3.5.1. Tested Object

The possibility of conducting AE testing of roller bearings of rotary kilns without their decommissioning was investigated (Figure 15). Using AE monitoring for rotating equipment is an alternative solution to vibrodiagnostics [19,20]. In the case of slowly rotating elements, the use of the AE method is more effective for the diagnosis of rolling bearings, rotating shafts, rollers and others [21,22]. Experimental work was carried out at an alumina plant in the sintering and calcination shops and also in the repair shop.

**Figure 15.** Rotary kiln on paired roller bearings. 1: kiln; 2: roller bearings; 3: support foundation.

The kiln is a cylinder with a diameter of up to 5 m and a length of up to 185 m, installed with a slope. The kiln is supported by 7–8 pairs of roller bearings. Each pair of roller bearings consists of 2 support blocks, mounted on a common foundation. Each block consists of a shaft pressed onto it by a supporting roller and 3 bearing assemblies mounted in 2 bearing housings (Figure 16). The rotation period of the kiln is 50 s; the rotation period of the support roller is 12.5 s. The lifetime of the monitored supports was from 1 to 13 years.

Roller support is a complex load carrying element of the kiln. In case of its breakdown, an emergency stop of the equipment occurs, followed by the replacement of defective components. Of the 200 supporting blocks at the plant, on average 5 blocks per year are replaced because of broken bearings and 1 more block due to the fracture of the shaft. Therefore, it is necessary to ensure the detection of defects of all its elements such as shaft, roller and bearings.

The roller is a part of the support block and is in direct contact with the kiln. Thus, the main defects on it are scoring and metal chipping from rolling contact under load.

**Figure 16.** Roller support.

The shaft is a cast steel component. Since roller bearings are operated under unfavorable temperature conditions, with a high level of vibration and under cyclic loading, the most likely defect of the shaft is the formation of fatigue cracks. Since the shaft is a monolithic all steel construction with numerous protrusions and thickness differences, it is difficult to carry out the NDT of shaft by traditional scanning methods, even for roller bearings in decommissioned state. The main bearing elements include an outer ring, an inner ring, cylindrical rolling bodies and a separator. Destruction of the bearing occurs for 3 main reasons: incorrect mounting of the bearing, insufficient quantity or low quality of lubricants, as well as fatigue failure of the bearing. Due to the low angular velocity (1 rotation in 12.5 s), a well-known method of vibrometry is not quite suitable for testing the roller bearings and AE is expected to provide a good solution.
