The hoisting article in the August issue of International Mining magazine notes the availability of ABB’s new service product, Hoist Performance Fingerprint, to help its customers assess and optimise the condition of mine hoists. Here ABB’s Slobodan Vidmar offers some top tips to ensure mine hoists remain as effective as the day they were installed
“A mine hoist is the most critical machine in an underground mine, taking people and material below ground quickly and safely. It must work reliably at all times, while operating at maximum efficiency with optimum capital and maintenance cost.
“Whether your mine hoist is friction, single- or double-drum, keeping it up and running requires the skills of electrical and mechanical engineers. Electrical considerations include aging hoists suffering speed drift, deteriorating control components such as capacitors and thryistors and protection devices not working. Mechanical considerations include the brake unit’s air gap being bigger than recommended – for certain brake types, an air gap increase from 2 mm to 4 mm results in 30 percent lower brake force.
“With over 3,000 installations around the world, there are very few catastrophic accidents considering how many hours these machines work. Their safety record is impeccable – but mine hoists do break down, become slower, suffer erratic rides and use more energy than the day they were first commissioned.
“While poor maintenance can be blamed, it is normally due to a lack of knowledge of how the electrical and mechanical systems integrate. This is further exacerbated by an increasing shortage of on-site skilled employees capable of coping with the new, sophisticated technologies.
“For example, current maintenance practices generally do a good job of looking after separate devices. Rarely, however, is the “cause and effect” of the entire system considered. And yet, this is essential. Electrical issues affect mechanical and vice versa and treating one in isolation could introduce other problems.
“The starting point of any mine hoist appraisal is to understand what you are trying to achieve.
“The biggest concern is failures that do not reveal themselves unless tests are carried out – or a failure actually happens.
“Consider a mine hoist with 18 brake units. Standard tests used today normally show that the overall system satisfies local mining regulations. What they fail to reveal is which brake units are working harder than others and which, therefore, are suffering a decrease in operating life. What is needed are tests on each individual brake unit.
“A mine hoist is a hugely complex machine, spread over a large area often having a 50 t payload and travelling at 60 km/h. As such, some areas are not inspected regularly, leading to concerns over the hoist’s protection devices. Any failure is likely to be serious.
“It is not unusual for end-of-line protection devices to go undetected, primarily because checking those means disabling all the fail-safe devices that go before within the chain, which can be a time-consuming and daunting experience. Are you sure the final protection device is going to do what it’s designed for? By testing it, we are proving it does work.
“A common cause of poor performance is man-made. Often, during troubleshooting, in an attempt to adjust the duty cycle, the hoist’s production cycle will be slowed until the problem is fixed. “However, troubleshooting takes time and the maintenance crews are replaced by another shift. As an oversight, the production cycle may stay at this reduced rate and consequently, over time, may result in inevitable production losses. In mining, small delays in duty cycle can create a loss of production in excess of hundreds of thousands of dollars per year.
“Mine hoist reliability can be improved by regular assessment and testing to prove the hoist is running at its optimum level.
“Among the most common causes of unplanned downtime includes a build-up of dust and dirt under the switches and a failure of latch arms on the skips for the same reason. On the friction hoist’s production cycle, changes and over-wind trips are often the result of excessive friction insert wear.
“The cost of breakdown repair is up to seven times more expensive than planned repair. Planned maintenance should include regular inspections, assessments and testing to verify the reliability of the protective devices.
“A good starting point is to have a structured audit of the entire hoist system, covering electrical and mechanical components. Data is collected from up to 20 pre-defined points. These are various control signals used for analysis to determine hoist performance and include speed, torque, brake pressure and current
“This will identify ways to optimise the hoist’s condition, performance and safety. It will also identify abnormal conditions, which, if not corrected, could cause severe production problems or permanently damage the equipment.
“A comprehensive written report can be sent to mine inspectors as proof of compliance, as well as for maintenance planning and budgeting, particularly with respect to setting aside funds for expensive repairs.”
Hoist Performance Fingerprint (HPFP) consists of a structured audit of complete hoist systems – simultaneously covering all electrical and mechanical parts to ensure reliable and safe hoist operation.
“By using HPFP, hoist owners are now able to reduce the operational cost and at the same time increase production and business sustainability. Launched first in Australia, a majority of hoist sites have subscribed to the service – with some even opting for quarterly contracts.”