If a small steel ball falls through the oiling port into the main bearing of an internal mixer, it may obstruct the bearing's moving parts, damage contact surfaces, and increase resistance in the main drive. In severe cases, the motor may become overloaded or the drive may seize. Stop the machine immediately, isolate the power supply, and have qualified maintenance personnel inspect the equipment. Do not restart the machine until the fault has been resolved.
1. Why a steel ball can damage the main bearing
1.1 Interference with bearing clearances
The source reference gives a radial clearance of 0.1 to 0.2 mm and an axial clearance of 0.05 to 0.1 mm for the main bearing. The oiling port diameter is stated to be 6 to 12 mm, with the steel ball typically sized to fit the port.
If the ball enters the bearing assembly, it may become trapped in the raceway, bearing cage, or space between the shaft journal and bearing housing. This can obstruct normal bearing movement and sharply increase the resistance of the main drive.
1.2 Scratches on bearing contact surfaces
Bearing steel balls typically have a hardness above HRC 60. If the ball becomes trapped between moving or mating surfaces, it can scratch the shaft journal and bearing raceways.
These scratches can affect the accuracy of the mating surfaces. If the machine continues to operate, the damage may contribute to excessive vibration and abnormal noise.
1.3 Lubrication system contamination
If the machine continues running with the steel ball trapped inside, the ball may be crushed under the applied load. Metal fragments can then enter the lubrication circuit, contaminate the oil, and cause abrasive damage to other bearings and components, including hydraulic control valves.
The resulting damage may extend beyond the original bearing and increase the scope of the repair.
1.4 Oil seal damage
The steel ball may also become trapped at the lip of the bearing's radial shaft oil seal. This can damage the sealing edge and allow lubricating oil to leak from the assembly.
2. Repair procedure
2.1 Stop the machine and secure the work area
All repair work should be performed by qualified maintenance personnel with the required authorization.
First, disconnect the main power supply, lock the electrical cabinet, and attach a "Do Not Energize" warning tag. Allow the equipment to cool completely before dismantling any components to avoid burns from hot surfaces.
Do not attempt to remove the ball while the machine is running or while hazardous stored energy remains in the system.
2.2 Locate and remove the foreign object
Remove the main bearing end cover and radial shaft oil seal as required by the equipment structure. Drain the remaining lubricant from the relevant housing, then use a borescope or flashlight to inspect the bearing assembly and identify the ball's position and quantity.
If the ball is accessible and lodged only in a gap in the bearing cage, a fine copper hook may be used to remove it carefully. Avoid scratching the shaft journal or bearing raceways. If the ball cannot be removed without force, further dismantling may be necessary.
2.3 Dismantle the bearing and assess the damage
Use a suitable bearing puller to remove the main bearing. Do not strike the assembly with a hammer or force it apart with a pry tool, as this may damage precision mating surfaces on the shaft journal.
Clean the bearing's inner and outer rings, cage, and shaft journal with anhydrous ethanol or kerosene, where compatible with the component materials and applicable safety requirements. Dry the parts with a lint free cloth and inspect the contact surfaces.
The supplied reference suggests the following repair criteria:
For scratches with a depth of 0.02 mm or less, light polishing with metallographic abrasive paper may be considered.
For scratches deeper than 0.05 mm, replacement of the shaft journal or bearing housing assembly is specified.
These thresholds are reference values, not universal acceptance criteria. The bearing manufacturer's requirements and the actual damage should be assessed before attempting to restore a precision surface.
2.4 Clean the lubrication system and prevent further contamination
Remove any remaining steel fragments from the main drive housing and lubrication passages. Replace the lubrication filter element and refill the system with the lubricant specified for the equipment. The original reference gives L-CKD 150 to 220 industrial gear oil as an example, but the correct grade must be confirmed against the machine's lubrication requirements.
To reduce the risk of another foreign object entering through the oiling port, the source recommends installing a stainless steel filter screen with an opening size of 5 mm or less. Before fitting a screen, confirm that it will not obstruct the required oil flow or interfere with the lubrication arrangement.
2.5 Reassemble the bearing and conduct a no-load test
Reassemble the bearing according to the equipment manufacturer's instructions and adjust the radial and axial clearances to the specified values.
Tighten the end cover bolts to the required torque. The source reference gives a general range of 80 to 120 N·m, but the machine manual should determine the actual setting.
Turn the shaft manually to check for binding before restoring power. Once the assembly, lubrication system, and safety devices have been checked, conduct a no-load test run for 30 minutes as suggested in the source reference.
During the test, monitor the main motor current and bearing surface temperature. The supplied reference gives a normal operating temperature of 70°C or lower. Confirm the applicable temperature limit against the equipment manufacturer's specifications. Stop the test if abnormal noise, vibration, rising temperature, or other signs of a fault appear.
The machine should return to production only after the bearing operates normally and the repair has been verified.
3. Custom machining for internal mixer components
When a steel ball damages a shaft journal or bearing housing, the repair may require component replacement rather than simple cleaning. The replacement part must match the original drawing, dimensions, material requirements, and mating conditions.
Hansheng Automation can assess custom machining of suitable shafts, metal rings, and other precision mechanical components based on customer drawings and technical specifications. Our machining services also cover non-standard components for rubber processing, tobacco packaging, and filling machinery, as well as precision parts for yacht equipment and RC racing cars. Customers can provide drawings and dimensional requirements for a manufacturing assessment.
