Method for Controlling Rotary Kiln Tire (Shell Pad) Clearance

Note: Tire clearance refers to the gap between the inner circle of the tire and the outer diameter of the shell pad, not the contact clearance between roller and tire.

Reference control values: Cold condition: 10～20 mm; Hot operating condition: maintain 2～5 mm.

If the clearance is too small, thermal expansion will lock the kiln shell. If excessive clearance exists, the kiln shell will undergo elliptical deformation, refractory bricks are prone to crushing, impact noise occurs at the tire, and pads wear rapidly.

1. Core Adjustment Method: Adding, removing or replacing pads (most common on-site)

1. Excessive clearance (large slip caused by wear)

Shut down the kiln and wait for full cooling. Remove tire stops and thicken pads to reduce the diametral clearance between tire and pads. When the pad thickness increases by approximately 10 mm, the diametral clearance reduces by about 5 mm.

Caution: Pads shall be added or removed uniformly around the circumference. Do not thicken pads on one side only, to prevent shell ellipticity and localized stress concentration.

2. Insufficient clearance (risk of locking under hot condition)

Reduce pad thickness or replace with thinner pads to increase reserved cold clearance to compensate for thermal expansion of shell at high temperature.

3. Pad types: Removable pads and fixed pads. Pads with wear-resistant layer are preferred to reduce wear rate.

2. Clearance Monitoring Methods (to judge whether adjustment is required)

1. Roll-difference method (slip measurement, online dynamic measurement)

Mark the tire and kiln shell. Measure relative slip of marks in one full kiln revolution to convert to diametral clearance. This is the mainstream detection method for cement rotary kilns.

2. Cold condition measurement with feeler gauge / inside micrometer

After shutdown and full cooling, take multiple uniform measurements along circumference and record original clearance data.

3. Auxiliary infrared temperature measurement

Monitor temperature difference between shell and tire for theoretical calculation of reserved expansion clearance:

\delta=\Delta T\times D\times K

\Delta T = temperature difference between shell and tire; D = outer diameter of pad; K = steel thermal expansion coefficient of 0.000012.

3. Process Operation Control (control clearance and reduce wear during daily operation)

1. Baking and heating control

Do not apply rapid heating rate. Heat the shell and tire slowly and synchronously to minimize instantaneous temperature difference, avoiding local jamming and impact wear of pads. Strictly follow the baking curve for new kilns or after major overhaul.

2. Shell temperature control

Prolonged over-temperature operation is forbidden. Excessively high shell temperature increases shell expansion and changes clearance; over-temperature accelerates pad wear and continuously enlarges clearance.

3. Internal lubrication of tire

Apply graphite-based high-temperature grease on contact surface between tire and pad to lower sliding friction, slow pad wear and delay clearance enlargement.

4. Stabilize thermal system to avoid drastic shell temperature fluctuation caused by large-scale falling of coating.

4. Permanent Solutions during Overhaul (severely worn pads, elliptical tire bore)

- If pads are heavily worn and fail quickly after repeated shimming: Replace the complete set of pads, machine pad outer diameter to restore designed cold clearance.

- If tire inner bore is elliptical and heavily worn: Repair tire inner circle by machining or replace the tire when necessary.

5. Safety & Adjustment Key Points

1. Pad adjustment must be carried out after kiln shutdown and complete cooling with lockout-tagout. Working inside tire area under hot condition is strictly prohibited.

2. Follow the principle of small adjustment in multiple steps. Do not make large single adjustment. Re-measure cold clearance after adjustment, and recheck hot slip after kiln baking.

3. Clearance requirements differ at each roller station: Stations near burning zone operate at higher shell temperature, so reserved cold clearance is generally larger than that at kiln outlet stations.

4. Control target for hot clearance: After shell thermal expansion, the tire can slide slightly relative to shell, without locking or heavy impact.

6. Common Fault Diagnosis

- Hot clearance close to 0 mm: Shell expansion locking, shell cross-section deformation and squeezing damage of lining bricks.

- Hot clearance＞6 mm: Excessive slip, periodic banging impact from tire hitting pads and excessive shell ellipticity.