “design factor” ensures safety if they are correctly installed (Figure 1), correct? They have been installed wrongly so many times in so many places that some companies don’t allow their use. Wedges are used to attach blocks, overhaul balls, and boom rope dead-ends on cranes.
End connection efficiency is the ratio between the rope’s breaking strength and the actual failing force of the assembly. End connections, like wedge sockets, Flemish eyes, fold backs, and “hand tuck” splices, have reduced capacities due to uneven wire loading. Poured sockets can produce 100% efficiency.
All sockets have a significant advantage, the steel fitting that forms the eye resists abrasion, corrosion, and wear, thus reducing connection failure. Poured sockets were mostly employed as pendants and standing ropes on cranes. However, along with rope stops (swaged), poured sockets are increasingly being installed on hoist rope by crane manufacturers (as pictured) to ensure the attachment technique on their crane is consistent. Herein lies the objective of this article: reliability – of the fabricated socket.
One problem (Figure 2), here, the “wire brooming,” is too long and extends beyond the height of the zinc. Wires should be below the surface about ¼ inch for corrosion protection. To me, this represents a “rookie” mistake in measuring the length of the socket correctly. What else did they mess up?
The WRTB Wire Rope User’s Guide provides precise steps as to how to pour sockets. First, only pure zinc (not babbitt or other nonferrous metals, that have higher flow temperatures) are used. Zinc flows nicely at 920 degrees Fahrenheit, critical to preventing structure altering of the Zinc or steel wires.
Strict steps are followed to prepare the torch and the crucible pot. These steps, measuring, cleaning, and pyrometer temperature readings of the zinc about 925 and the socket at 150 degrees. The brooming of wires, positioning of the rope, seizing wire at the base of the socket, and more. You could get it “wrong,” then no 100% efficiency.
In Figure 3, is a reoccurring problem we find with sockets. The seizing wire installed to limit the broomed wire length should be removed after completion to expose the wires for inspection of acid pitting, broken wires, corrosion, and as shown here, pulling out of the socket. We cut and slid down the seizing wire to expose the area.
A lot of skill is necessary to pour zinc sockets. The epoxy socket was first marketed as being easier to fabricate. Mix it up in a cup, and pour the compound in the socket at room temperature. Much less skill and technical knowledge is required. However, later instructions were much more detailed. One example is necessary, proper “brooming” methods.
The socket in Figure 2, being deficient, was replaced. The epoxy filled socket in Figure 4 was in service one month, and the epoxy started chipping; the fabricator informed the crane owner; those missing chips of plastic didn’t matter to the holding power of the socket on this 137-ton port crane! Okay, you’re the crane inspector. What do you think we should do to Certify the crane? WRN
DENNIS J. O’ROURKE, CSP, is the Director of National Crane Services, Inc. He has over sixty years’ experience in the industrial, maritime, and construction fields working with heavy equipment and material handling devices. As a safety engineer, Mr. O’Rourke has developed and/or presented hundreds of safety-training programs for all representative elements of government and industry. (dennis@natlcrane.com)