The Life Span Of A Crane
Today we talk about the life span of crane through the fatigue design.
At the whole life span of a crane , although it is necessary to go through a number of overhaul, each overhaul will be replaced during some wear parts, such as wire rope, rolling bearings, open gears, wheels, brake friction pads, brake wheels and electrical equipment, etc., and the replacement of these local parts and components does not mean that the whole crane working life of the end; but in a crane used for many years, once its structure constantly appearing Serious fatigue cracks, up to the crane or trolley can not work normally, it is considered that the whole crane can no longer work safely.
Therefore, the life span of a crane mainly depends on the working life of its metal structure without fatigue cracks (the life of the crane structure in China is delineated between 15 and 50 years, generally 30 years).
The classification of crane working level is currently based on the fatigue design theory of metal structure.
Currently there are three main methods of fatigue design:
(1) infinite life design
Requirements for the structure in the infinite length of the use of the period does not occur fatigue damage, the structural stress should be less than the fatigue limit. Crane structure is loaded with changes, the fatigue resistance of the material at this time is generally measured by the relationship between the alternating stress and the number of damage cycles N curve (i.e., S-N curve). S-N curve of steel materials have a horizontal asymptote, the transition point is roughly 106-107, it is generally believed that, as long as after 107 cycles are not destroyed, it can withstand an infinite number of cycles. Steel material corrosive media in the S-N curve does not have a horizontal asymptote, but there is also a gradual flattening of the section, can still be 107 cycles as the failure stress, which is known as the condition of fatigue limit. This is the first way to judge the life span of a crane.
(2) Finite life design
The theoretical basis for crane work level grading is the concept of finite life design – linear cumulative damage theory.
Crane fatigue life estimation principle is that the formation of fatigue damage is considered to be the material under the action of alternating loads first local slip and yielding, followed by cracks and gradual expansion, eventually leading to fatigue damage.This is the second way to judge the life span of a crane.
(3) Damage tolerance design
Damage tolerance design is based on the principle of fracture mechanics to deduce the mechanical parts or structural components in the crack will be how to expand, to predict in how much time to ensure the safe use of the crane design method. That is, allow mechanical parts or structural components of a certain degree of cracks, and to ensure that the next inspection before the safe use.This is the third way to judge the life span of a crane. This method of structural design of cranes is still under study.
At present, we judge the life span of a crane according to the above standards.However,in actual work,it is very complicated to judge the life span of a crane.
