One way of identifying potential problems with all types of cranes is to follow guidelines laid down in the international standard ISO 12482-1 which covers crane condition monitoring (CM). The purpose of the standard is to define the limits of the crane design and to point out the necessary steps to be taken in order to keep the crane in safe working condition. CM must include all parts of the crane in which a deterioration can affect safe handling and must include the following main groups:
• load carrying structures
• mechanical systems
• hydraulic systems
• pneumatic systems
• electrical systems
• safety systems
Different parts and components will reach their design limits at different times. The crane manufacturer can give information about when, where and how the assessment must be done with respect to the number of crane cycles, load spectrum and classification groups. If the crane manufacturer is no longer on the market or has not taken the above list of points into consideration, available documentation must be completed with new calculations and other essential information to make a safe analysis.
Condition monitoring assessments need to be carried out by skilled engineers (qualified to ISO 9927-1) who are familiar with that kind of crane inspection. The crane user has to estimate how many hours each separate mechanism has been working.
Furthermore, knowledge of the number of load cycles and load spectrum are essential to be able to judge the crane’s remaining life expectancy.
If the crane is designed in accordance with an ISO standard, information can be found on the limits of life time, load cycles and load spectrum in ISO 430 1-1.
Provisions for assessment
To determine a crane’s remaining life-time, different safety factors must be used depending on the way the figures were obtained:
1. Working conditions and hours recorded by a collecting instrument giving working time and relative load are to be multiplied by 1.0 – or in other words, taken as read.
2. Log-book documentation together with suitable counters is to be multiplied by 1.2.
3. Log-book documentation without counters is to be multiplied by 1.4.
4. Where previous operating conditions are not known the expected time must be multiplied by 1.5.
Additionally, it is supposed that the nominal load spectrum factor is 0.25. One year is 250 working days at one hour a day.
Condition monitoring also includes a very careful visual inspection of all parts and functions which are essential for safe handling. That means that some parts must be opened and checked using the necessary test methods. It may be necessary to cut up box girders to carry out internal inspection for cracks and other defects in critical points. The welds under the rail and in the bottom plates are the main points.
Discovering cracks during an inspection can be seen as a good and bad thing but generally they can be repaired, even making the structure stronger in most cases. Most sensitive to cracks are fillet welds. There are design examples and tables for maximum allowable tensions in both FEM and DIN-standards.
Mechanisms with rotating shafts are the most difficult objects to inspect. It is easy to see pitting or deterioration in the flanks of gear teeth and it is also easy to check bearings, but cracks in shafts are very difficult to identify. And because of the high number of rotations (load cycles), it is almost impossible to find them before failures occur. The only, and least unsafe, way to check rotating shafts is to look at the design drawings and make a calculation from there. Stress concentration factors are the dangerous points where there is most likely to be a failure. The radius between two different diameters, keyways and similar areas are the most difficult to assess. Rotating parts must be replaced when their calculated lifetime is over.
One of the most important points is to take into consideration experience gained from other examples with the same or similar design. It is also clear that some points in older design rules and standards specified designs that were too weak or where material quality was too low. Rebuilding and repair work can also cause damage through bad workmanship.
Many crane failures are caused by bad workmanship. Experience of faults and failures in old cranes is of great value when attempting to estimate remaining lifetime. If, for example, fatigue cracks are repaired by welding, the welder’s competence is of primary importance. Many welders are ready to do the job but very few have the right competence. Even if the most highly qualified welder grinds out the whole crack and makes a perfectly neat weld seam, your crane might still not be safe.
Because of heat introduced from the welding process there will be heat-affected tensions and deterioration of the material in the heat affected zone. This can generate new cracks in a very short time.
Secondly, these tensions are so strong that there can also be deformation and distortion which affects other parts such as mechanisms and bearings in the vicinity of the repaired part. That means that if the steel construction part is deformed, the bearings or mechanisms could fail in a very short time if tolerances have changed.
Cranes do not have eternal life
Condition monitoring of cranes is useful in finding cracks and other faults but most important is what action is taken after the faults have been discovered:
• Can the part be replaced? If so, how?
• Is it possible to repair? If so, how?
• Is it possible to weld? If so, how?
• Can it be made stronger? If so, how?
• How is the remaining life expectancy affected?
All questions relevant to the crane must be examined and answered in a professional manner. It is of utmost importance that condition monitoring is started before individual crane components reach the limit of their calculated life expectancy.
Many factors must be considered when deciding on a repair strategy. On old cranes that use, for example, riveted connections in their steel construction, these should not be welded or otherwise heat treated unless the material is tested and found to be of the right quality. The same goes for older welded cranes due to embrittlement by ageing of the material. If cracks or deformations are found in such parts these must be exchanged. The new part must then be attached by rivets or bolted connections.
Steel grades
New cranes made of modern grades of semi-killed or killed steel (see explanation in box below), with acceptable elongation, are at least possible to weld, heat treat or even straighten without unacceptable forced ageing of the material. The original lifetime calculation for a steel construction or mechanism of a crane is based on statistical calculation.
A small crack in low tensile, not brittle, steel is usually easy to handle because the tension levels are low and the crack can be relatively long before the final failure. It is much more complicated if the crack is found in a high tensile, brittle steel, or the tension levels are high. In these cases the critical length of the crack might be very short.
