3.2 Load Weight, Center of Gravity, Sling Angle, and Hand Signals
Key Takeaways
- Tension in each leg of a sling equals the share of the load divided by the sine of the angle the leg makes with the horizontal, so tension rises as the angle decreases.
- At a 60-degree sling angle each leg carries about 1.155 times its share of the load; at 45 degrees about 1.414 times; at 30 degrees exactly 2 times.
- The hook must be placed directly over the center of gravity or the load will swing and rotate when it lifts off.
- Steel weighs approximately 490 pounds per cubic foot, which is the basis for most field load estimates.
- Only one person gives signals to the operator at a time, but anyone may give the stop signal, and the operator obeys a stop signal from any source.
Estimating the load
You cannot rig what you cannot weigh. Three sources, in order of preference: the equipment nameplate or shipping documents, the fabrication drawing, and finally a field calculation from volume and density.
| Material | Approximate density |
|---|---|
| Steel and cast iron | 490 lb/ft³ (cast iron slightly less, about 450 lb/ft³) |
| Aluminum | 165 lb/ft³ |
| Copper | 555 lb/ft³ |
| Concrete | 150 lb/ft³ |
| Water | 62.4 lb/ft³ |
For a solid steel shaft, volume in cubic feet multiplied by 490 gives a usable estimate. Always round up and add for attached piping, trapped process fluid, ice, or mud. A gearbox full of oil is heavier than its dry weight on the nameplate.
Center of gravity
The center of gravity is the point at which the entire weight of the load can be considered to act. Two rules follow:
- The hook must be directly above the center of gravity at liftoff. If it is not, the load will swing until it is, which is why an off-center pick rotates the moment it leaves the ground.
- The center of gravity should be below the attachment points for stability. A load slung below its center of gravity is inherently unstable and can capsize.
A motor-and-base skid, a horizontal pump with a heavy drive end, or a vessel with an internal weir all have an offset center of gravity. Adjust by using unequal sling lengths or a chain-shortening device so the hook sits over the center of gravity while the load hangs level.
Sling angle: the calculation that appears every cycle
The sling angle is measured between the sling leg and the horizontal plane of the load. As the angle decreases, the horizontal component of the force increases and the tension in each leg rises sharply.
where T is tension per leg, W is the load weight, n is the number of legs sharing the load, and theta is the sling angle measured from horizontal.
| Sling angle from horizontal | Load factor per leg | Tension in each leg of a 2-leg sling on a 4,000 lb load |
|---|---|---|
| 90° (vertical) | 1.000 | 2,000 lb |
| 60° | 1.155 | 2,310 lb |
| 45° | 1.414 | 2,828 lb |
| 30° | 2.000 | 4,000 lb |
| 15° | 3.864 | 7,727 lb |
Read the last row carefully: at a 15-degree sling angle, each leg of a two-leg sling carries nearly twice the total weight of the load. This is why 30 degrees is generally treated as the minimum acceptable sling angle and why riggers are taught to keep angles at 60 degrees or steeper whenever the headroom allows.
A field shortcut using measurements rather than trigonometry:
where L is the sling leg length and H is the vertical height from the load attachment point to the hook. For a 4,000 lb load on two 10-foot legs with 7 feet of vertical height: 2,000 multiplied by 10, divided by 7, equals 2,857 lb per leg.
Hitch types and their effect on capacity
| Hitch | Typical capacity relative to vertical rating | Notes |
|---|---|---|
| Vertical (straight) | 100% | Single leg, in-line pull |
| Choker | About 75% at a 120° or greater choke angle | Grips the load; never force the choke tight with the hoist |
| Basket | Up to 200% when the legs are vertical | Falls off rapidly as the sling angle decreases |
The choker derating exists because the sling is bent sharply against itself at the choke point. Never use a choker hitch to lift a bundle whose individual pieces can slide out.
Standard hand signals
Signals coordinate the lift when the operator cannot see the load. Rules first, then signals:
- One designated signal person communicates with the operator at a time.
- Anyone may give the stop signal, and the operator must obey a stop signal regardless of who gives it.
- If the signal person loses sight of the load or the operator, motion stops.
- Radios do not replace the requirement that signals be clear and confirmed.
| Signal | Motion |
|---|---|
| Hoist | Forearm vertical, forefinger pointing up, hand moving in a small horizontal circle |
| Lower | Arm extended downward, forefinger pointing down, hand moving in a small horizontal circle |
| Stop | Arm extended, palm down, arm swung back and forth horizontally |
| Emergency stop | Both arms extended, palms down, arms swung back and forth horizontally |
| Dog everything | Hands clasped together in front of the body |
| Move slowly | One hand placed motionless in front of the hand giving the motion signal |
Two final discipline rules that appear as items: never ride the load or the hook, and never allow anyone to pass or stand under a suspended load. Control load rotation with tag lines, kept long enough that the handler stays clear of the swing path.
A 6,000-pound skid is lifted with a two-leg bridle at a 30-degree sling angle measured from horizontal. What is the tension in each leg?
A horizontal pump and motor skid rotates sharply as soon as it leaves the floor, even though both slings are in good condition. What is the most likely cause?
A millwright who is not the designated signal person sees a pipe stand about to be struck by a suspended motor and gives the stop signal. How should the crane operator respond?