Introduction
This case study deals with finding bearing damage on an electric rope shovel with HD technology.
The customer is a major mining maintenance contractor working with repairs of mining equipment, such as electric rope shovel components. In conjunction with servicing crowd drives, the contractor’s maintenance team follows manufacturer specifications regarding vibration data acquisition to assess equipment condition. Based on previous and current measurements according to instructions, a dedicated analysis team determines whether the crowd drive is good to go for another period of time.
However, as this case study shows, the manufacturer’s recommended vibration measurements alone are not enough to capture all potential gear and bearing damage while the drive is being serviced. Furthermore, some of the manufacturer’s recommendations are difficult to fulfill for safety reasons. Thus, there is a risk that the crowd drive is sent back to the mine from service with undetected damage or problems. That, in turn, may, in effect, shorten the drive’s expected working life, causing dissatisfaction with the maintenance service and, potentially, economic losses for all parties.
The crowd drive is a critical component for the functionality of the rope shovel and the mining company’s production schedule. Since there is generally no redundancy for this massive machine, fulfilling several smaller excavators’ work, a shovel breakdown can cause a complete production stop.
Image 1 Depending on brand and model, rope shovels may be able to handle up to 138 m3 of material in a single dig.
A challenge in this kind of machinery is its technical complexity, with eight bearings and six gears, all crammed into a small space. This can make it difficult to distinguish the various signals from each other. Moreover, the gearbox is a reducer, so the rotational speed gets lower with each stage, going from 900 RPM on the input shaft to 23 RPM on the output shaft.
These factors motivated the maintenance contractor to seek a more complete solution for quality control of its maintenance services, and therefore contacted the SPM distributor in Chile, who trained the relevant staff in HD condition monitoring technology and ISO standards. Applying shock pulse and vibration monitoring with HD technologies, the contractor obtains very clear measurement results and distinct fault symptom matches, as compared to relying only on the vibration measurements recommended by the manufacturer. As a result, the contractor can now justify with confidence any recommendations they give to their mining customer, the customer’s trust in them has increased thanks to the more reliable diagnostics, and the shovel manufacturer approves and even recommends measurement with equipment and methods from SPM.
Image 2 Electric rope shovel.
Conclusion and summary
In this case study, the conclusion is clear. HD condition monitoring technology overcomes the challenges of reliably determining the mechanical condition of crowd drives and detects very early failure stages at each RPM.
The training and use of HD technologies make service and quality control traceable, more efficient, and diagnostics more precise for the maintenance contractor. It also makes it easier for them to demonstrate to all parties involved the benefits of using supplementary technologies, such as enabling some repairs under warranty, such as faulty installation of gears and bearings.
Application description
Electric rope shovels are used for digging and loading earth or fragmented rock and for mineral extraction on larger scales. The shovels are among the most critical assets in surface mining operations, oftentimes with extreme downtime costs.
Driven by an electric motor, crowd-pushing machinery controls pushing or picking up the dipper. Through a gear train, it drives a drum that winds the steel cables going to the seat of the dipper.
The primary motions of the shovel include the following:
– Hoisting, where the bucket is pulled up through the bank of the dug material
– Crowding, where the dipper handle is moved in or out to control the digging depth or position the dipper for dumping
– Swinging, where the shovel shifts between digging and dumping
– Propelling, where the entire machine is moved to a different location or dig position.
Located inside the turret, the crowd drive is connected to the gear via cable wires. The hoist transmission is in the back part of the turret, and at the turret base is the swing transmission, connected via the gear ring. The propelling transmission is at the bottom of the machine, adjoining the crawler.
Figure 1 The location of the various transmissions (described in more detail below).
Figure 2 Example of an electric rope shovel.
Four phases make up the shovel’s work/digging cycle:
1. Digging,
2. Swinging,
3. Dumping,
4. Returning.
During digging, the shovel operator crowds the dipper into the bank, hoists it, and then retracts the filled dipper from the bank. The swinging phase begins as soon as the dipper is vertically and horizontally clear of the bank. To position the dipper over the designated haul unit (e.g., a truck), the operator controls it through a planned swing path and dump height. To dump the load, the dipper door is opened while maintaining the correct dump height. In the return phase, the dipper is swung back to the bank and lowered into the track position. The dipper door is then closed.
Image 3 The crowd application (left) and gears (right). Images: CMS Condition Monitoring Solutions Ltda.
Figure 3 Side and top views of the crowd drive. Measurements are taken in positions 1-7.
Background
The technical complexity of the drive presents some challenges when it comes to obtaining reliable condition measurement results. Each reduction stage has a different RPM, and each time a new measurement is taken, the rotational speed is lower than in the previous measurement. Thus, the measurement configuration needs to be different for each axis. Gears and bearings are cramped together in a small space, so multiple fault symptoms – originating from the various components – might appear, and there might be crosstalk from neighboring parts.
This case study shows how HD condition monitoring technology is used to supplement the manufacturer’s instructions concerning vibration measurements on a crowd drive in a test rig located in the contractor’s workshop. The measurements were taken after a drive overhaul. Each component of the gear drive, i.e., bearings and gears, was inspected and repaired, then vibration measurements were taken as instructed as a means of quality control. All drive components were thus as good as new, and vibration readings according to instructions did not indicate any fault symptoms.
