News Date:2021-08-06 11:05:33
Kiln Tyre
The kiln tyre itself is usually a single steel casting, machined to accurately circular dimensions and with a mirror-smooth texture on all surfaces. Early tyres were occasionally produced as half-sections that could be easily assembled and replaced, but this was very soon abandoned because of the resulting rapid and erratic wear at the joints.
In the standard design, the tyre was mounted loosely on the kiln shell. Inevitably, the tyre is cooler than the kiln shell, and so a small gap allows differential expansion to take place. The gap is usually designed to be about 0.2% of shell diameter at normal operating temperature. The kiln tube bears down upon the inside of the tyre through smooth-surfaced chairs which also have lugs bracketing the tyre, preventing it from slipping along the kiln axially. The spacing of the chairs also reduces the amount of heat conduction from the kiln shell to the tyre. The tyre needs to remain relatively cool because so large a casting would be unlikely to survive a large radial temperature differential during heating up of the kiln.
Support Rollers
The rollers are mounted on a massive cast iron or steel base plate which provides the inward horizontal forces on the rollers and distributes the weight of the kiln over the pier. The spacing between the rollers has to be small enough to prevent large horizontal forces, but large enough to keep the kiln laterally stable. Rollers are designed to subtend 60° at the tyre centre, and this seems always to have been the case. Minor adjustment is allowed so that the kiln can be kept aligned (i.e. to keep the centres of the tyres co-linear) as small changes take place, such as wear of the tyre or settlement of the pier. The roller outer face is made wider than that of the tyre, mainly to allow for contraction of the kiln during shut-down. This poses a problem: if the tyre remains in one position relative to the roller, wear or plastic deformation causes a depression to form on the roller face. It is therefore normal practice to deliberately make the kiln “float” (i.e. regularly move uphill and downhill across the rollers) so that wear is evened out. Because the kiln slopes (typically 1.5° to 3.5°) it has a natural tendency to slip downhill as it turns. From the earliest times, this tendency was compensated by “cutting” the rollers – skewing their axes by a very small angle so that an uphill screw action is imparted to the tyre. This action relies upon the friction between the tyre and roller surfaces, and operators could therefore make the kiln move up or down by adjusting the amount of friction. As a further precaution to prevent the kiln from falling off its rollers, thrust rollers bearing upon the side of the tyre are used. These are usually located on the roller beds nearest the drive, where movement most needs to be restricted.
Girth Gear
Until the advent of frictional drives (see below), kilns only ever had one turning gear and this supplies all the torque to turn the kiln, so in the case of a long kiln, it is usually positioned somewhere near the middle (strictly speaking, the centre of mass) to minimise the amount of torsional distortion produced in the shell. Preferably a relatively cool section of the kiln is chosen. The gear is placed near to a tyre so that it is accurately aligned with the kiln axis, with minimal wobble. It is normal for the nearby tyre to be fixed in position with thrust rollers, so that as the kiln expands on warming up, the turning gear position remains fairly constant, while the nose and tail of the kiln expand outward. The pier of the nearby tyre is usually extended to include the pinion mounting bed, the gearbox and the motor, although on early kilns it was common to mount the motor on the kiln house floor, and connect it to the gearbox with a flat belt. In the case of shorter dry process kilns with preheaters, it has been normal practice to locate the turning gear next to the rear tyre, at the coolest part of the kiln.
