Print5.2.5: Maintenance
The level of maintenance required to keep the equipment operating properly and safely varies by the mining method and the particular equipment in use. Usually, the idea is to minimize breakdowns during a production shift and the need for emergency maintenance. Instead, preventive maintenance is practiced to reduce failures, and scheduled maintenance is planned for nonproduction shifts. In some mines, there will be two eight-hour production shifts followed by an eight-hour maintenance shift every day. In other mines, there will be two ten-hour production shifts, followed by a four-hour maintenance shift. And in yet other mines, a week of production will be followed with a weekend of maintenance.
The value of a minute is often underestimated by those just learning about the industry. Given the capital cost of mining equipment, and the cost of labor and so forth, a delay can easily cost hundreds of dollars per minute and exceed a thousand dollars a minute in certain circumstances. As such, maintenance is a critical auxiliary operation.
There is an important metric that is used to keep track of how well the mine is doing with its maintenance. The metric is known as availability, which is defined as the amount of time that the equipment was usable divided by the amount of time that we had planned to use it. For example, suppose that we planned to install roof bolts for six hours yesterday. A hydraulic hose ruptured, and it took 30 minutes to obtain and replace the hose. The availability of the roof bolter yesterday is: Instinctively you may feel that this is a pretty good number. After all, if it were an exam, you would have gotten an “A”! Let’s examine this a bit more closely.
If we have a production system, we need all pieces of equipment functioning. Otherwise, a delay in one piece, like the roof bolter, will prevent other equipment from moving into that workplace. As a result, the other equipment will be delayed as well. Mathematically it can be shown that the availability of a system of equipment is equal to the product of the individual availabilities of the machines. Let’s imagine that we have the following group of equipment in our production system, along with the availabilities as shown. What is the availability of this production face?
- Jumbo drill 90%
- Scaler 96%
- Rock Bolter 92%
- Wheeled loader 92%
- Mine truck #1 88%
- Mine truck #2 91%
- Feeder-breaker 98%
- Conveyor system 94%
Can you believe it? 54%! In other words, at this production face, they are mining only half the shift! And if you really want to have an anxiety attack, let me tell you that we haven’t accounted for other mining delays – unexpected rock falls or an explosives misfire, for example! We can drop down close to 40% overall availability if we aren’t careful. Clearly, there’s room for improvement – better engineering, better maintenance, and so on!