Kiln operation – friction, torque, rotation.

Kiln load is rotating on roller shaft with liners. It is not possible to run the kiln without this because the load by itself is to high ant it would be to expensive do design, produce and operate. There is of course some relation between bearing liners (its friction) and rotation – we cannot increase the pressure infinitely. There is always limit of load on a liner.

Kiln geometry

The liner friction and rotation are parameters linked each other, because almost all torque is used to move the material inside the kiln and only a small part is covered to heat shaft and liner contact. But sometimes, especially in case of severe overheating this torque can increase very fast. As a symptom when main motor power will increase by 20%, all bearing inspection should be performed. This situation does not include process reasons.

Liner temperature is also depend on the load applied to the shaft, but other factors are possible as well. The main parameters that affect the load are: alignment of kiln tires, shell axis deformation (both permanent ad thermal  types), lubrication, quality of geometry and precision of components production.

Bearing liner design

Generally bearing liner is has 4 functional points: outside and inside flange, outside and inside diameter. The inside one is divided in 3 areas: oil pocket, oil film creation surface and contact area with the shat.

Typical (new) bearing linear inside diameter area proportion:

  • Oil pocket – from 5 to 15o
  • Contact surface – from 10 to 15o
  • Creation of oil film – the rest.

In most cases bearing liner is perpendicular to kiln axis and load distribution. There are some exceptions. For example KHD design – not perpendicular.

For normal operation the shaft and liner diameters should be properly fitted to each other. There is couple of method to check this fitting. One of them is “blue mark” method which show the real contact area. The axial contact should be quite equal and continuous at least for 80% of all  contact surface. With on the other hand cannot exceed 15o angle.

The contact angle cannot by higher because of thermal expansion is very dangerous for operation  – this is at short term reason. When the kiln starts after stoppage everything is getting hotter and hotter. Tire will heat the roller, than the shaft. Finally the shaft (aprox. After 48 hours) will be bigger in diameter. At the same time, due to water cooling the liner will stay the same. So eventually the oil film thickness will be lower (caused by shaft expansion) and contact will increase.



At long term – mechanical wear will be faster and will create other damages. During years of operation the 15o angle contact area will increase and expands up to 20 – 30o. In case of any problem there is still some spare limit where the liner can still work correctly. When the contact area is too high on the beginning, each unexpected incident can lead to severe failure.