Rigging hardware: shackles, hooks, eyebolts and spreader bars

Slings rarely work alone. They are joined to loads and hooks by hardware: shackles that connect sling eyes to lugs, hooks with safety latches, eyebolts screwed into equipment, master links that gather several legs, and spreader bars that hold slings apart. Each piece has its own rated capacity, its own rules for how it may be loaded, and its own ways of failing. A rig is only as strong as its weakest part, and the weakest part is often a small piece of hardware that was loaded the wrong way.

On Kingsway Tower the rooftop make-up air unit for the residential corridors arrived with four lifting lugs welded to its base frame and a manufacturer's drawing showing a spreader bar arrangement. Hector Ruiz walked Maya and Sam through the rig the day before the lift: shackles on each lug, sling legs up to a spreader bar, two more legs from the bar to the crane hook. "Every piece in this rig has a tag or a stamp," he said. "If I can't read it, it doesn't go on the hook."

Hardware must be rated and identified

Rigging hardware used for lifting must be made for lifting, rated by its manufacturer and permanently marked. A shackle, for example, is normally forged with the manufacturer's name or mark, its size and its working load limit, and many are also marked with a grade or traceability code. Hardware from a farm supply or hardware store that is not rated for overhead lifting, such as a cast hook or an unmarked shackle, has no place in a lifting rig. Hardware standards such as ASME B30.26 for shackles, eyebolts, links and turnbuckles, and ASME B30.10 for hooks, set the marking, inspection and removal rules that most North American manufacturers follow.

As with slings, the working load limit is the breaking strength divided by a design factor, commonly 5 to 1 or 6 to 1 for hardware. And as with slings, that margin is not available for you to use.

Shackles

Shackles are U-shaped connectors closed by a pin. The two shapes are the chain or D shackle, with straight sides, which is designed for an in-line pull, and the anchor or bow shackle, with a rounder body that accepts slings or loads from a range of directions. The two common pins are:

• Screw pin: the pin threads into one ear of the shackle. Quick to use, but the pin can unscrew if the shackle rotates, for example when a sling runs over it. Screw pins must be fully seated, with the collar against the ear, and on long-term or rotating connections are moused (secured with wire) so they cannot back out.
• Bolt type: a bolt with a nut and a cotter pin. More secure for permanent or long-term rigging and for connections where the pin might rotate. The cotter pin must always be installed.

Rules for shackles that Hector drills into every apprentice:

• Never replace a shackle pin with a bolt. A bolt is not made of the same steel and will not have the same strength. Use only the manufacturer's matching pin.
• Load the shackle in line with its body, along its centreline. Side loading, where the force pulls across the shackle at an angle, reduces its capacity. Manufacturers publish reduction tables; a common pattern is about 70 percent of the rated load at 45 degrees and about 50 percent at 90 degrees. Some shackles may not be side loaded at all.
• Centre the load on the pin and bow. Spacers can be used to keep a narrow lug or sling centred on a wide pin. Never pack the pin with washers that let the load sit off-centre.
• When a shackle is used in a choker hitch, keep the running part of the sling against the bow, not the pin. A sling sliding over the pin can turn it and unscrew it.
• When a shackle connects two sling legs on a hook, put the legs on the bow and the pin on the hook, so the legs do not spread the shackle.

Hooks

Crane hooks, hoist hooks and sling hooks are forged, rated and marked. Most hooks used in construction have a spring-loaded safety latch that closes the throat so a sling cannot jump out if the load goes slack. The latch is not a load-bearing part: it only keeps the sling in place. Rules for hooks:

• Seat the load in the bowl, the bottom of the hook, never on the tip. A tip load acts on a long lever and can bend or break the hook at a fraction of its rated load.
• Make sure the latch closes completely. A latch that is bent, missing or held open must be repaired or the hook removed from service, unless the site allows a specific latch-less hook for a specific purpose with special precautions.
• Do not overcrowd a hook. Too many sling eyes on one hook can force the latch open or load the tip. Use a shackle or master link to gather several legs.
• Watch for opening and twisting. A hook whose throat has opened, usually judged against the original opening or marks stamped by the manufacturer, or whose tip has twisted out of the plane of the body, has been overloaded and must be removed from service. The hook standard and the manufacturer give the limits, often around 5 percent of throat opening and 10 degrees of twist.

Many crane hooks swivel so the load can rotate without twisting the hoist rope. On a crane hook, the rigger does not adjust or repair anything; any concern goes to the operator and the lift director.

