hydrostatic bearings in ball mills and sag mills

Hydrostatic bearings play a crucial role in large rotating equipment like ball mills and SAG (Semi-Autogenous Grinding) mills, providing superior load support, reduced friction, and enhanced reliability compared to conventional hydrodynamic bearings. Here’s how they are applied and their benefits:

1. Role of Hydrostatic Bearings in Mills
Hydrostatic bearings support the mill’s trunnion (hollow shaft) by maintaining a high-pressure oil film between the bearing surface and the journal (rotating part). Unlike hydrodynamic bearings (which rely on shaft rotation to generate pressure), hydrostatic systems use external pumps to create an oil film even at low speeds or standstill.

# Key Advantages:
– Eliminate Metal-to-Metal Contact: Prevents wear during startup/shutdown when hydrodynamic lubrication fails.
– Higher Load Capacity: Supports extreme weights (e.g., large-diameter SAG mills can weigh hundreds of tons).
– Stiffness & Stability: Maintains precise alignment under dynamic loads (ore impacts, uneven grinding).
– Lower Friction Losses: Reduces energy consumption compared to sliding-contact bearings.
– Extended Lifespan: Minimizes wear, reducing maintenance downtime.

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2. Design Considerations
# (a) Oil Supply System
– High-pressure pumps (~100–300 bar) deliver oil through recessed pockets in the bearing pads.
– Flow control valves ensure even pressure distribution across all pads.

# (b) Bearing Geometry
– Multiple segmented pads surround the trunnion for uniform load distribution.
– Tilting pad designs compensate for misalignment.

# (c) Safety Systems
– Redundant pumps and filters prevent failure.
– Temperature/pressure sensors trigger alarms if oil film integrity is compromised.

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3. Comparison with Hydrodynamic Bearings
| Feature | Hydrostatic Bearing | Hydrodynamic Bearing |
|———————–|—————————————-|————————————-|
| Oil Film Formation | External pumps (works at zero speed) | Requires rotation (~10–15% of rated speed) |
| Startup/Shutdown | No wear | Risk of boundary lubrication damage |
| Load Capacity | Higher | Limited by speed-dependent pressure |
| Complexity/Cost | Higher initial cost | Simpler, lower cost |

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4. Applications in Ball Mills & SAG Mills
– Large-Diameter Mills (>8m): Hydrostatic bearings