Rollers inclination

Support roller is a cylindrical components that allow the kiln to work in rotary condition. Properly maintain rollers are clean, free from oil, dust, shiny surface, correctly aligned (where the axis of roller is parallel to kiln axis).

Each two rollers carries load (weight) of kiln up to thousand tons. That is why even the smallest surface deviation can lead to significant adverse effect to the entire kiln operation.

The rotary kilns feature is lateral movement – axial movement of the shell. Main reason of this is thermal expansion of shell which reposition tires on the stationary rollers. The problems appears when the running surfaces are not perfectly cylindrical. Any axial position changes bringing additional excessive stress to working components. This additional load is transferred from shell through the shims and tires to support rollers and bearings and finally to concrete foundations. All  parts in this chain of causes have to absorb excessive stress.

When the load on specific roller is above design tolerance various problems piles up like worn side faces of tires, stop-block damage, shims wear, weld cracks etc. Even when the tire position on the roller is stable the irregular running surfaces can create enormous additional pressure. All these additional energy has to be absorb somewhere (rollers or tires or shell). But the girth gear and pinion (drive system) must work harder to rotate the kiln. The negative effect is not only higher power consumption but mainly faster wear of drive component. Possible failures coming from worn roller surfaces are: hot bearing, damaged rollers’ shaft, cracks on the shell or welds, axial run-out of tires. This symptoms can occur one-by-one or be present together.  There are also many other points (factors) that can lead to mention above symptoms, but not correctly maintain running surfaces are usually most significant.

When the rollers and tires running surface are not clean (full of oil or material, dust) it is very difficult to get proper geometrical position and balance of the kiln at the same time. Oil on the rollers surface will move the kiln downward so the hydraulic pressure on thrust roller will be higher. To compensate this excessive force the skewing of rollers must be changed (the wear of surfaces will be higher). Same situation with the material existing on rotating surfaces which work opposite way than oil. The friction is higher and the wear of components as well.

Both roller surface and tires should be clean of oil and dust. Very big pressure on the oil at the contact point push it into the surface creating microscopic cracks. These small gaps will eventually enlarge its size and grow deeper and deeper. They can even join with one another creating even bigger irregularities. This conditions is known as “oil spalling”.

Another concept which is linked to existence of foreign objects on the contact point of two running surfaces is “pitting” and “denting”. The foreign bodies in this case are dented by the extreme pressure. This bodies in extreme cases can even become embedded in the surface. This parts are stressed very high and can lead to cracks.

Cracks on the roller is very important symptom. Generally it occurs when the contact between running surfaces are not correct or surfaces are worn out (mushroomed edges). When any crack was found, the whole roller should be cleaned from oil and dust and one of the non-destructive detection method should be used. Repairs may by possible depending on the type and the reason of crack existence. But generally in most cases it is only a temporary solution. The roller with cracks should be replaced.

When something is wrong with two running surfaces (alignment, cleanliness, material problems) some visual wear patterns appears on it. This patterns gives a lot of useful information to experienced diagnostician. Analysis of wear pattern should be performed compared with other measured parameters.

Normal operation of the rotary kiln can lead to uneven wear. The reason is that the rollers is wider then  tire to accommodate the axial movement. Tires is moving across the roller with full contact. This solution (design) reduces hertz pressure. But even correct balance, most of the time the tire work on the middle part of roller and the wear on will be higher. After years of operation running surfaces become concave. Improper roller skewing accelerates the wear. Support rollers generally should be adjusted parallel to kiln axis to easily control the balance and pressure.

Another cause can be different position of rollers (not parallel to each other). This maladjustment may create tapered wear shape. This pattern can be created even if one of the rollers is out of slope. Important information is the roller never has only a taper, but wearing process is irregular. Generally main reason of tapered shape is not correct skewing or inclination of rollers.

We can also distinguish the horizontal or diagonal patterns called “washboard” or “timing marks”. Generally this in an imprint appears over time and as a result of misalignment of girth gear and pinion meshing. The reason of that kind of mark is too big value of root clearance or in most cases too little where the teeth hits to bottom part of meshing also very often causing high vibration. This is another part of chain of causes – vibration creates higher wear and higher vibration (cycle) leading quickly to mechanical damage.

Daily maintenance inspection check should include looking for cracks, spalls, fractures and contact conditions, shaft condition, lubrication efficiency. Also the wear patterns should be analyzed. As a roller surface lubricant the dry graphite should be used. The cleaning should be carried out as well as a standard routine. General housekeeping is very important in well maintain kiln unit and allows to avoid many future problems with operation. Regular inspection guarantee long and trouble free kiln service life.