Making Use Of Compact Plant In Quarries
Mike Isherwood examines some of the latest developments in Bobcat compact equipment and outlines the benefits of these small, versatile machines in quarry applications
Skid-steers and mini-excavators are the mainstay of the compact equipment market worldwide. Both types of product have applications in quarries, particularly in secondary breaking, site/plant clean-up and material-handling work.
The versatility of skid-steers and mini-excavators can be fully utilized by combining the machines with a wide range of attachments. There are now well over 60 attachments available, including various sizes and types of buckets, pallet forks, grapples, augers, rock-picking attachments, hydraulic breakers, hydraulic trenchers etc. These attachments allow compact equipment to perform numerous tasks, increasing productivity while reducing the costs associated with renting or purchasing speciality equipment designed for only one specific task.
This article examines some of the latest developments in skid-steers and mini-excavators that may offer new opportunities for quarry applications.
Choice of chassis and boom arms
Today there are basically three types of skid-steer in a range of sizes and capacities to help the customer choose the right machine for their application. The simplest, traditional design offers a chassis with conventional wheelbase and radial boom arm for maximum manoeuvrability in tight areas.
Moving up the capacity range, there is a second type of skid-steer that has a longer wheelbase for a faster travel speed and greater lift capacity. These machines are well suited to applications requiring a lot of lifting and carrying, especially where there are significant distances between applications. With the third type, the longer wheelbase is combined with vertical lift path boom arms for maximum efficiency in lift-and-carry as well as material-handling applications.
Wheels vs tracks
While wheeled undercarriages offer the greatest overall flexibility and meet most operating conditions, several manufacturers have now developed compact tracked loaders based on the skid-steer frame, for those ground conditions where extremely low ground pressure and high traction are required.
Compact tracked loaders are high-performance machines equipped with dedicated rubber tracks to provide superior traction and flotation, especially when operating on very soft or muddy ground and sand, as well as on uneven terrain. Indeed, these machines perform so well in tough environments that they can significantly extend the work season far into the winter when these conditions are most commonly encountered.
By distributing weight over a larger area, the compact tracked loaders also offer a smooth ride with the high stability and manoeuvrability needed for grading and backfilling on rough terrain. Thanks to their excellent traction and low centre of gravity, they are also suitable for climbing and working on steep grades, as well as for general operation anywhere on uneven terrain.
With compact tracked loaders, there is a choice of size and boom configurations — radial and vertical lift path models — similar to those in wheeled skid-steers.
To help match the user to the loader, several skid-steer manufacturers now offer a choice of control systems. Bobcat are unique in offering four different control systems. The company have launched their new Selectable Joystick Control (SJC) giving a choice between ‘ISO’ and conventional ‘H’ hand control patterns on the same joystick, simply by pushing a rocker switch in the cab.
When the SJC switch is in the ISO position, the left joystick operates the drive function and the right joystick controls the lift and tilt function. In the skid-steer or ‘H’ pattern, the left joystick controls the left wheels and lift arm functions, while the right joystick controls the right wheels and the attachment tilt function.
Now available for all Bobcat skid-steers from the S150 and larger, as well as the Bobcat compact tracked loaders from the T190 upwards, SJC potentially provides important benefits for quarry operators and contractors, as the ISO controls offered by the SJC system are similar in concept to the joystick controls found on much larger quarry plant, such as excavators.
Quick-change attachment systems
On most compact loaders, quick-change attachment systems allow the units to be transformed quickly and effortlessly into multi-task machines. These systems ensure that the attachment can be fitted securely and safely to the machines in seconds.
One of the major advances in this area is a time- and effort-saving hydraulically assisted attachment system from Bobcat, the Power Bob-Tach. Intended to extend the tool-carrier capability of the loaders, the system is activated by a rocker switch on the control panel of the loader. For solely mechanical attachments, such as buckets, dumpers and pallet forks, the driver does not have to leave the cab at all to change attachments. For hydraulically powered attachments, the system is still a time-saving device, but the operator needs to stop the engine and leave the cab to connect up the hoses for the attachment to the hydraulic system of the loader.
