A quarry or mining manager has basically three options in selecting a new loading tool when focusing on discontinuous mining and leaving bucket wheel excavators, surface miners, conveyor belts, etc apart. These alternatives are wheel loaders, hydraulic excavators – either in front shovel or backhoe version – or rope shovels. However, the deployment of rope shovels is restricted to huge mining operations with some geographic exceptions like China, India or the former CIS countries, where electric shovels from local manufacturers in the 5 to 10m3 class are still quite common and can be found in quarrying operations as well. The ideal working environment for an electric shovel is a combination of a high working face, which is compulsory for an appropriate bucket fill, together with a well-blasted material that is not too hard to dig. The power supply cable at its rear end is not much of a handicap if the operation is more or less stationary. Especially at the top end of the range, above 30m3 bucket capacity, the rope shovels are still competitive today, if they find their specific working conditions.
Rubber-tyred loaders distinguish themselves by their mobility. They can change the loading area easily within a short time to adapt to various production requirements. This becomes relevant in particular in selective mining applications where the source of raw material within the deposit, and therefore the loading area, has to change several times each shift to serve quality requirements.
Another benefit is the capability for ?load and carry? operation. In this case, loading and hauling is executed by a single machine, which is more efficient when the hauling distance is very short, in contrast to the use of a dump truck, which would spend most time at the loading and dumping area instead of performing transport labour. This method is often used in connection with in-pit-crushing when mobile or semi-mobile crushers are in operation. The maximum economic distance depends on the rolling resistance and gradient and can be determined between 150m and possibly 200m. It is, again, the ground conditions, which limit the general feasibility of a rubber tyred loader application, as a flat and even surface in the loading zone is an essential precondition. Another requirement is sufficient space for manoeuvring, compared to hydraulic excavators, which can fill the bucket and dump on the spot.
Up to 50m3
Hydraulic excavators are used in the construction and mining industry and are offered in a wide range of different sizes and versions, including mobile and tracked units. A variety of attachments are available to adapt for multiple purposes, such as hydraulic breakers, hammers and grabs or special shaped buckets. The entry into the quarrying industry starts with an operating weight of at least approximately 45 tonnes; this market has certainly seen an influx of a lot of manufacturers recently. The very large and mining-proof machines, however, can be delivered only from a handful of suppliers.
The first hydraulic excavators were launched in the early 1960s and just a short while later they were introduced into the extraction business too. They grew rapidly in weight and units with a standard bucket volume of between 20m3 and 30m3, establishing themselves in open pit mining between 1980 and 1990. Today, hydraulic shovels have nearly caught up with electric shovels, with the Terex O&K RH400 serving as the market?s flagship, featuring a 1000 tonnes operating weight and 100 tonnes bucket payload per cycle (Fig. 1).
The human factor
Apart from the service weight, which is the major technical specification for the classification of a hydraulic excavator, there are some more important performance data, such as bucket capacity, engine output and digging forces. However, a lot of other factors influence the outcome at the end of the day, measured in loaded tonnes or bank cubic metres.
First of all, each technology is only as good as the human interface behind it, which is, of course, the operator. This means that a comfortable working environment will return in added productivity as well as proper training. For that reason, features like ergonomic seat and controls or a radio are regular features in the cab. Noise insulation and the compliance with the standards for operator vibration are important. An additional second air conditioning system is installed even redundantly in some units, when they are delivered to hot climate countries such as Australia. Yet the RH400s at Syncrude?s oil sand operations in Canada have an additional amenity cab behind the regular one, providing a microwave oven, wash basin and refrigerator. Many suppliers offer special training courses, including classroom lessons and on-the-job training, where experienced operator instructors teach the ?secret tricks? of most productive hydraulic shovel operation in addition to the normal briefing after the commissioning of a new excavator.
Shovel and truck match
One question that always comes up when discussing shovel and truck applications is the best match of loading and hauling equipment. In general, the number of passes to completely fill a truck should be in a range of between three and seven buckets; each rule, of course, has its exceptions. Basically, an extended hauling distance allows for a larger truck relative to the size of the shovel, since the increased percentage of unproductive waiting time of the truck at the loading area (more passes required) is balanced by a longer haul.
