Mill End Cover / Mill Head
Ball Mill & Rod Mill End Covers — Medium Carbon Alloy Steel (ZG35CrMo / ZG42CrMo / ZG45CrMo) / Low Alloy Steel (ZG30CrMnSi / ZG40Cr) — For Feed End & Discharge End
Mill End Cover / Mill Head
Material Specifications & Selection Guide
| Grade | Material | Tensile (MPa) | Yield (MPa) | Elong. (%) | Application |
|---|---|---|---|---|---|
| ZG230-450 | Carbon Cast Steel | 450 | 230 | 22 | Small mills, low load |
| ZG270-500 | Carbon Cast Steel | 500 | 270 | 18 | Standard ball mills |
| ZG310-570 | Carbon Cast Steel | 570 | 310 | 15 | Medium mills, high torque |
| ZG35CrMo | Cr-Mo Alloy Cast | 690 | 490 | 12 | Heavy duty, fatigue resist |
| ZG42CrMo | Cr-Mo Alloy Cast | 790 | 590 | 10 | SAG mills, extreme load |
| ZG40Cr | Cr Alloy Cast | 650 | 450 | 14 | AG mills, balanced strength |
| Component | Material | Hardness | Process | Feature |
|---|---|---|---|---|
| Main Body | ZG270-500 / ZG35CrMo | 170-220 HB | Sand casting + NDT | Structural strength |
| Trunnion | ZG35CrMo / ZG42CrMo | 200-250 HB | Centrifugal cast | Bearing seat precision |
| Gearing Rim Seat | ZG35CrMo | 180-230 HB | Machined + keyway | High torque transfer |
| Feed Opening | Wear plate overlay | 48-55 HRC | Weld-on Mn13 plate | Abrasion resistant |
| Bolt Holes | Drilled + tapped | — | CNC machined | Precision fit |
| Flange Face | Machined finish | — | Ra 3.2-6.3 | Sealing surface |
| Mill Type | Mill Dia. (m) | Cover Dia. (mm) | Weight (kg) | Material | Trunnion Dia. |
|---|---|---|---|---|---|
| Ball Mill (Small) | 1.5-2.4 | 1,800-2,800 | 800-2,500 | ZG230-450 / ZG270-500 | 300-500 |
| Ball Mill (Medium) | 2.4-3.5 | 2,800-4,200 | 2,500-8,000 | ZG270-500 / ZG310-570 | 500-800 |
| Ball Mill (Large) | 3.5-5.0 | 4,200-6,000 | 8,000-25,000 | ZG310-570 / ZG35CrMo | 800-1,200 |
| SAG Mill | 7.0-12.2 | 8,000-13,000 | 25,000-80,000 | ZG35CrMo / ZG42CrMo | 1,200-2,000 |
| AG Mill | 6.0-10.0 | 7,000-11,000 | 20,000-60,000 | ZG35CrMo / ZG40Cr | 1,000-1,800 |
| Rod Mill | 2.0-4.0 | 2,400-4,500 | 1,500-12,000 | ZG270-500 / ZG35CrMo | 400-900 |
| Cement Mill | 3.0-5.0 | 3,600-6,000 | 6,000-22,000 | ZG310-570 / ZG35CrMo | 700-1,100 |
| Raw Meal Mill | 3.5-5.5 | 4,200-6,500 | 8,000-28,000 | ZG310-570 | 800-1,200 |
Selection Quick Reference
- Small ball mills / rod mills (1.5-2.4 m diameter): ZG230-450 or ZG270-500 cast steel — sufficient strength for small mills up to 500 kW drive power, trunnion diameter 300-500 mm, cover weight 800-2,500 kg
- Medium ball mills / cement mills (2.4-3.5 m diameter): ZG270-500 or ZG310-570 — balanced strength and cost for medium-duty mills, trunnion diameter 500-800 mm with precision bearing seats
- Large ball mills / raw meal mills (3.5-5.0 m diameter): ZG310-570 or ZG35CrMo — high strength for heavy loads, trunnion diameter 800-1,200 mm, requires full NDT (UT/MT) inspection
- SAG mills (severe impact + fatigue, 7.0-12.2 m): ZG35CrMo or ZG42CrMo mandatory — Cr-Mo alloy provides fatigue resistance under cyclic loading from large ore impact, trunnion 1,200-2,000 mm
- AG mills (autogenous grinding, 6.0-10.0 m): ZG35CrMo or ZG40Cr — withstands high torque without grinding media impact, precision machined for girth gear alignment
- All mill types — feed end protection: Wear-resistant overlay (Mn13 plate or hardfacing) on feed opening — prevents abrasion from falling feed material, extends end cover service life
Certifications & Authorizations
Quality you can verify. Partners you can trust.
Custom OEM / ODM
From drawing to delivery — one-stop customization, no minimum order
Send Drawing
Upload your technical drawing (PDF, DWG, STEP, IGES) or share sample photos with dimensions
Engineering Review
Material recommendation, casting process design, DFM analysis — free quotation within 24 hours
Sampling & Test
Prototype production with full inspection: hardness test, spectrometer, dimensional check
Production & Ship
ISO 9001 certified. 15-25 days standard lead time. Global shipping with full documentation
Frequently Asked Questions
This is the single most dangerous mistake in mill component specification. End covers and liners serve completely different engineering functions — confusing them leads to catastrophic failure:
The end cover’s structural role:
- Supports the entire weight of the mill shell (20-200 tons) + grinding media charge (30-400 tons of balls/rods) + material being ground + dynamic impact loads from tumbling charge — all transferred through the end cover to the trunnion bearing.