To disconnect an attachment, the operator simply rocks the switch in the other direction and the levers are unlocked hydraulically. When working with mechanical attachments, the operator can quickly reverse and drive off to hook up the next attachment.
Zero tail swing
One of the most important advances in the mini-excavator market is the entry of short-radius or zero tail swing (ZTS) excavators. The ZTS design eliminates the tail swing associated with conventional excavators, ensuring that the rear of the machine does not extend beyond the width of the undercarriage when slewing. These ultra-compact machines provide superior manoeuvrability and flexibility, making them ideal for use in tight spaces, increasing site safety and minimizing the potential for damage to the machine and its peripheries.
With the proliferation of zero tail swing machines, Bobcat felt that the company’s new 430 and 435 mini-excavators should take the design and capabilities of the zero tail swing excavator a stage further. The 430 and 435 ZTS mini-excavators therefore introduce new concepts offering important advantages over other ZTS machines on the market.
For example, customers can purchase for the first time, not only conventional drive versions of these ZTS machines, but can also benefit from the Bobcat FastTrack drive system, the industry’s first all-hydrostatic drive system, providing increased torque, precise manoeuvring, smooth acceleration and significantly higher travel speeds.
FastTrack drive system
Utilizing two separate closed loops of piston pumps and high-torque axial piston drive motors to provide hydrostatic power independently for both the right and left tracks, the FastTrack system is an advance over the conventional hydraulic drive systems found on other excavators, where all the hydraulic functions of the machine are supplied from a central hydraulic control valve.
The two hydrostatic pumps are belt driven like the proven technology in Bobcat skid-steers, splitting power infinitely between the right and left tracks. This significantly increases productivity in grading and backfilling and enhances the ability to move quickly from one part of a site to another. It also provides better torque control for levelling and dozing, smoother turns under load and more gradual turns on surfaces such as asphalt and concrete.
The FastTrack drive system allows full track power to be available on each track at all travel speeds. The 430 FastTrack model offers a top speed in the low range of 2.6 miles/h (4.2km/h) and 5 miles/h (8km/h) in the top range. Similarly, the 435 has a maximum travel speed in the low range of 2.8 miles/h (4.5km/h) compared to 5.4 miles/h (8.7km/h) in the high-range mode.
With top speeds of 8km/h and 8.7km/h, respectively, the 430 and 435 FastTrack models are nearly twice as fast as their nearest competitors, allowing the machines to move faster both on job sites and between job sites.
One of the main differences between the FastTrack system and a conventional system is that the drive transmission has no impact on the hydraulic flow available for the attachment. In contrast to other excavators, maximum flow is available for attachments on the 430 and 435 even when driving at maximum travel speed.
All the 430 and 435 models feature a new, highly advanced load-sensing hydraulic system. The move from an open-centre control valve to load-sensing provides operators with smoother control and improved workgroup balance. The load-sensing system facilitates this by ensuring a movement will never be stopped since each spool has a flow regulation that depends on the spool position and not on the load applied.
As the operator does not have to compensate for changes in flow or load conditions, the excavator is much easier to control. By preserving motion under any load or flow, the system also produces predictable movements.
The new load-sensing system provides precision at all engine speeds, providing a more ‘natural’ behaviour. The higher precision and better control produces faster cycle times and finer control for precision work. The system also reduces fuel consumption.
At the centre of the load-sensing system is a closed-centre valve that includes an auxiliary port relief. The inclusion of this port relief simplifies the installation of Bobcat’s special hydraulic clamp attachment. The enhanced excavators are, in effect, ‘clamp ready’ to be equipped with the dipperstick-mounted hydraulic clamp attachment, which enhances the ability to pick up blocks of debris or asphalt and load them into waiting trucks and trailers.
The introduction of load sensing has been accompanied by the use of a new dual-outlet piston pump offering increased pump flows to remove the need for an additional gear pump. This increases the overall reliability of the machine. The move to the new pump has also allowed the hydraulic system relief pressure to be raised from 172 bar to 270 bar.
As well as improved machine performance, the combined use of the closed-centre control valve and the new dual-outlet hydraulic piston pump has allowed the size of the cylinders to be reduced, lightening the work group and significantly improving the balance and stability of the excavators.