We received a letter from Bob Bailey concerning safety-wiring procedures. He correctly pointed out that in a recent car feature safety wire had been installed incorrectly. After speaking with Bob it quickly became apparent that with 29 years overhauling nuclear submarines and working as a quality analyst this was a guy who knew what he was talking about. We asked Bob to share his expertise and he graciously agreed. Here's what he had to say:
"The primary purpose of safety wiring is to prevent complete loss of a fastener. It isn't very effective in preventing minor relative nut and bolt rotation since some rotation can still occur even with the best of tying techniques (rotation of only a few degrees can reduce fastener preload by 50 percent or more), so safety wiring should only be done on fasteners that thread into one of the parts and not those using nuts.
"Fasteners may be drilled in various configurations. Small fasteners will generally have one hole through the head (Figure 1). Fasteners may be drilled through each of the hex flats (Figure 2). Large fasteners are usually the only ones that will be drilled through the corners (Figure 3). If you are drilling your own fasteners be sure to deburr the holes so that the wire is not pulled tight against a sharp edge. When it is necessary to wire hex nuts on studs they must be drilled as shown in Figure 3."
1a. Safety-wire sizes should be as follows: 0.032-inch wire for general purpose, 0.020-inch may be used on parts with hole diameters of less than 0.045 inch, parts having a hole diameter between 0.045 and 0.062 inch and having a spacing between fasteners less than 2 inches, closely spaced fasteners of 1/4-inch thread diameter and smaller.
1b. The best material to use for all general safety wiring applications is monel (a nickel-copper alloy) for temperature environments up to 700 degrees F and inconel (a nickel-chrome-iron alloy) for temperature environments up to 1,500 degrees F. However, in general automotive applications readily available stainless steel wire is completely satisfactory. All torquing operations must be completed before installing safety wire. Never loosen or over-torque the fastener to achieve better wire alignment.
1c. There are essentially two methods of safety wiring, the double-twist method (upper example in Figure 2), and the single wire method (lower example in Figure 2). The process of twisting the safety wire can be accomplished either with commercially available wire twisting pliers or by hand. In either case the frst twist next to any fastener head should by made by hand to ensure no gap occurs between the twist and the fastener head.
2. The double-twist is the preferred method for most applications. In a closely spaced, closed geometrical pattern (fasteners form a triangle, square, circle) the single wire method may be used. "Closely spaced" is considered to be no more than 2 inches between the fastener centers. This method is less preferred because an impact or injury of any kind can weaken or break the wire, leaving all fasteners in the pattern at risk. When using the single-wire method use the largest wire that can pass through the holes. Parts should always be wired in a manner that puts the wire in tension if any part should rotate in loosening direction (next figure). Always cut plenty of wire to accomplish the task for either method.
3. The safety wire should always be installed and twisted so that the loop around the fastener head stays down and does not tend to come up over the head leaving a slack loop. The closer the wire fts around the head, the less chance it has to lift over the head. The installed safety wire must be tight but never overstressed. Do not twist the wire too tightly. If the wire is being passed through a thin-head bolt it is acceptable to pass the loop over the head rather than around the head. After locking the last fastener a pigtail of 1/4- to 1/2-inch length is formed to terminate the installation. When working with righthand fasteners make all twists in a clockwise direction except when forming the pigtail. The pigtail should be twisted counterclockwise and bent back and under itself to prevent becoming a potential snag to fesh, clothing, or anything else. When wiring a single fastener it will be necessary to wire to an adequate anchor point. An anchor point may be an angles hole, a structural rod, or pipe. The anchor point must be an immovable location the fastener or plug it is installed into. See various examples below.
4. Never reuse a piece of wire that has already been installed. If too much wire is twisted to connect the next fastener, cut it out and start over. Don't untwist the wire as this tends to harden the wire and weaken it further. If an installed safety wire is discovered to be broken, recheck the torque/tightness of all fasteners in the group before installing new wire. When multiple fasteners are 4 or more inches apart, three fasteners is the maximum number to be wired together. When it does become necessary to install into lefthanded fasteners, reverse the twisting directions described above.