The hoisting system is the heart of underground mines

The hoisting system is the heart of the business for underground mines; if the hoist malfunctions, production comes to a halt; therefore simplicity of maintenance is as crucial as operational reliability.

When a mine is designed the hoisting system is designed with a capacity exceeding the mine's expected throughput. The lifespan of the mine is however often decades with plenty of parameters changing over the years from ore quality to the depth of the mine; even on a daily basis there are variations in throughput. The daily average very rarely is linear. This means that the designed capacity of the hoisting system might meet the average throughput, but not the peaks. A hoist completing one cycle is lifting one skip from 900 metres up to ground level in 120 seconds; if the underground mining production exceeds the hoist capacity by 5% for a peak lasting a couple of hours then a bottleneck is eminent at the hoist system.

If it is repetitive and mundane; simplify!

While the majority of underground mines are operated with a traditional scroll discharge system for skips, the shaft system of new mines is designed with far more efficient hydraulic or pneumatic discharge mechanisms.

Mechanical design engineer in FLSmidth, Francois Koekemoer, explains that it was the frustration of looking at the various types of solutions that are currently on the market that gave him the initial impetus to develop a far simpler alternative: "Two years ago I was looking at a hydraulic design for a hoisting system at a platinum mine in South Africa - it just seemed too complex and costly to convert electrical energy via hydraulics into linear motion for such a mundane repetitive task of opening and closing a skip door; a large oil reservoir is required and a comprehensive list of multiple control valves, accumulators and coolers all add up to specialized maintenance, fire hazards and increased capital expense. Surely it must be possible to design a more cost effective solution, I thought. Initially I went for a rack and pinion gear design that led to a crank motion concept that would be ideally suited. I then sketched out some concepts and when I had the Crank Type Dump Mechanism (CTDM) I knew it was solid," Koekemoer explains: "The velocity and torque profiles generated by the crank motion are perfect for this particular application, where the opening and closing of a skip is required. Where possible the design makes use of standard components, such as motor and gearbox, which are off the shelf and not bespoke."

Payback of less than 6 months

The originality of the CTDM is the system’s operational simplicity. "The scroll type is super robust and requires almost zero maintenance but is very inefficient with regards to the hoist production cycle. When replacing a scroll option with the CTDM we expect a production increase of 10-15% depending on the current hoisting cycle. The hydraulic type is more efficient than the scroll type, but not as robust due to the complex control and sensitive associated hydraulic components. The CTDM, however, delivers the same efficiency as the hydraulic system, but it is robust and requires very little and very simple maintenance," says Koekemoer.

The CTDM technology is a simple mechanical system that comprises a gearbox assembly, and swing arms; all well-known components. The system does not require speed control as the crank motion naturally produces acceleration, constant speed and deceleration through an opening or closing cycle.

"The CTDM is significantly cheaper in total cost of ownership than the hydraulic system; the low level of complexity makes it possible for the operators to maintain the CTDM themselves without the need for specialists. Also the CTDM only uses power when actuated, this compared to a hydraulic unit which has pumps running continuously to charge accumulators and therefore generating heat," says Koekemoer. Both as a retrofit or as a new installation the CTDM can be custom-designed to accommodate the type of skip used as well as shaft specific parameters. The payback for a retrofitted CTDM can be in as little as six months.