Ball Mill Main Shaft

Detailed Introduction, Manufacturing Process and Inspection Process of Ball Mill Shaft

I. Functions and Structural Characteristics of Ball Mill Shaft

• Torque transmission: Transmit the torque output by the motor through the reducer to the cylinder, driving the cylinder to rotate (rotational speed: 15-30 rpm);

• Load bearing: Bear the total weight of the cylinder, grinding media (steel balls) and materials (up to thousands of tons for large ball mills);

• Sealing connection: Cooperate with the sealing device at the feed and discharge ports to prevent material leakage.

• Shape: A stepped hollow cylinder (hollow structure reduces weight), with journal (matching bearings) at both ends and flange bolt connection with the cylinder in the middle;

• Dimensions: Shaft diameter of small and medium-sized ball mills is usually 300-800 mm, while that of large ones can exceed 1500 mm; length matches the cylinder (2-10 m);

• Material: Requires both high strength and toughness. Small and medium-sized shafts use 45# high-quality carbon steel (low cost, good machinability), while large or heavy-duty models adopt 42CrMo alloy structural steel (tensile strength ≥ 800 MPa, impact toughness ≥ 60 J/cm²).

II. Manufacturing Process of Ball Mill Shaft (Taking 42CrMo Material as an Example)

1. Raw Material Pretreatment and Forging

• Raw material selection: Use 42CrMo round steel with diameter 300-1800 mm, accompanied by material certification (chemical composition: C 0.38-0.45%, Cr 0.9-1.2%, Mo 0.15-0.25%);

• Forging process:

• Heating: Heat the round steel in a natural gas furnace to 1100-1150℃ (austenitizing temperature) and hold for 2-4 hours (adjusted according to diameter);

• Forging: Adopt free forging or hydraulic press die forging, with staged upsetting and drawing to ensure the forging ratio (length-diameter ratio) ≤ 3.5 and eliminate internal porosity;

• Post-forging treatment: Cool slowly to 600℃ and then air-cool to avoid cracks caused by excessive temperature difference; control forging allowance at 15-20 mm.

2. Heat Treatment (Key Process Determining Mechanical Properties)

• Normalizing: Heat to 860-880℃, hold for 3 hours, then air-cool to refine grains, eliminate forging stress, and reduce hardness to 220-250 HBW;

• Quenching and tempering:

• Quenching: Heat to 840-860℃, hold, then oil-cool (cooling rate ≥ 50℃/s) to ensure core hardening;

• High-temperature tempering: Hold at 580-620℃ for 4 hours, then air-cool to achieve final hardness of 280-320 HBW, balancing strength and toughness;

• Flaw detection: Conduct 100% ultrasonic testing (UT) after quenching and tempering (compliant with JB/T 4730.3 Level I), with no cracks or flakes allowed.

3. Rough Machining and Semi-Finishing

• Rough machining:

• Turning: Turn the outer circle and end face on a floor lathe or CNC lathe, leaving 8-10 mm machining allowance;

• Drilling and boring: Bore the hollow inner hole (diameter 1/3-1/2 of the shaft diameter) to remove internal stress concentration areas;

• Semi-finishing:

• Finish turning: Further turn the journal and flange end face, leaving 2-3 mm grinding allowance, and machine a process table at the journal (for subsequent positioning);

• Milling: Mill flange connection holes (evenly distributed circumferentially, 8-32 holes, diameter φ20-φ60 mm).

4. Precision Machining (Ensuring Assembly Accuracy)

• Grinding:

• Journal grinding: Grind the journal matching the bearing on a universal cylindrical grinder (tolerance IT6, surface roughness Ra ≤ 0.8 μm), ensuring cylindricity ≤ 0.005 mm/m;

• End face grinding: Grind the flange end face (flatness ≤ 0.02 mm/m) to ensure perpendicularity to the axis ≤ 0.01 mm/100 mm;

• Precision inspection: Use a coordinate measuring machine to check key dimensions (e.g., journal diameter, flange thickness) with deviation controlled within ±0.02 mm;

• Surface treatment: Phosphating treatment on the journal surface (for rust prevention), and primer + topcoat on non-mating surfaces (total thickness ≥ 80 μm).

5. Final Inspection and Trial Assembly

• Cleaning: Clean surface oil and iron filings with diesel;

• Trial assembly: Preassemble with the bearing seat and cylinder flange, check the fit between the journal and bearing inner ring (interference 0.01-0.03 mm), and the position degree of flange connection holes (≤ 0.1 mm).

III. Inspection Process of Ball Mill Shaft

1. Raw Material and Forging Inspection

• Chemical composition analysis: Use a direct-reading spectrometer to detect 42CrMo composition, ensuring Cr and Mo contents are within standard ranges;

• Forging quality inspection:

• Macrostructure: Etch test (10% nitric acid alcohol solution), with no shrinkage, scab, or cracks allowed;

• Mechanical properties: Take forging samples for tensile test (tensile strength ≥ 800 MPa, yield strength ≥ 600 MPa) and impact test (-20℃ impact energy ≥ 40 J).

2. Heat Treatment Inspection

• Hardness testing: Measure hardness at multiple points on the journal and flange with a Brinell hardness tester, ensuring hardness of 280-320 HBW (uniformity deviation ≤ 20 HBW);

• Metallographic structure: Inspect the quenched and tempered structure (tempered sorbite, grade ≤ 3) with no network carbides or free ferrite.

3. Machining Accuracy Inspection

• Dimensional accuracy:

• Journal diameter: Measured with a micrometer (tolerance IT6, e.g., φ500 mm journal allows +0.03-+0.05 mm);

• Inner hole diameter: Measured with an internal dial gauge, tolerance H8 (ensuring fit clearance with the feed pipe);

• Geometric tolerance:

• Radial runout: Measured on a precision lathe or deflection instrument, ≤ 0.02 mm/m at the journal;

• Straightness: Detected with a level, full-length deviation ≤ 0.05 mm/m.

4. Final Product Inspection

• Surface quality: Visually or by penetrant testing (PT) to check the surface, with no scratches or bumps (depth ≤ 0.5 mm);

• Dynamic balance test: For shafts with rotational speed ≥ 30 rpm, conduct dynamic balance testing (unbalance ≤ 50 g·mm/kg);

• Hydrostatic test: Perform a 0.5 MPa hydrostatic test on the hollow shaft inner hole, with no leakage for 30 minutes (to ensure sealing performance).

IV. Summary