- Subject to rotating bending fatigue: as the mill rotates (10-25 rpm), the load direction continuously changes. Over 25 years at 15 rpm = approximately 200 million load cycles. The end cover must resist crack initiation over this entire fatigue life.
- A failed end cover results in mill collapse — the entire rotating assembly drops onto the foundation. Cost: $1-5 million for mill replacement + 6-12 months lost production. No liner failure can cause this scale of damage.
Why Mn steel and high-Cr iron are dangerous for end covers:
- Mn steel (Mn13Cr2 etc.): As-cast toughness is excellent (80-200 J/cm²), but Mn steel has very poor fatigue strength — it work-hardens under cyclic loading, developing surface cracks that propagate. This is why Mn steel is used for liners (where cracking is acceptable) but never for structural castings.
- High-Cr cast iron (Cr15-Cr26): Extremely brittle under tensile stress. Impact toughness of 4-8 J/cm² vs 30-50 J/cm² required for end covers. A SAG mill end cover cast in high-Cr would shatter the first time the mill starts under load.
- Correct material: Medium carbon Cr-Mo steel (ZG42CrMo, ZG45CrMo) — these grades are specifically designed for thick-section structural castings with high fatigue strength (320-430 MPa at 10^7 cycles) and adequate toughness at sub-zero temperatures.
Bottom line: An end cover costs $8,000-80,000. A mill costs $500,000-5,000,000. Never, ever substitute material grades on an end cover to save money.
Mill end covers are the most quality-critical castings in any grinding circuit. ZHILI’s quality program is built around detecting the three defects that cause end cover failure:
Defect #1 — Internal porosity & shrinkage cavities (detected by UT):
- End covers are thick-section castings (40-250mm wall). Without proper riser design, the casting shrinks during solidification, creating internal cavities. These act as stress concentrators that initiate fatigue cracks.
- ZHILI process: FEA solidification simulation before casting to optimize riser placement and size. 100% ultrasonic testing (UT) at 50mm grid spacing across all critical sections — any indication >2mm FFH (flat-bottom hole equivalent) is rejected.
Defect #2 — Surface cracks (detected by MT):
- Hot tears during solidification or grinding cracks during machining create surface-breaking defects. Even a 3mm surface crack in the trunnion fillet radius (the highest stress zone) will propagate to failure within months.
- ZHILI process: 100% magnetic particle inspection (MT) on all surfaces after machining — especially trunnion fillets, bolt hole edges, and flange faces. Fluorescent wet method for highest sensitivity.
Defect #3 — Non-metallic inclusions & chemical segregation (verified by destructive testing):
- Each heat is poured with separately cast test blocks. Mechanical testing: tensile (yield + UTS + elongation), Charpy impact at -20 C, Brinell/Rockwell hardness. Chemical analysis by optical emission spectrometer.
- For SAG mill end covers, additional radiographic testing (RT) is performed on high-stress zones plus weld repair areas.
Documentation package for every ZHILI end cover:
- EN 10204 3.1 material certificate with full chemical composition and mechanical properties
- UT report with grid map showing all indications
- MT report with photographic documentation
- Dimensional inspection report (CMM on all machined surfaces)
- Heat treatment chart (time-temperature curve from furnace recorder)
- Weld repair map with procedure qualification record (if any repairs performed)
Third-party inspection by SGS, Bureau Veritas, or client’s own inspector is welcomed and encouraged. ZHILI has never had an end cover fail in service due to casting defects in 30+ years of manufacturing.
Yes. ZHILI has been reverse-engineering mill end covers for 30+ years. We produce exact-fit replacements for all major OEM brands. The process requires precision because three interface surfaces must match OEM exactly:
1. Shell flange interface — where the end cover bolts to the mill shell:
- Bolt circle diameter (+/-0.2mm tolerance across diameters up to 5 meters), bolt hole quantity and diameter (+H8 tolerance), flange face flatness (0.05mm/m), flange thickness and spigot diameter. A single misaligned bolt hole means the end cover cannot be installed.
2. Trunnion journal — where the end cover rides in the main bearing:
- Journal diameter tolerance: IT6 (h6 for bearing fit). Surface roughness Ra 0.8µm or better. Runout <0.02mm relative to the centerline. The journal surface is the bearing surface — any imperfection causes bearing overheating and failure.
3. Trunnion bore — where the feed spout or discharge cone passes through:
- Bore diameter, concentricity with journal OD (<0.05mm TIR), length, and any internal liners or wear protection features must match.
ZHILI reverse-engineering workflow:
- Option A — OEM drawing available: We cast to the drawing with our material grade recommendation. 6-8 week delivery.
- Option B — Worn end cover available for measurement: We 3D laser scan the existing end cover to capture all interface geometries, then reproduce in CAD with wear allowance restored. 8-10 week delivery.
- Option C — Mill on-site measurement: Our engineer visits your plant to measure the shell flange bolt circle, trunnion bearing housing, and other critical interfaces. From these measurements we build the CAD model. 10-12 week delivery.
Material upgrade recommendation: If your existing OEM end cover is ZG35CrMo, we often recommend upgrading to ZG42CrMo for the replacement — this provides 10-15% higher fatigue strength for minimal cost increase (~10%). If you’ve experienced end cover issues, upgrading one material grade is the single most effective improvement you can make.
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