In the mining world, you often hear screen suppliers mention “G-force” as a critical performance metric. But what exactly is G-force, and why does it matter?
Understanding G-Force
G-force in screening systems is determined by two key factors: the speed at which the offset weights rotate and the stroke length of the machine’s vibration. For instance, consider a Brute Force machine operating at 800 cycles per minute (CPM) with an 11 mm peak-to-peak stroke. This combination produces roughly 3.93 G’s. When the machine is unloaded, it might maintain that 11 mm stroke; however, as the material load increases, the stroke can drop, say, to 9 mm. This reduction lowers the G-force to around 3.22. Essentially, the heavier the load, the more the stroke drops and therefore the G-force is reduced. For a Brute Force unit, the only way to adjust the travel rate on the fly (the rate at which material moves) is by changing the speed, since the stroke is fixed mechanically. You could change the stroke mechanically, but this would require stopping the machine, adjusting the centrifugal weights and then re-starting.
The Design Challenge: Handling Higher Gs
High G-forces imply greater stresses on the machine, which in turn require a more robust design. To handle these higher stresses, additional steel weight is often needed to further increase the strength of the machine to withstand these added stresses. However, increasing the machine weight results in a need for even more centrifugal force and a requirement for increased horsepower. This cycle can lead to continuously escalating stresses on the machine, a challenge common in Brute Force designs. Moreover, Brute Force drive mechanisms are typically connected at only two points (one on each side), which further concentrates the stress.
In contrast, Two-Mass machines offer significant design advantages. With multiple attachment points, the drive is more evenly distributed, and the system experiences significantly reduced stresses at each given point. Add this to the much lower centrifugal force requirement due to the sub-resonnant natural frequency design and the stresses are lowered even more. This reduction in centrifugal force leads to much lower horsepower requirements. With all these lowered stresses on the unit, you achieve much longer machine life and improved reliability. Plus due to the much lower centrifugal force required, you are able to and get up to speed rapidly, as well as shut down rapidly to dramatically reduce isolation bounce and loading on the structure.
Stroke vs. G-Force: What Makes a Screen Effective?
There is a common misconception that higher Gs are essential for better screening performance. In reality, it’s not the G-force but the stroke that makes a screen effective. Optimal screening is achieved through stratification: The process by which material is separated by particle size, allowing the finer materials to settle and be released through the screen panel openings. Maximizing efficiency means keeping the material on the unit as long as possible while maintaining the highest practical stroke.
VFD-Controlled Two-Mass Machines: Consistent Performance Under Load
Two-Mass machines with a Variable Frequency Drive (VFD) operate quite differently from their Brute Force counterparts. When applying General Kinematics’ extensive experience in sub-resonant design and tuning to customized industry-specific requirements, couple this with 100+ patents in Two-Mass technology. The VFD enables precise control over both speed and stroke, as speed increases, the stroke naturally increases, and as speed decreases, the stroke reduces accordingly. Importantly, when a Two-Mass machine is loaded with material, the natural tendency is for the stroke to increase. With the VFD, we can make very fine adjustments in frequency, ensuring that the stroke remains constant regardless of the material load. This level of control keeps the G-force nearly constant, typically fluctuating only slightly between about 3.5 and 3.4 Gs.
Conclusion
While both Brute Force and Two-Mass units may operate within similar G-force ranges, the critical difference lies in the ability to maintain a consistent stroke under varying loads. Brute Force machines lose stroke when loaded, reducing efficiency and throughput. Two-Mass machines, on the other hand, preserve stroke consistency, which translates into higher capacity, better screening efficiency, and smoother operation.
