By their design, hydraulic components are explosion-proof, making the powerfully dense technology a natural choice in dangerous mining applications.
By Josh Cosford, Contributing Editor
Hydraulics in mobile mining machinery is unique. Just as any intelligently designed hydraulic circuit must consider efficiency, performance and reliability, so too must the hydraulics within the mining industry. However, the consideration for safety in mining is often the top priority, and therein lies the unique nature of hydraulics in this industry.
Mining as a profession makes any top 10 list describing the most dangerous in the world. Some of the top dangers in mining are unrelated to machinery, such as dust, vibration and noise, but mining is a machinery dependent industry, and a great majority of those machines are hydraulically powered. So, what makes mobile hydraulics in mining machinery different from construction or forestry?
Researchers at the National Institute for Occupational Safety and Health in conducted their study, Machine-related injuries in the US mining industry and priorities for safety research1, and found machinery related accidents accounted for 41% of all severe accidents in mining operations, in regards to being struck by or caught in the machine.
Although being struck by a vehicle such as a dump truck that happens to be equipped with hydraulics is unfortunate, it often is not the hydraulics in itself that causes the harm; the dangers related to machinery occur on so many levels. Hydraulic machine-related accidents occurred in high volume with conveyors, haulage, milling, dumping (from both loaders and shovels), tunnel-boring, crushing and forklifts. The danger potential comes from so many directions in mining.
Aside from injury related to mostly hydraulically powered machinery, explosion-related injury and death have played an unfortunate role in mining’s history, especially with coal. The worst year on record was 1907, when more than 3,200 deaths were attributed to explosions. Data as recent as 2012 show that explosion-related deaths have dropped to less than 20 per year, an extraordinary achievement. Hydraulics played a massive role in the reduction of explosion-related injury and death, but I’ll get to that later.
Benefits of hydraulics are huge in mining
So, what makes hydraulics so good for mobile machinery in mining? To start, hydraulics are perfect for mining for the same reason it’s perfect for any other industry; power density, controllability, reliability, and serviceability. One could be so bold as to claim mining machines are hydraulics raison d’être. “Big Bertha,” the CAT 6090 and the Belaz 75710 are all mammoth machines of legend, capable of workloads no other machines can match. And they wouldn’t be possible without hydraulics.
Mining is big business requiring the processing of earthen material on a scale difficult to describe in words. Just have a look at the CAT 6090, which is driven by a 4,500-hp engine running eight variable displacement piston pumps each flowing 250 gpm, which doesn’t include the six swing pumps flowing 130 gpm each. The total volume of hydraulic fluid in the machine is almost 3,500 gallons. Operating at over 5,000 psi, its bucket payload is 103 tons – or about a hundred 1990 Honda Civics.
The hydraulic circuit on the CAT 6090 is as complex as any injection molding machine or hydraulic press, if not more. There is closed-loop control of pressure, flow and temperature, and with horsepower limiting pumps. The focus of the hydraulic circuit is productivity and efficiency, an approach that ensures a high level of output even in extreme conditions. For example, even if the 1,000+ gpm cooling circuit with hydraulic motor-powered coolers isn’t enough to keep hydraulic fluid within an acceptable range, the sophisticated control system will reduce pump flow to lower input power, thereby reducing heat load.
As you can imagine, such a massive machine capable of astounding productivity has potential danger. This is where the inherent qualities of hydraulics are put to play to improve controllability and safety. The track’s wheel motors use hydraulic retardation valves to prevent overspeed while traversing downhill, enabling the 1,400-ton excavator to descend a 44% grade without running away. And although not a function of hydraulics alone, the 6090 has sophisticated kinematic control, enabling functions such as constant-angle bucket guidance, automatic roll-back prevention and constant boom momentum control.
Mining machinery is made safe not only by the techniques applied to circuitry but also in the quality put into component construction. I’ve done design work on the hydraulic cylinders used in mining applications, and you’d be shocked at the level of quality control that goes into a traditionally commonplace component.
The first step in quality control is to ensure the steel alloy is pure and free from defects. The raw material is sent for third party MTR testing, which confirms grade, hardness and mechanical test properties. Once the raw material returns from testing, the rods are machined and sent back out for further non-destructive testing.
The magnetic particle test is performed on the threads, and the integrity of the entire rod is confirmed using ultrasonic scanning. If welding was involved to join the barrel to the caps, or perhaps to attach a rear clevis, the weld joints are coated with a penetrating dye. Welding defects, such as excessive porosity, show up from the dye. Should the welded components fail this DP test, the parts are rejected and remanufactured.
Safety in mining must be considered at every conceivable level, from manufacturing of machine components right through to installation and implementation. Coal mining, especially when underground, is perhaps the most extreme of mining conditions. Underground mining has its own series of challenges; the space constraints are obvious, making the close proximity of machines and personnel an obvious hazard.
If you’ll recall, I mentioned hydraulics played a part in reducing the coal mine explosion-related deaths from thousands to just a handful in the past hundred years. Hydraulic energy transmission occurs without the requirement for electrical energy transmission. Early electrical components were veritable Zippos in a mine, and would ignite coal dust easily. The term explosion-proof appeared to describe components that would not cause ignition via a spark. Obviously, a bit of a misnomer – if you throw a ton of C4 at a solenoid valve, it’ll explode no matter its explosion proof rating. Perhaps the phrase “electrically inert,” would be more appropriate, but I digress.
It goes without saying, hydraulics are explosion-proof by their very nature. Hydraulic machinery was pivotal in mining to ensure productivity was high while remaining safe. Modern hydraulics with electronic controls are able to be used in mining with no safety concern thanks to the advent of explosion-proof electrical components. These are coils and switches with sealed electrical connections and contacts that are not exposed to exterior ambient air, where a spark could ignite coal dust.
Regardless if for power or safety, hydraulics will always play a lead role in mining. As I’ve mentioned many times, there are markets electrification will never supplant hydraulics, and mining may be the best example of that. The loads and forces involved in mining make hydraulics a perfect match, and the chase for productivity and profitability will not rest as mining becomes more competitive. I imagine CAT is working on a 6100 hydraulic shove as you read this.