Conversely, a short distance to the dump or crusher involves a quick loading of the haul unit. In any case, a key consideration is whether the productivity is truck- or excavator-driven. A high level of utilisation of the truck is supported by a minimised idle time when being loaded; high level of utilisation of the hydraulic excavator calls for larger trucks to keep the fraction of unproductive spotting time between leaving and arriving truck low. A hydraulic shovel, for instance, or any other loading tool, achieves the best possible utilisation when dumping into the hopper of a mobile crusher, since the spotting time is reduced to zero, except for moving the crusher now and then.
The best overall approach is the consideration of the entire scenario; simply put, the costs per tonne, isolated for excavator and for truck, have to be investigated based on different numbers of loading cycles. Both individual cost curves are summed up afterwards to find the lowest total cost, which reveals the best match. Needless to say, any equipment combination should work with an even number of passes only, since half-filled buckets are a waste of time and energy.
Alongside the utilisation question, each set-up must also reflect on the following issue – if the excavator and therefore its bucket is comparatively large, the impact on the truck and its body and suspension while dumping is also higher. Nevertheless, the hydraulically controlled bucket clam function for face shovels, or the backhoe bucket curl operation, allows for smoother discharging than with a rope shovel, where the dipper door is unlatched rapidly and the payload is released in one rush. Yet a smoother and careful dumping process takes longer in any case, and an oversized excavator lessens the chance for the operator to fill the truck exactly to its rated payload as a result of fewer passes by as many as one or two. The backhoe cannot load a truck from the backside, since the bucket might be wider than the truck body. A different arrangement from the side of the truck towards the excavator is necessary.
The other extreme – a far too small hydraulic excavator in comparison to a truck – can be handled better when using the backhoe version. Due to its position above the truck the visibility for the operator is not much affected, and thanks to the larger working range of the attachment it is, in most cases, possible to load the truck body evenly. The front shovel type faces more difficulties with an oversized haul unit. The machine has to be very close to the truck to place the bucket over the body for discharging. The cylinders are endangered to hit the side walls at the same time, calling for a careful and, therefore, slower operation. Another risk is the small clearance of the counterweight when slewing back to the working face or even applying double-side loading.
Face shovel or backhoe?
A general answer cannot be given, and preference for one of the two attachment options sometimes depends on individual attitude or experience of the responsible mine management. Some mining contractors, for instance, prefer a backhoe, since its versatility creates a better potential for follow-up jobs when the machine has to be moved to a different site after a couple of years. In any case, there are also operational attributes which characterise the application of a face shovel or a backhoe.
When working with a front shovel attachment, excavator and truck position are mandatory at the same level with the loading unit in front of the digging face. This provides a decisive advantage for the operator, especially when it is not a pure loading or easy digging application. The working face is clearly visible from the cab and helps the operator to find the best point for bucket penetration, such as fractures or layer boundaries. The position of the loader on the truck level allows also for cleaning of the surface to prevent tyre damage, though the shovel should concentrate on loading the haulers and these auxiliary jobs can be carried out by support equipment such as a dozer. The physical digging forces of a shovel are generally higher in comparison to a backhoe with the same bucket capacity, which is one reason that they are the preferred choice for hard rock applications. Another important cause is the positioning in front of the blasted pile, where the front shovel unit can start digging immediately. A backhoe machine has to prepare a ramp to get onto the material and travelling on a shot hard rock pile involves high wear to the undercarriage components, particularly when the machines are larger and therefore heavier. Nevertheless, a backhoe could sometimes be the better alternative for a free digging operation, since the working mode of the bucket is more of a scratching type and the material is ?clamped? between bucket and tracks. A front shovel tends to compact the material while penetrating.
Many quarries are facing growing problems with their blasting operations since the boundaries of the pit approach residential districts or the other way round. This involves annoyance or even danger caused by flying rocks, vibration, dust and noise emissions. In this case, people look for alternative mining methods to loosen the material without any blasting at all.
Terex O&K delivered a 265-tonne RH120-E backhoe excavator specifically designed for that purpose to a huge cement factory in southern Germany (Fig. 2). But instead of using a quick hitch to interchange between ripper tooth and bucket, O&K favours a combined tool, a ripper bucket. This system allows for the loosening and loading of the material in one action. That particular RH120-E is equipped with a special and extremely narrow 9m3 bucket instead of the standard 15m3 bucket and is reinforced with a centre rib which has the shape of a ripper tooth. A production test had to be performed over an eight hour period after the commissioning and the target was set to 850 tonnes per hour in the toughest zone of the deposit. The result ended up in 850 tonnes per hour, achieving an average bucket fill of 81 per cent, which is very good for a free digging application. The specific powder factor for previous blasting in that area had been between 120 and 140 grams of explosives per tonne.
