April 1, 2011
by Craig Eller and Kevin Gildea, P.E.
With the ongoing advancements in maintenance technology, most industries are pursuing initiatives related to condition monitoring, which moves them away from the traditional time-based actions. The aggregates industry is no exception, and there are many opportunities to adopt more sophisticated, less invasive maintenance practices. The use of technological advancements, such as continuous fault monitoring and parameter trending on rigid frame haul trucks, improve truck performance and lower life-cycle costs. The use of new technology is most effective, for any machine, when a solid foundation of best practices is implemented. These best practices usually involve consistent inspections and carefully documented results. Most, if not all, maintenance programs can find areas to improve on these basics, while preparing for further evolutions in equipment optimization techniques.
Improve what you already do
There are many ways to monitor a system or component and accurately predict when it approaches the end of its service life. Some of these methods are simple, and most maintenance programs have been engaged in these practices for some time. However, it may be helpful to step back and ask if these simple steps are being done properly and are making the most of the available data.
In general, equipment maintenance programs include the following practices:
Oil Analysis. An oil program is only as good as the sample. With clear guidance, two common pitfalls can be avoided.
• If there is a sample line, effectively purge the residual oil before the sample is drawn.
• For mobile equipment, draw the oil before any particles settle.
In addition, there are system arrangement considerations that can significantly impact the accuracy of a sample. Consider the following:
• Are the samples’ locations oriented properly in relation to the filter?
• Does the arrangement allow the technician to obtain a sample without introducing contamination (oil runs down the side of a sump and into the bottle, etc.)?
Pin and bushing wear assessment. Where possible, slop (or play) limitations in an axis of normal wear should be defined and easily measured. Much of this is established when the connections are designed, but a few simple measurements often can significantly improve the assessment capability.
Structure inspections. This most basic maintenance action becomes much more effective and efficient when guidance is provided on where and how to inspect (visual, non-destructive testing, etc.). Obviously, some areas of the structure are more critical and susceptible to cracking, while other areas experience low stress and do not require as careful an inspection. Additionally, to further improve the ability to monitor structural condition, cracking thresholds are necessary to explain what amount of cracking requires immediate repair and what can be scheduled for future planned down periods.
Collect basic data and use it
Based on the characteristics of the given equipment and operating environment, logical individual threshold values need to be clearly defined. When thresholds are exceeded, logical and realistic corrective action should be specifically explained.
If the thresholds and limits are not readily available, the original equipment manufacturer may have helpful information. Alternatively, initial numbers can be determined based on your crew’s experience with the equipment. If any of the sampling/inspection steps are currently included as part of the maintenance program, someone is making judgment calls on the appropriate corrective action. Thresholds and limits currently used by resident experts need to be collected, quantified where necessary, validated, and formally documented. This information will allow decisions to be made in a consistent manner and eliminates complete dependence on any one member of the maintenance crew.
Additionally, once initial thresholds and limitations are established, basic data analysis allows the numbers to be refined and improved. A comparison of the conditions observed that prompts a component removal or system disassembly can be compared to the actual condition found. With a reasonably small set of data, solid conclusions can be drawn.
For example, the iron (Fe) level in a bearing sump may be 25 parts per million (ppm) and shows an increase of 10 ppm from the last reading. The bearing is pulled from service, and the condition is considered excellent. Over time, it is determined that similar scenarios occurred with a few additional bearings of the same type in the same application. Therefore, a reasonable conclusion could be to raise the threshold to 40 ppm and see if any wear is noticed when bearings are disassembled and inspected.
Another example might include a measurement of 0.095 inches of slop on a particular pin/bushing connection. The bushing is almost completely worn through, and cracking is observed in the adjacent structure. If similar observations are documented for this same connection, it would be reasonable to lower this slop limit, perhaps to 0.08 inches.
The basic information needed to refine your inspection thresholds fall into two general areas:
• Document the condition observed when the decision was made to pursue corrective action; and
• Document the condition found upon disassembly.
These two basic sets of information allow a correlation to be made between what your threshold is indicating and what the condition actually is.
One note on thresholds or limits: Frequently, condition assessments are not simply good or bad, based on one reading or inspection. A trend is the better indicator. Establishing the baseline and then monitoring the trend sets a foundation for making fully informed decisions about if and when to pull equipment from service.
The basic purpose of these examples is to identify issues prior to an operational failure, allowing time to plan the corrective action to minimize production impact. Additional areas that may benefit from a simple, but carefully developed, inspection/assessment/action approach include electrical ground fault monitoring, suspension charge status, and bearing vibration. Each type of machinery, in its particular application, will have its own unique opportunities.
Most industries are shifting focus from maintenance to reliability, and most recently, to equipment optimization. The steps outlined in this article, and those that are appropriate for your specific application, are still critical to operational success.
Of course, with an investment in continuous monitoring technology, a maintenance program can be further improved with little to no downtime required for invasive, risk-introducing, maintenance steps. Until your operation is ready for that next step, however, implementation of these existing systems can yield improved equipment availability. AM
Craig Eller is Liebherr’s manager of maintenance and reliability programs and Kevin Gildea, P.E., is the product manager for its T 282 mining trucks.
The Evolution of Equipment Maintenance
Preventive maintenance programs: Avoid failures. Disassemble equipment and inspect it for general problems. Replace components based on time or cycles.
Predictive maintenance programs: Avoid failures and minimize maintenance downtime. Conduct precision inspections with thresholds or limits dictating specific corrective actions.
Equipment optimization: A 97-percent-plus proactive approach based on automated condition monitoring. Most recurring manual inspections/samples are eliminated. Analysis determines precise action based on production schedule and empirical failure trends.