Relations – dimensions and tolerances.

Rotary kiln alignmentAll rotary kiln dimensions are significant due to size of the machine. On one hand we have dozen of meters  on the other hundreds of tones. But even that the each component and whole kiln look very big and strong it cannot accept any defect. The truth is – the bigger it is the more attention as required. Each component should be aligned with very high precision.

Rotary kiln alignment. How important is the tolerances and how accurate the alignment have to be? The example is below:

  • Contact side of teeth of pinion and girth gear
    • Tolerance for normal operation is below 0,05mm
    • The warning for operator is when this value is higher than 0,1mm
    • Damage, tooth crack or other problems can appear when clearance is higher than 0,2mm
  • Skewing of support rollers (position of rollers in horizontal plane – in relation to kiln axis)
    • Normal operation tolerance – below 0,3mm
    • Warning, which will show us the first signs of irregularities – skewing value more than 1mm
    • Danger o operation, and higher level of mechanical wear – above 3mm.

Thermal expansion

As an example of thermal bending of metal component imagine one beam, 10m long and put 420oC temperature. The calculated thermal expansion will be 48mm.

And now take the same beam and put 220oC to it. The calculated thermal expansion will be 24mm.

As we can see the temperatures difference has big influence over metal expansion.

So will happen if we put the same temperatures to the shell – both at the same time?

As an example let’s take 10m shell length and 4m diameter. To illustrate the case we put same temperature 420oC on half of circumference and 220oC on another half. This kind of temperature distribution we can obtain due to uneven coating.

Taking the calculation of thermal expansion as a result we get mention above 48mm in one side of the shell and 24mm on the other. So the differential expansion for the shell will be 24mm. Due to kiln is resting on the piers what will happen? The result of this condition will be shell deflection.

This is of course approximate situation but allow to illustrate the problem. As a conclusion we can say that:

  • Uneven coating distribution can lead to difference in temperatures on circumference of kiln shell
  • As a consequences to differential expansion
  • And finally to kiln shell thermal bending.

Extreme situation if the bending will be very high (enough to lift kiln components), the rollers and tires running surfaces can even loose the contact.

So as a conclusion it is very important to keep the shell temperature equal  around whole circumference to avoid kiln shell bending. If the problem already exists – process parameter should be modified to get proper coating thickness.

The other reason that can create thermal bending:

  • When the rotary kiln in not properly rotated (especially during start-up and stoppage procedure)
  • When the kiln is not rotated after one incident.

How important is constant operation (not full stoppage during first problems) is the best ways to show in numbers. When the user stop the kiln very fast, thermal bending will be bigger and bigger. After that shell will be deformed permanently.  Situation mention below is coming from real case a one of the Cement Plant.

Due to some production problem the kiln operation was stop for only 10 minutes. At the same time the shell was exposed to tropical rain which cools only upper part of shell. The thermal bending become a permanent type – the real clearance between tire and roller at middle pier was 27mm. To correct this damage and restore normal kiln operation the correction cut and shell segments replacement were necessary.  Cost – more than 100.000 USD. Stoppage time – more than 12 days.


The temperature distribution is the most important kiln parameter. The dangerous for operation is not only maximum value of temperature but differential expansion (especially different temperature on kiln shell circumference). To avoid unexpected kiln stoppage auxiliary drive should always ready to operate. Also the mechanical and maintenance staff should be familiar with thermal expansion importance.