Regarding the support of the rotary kiln, the support rollers work with tyres and cushion plates to lift up the kiln shell, the fixing and limiting of the cushion plate rely on the stop blocks that welded to the kiln shell, the limiting of the tyres rely on the stop blocks that welded to the cushion plates.In general condition,the tyres and cushion plates can move in the restricted scope. In the kiln operation process, for some kilns, some cracks occur at the cushion plate circular fixed stop block welded seam, the crack causes analysis as below, as well as the prevention measures for the cracks.

1. The cause analysis of cracks

The cracks occur at the cushion plate under the tyre are mainly in the axial direction, the cracks origin at the external side of the stop block and develop from the external side to the internal side along the axial direction, although there are lots of causes for the cracks, a few leading causes as below:

1.1 Structure cause

In the design of the rotary kiln, the area of cushion plates account for 70% of the area of kiln shell, the stop blocks are welded to the kiln shell , the welded seam of stop blocks  (used for circular limiting) is in the axial direction. And a result, when the kiln shell is under the support force of the tyre, the kiln shell bends inward. Due to the large stiffness of welded seam, the coordinated deformation of the kiln shell is hindered. It bears the bending stress, the fatigue cracks occur once part strength limit is exceeded or under the long term pulse stress.

1.2 Force bearing cause

Though the load of the kiln shell is fully considered in the design process, in the actual operation due to the kiln calibration, the foundation imbalance and sedimentation occurrence or the bending of kiln and so on, the force bearing of one pier tyre will be out of range and it exceeds the nominal load.

1.3 Weld cause

Due to the improper welding operation, such as large current, too much heat input, the fault such as undercut occurs; due to the less current and low weld pool temperature, the non-mature rod melt-down occurs ; due to protection measure absence such as anti- wind absence, the blowhole fault occurs; due to improper weld rod selection , the stiffness of the welded seam is low. The above mentioned improper welding process will bring about poor quality of welded seam and concentration of stress, and furthermore the cracks will occur in the heat-affected area of welded seam when the welded seam is under the external force.

1.4 Improper operation cause

The improper operation includes many aspects. For example, when the kiln shell temperature is high and the kiln shell is cooled down by force, large temperature difference is created between the internal and external sides of the kiln shell, when  the temperature of the kiln shell surface between two stop blocks drops quickly and the temperature of internal kiln shell drops slowly, meanwhile there is no way to cool down the kiln shell section that under the stop blocks and cushion plates, at the moment the strength of welded seam is large, when the kiln shell surface between the stop blocks shrinks, the cracks will occur in the heat-affected area of the welded seam because the seam is under the shrink tensile force.

The tyres are running in the large gap condition for the long term, the strengthen effective from the tyres to the kiln shell will weaken, and the deformation of kiln shell will large in the operation process .

The lack of kiln coating protection, the kiln is running in non-aligning line for long term, the current fluctuates up and down and the kiln shell stress is unstable, the load of part of kiln shell is too large.

2.Prevention for the cracks

here are many causes for the kiln shell cracks, and it must be prevented, otherwise the safe operation of the kiln will be affected badly .

2.1Improve the fixed form of the cushion plate

Through observation, it can be found that the cracks occur easily at the cushion plate circular directional stop blocks welded seam, however in general condition cracks don’t occur at the axial directional welded seam, it may because the welded seam form is round or oval. The cushion late limit form in below graph is advised.

2.2 Ensure weld quality

The weld quality guarantee measures will be in place to ensure the quality of the welded seam; there is no bulge in the middle section of welded seam form, there will be concave in this section, as shown in below graph 2, in order to increase the toughness of the welded seam; the annealing will be done after welding to remove the welding stress.

2.3 Normal maintenance and operation

Measure the centre line of the kiln on time. Ensure the kiln isn’t off the centre line during its operation process. The extra load will not be added to the kiln. When the kiln is adjusted, each pier section load will be watched out. The load of all the kiln piers can’t exceed the load capacity. When the slippage volume is above 20mm, the cushion plate thickness will be adjusted in time to ensure the strengthen effect of the tyres to the kiln shell. The kiln coating will be protected, the kiln in non-aligned line running for long term will be avoided, the extra load to the kiln will also be avoided, and so on.