Proof Load Testing of Lifting Equipment
This week I would like to address a question about lifting equipment testing that was asked in a forum by a machinery safety expert.
Douglas asked: “Can you explain the relationship between proof loads, safety factors and guaranteed minimum breaking loads in terms of lifting equipment? Also, what guides the use of yield or ultimate strength to determine safety factors in structural design? For example, EN 12999 for loader cranes requires a static proof load test at 1.25 times rated load, but for the hoist rope, the safety factor (manufacturers guaranteed minimum breaking load vs rated load) should be x 5. The proof test load gives you no indication how close to breaking the hoist rope actually is. The rope should be really very relaxed at the x 1.25 proof load.”
We deal with these types of issues almost daily and here are my thoughts:
Yield strength is a material property describing the highest internal stress in a material that an item could achieve without permanently deforming.
Ultimate strength describes the internal stresses at a load that would cause failure.
Almost all codes require an engineer to use the yield strength for design except in cases like fall protection. In this scenario, it is common to see ultimate strength used in conjunction with a high loading scenario and the requirement to re-inspect after it’s been loaded. It is important to keep in mind that many high tensile materials such as quenched, tempered or high carbon steels have a narrow band between yield, ultimate and transition causing permanent bending to breaking very quickly.
The breaking load being five times greater than the working load is commonly called a safety factor. Safety factors are applied to devices in order to protect everyone from unforeseen events. They are usually derived from assessing the risk to human life and the confidence in a loading scenario. For instance it is common in below the hook lifting devices to have a 3:1 safety factor on the yield stress because the risk to human life is high and the loading is moderately controlled. Read more here: http://www.spartaengineering.com/rating-plates-and-below-the-hook-lifting-devices/
In the case with wire rope, it is common to see a 5:1 requirement because of the unknowns regarding exactly how the rope will be used and in what conditions. Also, wear and shock loading are not tightly controlled. In some scenarios in the oilfield such as with derrick design it is common to see safety factors as low as 1.7ish because the loading is well defined, the analysis is thorough and the problem well understood. They also tend to inflate the input-loading scenario higher than is realistic as well.
With regards to proof loading, I am not going to speak on the nomenclature because I think it is open for interpretation but the rationale behind testing to 1.25 is that it is well within the designed safety factor but also above the rated capacity of the system, thus being a good “proof” of loading. We wouldn’t want people testing to the breaking capacity (because of safety reasons) and we wouldn’t want people not testing to the full capacity of the crane. I think the 25% above full loading comes from possible errors in measurement more than anything else.
To summarize, it is a little grey and in many cases and left up to the engineer. The breaking load doesn’t necessarily correlate with the ultimate strength of the material. In many instances we define “failure” as occurring once permanent deformation happens. This will start as soon as the yield strength is exceeded.
Feel free to send me an email if you want more information or have other questions.