The production and cost benefits associated with having correct load balance on haul trucks and improving haul-road conditions are difficult to quantify on paper and therefore a lot of mines do not make it a part of their standard operating procedure/best practice—but it is a relatively easy change to make to yield lower cycle time, higher production and lower costs. The goal is to create an environment where the truck is operating as close to design parameters as possible. In this article, we will only focus on the design parameters of road conditions and load balance and how to utilize technology to monitor, analyze and improve these parameters.
My experience with open-pit haulage has been with Caterpillar mechanical-drive 240-ton trucks (793x) so this article will use the Caterpillar line of products in the examples of technology. However, the same best-practice strategies should apply to other brands of haul trucks and if your truck isn’t currently equipped with the technologies mentioned in this article, third-party technologies are available.
First, let’s look at the “perfect” environment for haul trucks—the environment that spec sheets are based on, for the most part.
- Truck weight when loaded should be distributed as follows:
- 1/3 of weight to front axle
- 2/3 of weight to rear axle
- 1/2 of weight to the right side
- 1/2 of weight to the left side
- Truck is traveling at 0% rolling resistance. The smoother the road, the lower the rolling resistance value. Zero percent is impossible to achieve in the real world; in general, 1% is great, 2% is average. A high rolling resistance causes reduced rimpull and reduced acceleration and therefore, longer cycle times and lower productivity.
- Truck box is optimally sized, based on density of material to optimize payload.
- Payload (Gross Vehicle Weight (GVW) minus Empty Vehicle Weight (EVW) equals Maximum Payload) follows the 10/10/20 rule; often misunderstood, the rule means no more than 10% of loads may exceed 10% above target payload and no loads may exceed 20% above target payload.
Based on the “perfect” environment described above, this means that anytime you deviate from the conditions, the truck is performing at a level slightly less than its potential. Translated into production/savings, it means that the changes you make to ensure proper payload weight/distribution and well-maintained haul roads will enable the following:
- Faster cycle times → increase productivity.
- Extended drive train, suspension, frame and tire life (See p. 145) → lower operating cost and increased machine availability.
- Reduced fuel consumption → lower operating cost.
- Reduce operator fatigue → promotes safety.
- Increased haul road maintenance → maximize support equipment utilization.
Use It, Improve It
The information above begs two questions:
- What technology tools can be used to help monitor and analyze the conditions?
- How do we improve conditions to better reflect design parameters?
To answer the questions, I will first introduce a tool, then explain how this tool can help in each of the four operating conditions of the “perfect” environment. Caterpillar has designed a system called Road Analysis Control (RAC) which is an on-board information provider source integrated with Cat’s VIMS (Vehicle Information Management System). RAC quantifies and monitors haul road conditions and identifies events at a range of severity that are damaging to:
- Machine performance
- Component and frame life
- Availability/Utilization
- Machine productivity
The RAC warnings can be local to the truck only, with real-time feedback to the operator; or with a wireless system in the pit, this information can be transmitted back to a dispatcher, engineer, or supervisor. However, to utilize the data back at the office, you will need proprietary Caterpillar desktop software.
To monitor and improve road conditions: Using RAC and activating a GPS unit with your dataset, you can then start to identify where haul road problems should be avoided/corrected and better allocate your resources—grader, operators and training dollars. Also, you can respond quicker to needed road repairs. With improved pit planning, haul profiles and haul road maintenance, you can see increased truck speeds (if you are not already at maximum for the corresponding road grade).
To monitor and improve load distribution: Because RAC is constantly measuring the strut pressures for correct load distributions, you can determine when the truck is not at 33%:66% front/rear and 50%:50% right/left loading. You can then identify the shovel operators that need further training.
To monitor and improve payload optimization: Within VIMS, there is a function called TPMS (Truck Payload Monitoring System). When the struts are properly calibrated (I would advise adding that as a maintenance item at the 500-hour preventative maintenance interval), TPMS will accurately provide payload weights. This data is stored and shown locally and/or streamed wirelessly to the office. VIMS and TPMS data require proprietary Caterpillar software to decode, review and analyze.
Using this data, you can then create charts to see what the payload percent curve/histogram looks like. With that, you can spot-train shovel operators. When trucks are loaded properly, you should maximize GVW minus EVW of your equipment (see spec sheets for GVW and EVW).
Depending on whether a mine is over-trucked or under-trucked and many other parameters such as cycle time, commodity materials grade, etc., it may or may not be worthwhile for the shovel to load a partial bucket to bring the truck up to maximum payload. This is why shovel/truck pairing is so important when purchasing equipment during the capex phase. Do the calculations to see what it shows on paper.
To monitor and improve truck box design: To review your truck box, first make sure it is loaded correctly: to avoid spillage, there should be slight extra room on the left, right and rear of the truck box. I have typically recommended a 1/1/1-ft margin space in training materials for shovel operators when they load a truck. This could change from site to site depending on uphill loaded road grade.
If you are loading the truck properly and your scales are calibrated, but you are still not able to maximize payload (GVW minus EVW), then consider a new truck box design (lighter material/shorter life but higher payload). Do the cost analysis to see if the shorter component life is worth the higher payload (at current commodity prices, the payback is generally available).
If trucks are constantly spilling but the truck scales say they are not overloaded, then consider installing “hungry boards” on the side to increase the carrying volume of the box—or redesign the box to increase volume while maintaining the same box metal/liner weight.
As with all technologies, success is based on the users. Unless operators and/or downstream users of this information are educated and unless there is a process to create/fix the source of the alarms, the technology will not yield cost savings or other benefits for the operation.
Vivien Hui is a mining engineer with operations experience in gold and copper mining, and author of an independent mining-technology blog at vivienhui.com.
