Wearing It Well
How monitoring shovel tooth wear can increase productivity
by Enoch Chow

Published in the August 2011 issue of World Coal Magazine


Operating a mining shovel with worn teeth reduces digging performance, increases energy usage, and increases the likelihood of breaking bucket teeth and adapters. In order to maintain peak performance and reduce shovel downtime, it is important to carefully monitor the wear of the teeth on the shovel bucket.

Replacement of the bucket teeth typically occurs at regular intervals, which is based on the wear rate estimated by the Ground Engagement Tools (GET) crew. However, tooth change-outs cannot always be planned, and it is the unplanned occurrences that can have a negative impact upon the productivity of a mine. According to a case study carried out at the Freeport McMoRan Morenci copper mine in Arizona, an unplanned change-out of shovel teeth can cost the mine 13.8 times more than a planned change-out¹. In this study, the estimated cost of each tooth was US$300, and the labor cost US$300. Assuming that all nine teeth are changed at once, this brings the total cost of a planned change-out to US $3000. This estimate also assumes the change-out coincides with a scheduled shovel downtime for other maintenance purposes, in which no additional lost production is incurred. Conversely, an unplanned change-out results in an average of 0.78 hours of lost production, which was estimated to cost US$38,368. Including the labour and material costs, the total cost of an unplanned change-out is US$41,368, compared to US$3,000 for a planned change-out.

By routinely monitoring the wear status of the shovel teeth, the study determined that reducing the tooth change-out interval from 255,000 to 200,000 t worth of shoveling would substantially reduce the probability of an unplanned change-out due to tooth breakage or wear. This would result in savings of around US$300,000/shovel/year for the copper mine. The analysis also identified that the outer four teeth on the dipper wear at a slower rate than the inner five teeth and could possibly remain in service for an additional 83,000 t. Adopting a change-out strategy that changes the inner and outer teeth would maximise the effective utilisation of the teeth.

Based on experience with various mines, the general consensus is that a failed tooth is defined as a tooth having less than 60% of the original length remaining. If a tooth is worn beyond the failure point, there are a number of potential consequences. One of the major concerns is that a tooth may break off during normal operation and end up in the crusher. If the tooth jams the crusher, removing it can be a very dangerous and costly procedure that can halt production for hours at a time. Another concern is that excessive wear on a tooth can also cause damage to the adapter. These can be difficult to repair, especially in scenarios where the adapter is welded to the dipper.

The useful lifespan of a tooth is affected by a number of factors including the tooth material, shape, and its location on the bucket². Shovel teeth are designed with a wedge profile to provide optimal digging effectiveness. As the teeth wear down, the wedge profile deteriorates which results in increased digging forces, energy usage, and fill times. These effects can also cause premature wear on other shovel components such as the shovel dipper, boom, and electrical drives.

A number of operational factors can also affect the useful lifespan of the shovel teeth, the most prominent being material type, blasting efficiency, and operator behavior. In an iron ore mine, for example, the shovel teeth can typically last one week; however, in the oilsands, it is not uncommon for a tooth to wear to the critical point in less than a day.


Motion Metrics International Corp. has addressed this issue with an innovative solution called WearMetrics™. WearMetrics™ is an automatic tooth-wear monitoring system that provides the status of each tooth on the shovel bucket in real-time by displaying the remaining length of the tooth expressed as a percentage of the original length. The WearMetrics™ system builds on the same platform used by the company's ToothMetrics™ missing tooth/adapter detection system, which has a proven track record of over 130 installations in 30 mines around the world since 2003. Both products can be installed on any type of mining shovel, including P&H and Bucyrus rope shovels, as well as Komatsu, Liebherr, Terex and other makes of hydraulic face shovels.


