Slack and tight rope monitoring in mine shaft conveyances

Reliable rope monitoring, improved shaft conveyance safety

The implementation of tight and slack rope systems is tried and tested preventative approach which reduces the chance of a rope break in shaft conveyance. However, vital factors have to be followed in the procurement of tight and slack rope solutions, as safety in shaft conveyances is paramount, the engineering team at GST Guduza advises industry. South African mines are the deepest in the world and continue getting deeper. The gold mining sector, in particular faces challenges of conveying people, equipment and rock up and down shafts with depths close to four kilometres. To maximise productivity, high speed conveyances are used, which travel at 15 metres per second (54 km/h). However, this, combined with the 24/7 nature of mining operations, means that the risk of serious incidents is dramatically increased so much so that the law for slack rope monitoring, was promulgated. That is why mines have to be well-equipped with proven slack and tight rope monitoring solutions.

Problems experienced in the shaft

The risk of serious accidents caused by rope failure in shafts should not be underestimated.
One of typical problems experienced in a shaft as the conveyance descends and ascends, is the possibility of colliding with an obstruction in the shaft, bringing the conveyance to an abrupt halt. A stop in the downward direction could cause the heavy steel rope to coil onto the roof of conveyance, overloading it to the point of break-away, causing it to fall free. In the upward direction, the jamming could cause the rope to stretch and possibly brake.
Other issues that have been reported are the shaft guides being offset or broken due to seismic movement. Problems also have even been experienced during the offloading in the region of the camel back, causing a slack rope condition. Furthermore, conveyance doors have been known not to be closed properly, and thus getting stuck on the steel structure, once again causing a tight/slack rope condition.

Various technologies used in tight and slack rope monitoring

In the past, various solutions have been implemented tight and slack rope monitoring which are worth analysing.
One of the solutions is a plate that is attached to the steel rope. The rope is attached in a way that in the event of a slack rope condition, it will bend the plate causing a switch to be depressed, sending a signal to surface to stop the winder.
In the past two and a half decades, more advanced solutions have been designed to cater for slack slope problem using electronic technologies.

The typical point-to-point radio communication between surface and the cage is existing surface technology. Although this works well above ground when the points are fixed, can cause frequency scatter and standing wave when moving down a shaft and cause dead spots, and therefore a communication possible glitch, which is a “no no” for safety critical equipment.

To avoid this situation, the conveyance’s steel rope is used to communicate this information from the cage to surface. This is done by mounting loadcells on the top of the conveyance measuring the actual weight of the conveyance, and with the software, calculate the warning and tripping points of a pending tight/slack rope condition. This information is then reliably sent to surface via the steel rope to the mine’s winder safety circuit. The winder is then brought to a halt in a controlled fashion before it can cause extensive damage. Other technologies have come onto the market using the indirect method of measuring the load of the conveyance and using software to determine tight/slack rope trips.

Tight/slack rope system with added functionality

The other danger that can occur with the moving conveyance is with the start stopping of the conveyance by a hoist driver who is situated in the driver’s cabin above ground, being “blind” to the actual happening on the conveyance itself. In South Africa at present, electronic signaling (bells) is used to instruct the conveyance driver when to start and stop the conveyance during its travels through the shaft. The Guduza system offers a solution for this.
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And so, in order to manage the risk during embarking/disembarking, at the different levels, additional functionality has been made available in the tight/slack rope system, monitoring gates, booms, clutch and breaks and then to interlock this information with the mine’s safety circuit.

Factors to consider in choice of a solution

Safety is the operative word in the mining industry, specifically in the shaft environment. Thus, the role of electronic equipment in enhancing safety is critical. However, equipment itself can fail, when least expected, causing a potentially dangerous situation. False security meaning that the system has become defective and is not reported as being faulty, leaves the driver/supervisor unaware of the potential safety risk. Guduza provides a safety function by providing a carrier signal travelling to surface, which when broken will automatically stop the conveyance allowing for an investigation to take place.
So, the following are the main critical factors to be considered when looking for a slack and tight rope monitoring solution:

Track record of the design/integration company
Thus, the equipment should be designed for the highest safety levels. In view of this, the track record of the design/integration company should be considered when searching for a solution. Guduza is a company that has a track record in the design of tight and slack rope equipment as well as field experience for 2,5 decades. See advertisement, Guduza’s shaft sheriff (CCS 3000).
The electronic coded key
Another safety consideration to be taken into account with tight and slack rope systems is the situation after a trip, where the cage must remain in that position until an investigation has taken place. This means that the driver must not move the conveyance until the investigation has been carried out.
To manage this situation, a senior person, for instance, the supervisor or engineer can then reset the system. These levels of security should be in place when reviewing possible systems. The electronic coded key, is the ideal way to go, not allowing any duplication of keys apart from at the factory.
Logging of events
Logging of events is crucial for the investigation of any accident and should be taken into consideration when searching for a solution. Another logging feature that is advantageous is the accelerometer, monitoring the vibration in the X, Y and Z axes and logging the data on a real time basis. It also provides predictive information regarding the movement of the guides and thus can prepare in advance of a pending problem.
Safety vs Productivity
Insuring safety in mines is a large cost factor, however, if the same safety equipment can be used for production monitoring purposes as well, it becomes very cost effective. The information used for safety, tight and slack rope systems can be used for productivity purposes, for example, driver evaluation, counting of loads coming to surface, the speed per trip and more.

Tight and slack rope monitoring systems to gain traction

The use of automatic conveyor systems (operate like and elevator in a high rise building) in the mining industry, a few of which has been operating in South Africa for more than two decades, should start gaining traction in the next five to ten years.
There is no question that the implementation of tight and slack rope systems is a tried and tested preventative approach which reduces the chance of a rope break. However, it has to be acknowledged that rope breaks can still occur. And to manage this eventuality, The E-FAS (Emergency Fall Arrest System) has been designed to arrest the falling cage, which, thus far, has been used at various mines around the world over the past 8 years.