Ball Mill Feed end cover

Detailed Introduction, Manufacturing Process, and Inspection Process of Ball Mill Feed End Covers

I. Functions and Structural Features of Feed End Covers

• Material guiding: Through the internal feed screw or conical structure, uniformly introduce externally conveyed materials into the cylinder to avoid local accumulation;

• Sealing protection: Cooperate with sealing devices (e.g., labyrinth seals, felt seals) to prevent dust and grinding media from leaking out of the cylinder;

• Structural support: As a closure for the cylinder end, it is welded or bolted to the hollow shaft to jointly bear radial and axial forces of the cylinder.

• Shape: Mostly disc-shaped or special-shaped with flanges, with a central feed port (connected to the hollow shaft inner hole) and a flange at the edge for bolted connection to the cylinder;

• Material: Requires both strength and toughness. Small and medium-sized ball mills commonly use Q235B carbon steel, while large or heavy-duty models adopt Q355B low-alloy steel (yield strength ≥355MPa). The wall thickness is typically 20-60mm depending on the diameter;

• Design details: The inner side is often welded with feed screw blades (material ZGMn13 for wear resistance), and the outer side is machined with a step surface (roughness Ra≤3.2μm) matching the sealing device.

II. Manufacturing Process of Feed End Covers (Taking Large Q355B End Covers as an Example)

1. Raw Material Pretreatment and Cutting

• Raw material selection: Use Q355B steel plates with a thickness of 20-60mm, accompanied by material certificates (chemical composition: C≤0.20%, Mn 1.2-1.6%, Si≤0.55%). Mechanical properties (tensile strength 470-630MPa, elongation ≥20%) must be verified;

• Cutting:

• Cut according to the expanded size of the end cover (including flange edges) using CNC flame cutting. The perpendicularity of the cutting surface is ≤1mm/m, with no cracks or burrs on the edges (checked with a 10x magnifier);

• Reserve a 5-10mm machining allowance for the central feed port, and pre-mark the positions of flange bolt holes.

2. Forming and Rough Machining

• Forming:

• Small and medium-sized end covers can be directly formed by CNC cutting; large end covers (diameter ≥3m) require local heating of the steel plate to 300-400℃ and pressing with a press to avoid cold working deformation;

• Rough turning:

• Roughly turn the end face and flange joint surface on a CNC vertical lathe, leaving a 3-5mm finishing allowance;

• Roughly bore the central feed port, with a 2-3mm grinding allowance for the diameter, and control the inner hole surface roughness to Ra≤12.5μm.

3. Welding and Heat Treatment (Key Processes)

• Component welding:

• If the end cover needs to be welded with feed screws or reinforcing ribs, use submerged arc automatic welding (wire H08MnA, flux HJ431) with a welding current of 500-600A and voltage of 28-32V. Ensure the weld leg height is ≥8mm;

• Immediately perform post-weld heat treatment at 250-300℃ for 2 hours after welding to eliminate welding stress and prevent cracks;

• Overall quenching and tempering (optional for large end covers):

• For end covers bearing heavy loads, heat to 850-870℃, air-cool after insulation (normalizing) to refine grains, and control hardness to 180-230HBW to ensure machinability.

4. Finish Machining

• Finish turning the flange surface:

• Finish turn the flange surface connected to the cylinder on a CNC vertical lathe, with flatness ≤0.05mm/m and surface roughness Ra≤3.2μm, ensuring tight fitting with the cylinder flange (gap ≤0.1mm);

• Machine flange bolt holes (12-36 holes depending on diameter, aperture φ20-φ50mm) with a positional tolerance of ±0.1mm and cumulative hole distance error ≤0.2mm;

• Finish boring the feed port:

• Finish bore the inner hole of the central feed port, with the tolerance of the part matching the hollow shaft controlled to H7 (e.g., a φ300mm inner hole allows +0.03-+0.07mm) and surface roughness Ra≤1.6μm;

• Machine the sealing step surface (matching the sealing device) with a step height deviation of ±0.05mm and perpendicularity ≤0.02mm/100mm.

5. Accessory Assembly and Surface Treatment

• Weld the feed screw: Weld ZGMn13 wear-resistant screw blades to the inner side of the end cover, grind the weld after welding to ensure the screw surface is smooth with no protrusions (≤1mm);

• Surface treatment:

• Non-machined surfaces are sandblasted for rust removal (up to Sa2.5 grade) and coated with epoxy primer (thickness ≥60μm);

• Machined surfaces are coated with anti-rust oil (e.g., 20# machine oil) to prevent corrosion.

III. Inspection Process of Feed End Covers

1. Raw Material Inspection

• Chemical composition analysis: Use a direct-reading spectrometer to detect C, Mn, and Si contents in Q355B steel plates to ensure compliance with standards;

• Mechanical property sampling: Take samples from the same batch of steel plates for tensile tests (tensile strength, elongation) and impact tests (-20℃ impact energy ≥34J) to verify material toughness.

2. In-Process Inspection (Key Nodes)

• Inspection after cutting: Check the cutting size deviation (≤±3mm), and observe the cutting edge with a 10x magnifier for no cracks or delamination;

• Welding quality inspection:

• Visual inspection: Welds must be free of pores and slag inclusions, with undercut depth ≤0.5mm and length ≤10% of the total weld length;

• Non-destructive testing: Perform 100% penetrant testing (PT) on the welded part of the flange and end cover body to ensure no surface cracks; key welds of large end covers require 20% ultrasonic testing (UT) compliant with JB/T 4730.3 Grade II.

3. Dimensional Accuracy Inspection

• Flange surface accuracy:

• Measure the flange surface with a flatness tester, ensuring flatness ≤0.05mm/m;

• Measure the flange thickness with a micrometer, with a deviation of ±0.5mm;

• Feed port and step surface:

• Inner hole diameter: Measure with an internal dial gauge, ensuring tolerance meets H7 requirements (e.g., the measured deviation of a φ300mm inner hole must be within +0.03-+0.07mm);

• Step surface perpendicularity: Detect with a dial gauge on a precision vertical lathe, with deviation ≤0.02mm/100mm;

• Bolt hole position: Detect hole positions with a coordinate measuring machine, with positional tolerance ±0.1mm and cumulative hole distance error ≤0.2mm.

4. Final Inspection of Finished Products

• Sealing performance test: Test-fit with standard sealing devices, check the fit gap with a feeler gauge (≤0.05mm) to ensure no looseness;

• Assembly compatibility: Preassemble with the cylinder flange and hollow shaft, check bolt hole alignment (all holes allow free insertion of bolts), and ensure the flange fitting surface gap ≤0.1mm (checked with a feeler gauge);

• Appearance quality: Visually inspect the surface for no scratches or deformation, ensure the screw blades are firmly welded with no obvious protrusions.