Hoisting consistency

As the costs of mining creep ever upwards miners are increasingly recognising the bene­fits of predictive maintenance. 
Because in this capital intensive industry, consistent uptimes, that is the equipment working when it is meant to be working, is key to profitability in these slimmer times. 
One of the commonly used maintenance and monitoring techniques is shock pulse measurement and vibration analysis. 
Both are widely used for monitoring the ongoing condition of industrial equipment, as the two work together to provide a reliable solution for predictive maintenance of mining equipment.  
At its Renstrom mine, Swedish miner Boliden recently implemented  high definition shock pulse measurement technology in conjunction with vibration analysis to monitor the mechanical condition of its multi-rope friction hoist, which was recently converted from a tower-mounted setup into a ground-mounted  hoist system. 
The shock pulse technology is useful for detection of bearing damages in industrial environments, while vibration technology is optimal for low frequency related fault conditions like unbalance, loose gears, and misalignment. 
Hoist performance is vital to underground mining operations, and the ongoing tough operating conditions and high availability requirements make it critical to constantly know the mechanical condition of the hoist at all times. 
The costs of hoist failure or having one out of commission due to unexpected downtimes are well known. 
In 2009 a hoist failure at BHP's Olympic Dam mine caused a skip loaded with ore to fall down its primary Clark Shaft, which brought the majority of the operation's ore to the surface. 
As the loaded skip fell it caused another on a linked cable to fly up the shaft, damaging the headframe. 
BHP put the accident down to a failure of its computer braking system. 
At the time a BHP spokesperson, Kelly Quirke, told Australian Mining that "detailed investigations by the company, with a third party independent expert, have found that a fault in the logic of the braking system was the root cause of the accident," she said. 
"The fault prevented the system's breaking mechanism from engaging fully, which in turn allowed the hoisting system to freefall to the bottom of the shaft." 
This single incident, due to one hoist failing, saw production fall by 75 per cent at the shaft, and cost the company close to $200 million in 'idle capacity costs', and forced it to use its secondary Whenan shaft. 
Constant condition monitoring is crucial. 
To ensure peak performance at its own Renstrom mine, Boliden installed an online conditioning monitoring system which provides real time condition based information on the status of its critical components. 
Mats Johansson, the mainten­ance manager at Renstrom, explain­ed that "the mine hoist is critical equipment for us and the heart of the business; if the hoist malfunctions production more of less comes to a halt". 
"On this type of equipment we need good control and that makes online monitoring necessary." 
The site's hoist is made of up a frequency controlled motor, a two-stage gearbox, a drum, the wires, and two skips. 
The skips themselves are mounted at the tail end of the wires with a lifting capacity of around five tonnes each. 
It has a 400kW ABB motor, with gearbox that has a ratio of 13.609:1 
When it was installed last year Intellinova Compact, with SPM HD shock pulse measurement technology, was implemented during the commissioning phase to provide a baseline from the new hoist. 
Fifteen transducers are used to cover the motor, gearbox, and drum; ten shocks pulse transducers to monitor bearing condition, and four vibration transducers to detect low frequency vibrations such as gear mesh frequencies in the gearbox or unbalance in the drum. 
Additionally, RPM is measured on the motor drive shaft at one pulse per revolution. 
Using shock pulse transducers with the SPM HD technology provides a capability to detect bearing conditions.  
The multiple gear-mesh frequencies in a gearbox significantly affect normal vibration transducers, making the spectrum and overall values very hard to interpret. The shock pulse transducer is not affected by normal mesh frequencies – as long as the gear teeth are in good condition and no impacts are emitted – however is there are any irregularities in the contact surfaces between the gearbox teeth it will trigger an immediate response with easily interpreted readings. 

Measuring maintenance 

One of the problems of setting up predictive maintenance programs for hoists is the limited measurement 'window' created by the hoist's cycle times. 
In shallow mines short cycles may be a challenge in terms of very limited measuring times, reducing the potential to create accurate measurements, however this was not a problem at the Renstrom mine. 
 For example a hoist completing one cycle – lifting one slip from 900 metres up to ground level – in 120 seconds. 
It accelerates quickly and the drum reaches a stable 51.3 RPM. Based on the 120 second cycle time at 51 RPM, one hoist cycle equals 102 revolutions of the drum.  
Selecting a measuring time of 20 revolutions for all measuring points on the slowest shaft ensures enough readings without using triggered measurements. 
Even if some measurements are lost if the window happens to hit during a rapidly changing RPM due to acceleration or slowing, most of the readings will be taken during stable RPM conditions. 
The parameters measured and trended are HDm for all shock pulse measurements and vibration velocity RMS.  
The measurement interval is set to thirty minutes for each measuring point. The long cycle time of 120 seconds gives plenty of time to measure, resulting in a number of readings per day on all measuring points.  
During the first six months of measurement at Renstrom – from July 2013 to January 2014 – a number of observations were made, such as readings showing that gearbox data provided by the supplier was incorrect – as the readings did not match the expected results according to the gearbox specifications. 
This, on top of other data such as system performance, has provided full control of mission critical application. 
According to Boliden "shock pulse monitoring integrates perfectly with vibration analysis, providing an ideal  reliability solution for all types of hoist monitoring systems as well as other critical mining equipment".

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