When mentioning the better visibility of the working face for the shovel operator, it is necessary to point out that, on the other hand, the truck body is better in sight with a backhoe machine, at least when the truck is on the lower level, which is the regular case (Fig. 3). This fact eases a well-balanced load distribution inside the body. The slew angle of the backhoe is usually in a range of between 30 and 60 degrees and facilitates quicker loading cycles. The slew angle of the front shovel averages from 60 to 90 degrees.
The workable bench height with a front shovel attachment is generally higher in comparison to the backhoe. This results in increased floor space required for backhoe operation when mining down to the same depth.
The height of the working face for a front shovel depends on the material and its behaviour. For in situ and sticky material, which creates vertical faces, the height should not exceed approximately 75 to 80 per cent of the maximum vertical reach of the bucket due to the restricted working radius at the top end of the digging range. As a rule of thumb, the ideal height under these conditions is equal to the roof top of the cabin (Fig. 4). The higher the working level, and the closer the undercarriage and the basic unit itself is to the face, the greater the risk of damage by falling rocks. However, if the material has the tendency to roll down like in a blasted rock pile, the slope is flatter and does not create overhangs. Here the feasible height is definitely increased (Fig. 5).
The bench height for a backhoe operation should correspond to the stick length of the excavator or truck body loading edge respectively. With the ?double benching? mining method, however, it is possible to utilise the backhoe?s full working range and to expand the effective bench height at the same time (Fig. 6). For this purpose, the excavator is located on a level of approximately one third of the entire face height. During the first phase, the digging section is located behind or aside the machine and above excavator level. For this purpose, the material is scratched down the face and the bucket gets filled in a little trench at the bottom afterwards. Having worked out a certain distance this way, on the way back, the portion on the lower level is removed in the second phase by means of a traditional backhoe operation. Nevertheless, the material parameters must allow for this working mode which is not practical in severe conditions. The possibility to loosen the material above the excavator level is the limiting factor.
The influence of properly arranged spotting of the dump trucks is very often underestimated. Under consideration of a constant excavator loading time, the productivity of a shovel and truck operation can be boosted by 10 per cent if only the spotting time is minimised from one minute to 45 seconds, as shown in the following example with an RH200 (525t, 26m3) in combination with a 190 tonne truck:
Load per bucket = 26m3 bucket volume x 1.8t / m3 material density x 0.95 fill factor
Load per bucket = 44.5t
Load per truck = 44.5t load per bucket x 4 loading cycles
Load per truck = 178t
Loading time = 3 full loading cycle x 30s cycle time + 5s only dumping
Loading time = 95s
Productivity = 178t load per truck / [(95s loading time + 60s spotting time) x (1 h / 3600s)]
Productivity = 4134t/h
Of course, quick spotting requires sufficient space in the loading area and might not be constantly achievable. But wherever applicable, production managers should sensitise their drivers to position the truck in a way that it gets under the bucket in just one step without time-consuming manoeuvring. If a sufficient number of trucks are available, double side loading minimises the spotting time, but this method requires permanent support of a dozer for surface clean-up (Fig. 7). Needless to say, the excavator operator can contribute to better productivity.
Alongside a good bucket fill, it is important to accomplish fast cycle times. Therefore a small slew angle should be maintained. The penetration height into the face should be as high as possible in order to keep the lifting effort low. When digging a blasted pile instead it might be easier to penetrate at the bottom. This is easy to follow for everybody who ever had to shovel a couple of cubic metres of gravel into a hand barrow. Yet during the changing of loaded and empty truck, the operator of the hydraulic excavator can switch to the most unfavourable conditions – large slew angle and low penetration level – since a loading cycle under these circumstances is usually still quicker than the spotting of trucks. The excavator will present its full bucket before the empty truck is in position.
The influence of individual aspects on productivity and efficiency might appear negligible, as long as they are perceived isolated. But the total sum combined can make a tremendous difference in the performance of a hydraulic excavator. Steady monitoring of the application and continual improvement is the key to a successful operation.
This article was originally published in the November 2007 issue of Quarry Management, the journal of the Institute of Quarrying (United Kingdom). With thanks to Dirk Tegteimer, product manager, Terex O&K, and Stephen Adam, editor of Quarry Management.