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Safe Sling Guidlines

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CHOKER HITCH configurations affect the rated capacity of a sling. This is

because the sling leg or body is passed around the load, through one end attachment

or eye and is suspended by the other end attachment or eye. The contact of the sling

body with the end attachment or eye causes reduction of sling efficiency at this

point If a load is hanging free, the normal choke angle is approximately 135 degrees.

When the choke angle is less than 135 degrees, an adjustment in the choker rated

capacity must be made (Figure 3).

 

Extreme care should be taken to determine the choke angle as accurately as

possible. As indicated in Table 1, the decrease in rated capacity is dramatic.

 

 

Figure 3. Choker hitch rated capacity adjustment, for wire rope slings in choker hitch when angle is LI^S than 135 degrees. Choke angles greater than 135degiees are unstable and should not be used.

TABLE 1

CHOKER HITCH - RATED CAPACITY ADJUSTMENT IWRC AND FIBER CORE ROPE

Slings with the choke angle greater than 135 degrees are not recommended

since they are unstable.

 

NOMINAL SPLICE EFFICIENCY is the efficiency of the sling splice. Any

time wire rope is disturbed such as in splicing an eye, the strength of the rope is

reduced. This reduction must be taken into account when determining the nominal

sling strength and in calculating the rated capacity. Each type of splice has a differ

ent efficiency, thus the difference in rated capacities for different types of slings.

Nominal splice efficiencies have been established after many hundreds of tests over

years of testing.

 

D/d RATIO is the ratio of the diameter around which the sling is bent divided

by the body diameter of a single part sling (Figure 4), or the component rope diame

ter in a multi-pzirt sling. This ratio has £in effect on the rated capacity of the sling

only when the sling is used in a basket hitch. Tests have shown that whenever wire

rope is bent around a diameter, the strength of the rope is decreased. Figure 5

illustrates the percentage of decrease to be expected. This D/d ratio is applied to

wire rope slings to assure that the strength in the body of the sling is at least equal

to the splice efficiency. When D/d ratios smaller than those listed in the rated

capacity tables are necessary the rated capacity of the sling must be decreased.

Figure 4. When D is 25 times the body diameter (d), the D/d Ratio is expressed as 25/1.

 

RATED CAPACITY is the maximum static load a sling is designed to lift using

new, unused rope. The tables give rated capacities in tons of 2000 pounds. Rated

capacities contained in all the tables were calculated using component rope strength

as a basis. Due to rounding of numeric values, rated capacity values for 2,3, or 4 leg

slings may not be even multiples of single leg values. Rounding also accounts for

small differences in values between tables in other publications and the tables in this manual. These small differences should not be construed to be in error. The purpose of this manual is to define wire rope slings in keeping with good practice and sound engineering design. Many areas of design apply to all types of slings. Specific design criteria will be covered in the appropriate sections. Rated Capacities shown in this manual are for wire rope slings fabricated from new wire rope.

 

DESIGN FACTOR is a number which is divided into the nominal strength of a sling to arrive at a rated capacity. A design factor is necessary to allow for conditions such as wear, abrasion, damage and variations in load which are not readily apparent. Design factors have been established which allow the sling to give the most efficient service to the user. Rated capacity tables contained in this manual are based on a design factor of five (5). Other design factors may be applied for engineered lifts; however, the sling manufacturer should always be consulted. Sling Rated Capacity is based upon the minimum breaking force, formerly called nominal (catalog) strength, of the wire rope used in the sling and other factors which aflect the overall strength of the sling. These other factors include splicing efficiency, number of parts of rope in the sling, type of hitch (e.g., straight pull, choker hitch, basket hitch, etc.). diameter around which the body of the sling is bent (D/d) and the diameter of pin used in the eye of the sling (Figure 1).

 

Figure 1.

 

SLING ANGLE is the angle measured between a horizontal plane and the sling leg or body. This angle is very important and can have a dramatic effect on the rated capacity of the sling. As illustrated (Figure 2B), when this angle decreases, the load on each leg increases. This principle applies whether one sling is used to pull at an angle, in a basket hitch or for multi-legged bridle slings. Sling angles of less than 30 degrees shall not be used.

 

Figure 2B. 

SLING ANGLES in this manual are measured from the horizontal plane. If the

horizontal angle is used you must use the trigonometric sine of the horizontal angle.

When the vertical angle is used you must use the trigonometric cosine of the vertical

angle (Figure 2C)

 

Figure 2A.

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