Using a rugged camera mounted on the boom of a rope shovel or on the stick of a hydraulic face shovel, WearMetrics obtains a clear view of the shovel dipper and its teeth, which is transferred to the embedded CPU installed in the shovel operator's cab. The camera bracket assembly on a Bucyrus rope shovel is shown in Figure 1. As the mine pit is an open environment subject to varying environmental conditions such as dirt, dust and lighting conditions, a key challenge is the ability to deliver consistent results. To counter lighting variations, a specially-designed long-life high-intensity rugged LED light is installed alongside the camera to illuminate the dipper during night operations. When the view of the teeth is blocked by dirt, dust or shadows, advanced object recognition algorithms continuously process the incoming video to exclude these images and select only optimal images for tooth-wear analysis. During typical shovel operation, a new image is obtained every 2-10 minutes, which is more than sufficient to extract tooth wear trends at high accuracy.

When a suitable image is obtained, a photogrammetric algorithm is used to automatically locate the lip shroud and the tip of each individual tooth on the dipper to determine the length of each tooth, with respect to the lip shroud as shown in Figure 2. This information is provided directly to the shovel operator via a touch screen display installed in the cab of the shovel and is stored as a .CSV file on an onboard industrial-grade CompactFlash (CF) card for the GET engineers, supplemented with periodic (such as daily) summary reports. Images of the shovel bucket are also periodically logged so that the tooth wear can be visually verified.


By using the WearMetrics solution, the GET and mine maintenance engineers can actively plan tooth change-outs and develop change-out strategies to maximise the effective utilisation of the shovel teeth. Many mines have also indicated that they intend to use the system to compare the wear packages of different manufacturers in order to select the most effective configuration for their mine. When correlated with other data sets, such as GPS positioning data, the wear rate of the shovel teeth can also be an indicator of the blasting performance: an increase in the tooth wear rate could indicate that the blasting pattern is not as effective. This information could also be used to analyse the behaviour patterns of different operators. For example, different operators may engage the dipper at different angles, resulting in different digging efficiencies and variations in wear over time. When correlated with the high-precision dipper positioning data, provided by Motion Metrics' shovel arm geometry System (AGS), this relation between wear and operator behavior can be investigated in greater detail.


In a recent WearMetrics installation on a Komatsu PC8000 hydraulic face shovel at a Chilean copper mine, a six-day study was conducted to evaluate the performance of the system. The system obtained 3848 images of the bucket teeth over the six-day period, an average of 26.7 measurements/hour, which would be impractical if traditional measurement methods were used.

The Komatsu shovel bucket had six teeth and at the beginning of the study, tooth #5 was replaced with a new tooth. Analyzing the data collected revealed that the new tooth #5 decayed rapidly, over eight centimeters in the six-day period, while little change was observed on the other teeth. At the end of the sixth day, tooth #5 was roughly the same height as teeth #1, #2, and #6, which are the outer four teeth.

A conclusion that could be drawn from this study is that when only a single tooth is changed, it will decay at a more rapid rate than the other teeth until an equilibrium state is reached. This would suggest that rather than replacing an individual tooth, a change-out strategy should be implemented, that consists of replacing groups or pairs of teeth at the same time. Such a practice would maximise the effective lifespan of a tooth. However, it should be noted that every mine presents different conditions which could result in an alternate conclusion; therefore, this recommendation should be considered on a case-by-case basis.

Tooth-wear monitoring is a critical but often overlooked aspect in maximising shovel uptime. An unplanned change-out of the shovel teeth can result in a significant loss in production due to unexpected downtimes. Therefore, it is important that the tooth wear is carefully monitored so that tooth change-outs can correspond with scheduled downtimes. The WearMetrics system eliminates the labour and time intensive process of manually measuring tooth-wear by providing a machine-vision based automatic monitoring solution. Using the system, it is possible to actively plan tooth change-outs, analyse blasting effectiveness and operator behavior patterns, and compare different wear packages and configurations to maximise shovel productivity and minimise maintenance costs.

References

1. KNIGHTS, P. F., "Optimal replacement intervals for shovel dipper teeth",
   International Journal of Mining, Reclamation and Environment 23.3 (2009), p. 157.


2. DEL VALLE, V., PARSONS, L., PATNAYAK, S., TANNANT, D.D., WONG, J., "Operator and dipper tooth influence on electric shovel performance during oil sands mining".
   International Journal of Mining, Reclamation and Environment 22.2 (2008), pp.120 -145.

Enoch Chow is the marketing manager with Motion Metrics International Corp. For more information, visit www.motionmetrics.com