Eyebolts and hoist rings

Eyebolts screw into threaded holes in equipment so the equipment can be lifted. They come in two main forms, and the difference matters:

• Plain shank eyebolts have no shoulder. They may be used only for a vertical pull in line with the bolt. Any angle bends the shank and can snap it.
• Shouldered eyebolts have a collar that bears against the surface of the equipment. They can take an angled pull in the plane of the eye, but only at a reduced capacity, and only when the shoulder is fully seated flat against the surface. Manufacturer tables commonly drop the capacity to about a third or less at 45 degrees and to about a quarter at 90 degrees.

Two more rules: the eye must be turned so the sling pulls in the plane of the eye, never across it (use shims or washers only as the manufacturer allows to align it), and the bolt must engage enough thread. A common guideline is thread engagement of at least one and a half times the bolt diameter in steel, more in softer metals, but the equipment maker's instructions govern. Never use an eyebolt that is bent, has damaged threads or will not seat.

Where a lift needs angled pulls on a threaded point, a swivel hoist ring is usually better than an eyebolt. Hoist rings pivot and swivel to line up with the sling and are rated for angled loads, but they must be torqued to the manufacturer's value and inspected like any other hardware.

Equipment lifting lugs are another matter: they are designed by the equipment manufacturer for a particular lift arrangement, often vertical or within a stated angle. The make-up air unit's drawing called for a spreader bar precisely because its lugs were designed for vertical pulls. Following the drawing is part of the rigging.

Master links, rings and turnbuckles

Master links gather two or more sling legs onto a single hook. They are sized so the hook can seat the link in its bowl and the legs can hang without crowding. Sub-assemblies of master links let four-leg slings sit correctly. Turnbuckles adjust the length of a rigging line or a tie and must be locked with lock nuts or wire so they cannot turn under vibration; they are loaded only in straight tension. Wire rope clips, used for some terminations and guy lines, must be the right size, the right number and installed with the saddle on the live (load-bearing) part of the rope. The old saying is "never saddle a dead horse": the U-bolt goes on the dead end, the saddle on the live end. Clipped terminations are not used for general slings.

Spreader bars and lifting beams

A spreader bar is a rigid bar or tube that holds sling legs apart. Legs run from the crane hook down to each end of the bar, and more legs run from the bar down to the load. The bar takes the inward squeeze that angled slings would otherwise put on the load, and lets the lower legs hang vertically onto lugs that were designed for vertical pulls. In a true spreader bar, the bar is mostly in compression.

A lifting beam is similar but hangs from the hook at one or two points and carries the load in bending, with the load attached below. Lifting beams can reduce headroom, because there are no upper sling legs, and are used where height is limited.

Both are below-the-hook lifting devices. They must be designed for lifting (typically by an engineer to a below-the-hook design standard), marked with their rated capacity and their own weight, and inspected like other rigging. A length of pipe or a scrap beam picked up from the yard is not a spreader bar. Remember too that the weight of the bar and its rigging is part of the load on the crane.

Worked example: checking hardware in the make-up air unit rig

The program below checks each piece of hardware in a rig against the force it will see, applying a side-load reduction to shackles where needed. The reduction percentages are typical example values; always use the manufacturer's table for the actual product.

# check hardware against the force it carries, with example side-load reductions (kg)
side_factor = {0: 1.00, 45: 0.70, 90: 0.50}     # example shackle reductions
rig = [  # (item, rated WLL kg, force kg, side-load angle deg)
    ("lower shackle A", 2000, 1475, 0),
    ("lower shackle B", 2000, 1475, 0),
    ("upper leg shackle", 3250, 1807, 0),
    ("master link", 6700, 3130, 0),
    ("tag-line shackle (side pull)", 500, 300, 90),
]
for item, wll, force, ang in rig:
    allowed = wll * side_factor[ang]
    ok = "OK" if force <= allowed else "OVERLOADED"
    print(f"{item:29} allowed {allowed:6.0f} kg  force {force:5} kg  {ok}")

lower shackle A               allowed   2000 kg  force  1475 kg  OK
lower shackle B               allowed   2000 kg  force  1475 kg  OK
upper leg shackle             allowed   3250 kg  force  1807 kg  OK
master link                   allowed   6700 kg  force  3130 kg  OK
tag-line shackle (side pull)  allowed    250 kg  force   300 kg  OVERLOADED

The side-pulled shackle in the last line is the kind of mistake the check is meant to catch: a shackle big enough for a straight pull can be overloaded when the pull comes from the side.