Tough. Brutal. Severe. These are the standard conditions mining equipment designers have to consider. Operation is on rugged terrain in contamination rich environments. Loss of productivity is an expensive reality as operator fatigue sets in over a long shift.
To overcome these issues and the resulting high maintenance costs due to equipment wear and tear, mining suspension systems are now receiving increased scrutiny from owners and operators.
A stable suspension
Mining scalers are used in underground mines to knock down loose aggregate. Scalers are considered mining’s most demanding suspension application because of the tremendous downward pressure exerted when the rock breaks loose and the 90,000-lb. piece of equipment drops to the ground with a force of about 3 Gs. The resulting wear and tear on a scaler and its suspension system (which traditionally consisted of air bags) is so severe that $100,000 in parts maintenance is generally required every year. When the engineering and marketing teams from Breaker Technology Inc. (BTI), Solon, Ohio, set out to develop their all new QS Series Scalers, Figure 1, they knew a stable suspension system would not only extend equipment life and increase operator comfort, it would give them a huge competitive advantage.
“The mine face is often so tight that the miner with conventional equipment has to keep clawing with downward pressure to loosen the rock,” notes Bill Aldridge, BTI’s U.S. mining manager. “If he doesn’t release the pressure when the rock lets go, the whole unit comes crashing down, lifting one of the axles one to two feet off the ground. The air bag suspension units just weren’t holding up to these demands. So, for our new scaler, we felt it was extremely important to design and develop a suspension system that would maximize unit stability by fully absorbing the shock.”
BTI’s engineering manager, Brad Toole, noted that operator comfort was a significant issue. “We learned from users that the operator would not generate any more productivity than his body could withstand. As soon as the operator would start to feel fatigued, he would work the machine less. Therefore, one of our goals was to build in more operator comfort to gain higher productivity for our customers.”
The key was to design a platform that would be as stable as possible, a difficult task when dealing with the weight and the balance to hold it.
Further, BTI market studies indicated the need to protect the axles, as competitive units had experienced axle failures. One maintenance specialist noted, “all scalers are designed to break, it is just a matter of how long.” To avoid potential axle breakage, BTI’s design criteria specified that the axles must withstand the force of dropping to the ground from up to two feet in the air on a 90,000-lb. gross weight vehicle, or 45,000-lb. per axle.
Hydraulic accumulators as “gas struts”
Toole first looked at air bag and spring suspension technology and found them lacking. Remembering hydraulic accumulator technology in other applications, Toole checked with Parker’s Hydraulic Accumulator Div. about a possible accumulator suspension solution for the new QS45 Scaler. After looking at the performance requirements, Parker engineers were convinced that their new Accumulator GAStrut technology would work.
The gas strut design, Figure 2, fully integrates cylinder and accumulator technologies into a compact, robust piston design in a virtually leak-free 4000-psi nitrogen gas chamber. The term “gas strut” was applied because the cylinder acts as a strut in the suspension system with pressurized nitrogen gas serving as the active energy shock absorber.
Hydraulic accumulators store energy in the form of pressurized gas. The higher the pressure, the more stored energy in the gas. The stored energy acts like a spring to absorb shock — a perfect fit for mobile equipment with very demanding high force loads.
The gas strut differs from traditional accumulators in that it combines the energy storage capability of an accumulator with the functionality of a cylinder. For the BTI application, the cylinder serves as a strut that flexes as a regular suspension unit. The cylinder is attached by a bolted-on trunnion mount that allows for rotation, Figure 3. Inside the cylinder, a piston with a custom designed seal separates the two chambers. Pressurized nitrogen is on the cap end. Nitrogen is used because of its fast response and inert nature. The rod end is open to atmosphere via a filter.
For the BTI QS Series Scaler, the nitrogen gas in each of the four gas struts (two on each axle) is precharged (pressurized) to 4000 psi, five times greater than anything else on the market. Each gas strut has a rating of more than 100,000 lb. While this may sound extreme, it is appropriate for a scaler that experiences 3 Gs as it drops to the ground, or when the vibratory pick scales along mine walls to knock off debris. The reality is that this is a 90,000-lb. mobile vehicle placing three times its weight on the axles. By integrating cylinder and accumulator technology into the four gas struts, the result is a robust, stable suspension design that combines significant load carrying with a high-pressure energy absorption capability.
“Rock and roll” eliminated
“The new gas strut technology,” notes Aldridge, “has eliminated the abuse problem. Our QS45 Scaler is in the heaviest class at 90,000-lb. The stable suspension, coupled with a small footprint, has eliminated the previous ‘rock ’n roll’ operation that causes scalers to come off the ground. In this type of mining operation you only want to set the unit up once. From that position, the miner can go 45° to the left or right. When you do that with a 23,000-lb. boom, you have a lot of extended weight and movement. Scalers with conventional air bag suspensions will come right off the ground when working opposite sides of the mine.”