Why 'Run-to-Failure' is Costing Ghanaian Mines Millions
Charles Duah
Lead Reliability Engineer & Principal Consultant
At open-pit gold operations across the Tarkwaian and Birimian belts (Tarkwa, Obuasi, Ahafo, Damang, and Chirano), Heavy Mobile Equipment (HME) operates on steep haul profiles in high-silica laterite dust and heavy seasonal rainfall. Under these duty cycles, OEM handbook intervals written for North American or Australian pits rarely match actual component survival curves.
When maintenance planners do not recalculate Planned Component Replacement (PCR) intervals from local life data, fleets default to run-to-failure. On a CAT 785C/793D haul truck or hydraulic face shovel, running a wear-out component to functional failure turns a scheduled 16-hour bay swap into a multi-week outage waiting on sea freight through Tema or Takoradi port.
The Unit Economics of Unplanned HME Failures
When a primary loading unit or haul truck fails mid-shift, the direct repair invoice is usually the smallest cost component:
- Lost Haulage & Mill Feed: A downed CAT 785/793 haul truck or shovel costs $3,500 to $6,500 per hour in deferred ore movement. If a primary crusher or SAG/ball mill comminution circuit is starved or trips offline, losses reach $60,000 to $120,000 per hour.
- Emergency Air-Freight Premiums: Standard sea-freight replenishment through Tema or Takoradi port takes 6 to 16 weeks. Air-freighting a final drive or swing gearbox from Europe or the US adds 3x to 5x to the landed component cost.
- Secondary Component Destruction: Replacing a planetary pinion bearing proactively at its B10 life costs under $4,500 in parts and seals. Running that bearing to spalling failure liberates steel debris through the sun gear and ring gear, destroying a $95,000 final drive assembly.
Weibull Parameter (β, η) & B10 Preventive Replacement Table
In ReliaSoft Weibull++, we fit 2-parameter Weibull distributions to censored SMU-hour histories extracted from SAP PM and Maximo work orders. When the shape parameter β > 1.0, the failure rate increases with age, justifying a scheduled B10 replacement (the SMU age at which 10% of the population is expected to fail) coordinated with port lead times:
| HME Subsystem | β (Shape) | η (Char. Life) | B10 Life | Maintenance & Spares Task (6–16 Wk Tema Lead Time) |
|---|---|---|---|---|
| CAT 785/793 Final Drive Planetary Gear | 3.4 (Wear-out) | 18,500 hrs | 9,550 hrs | Scheduled PCR at 9,500 SMU hrs; pre-order planetary carrier 14 weeks ahead via Tema port; ISO 4406 oil particle count every 250 hrs. |
| Hydraulic Shovel Main Piston Pump (PC3000 / R9250) | 2.8 (Fatigue / Abrasion) | 12,200 hrs | 5,460 hrs | Case-drain flow & pump discharge pressure trending every 500 hrs; hard-time changeout at 5,500 hrs; retain 1 rotatable spare on site. |
| Twin-Turbocharger Cartridge (3516B / QSK60) | 2.3 (Thermal Cycling) | 9,800 hrs | 3,680 hrs | Preventive cartridge swap at mid-life top-end overhaul (3,500–4,000 hrs); inspect intake piping for laterite silica ingress. |
| Dump Body Hoist Cylinder Rod & Seal Assembly | 1.6 (Early / Mixed Wear) | 7,400 hrs | 1,820 hrs | Condition-based inspection of rod chrome scoring and wiper seals at 500-hr PM; install bellows boots during harmattan dust season. |
Deploying RCM-II and Condition Monitoring on Site
Preventing in-pit breakdowns requires coupling Weibull B10 hard-time limits with condition-based P-F interval tracking. Across West African open-pit fleets, structuring RCM-II (SAE JA1011) task lists around actual failure modes reduces unplanned HME downtime by 25% to 40% within 12 months.
Field Implementation Sequence (8 to 12 Weeks):
- Pareto & Crow-AMSAA Bad-Actor Screen: Pull 24 months of SAP PM or Maximo downtime notifications to isolate the 15% of subsystem codes driving 80% of fleet availability loss.
- Right-Censored Weibull Fitting: Separate true functional failures from preventative changeouts (suspensions) to compute accurate β and η parameters for each subsystem.
- P-F Interval Alignment: Set ICP-AES oil wear-metal sampling (Fe, Cu, Cr, Si) and vibration routes at less than half the minimum P-F interval so degraded components are flagged before catastrophic seizure.
- Min/Max Buffer Sizing in BlockSim: Calculate rotatable spare stock levels using local B10 demand rates and 6 to 16 week Tema port customs clearance distributions.
Gold mines cannot control LME or LBMA spot prices, nor can they eliminate port clearance delays. They can control whether a final drive is swapped in a planned 16-hour maintenance window or after a catastrophic in-pit failure.