Due to design there is a difference in circumference of tires and shell. Tire inside diameter is bigger than shell outer diameter. Because of this difference the tire rotates a little bit slower than shell. We can observe it on each revolution by linear movement between both surfaces. This movement is called “tire migration”, “creep”, or “relative movement”.
How we can measure tire migration?? The simplest way is to mark a line on both components – tire and shell (chair pad or any other part fixed to shell). After one revolution of the kiln we can observe line separation. By measuring this line separation we can read the value of creep. This parameter can be easily recalculated into under-tire clearance (gap). Creep divided by 3,14 (Pi) gives us the gap. For most kiln producers tolerance for under-tire clearance is between 3 to 6 mm in hot condition.
Under-tire parameter value is directly linked with temperature changes and also kiln axis position in vertical plane. The sudden temperature change decrease or increase the value of creep. It is strongly recommended to keep shell temperature stable around whole kiln shell circumference. There are many problems that can be caused by excessive tire migration:
- When creep is bigger – wear accelerates
- Ovality of shell is higher. This can create problems – brick failure, shell crack and deformation, stop-blocks, welds or chair pads cracking.
- Stop-block or retaining rings wear is faster and can create undercut of tire.
- Tire axial run-out (wobbling)
There are a direct connection between number and type of supports and its ability to distribute the unit load to the shell. The tires guarantee necessary strength to the shell by keeping its roundness. Due to design the shell naturally flattens at the top position, the tires must maintain the shell integrity by preventing flex.
When the chair pads are worn out the under-tire gap exceeds tolerance. This causing the bigger shell ovality. Higher value of gap and ovality means faster wear of rotating components (chair pads, stop-blocks, tire side surface, tire outer diameter). We can say that wear is getting bigger and bigger at each revolution. Besides too big value of creep can lead to crack on the shell segments.
Excessive wear also allows the shell to move through the tire which mismatches the tire to the roller and the gear to the pinion. This than increases the loads at the contact area of the tires and rollers, girth gear and pinion, and increases hertz pressure which leads to surface spalling.
When the wear of the tire will appear there will be difficult to perform adjustment to the tire. As a next step drive system or even failure is possible.