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The process of scraper winch pull force measurement and shows the results of the actual winch pull force in extreme stall situations as well as during the face and gulley cleaning. It was found that by slowing down the motor and causing it to slip, torque up to 3.5 times higher than the nominal can be developed, resulting in the same increase of winch pull force.
On the other hand, measurements of the actual force needed for cleaning of the face and the gully have shown that the currently used winches are too powerful. The extra torque that can be developed by the already too powerful winch motor poses a significant danger and necessitates the implementation of measures that will eliminate this risk.
Scraper winches in operation
The scraping of ore and moving it to the ore pass at all Anglo Platinum conventional mining operations is done by using 37 kW and 55 kW double drum single speed electric scraper winches.
A 37 kW winch and one scoop are used for cleaning the face, whereas the material in the ASG and centre gulley is moved by a 55 kW scraper winch with two scoops in tandem.
Winch specifications
There are many different versions of scraper winch machines in everyday use, but models derived from the old Joy-Sullivan design are the most common. This design has two drums and an electric motor in an in-line arrangement.
The motor drives the pull drum via a gearbox. The change of direction of the pull is done by applying a breaking force via clutch pads onto the surface of drums. The pull force and velocity vary with the effective drum diameter. These parameters are continually changing with the change of the amount of rope wound on the drum. When the rope velocity is the highest, the pull force is the lowest and vice versa.
Winch stall and AC motor behaviour
Scraper winches are powered by low voltage squirrel-cage alternating current electric motors. These machines are purposely designed and built to operate at larger slip angles, due to the variable loading that they experience during the cleaning/ore scraping process.
This is in contrast to the relatively stationary operating regimes of conventional motors that are used to power various pump or fan applications.
The winch motors operating point is, however, dependent on the load, constantly moving between points 1 and 2 on the torque curve. This means that, in extreme situations and for a relatively small amount of slip, the motor torque far exceeds the torque at its full load. This is experienced and interpreted by a winch driver as the winch having sufficient power to pull the scoop’ even when it is jammed somewhere or gets hooked onto the uneven footwall.
Gold Fields test
In an attempt to experimentally determine the winch pull forces delivered by 37 kW and 55 kW winches, Gold Fields, together with COMRO and two suppliers conducted tests in May 19933. Two winches were rigged up in the workshop of one of the suppliers as per the sketch in the Figure 2, with the load cell fitted in line with the rope.
Both winches were then subjected to breaking (stopping the pull drum) and the resulting pull force was measured.
The profile of measured winch force changes was recorded on an analogue recorder. The measurement system was calibrated with the accuracy of the measurement system found to be within 5%4. In conjunction with force measurements, during the test the currents that were drawn by the motors were also recorded.
The measurement results are shown in the Table II.
The Gold Fields report concludes that measured winch pull forces are substantially higher than those quoted by the manufacturer (113 kN to 170 kN vs. 35 kN and 193 kN to 228 kN vs. 69 kN). It was then recommended that users check their standard practices to ensure that ancillary equipment such as snatch blocks, pulleys, rope and anchors, has adequate strength.
Under certain circumstances, scraper winches that are used during face and gulley scraping have the potential to develop much higher pull forces than the rigging system, in particular the rope, can withstand.
On the other hand, it was found that the required pull forces measured during actual face and gulley scraping are lower than what the scraper winches can provide. This creates an unsafe situation, which may or already have resulted in an injury or even a fatality.
It is therefore necessary to make an attempt to either limit the winch motor torque and/or use less powerful winch motors.
Due to the slow reaction time of conventional motor overload and thermal overload devices, these are not effective methods of torque limiting. Therefore, downsizing of motor power would be the immediate recommended course of action.