Due to previous experience with newly-serviced components that looked healthy during recommended vibration measurement but still turned out to have defects appearing before the next planned maintenance occasion, the contractor decided to trial measurement with technology from SPM. HD technology measurements indicated damages that the recommended vibration measurements alone did not capture.
The handheld instrument Leonova Diamond was used to measure shock pulse and vibration levels and HD Enveloping to check the quality of the serviced drive.
The position and configuration of the measuring points on the drive follow manufacturer instructions (chiefly vibration velocity measurement in specific frequency ranges). Based on the results shown in this case study, we recommend applying the SPM HD and HD ENV techniques in addition to the manufacturer’s instructions on vibration measurements.
Image 4 Electric rope shovel dipper.
System setup
Measuring equipment
To measure all the measuring points described previously, a handheld Leonova Diamond instrument with all its vibration and shock pulse functionality activated was used.
The SLD144B-M8 vibration sensor and a magnetic base were used to take all vibration measurements. A shock pulse transducer with probe was used to measure shock pulses on each bearing. In some cases, the stethoscope function was used. The RPM sensor TTP10 was used to measure rotational speed and temperature – in some cases just to check and verify.
Measuring techniques
The contractor’s maintenance team follows a procedure laid out by the transmission manufacturer, stating that a set of tridirectional velocity measurements be done at 1000 Hz on all bearings. The instructions also indicate alarm levels for certain frequency ranges and the fault symptoms to look for. Acceleration measurement may be used, as well as other measuring techniques like shock pulse and enveloping, albeit the instructions recommend neither any particular methods for this nor frequency ranges or other input data.
To fulfill the procedure, vibration velocity measurement up to 1000 Hz was configured, along with all the band alarms and fault symptoms included in the instructions. Furthermore, acceleration, SPM HD shock pulse, and HD enveloping with a high enough frequency range to capture 3,5 x GMF of the fastest-rotating axis were also set up.
Image 5 Measurement assignments in the Condmaster software.
Condmaster setup
SPM HD
The standard configuration is 100 orders and 1600 lines. The symptom enhancement factor (SEF) was set to 1. The objective of this measurement is to capture only the bearing-related signals to enable in-depth analysis and diagnosis of bearing condition.
Image 6 SPM HD measurement assignment configuration.
Vibration velocity
This configuration was set to 1000 Hz, 6400 lines, with 4 lines averaging. The purpose of this measurement is to capture the machine’s general vibration condition, including gears and bearings and all its components, to fulfill the procedure recommended by the manufacturer.
Image 7 Vibration velocity measurement assignment configuration.
Vibration acceleration
This configuration was set to 10000 Hz, 25600 lines, and 4 lines of averaging. This measurement assignment is designed to capture high-frequency vibration that might originate from the bearings, gears, rubbing of components due to poor installation work, or manufacturing, for example.
Image 8 Vibration velocity measurement assignment configuration.
HD Enveloping
The HD ENV measurement configuration was set to 200 orders and 6400 lines. The symptom enhancement factor (SEF) was set to 1 and Filters 3 and 4. The purpose of this measurement is to capture all the signals coming from the gears and bearings and to do in-depth analysis of these components.
Image 9 HD Enveloping measurement assignment configuration.
Case description
Although designed for at least 25,000 working hours, one mining shovel crowd drive had reached only 7,000 hours when mine personnel detected vibrations on the drive foundation, indicating looseness. The mining company sent the crowd drive to the service contractor for overhaul and inspection. After refurbishing, inspection, and changing several components, the contractor’s maintenance team also performed the recommended vibration measurements as a part of the quality control process.
Supplementary measurements with HD technology indicated damage symptoms on one of the new bearings. It was particularly evident in the SPM HD and HD Enveloping readings on one of the lowspeed axis bearings. The maintenance engineers decided to disassemble the crowd drive for inspection, at which point they confirmed the bearing damages indicated in the HD technology measurements.
BSF (ball spin frequency) was detected on the final axis at 23 RPM with SPM HD and HD ENV, filters 3 and 4. The inspection showed pitting on the rolling elements where the fault was detected and some brinelling on the outer ring. These faults could not be seen with the naked eye but were clearly visible when using a microscope camera to evaluate damage severity.
Thanks to using HD technologies and detecting this very early stage damage in a brand new bearing, the contractor avoided costly downtime for the mine. Furthermore, the contractor can now provide more reliable analysis and diagnostics in the future, thus furthering its reputation as a trustworthy service partner.
Image 10 Detection of ball/rolling element spin frequency, as can been seen in Image 11 SPM
Image 11 BSF/pitting damage, as described above. Images: CMS Condition Monitoring Solutions Ltda.
Image 12 BSF/pitting damage as seen with HD Env, filter 3.
Economic justification
The contractor’s warranty for the mining company on the work of the shovel and all its components guarantees the machinery will run for 25,000 hours and/or about three years. The mining company pays the contractor about 600,000 USD every six months to ensure the accomplishment of the work hours.
The crowd drive failed at 7,000 hours and was sent back to the contractor for service. All the components were changed, and maintenance and reconditioning were carried out. In the quality control process, the HD condition monitoring technologies SPM HD and HD ENV detected damage on the rolling elements in one bearing. Thanks to these supplementary condition measurements, the contractor avoided the risk of sending a faulty drive back to the mine, in which case it might have had to be returned for service again before the six months were up, or, in the worst case – the drive could have caused a catastrophic failure.
In this win-win case, the condition monitoring investment paid for itself after a single instance of damage detection.
