Mechanical crankshaft and kiln stiffness.

What is exactly the crankshaft? As an example let’s look at three pier kiln.

The crankshaft is one of type of crank caused by mechanical bend where middle station (2nd pier) has some eccentricity (rotating) but 1st and 3rd pier are set correctly in one reference line.

The main reasons of crankshaft is brick lining failure, after which kiln stoppage procedure was not carried out. Too long operation with missing brick and too high temperature on the shell without bricks create shell bending.

Kiln stiffness

Rotary kilns are not all the same. Due to different design and supplies one kilns can be much more “sensitive” than others. “Sensitive” in this case means to misalignment or crankshaft formation.

One of kilns will be more flexible the other are stiff, where the reaction on each deviation will be severe. This stiffness can be calculated and it calls “kiln stiffness matrix”. It allows to connect the cause and consequences together (misalignment and crankshaft with periodical circumferential overloads on supports).

The best indicator (one of the first symptoms) is temperature difference on kiln shell circumference. If this difference in more than 90oC should be considered as dangerous. In this temperature difference shell can start bending process. Temperature below will create only some local deformations. This is also not good situation because local deformation can cause the brick problem as well and eventually can be the reason of crank.

When we compare axial and radial hot-spots (big difference on temperature) – the first one is much les dangerous for shell and kiln operation.

Another reason of mechanical crankshaft (occur less frequently) is erection of the kiln unit – especially wrong alignment of the shell. This problems occurs when executive company knowledge is not enough (lack of know-how), shell segments (old of new) are deformed or poorly chosen shell segments that have to be replaced.

So we already know what is crankshaft. The next question is: is it possible to detect the crankshaft when there is not visual indicators?

The answer is yes. The crankshaft cannot be visually obvious  because kilns are quite flexible machines  and the deformation is not visible. We call it “hidden crank formation”. Sometimes there are several that work in opposite phases.

The indication of crankshaft in the shell can be:

  • Periodic main motor amperage fluctuation – changes during rotation
  • Periodic noise during each kiln revolution.
  • Deflection of roller shaft.
  • Measurement of radial run-out of kiln shell
  • Indication of high value of radial run-out (eccentricity) on kiln inlet or outlet seals, or on the shell between two piers.

To detect crankshaft existence by rollers’ shaft deflection measurement, the data analyzed should cover at least 5 or 6 complete kiln shell rotations.