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How PM&I Can Help Prevent Delays

Choose Wire Rope?

How to Choose the Correct Wire Rope

Working Load Limit, Breaking Strength and Safety Factor

Choose Wire Rope
 
When choosing wire rope, first determine your Working Load Limit (WLL).
 
The Working Load Limit is the maximum load which should ever be applied to the wire rope, even when the rope is new and when the load is uniformly applied - straight line pull only. Avoid side loading. All wire rope ratings are based upon usual environmental conditions and consideration must be given to unusual conditions such as extreme high or low temperatures, chemical solutions or vapors, prolonged immersion in salt water, etc. Never exceed the Working Load Limit.
 
Working load limit differs from a wire rope’s Breaking Strength.
 
Breaking Strength is the average force at which the new wire rope has been found to break, when a constantly increasing force is applied in direct line to the product at a uniform rate of speed on a standard pull testing machine. Remember: Breaking Strengths, when published, were obtained under controlled laboratory conditions. Listing of Breaking Strength does not mean the Working Load Limit should ever be exceeded. Do not use breaking strength as a criterion for service design purposes. Refer to the Working Load Limit instead.
 
Numerical values published for Breaking Strength and Working Load Limit are very specific in one point: They refer to straight, inline pull or force and are obtained under laboratory conditions. There are, however, many applications where a straight line pull is not possible or even desirable. When a tackle block system is reeved, wire rope may be bent over many sheaves; multiple leg wire rope or chain slings involve differing lifting angles; angular loads on shackles or eyebolts alter Working Load Limits of the equipment used. All these and other factors influencing the Working Load Limit must be taken into account when systems are designed and used.
 
Another thing to keep in mind when choosing wire rope is the Design Factor (sometimes referred to as Safety Factory).
Safety factor is an industry term usually computed by dividing the catalog Breaking Strength by the catalog Working Load Limit and generally expressed as a ratio. For example: 5 to 1.
 
Government and industrial organizations such as OSHA, ANSI and ASME have established recommended design factors for various types of operations.  As an example: Wire rope used with cranes operates as running rope (which travels around drums or sheaves) and standing rope. For these applications, safety factors of 3.5 and 3, respectively, rate as sufficient, according to engineering standards in U.S. government safety reports. Other applications, such as a wire rope supporting a passenger lift, might require a higher safety factor.
 

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