The design factor is defined as the ratio of the nominal strength of wire to the total load it is expected to carry. Hence, the design factor that is selected plays an important part in determining the rope’s service life. Excessive loading, whether continuous or sporadic, will greatly impair its serviceability. Usually, the choices of a certain wire rope size will be based on static loading and, under static conditions, it is sufficient for its task. However, where a machine is working and dynamic loads are added to the static load, it is quite possible to exceed the material’s elastic limit.
As was noted in the earlier discussion, a “common” design factor is 5. Changes in the design factor from 5 to 3 its life expectancy from 100 to 60, a drop of 40%.
The breaking strength is the ultimate load registered on a wire rope sample during a tension test.
The nominal strength given has been calculated by a consensus, industry-accepted procedure, and manufacturers design wire rope to these strengths. When making design calculations, it should be noted that the given figures are static strengths. Designers should base their calculations on these strengths.
Minimum acceptance strength, 2-1/2% lower than published nominal breaking strengths, was established as the industry tolerance. It serves to offset variables that occur during that sample preparation and actual physical test of a wire rope. This tolerance is used in the basic rope governmental specifications.
Wire rope testing, whether it is performed for the purpose of determining grade or for adherence to specifications, requires the sample to be tested to comply with certain standard. For example: the sample’s length must not be less than 3 feet (0.91m) between sockets for ropes with diameters of from 1/8 inch (3.2m) through (77mm); on ropes with larger (over 3 inches) diameters, the clear length must be at least 20 times the rope diameter. The test is considered valid only if failure occurs 2 inches (51mm) or more from either of the sockets, or from the holding mechanism. The speed of separation between the testing machine heads as the tension is applied and increased shall not be greater that one inch per minute.