SLING SAFE WORKING LOADS

There are formulas for estimating the loads in most sling configurations. These formulas are based on the safe working load of the single-vertical hitch of a particular sling. The efficiencies of the end fittings used also have to be considered when determining the capacity of the combination.

The formula used to compute the safe working load (SWL) for a BRIDLE HITCH with two, three, or four legs (fig. 6-17) is SWL (of single-vertical hitch) times H (Height) divided by L (Length) times 2 = SWL. When the sling legs are not of equal length, use the smallest H/L measurement. This formula is for a two-leg bridle hitch, but it is strongly recommended it also be used for the three- and four-leg hitches.

NOTE: Do NOT forget it is wrong to assume that a three- or four-leg hitch can safely lift a load equal to the safe load on one leg multiplied by the number of legs.

Other formulas are as follows: Single-basket hitch (fig. 6-18): For vertical legs:

SWL = SWL (of single-vertical hitch) x 2. For inclined legs:

SWL = SWL (of single-vertical hitch) x H divided by Lx4.

Double-basket hitch (fig. 6-19): For vertical legs:

SWL = SWL (of single-vertical hitch) x 4. For inclined legs:

SWL = SWL (of single-vertical hitch) x H divided by Lx4.

Single-choker hitch (fig. 6-20):

For sling angles of 45 degrees or more:

SWL = SWL (of single-vertical hitch) x 3/4 (or .75).

Sling angles of less than 45 degrees are not recommended; however, if they are used, the formula is as follows:

SWL = SWL (of single-vertical hitch) x A/B. Double-choker hitch (fig. 6-21):

For sling angle of 45 degrees or more:

SWL = SWL (of single-vertical hitch) x 3 divided by4xHdivided byLx2.

Sling angles of less than 45 degrees:

SWL = SWL (of single-vertical hitch) x A divided byBxHdivided byLx2.

When lifting heavy loads, you should ensure that the bottom of the sling legs is fastened to the load to prevent damage to the load. Many pieces of equipment have eyes fastened to them during the process of manufacture to aid in lifting. With some loads, though, fastening a hook to the eye on one end of each sling leg suffices to secure the sling to the load.

Use a protective pad when a fiber line or wire rope sling is exposed to sharp edges at the comers of a load. Pieces of wood or old rubber tires are fine for padding. SLING ANGLE

When using slings, remember that the greater the angle from vertical, the greater the stress on the sling legs. This factor is shown in figure 6-22.

Figure 6-17.-Determination of bridle hitch sling capacity.

The rated capacity of any sling depends on the size, the configuration, and the angles formed by the legs of the sling and the horizontal. A sling with two legs used to lift a 1,000-pound object will have 500 pounds of the load on each leg when the sling angle is 90 degrees. The load stress on each leg increases as the angle decreases. For example, if the sling angle is 30 degrees when lifting the same 1,000-pound object, the load is 1,000 pounds on each leg. Try to keep all sling angles greater than 45 degrees; sling angles approaching 30 degrees are considered extreme] y hazardous and must be avoided.

STORAGE

Wire rope slings and associated hardware must be stored either in coils or on reels, hung in the rigging loft, or laid on racks indoors to protect them from corrosive weather and other types of damage, such as kinking or being backed over. Slings are not to be left out at the end of the workday.

CHAINS

Chains are made up of links fastened through each other. Each link is fabricated of wire bent into an oval and welded together. The weld usually causes a slight

Figure 6-18.-Determination of single-basket hitch sting capacity.

bulge on the side or end of the link. Chain size refers to the diameter, in inches, of the wire used to fabricate the chain.

In the NCF, never use a chain when it is possible to use wire rope. Chain does not give any warning that it is about to fail. Wire rope, on the other hand, fails a strand at a time, giving you warning before failure actually occurs.

NOTE: Although chain gives no warning of failure, it is better suited than wire rope for some jobs. Chain is more resistant to abrasion, corrosion, and heat. Additionally, use chains to lift heavy objects with sharp edges that could cut wire or are hot. When chain is used as a sling, it has little flexibility but grips the load well.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing