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How To Measure Wire Rope Diameter

The correct diameter of a wire rope is the diameter of a circumscribed circle that will enclose all the strands. It’s the largest cross-sectional measurement as shown here.

You should make the measurement carefully with calipers. The illustrations at left show the correct and incorrect methods of measuring a wire rope’s diameter.

Allowable Tolerance In Wire Rope Diameter

Wire rope is typically made slightly larger than its catalog (or nominal) size. The following chart lists the size tolerances of standard wire rope.

Metric conversion and equivalents

As we move toward metric measurements, it will become increasingly necessary to convert English units into SI – International System of Units – (or metrics), and vice versa. The following table and conversion factors are included in this handbook to help you.

Rope Diameter

In measuring rope diameter, the industry is leaning toward a “soft” conversion to metric during the transition period. For example, a 1” diameter rope converts to 25.4 mm in metrics. Using the soft conversion, this changes to the whole metric size that most nearly parallel the 1” size range, or 26 mm. In sizes smaller than 5/8”, the rope diameter is rounded to the nearest 0.5 mm.

Strengths & Weights

The following table gives the closest equivalent metric diameters for rope sizes up through 5 inches. Again, these metric sizes are based on the industry’s “soft” conversion.

Since rope minimum breaking force and weight per unit of length vary for different types and grade of ropes, the following conversion factors are given to help you convert the figures you need:

  • To convert rope weight in pounds per foot (lb/ft) to kilograms per meter (kg/m), multiply by 1.488
  • To convert rope minimum breaking force in tons (T) to kilonewtons (kN), multiply by 8.897; 1 lb equals 4.448 newtons (N)
  • To convert rope minimum breaking force in tons (T) to kilograms (kg), multiply by 907.2

Note: The Newton (a unit of force) is the correct unit for measurement of minimum breaking force in the SI system of units. We have included a conversion factor from tons to kilograms because a rope’s minimum breaking force is often referred to regarding kilograms (a unit of mass).

Nominal wire rope diameter

Inches Millimeters Inches Millimeters
5/16 8 2 1/4 58
3/8 9.5 2 3/8 60
7/16 11.5 2 1/2 64
1/2 13 2 5/8 67
9/16 14.5 2 3/4 71
5/8 16 2 7/8 74
3/4 19 3 77
7/8 22 3 1/8 80
1 26 3 1/4 83
1 1/8 29 3 3/8 87
1 1/4 32 3 1/2 90
1 3/8 35 3 3/4 96
1 1/2 38 4 103
1 5/8 42 4 1/4 109
1 3/4 45 4 1/2 115
1 7/8 48 4 3/4 122
2 52 5 128
Nominal rope diameter Tolerance Under Over
0 - 1/8" -0 + 8%
Over 1/8 - 3/16" -0 + 7%
Over 3/16 - 5/16" -0 + 6%
Over 5/16" -0 + 5%

Design factors

The design factor is defined as the ratio of the minimum breaking force of a wire rope to the total load it is expected to carry.

Use of design factors provides rope installations with reasonable assurance of adequate capacity for the work to be done throughout a rope’s service life. Considerations in establishing design factors include the type of service, the design of equipment and consequences of failure.

In most applications, the selection of a rope based on the proper design factor has been made by the equipment manufacturer. In an application where a different rope is to be used, or in a new application, check government and industry regulations for the required design factor. Different rope types on the same application may have different design factor requirements.

How To Use Design Factors

Standards and regulations require that design factors be applied to the rope’s minimum breaking force to determine the maximum working load. To determine the maximum working load for which an operating rope may be used, divide the rope’s minimum breaking force by the required design factor. This equation is the rope’s maximum working load. There may be other limiting factors in an application that make the maximum load the equipment can handle less than the rope’s maximum working load.*

Remember, an installation is only at the prescribed design factor when the rope is new. As a rope is used, it loses strength and becomes “used up.”

*NOTE: The rated capacity of a wire rope sling incorporates both a design factor and splicing or attachment efficiency.