• MINING INDABA
SAFETY CHALLENGE IN USE OF DOUBLE DRUM SCRAPER WINCHES
By Philip Coetzer , Head , SHER ( Safety , Health , Environment , Risk ) at Royal Bafokeng Platinum
In South Africa we still have many conventional gold and platinum mines where double drum scraper winches are used extensively , this is a necessity currently and it comes with many challenges .
The South African gold and platinum orebodies are mostly narrow and dip from horizontal to vertical . Where mining happens at a reef dipping angle where the blasted rock cannot be moved by a throw blast or gravity , it must be moved by means of a mechanical system from the blasted faces . This is the mining environment where most of the double drum scraper winches are used in South Africa .
The stoping width on conventional mines typically ranges from 70cm to 150cm . This is too narrow to apply a mechanised mining system for the stoping sections , as the minimum stoping width for mechanised mining with TMM drill machines and Load Haul Dumpers is around 150cm . Many conventional mines therefore use double drum scraper winches to move the broken rock , particularly from the reef horizon to a point where it can be moved by gravity , and then trammed by means of rail-bound equipment or conveyor belts to the stations , where it is hoisted or conveyed out of the mine .
Double drum winches in operation The double drum winch is a piece of machinery with two drums on either side , attached to a centre gearbox , which is fitted to a frame . During the installation of a winch , this frame is pinned onto the foundation with anchor bolts . This secures the winch at the point from which it will be working . The winch drums fitted to the frame are driven by a shaft with a planetary gear that drives sun gears , which turn the side shafts . In most cases , this main shaft is energised by means of an electric motor . The shafts connected to the sun gears enter the drums and are fitted at the end with a brake system arrangement . When it is activated by the opening of the friction pads , it locks the drum to turn along with the rotating shaft . Different brake calliper activation systems are available – hydraulic activation systems , compressed air systems or mechanical cam systems . A winch is a robust minimal maintenance and low-cost piece of machinery . Without double drum winches , high volume conventional narrow reef mining in its current form is impossible .
As part of the winch installation process , there are steel ropes that are coiled onto the drums in an underlay configuration . When the drum is locked and it turns , the scraper rope coils onto the turning drum . One drum will pull the scraper / s towards the winch position and the other will pull the scraper / s away from the winch position when activated . At the end of the pull rope ( steel scraper rope ) a scraper / s is / are attached with shackles . This allows the scraper / s to be pulled towards the winch when the operator activates the drum . The other drum also has a scraper rope that is coiled onto it . During the installation process this rope is pulled to the farthest point , where the scraper must return . At this point the scraper rope is placed over a return pulley ( snatchblock ). This snatchblock
26 • African Mining • January 2023 is attached to sling eyebolts that are secured to the rock in holes drilled for this purpose . The sling eyebolts are secured in the hole by placing a wedge into the hole , which will lock the eyebolt securely . This rope , now running over the snatchblock , is then attached to the back of the scraper . When the winch operator activates the drum to turn , it pulls the scraper away from the winch towards the snatchblock . The snatchblock in a gully is typically installed just beyond the tipping point of another scraper system . The scraper is then pulled over the broken rock and when pulled towards the winch it brings the broken rock along . After a blast , the blasted rock creates a muck pile in the stope panel . The scraper runs over this muck pile in the stope panel face and pulls the rock to the tipping point . The operation of the winch is done by activating the one friction drum and releasing the other . In doing this sequentially , the operator moves the scraper / s either towards the winch or back from the winch , with the one rope coiling up while the other is uncoiling .
Double drum winches come in different sizes , with different pulling strengths . Typically in the South African mining industry , 37kW , 55kW or 75kW winches are used . There are exceptional cases where even 100kW winches are installed , especially in very long pull gullies where the return wheel is placed in a stationary position as a permanent installation . For normal mining operations , the return wheel for the return rope is moved daily as the face advances . The scraper winch installation in a conventional stope is installed to pull the broken rock / blasted rock directly from the working face to a gully . Thus , during blasting , the scraper , return wheel and rope must be removed to a place where it will not be damaged by the blast . After the blast , they are replaced to scrape the broken rock from the mining face to the gully of the face . From here it then gets pulled by another scraper winch to the tipping point . Depending on the mining layout , a tipping point can be positioned at the point where there is an ore pass , directly in front of the winch , where the rock then gets scraped directly into an ore pass and dropped down to a lower level by gravity . From here it is drawn in a controlled manner into hoppers that transport it to the main station . In some mining layouts , the ore pass entry points are situated at points not directly in line with the gully , resulting in a requirement to install another winch to move the broken rock up or down to the dedicated ore pass position . This is typically called a centre gully .
The challenge Although installing all these winches , ropes , scrapers and return wheels is technically not difficult , sketching what is happening in the background is important in order to understand the complexity of what is happening in a working stope , gully and centre raise arrangement where double drum scraper winches are installed .
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