Introduction
The tire plant in this case study – manufacturing larger tires, such as for buses and trucks, among others – has been an SPM customer since 2011, stepping up its maintenance strategies over time and investing in condition monitoring equipment from SPM. The aim is to optimize production processes with as few unwanted stops as possible. Over the years, the plant has purchased some handheld Leonova Diamond instruments and has also expanded its portfolio of condition monitoring tools with online units. First out were the Intellinova Portable and Intellinova Standard systems, with Intellinova Parallel units also acquired later on to optimize the speed and accuracy of measurements.
Currently, the plant measures the mechanical condition of more than ten mixing lines with gearboxes driven by 1.5 MW motors. They are also measuring on more than fourteen so called extrusion lines and power stations. So far, the condition monitoring program covers almost 700 measuring points on a total of 190 machines.
The condition monitoring program at this tire plant has been very successful with several potential damages caught with the technology and measuring units from SPM, which has lead to large sav- ings over the years. The case described here is one of several.
The below case study involves a gearbox bearing failure caused by the passage of electric current. When an electric current passes through a rolling element bearing, electrical discharge takes place through the lubricant between the raceways of the inner and outer rings and the rolling elements. The heat, which is generated by the discharge, causes local melting of the bearing metal surface, ultimately leading to bearing failure.
Conclusion and summary
A bearing replacement was planned for February 2021, and after disassembling the bearing, damage on the outer ring and rolling elements was found, shown as typical signs of electrical current passage. There were also visible imprints on the gear teeth.
The conclusion was incorrect assembly of the coupling, with mechanical and electrical contact be- cause the motor shaft voltage was grounded via the gearbox, which is not the correct way to ground this application.
Following the bearing replacement, proper mounting of the coupling and grounding has been en- sured and measuring results are now back to normal levels.
Application description
The application is a four-stage Guibe gearbox. Guibe is a Spanish manufacturer, and the company specializes in designing and manufacturing customized mechanical reducers and gears for various sectors, especially the rubber industry. Driven by an electrical motor with 1.5 MW size and capacity, this power transmission is found in the mixing line.
Figure 1 Illustration of a Guibe gearbox.
Below pictures show and describe the set-up of the gearbox in mixing line ML25. The application is production-critical.
When it comes to condition monitoring, the gearboxes in the mixing lines are very challenging appli- cations, mainly for the following reasons:
- rapidly changing output speeds in a wide range of 5–45 RPM (see graphics below)
- the gearbox is at the optimum load for measurement for only a few tenths of a second at a time
- the speed profile depends on the actual rubber recipe, and may change a couple of times per day
- most gearboxes are under-the-hood, meaning they are not available for handheld measure- ment
- the environment is noisy, hot, and dirty (powdered carbon as part of the rubber compound)
Image 1 The gearbox operates in the 5-45 RPM range.
The speed profile of one of the recipes used in mixing the rubber compound:
- Emptying the mixer with minimal load – not suitable for vibration measurement
- Optimal load for measurement
Image 2 The Guibe gearbox.
Background
After the online measuring unit was installed in September, 2019, a higher vibration level was found on the motor’s axial direction. The vibration pattern was very clear, matching the 1st GMF (gear mesh frequency) symptom, and came from the gearbox transmission.
Further on, we recommended checking the axial settings (the clearance) of the coupling and the geometry of the shafts. Unfortunately, this was not possible to check during the measuring unit in- stallation because the motor-gearbox assembly was in operation.
Starting in June 2020, an increase in the HD ENV Filter 3 level was observed, with values showing changes of up to +30 dB. Two months later, the BSF symptom increased. The SPM HD shock pulse values increased with +25 dB and the VEL readings from 0.5 mm/s to 3.5 mm/s on the 1st GMF symptom (higher harmonics). This is not the typical case (usually HD ENV Filter 3 follows the shock pulse trend).
System setup
Measuring equipment
The application setup consists of the Intellinova Parallel online system together with eight DuoTech transducers (five × shaft DS + NDS; one bearing is inaccessible) on the gearbox, with an additional three DuoTechs on the motor (NDS, DS radial, DS axial) and one rpm sensor for best results.
Image 3 DuoTech sensor positions.
All transducers are installed directly on the bearing houses, as close to the bearing as possible. The gearbox has helical teeth, so it is possible to use both axial and radial directions for vibration meas- urements.
RPM measurement troubleshooting
When installing the speed sensor, a separate problem was solved. The only available option was to place an inductive sensor on the circumference of a 500 mm diameter coupling. The head of the M8 screw was chosen as the sensing point, which turned out to be insufficient – the sensor was not able to sense higher speeds even though its Fmax = 500 Hz. However, the calculation of the switching frequency of the inductive sensor is based on the pulse length, as shown below:
Figure 2 Switching frequency calculation. Source: www.balluff.com
Based on this calculation, our actual switching frequency was about 1.480 Hz (even if the maximum rotating fre- quency was 28.3 Hz). This was the reason why we changed the proximity switch to the diffuse infrared sensor BOS00K9 (Balluff) and painted part of the coupling black. The ad- vantage of this sensing principle is the large switching dis- tance, which is not sensitive to precise settings. This is a re- liable solution working today without a problem.
List of used items:
Measuring unit: Intellinova Parallel EN, 16 channels
Transducers: SLC144TD-M8, DuoTech
RPM sensor: Optical diffuse IR, Balluff BOS00K9
Cables: 2 x 0.34, PUR Unitronic FD CP (TP)
Connectors: M12, 90°, 90511
Image 4 Position of infrared sensor for rpm measurement.
Measuring technique(s)
All the high-definition measuring techniques are in use, both in vibration and shock pulse measurement.
The purpose of the techniques used is as follows:
SPM HD bearing problems – early warning
HD ENV, Filter 3 more advanced bearing surface damage
VEL RMS mainly GMF and other vibration-related problems
Condmaster setup
Image 5 Measuring techniques setup in Condmaster.
The main idea is to use triggered measurements and find a suitable time when the gearbox is under load, with relatively small changes in working speed. Thus, the RPM trigger type “RPM run down” is used in combination with “Max. fluctuation = 20%” enabled.
Image 6 RPM measurement is triggered when the rotational speed goes down.
The parameter settings for the analysis of the input part of the gearbox (1. + 2. gear) is standard, for shock pulse as well as vibration, capturing especially the GMF symptom. The highest GMF is around 200 Hz (see above for triggered measurements), so it is clearly visible, including harmonic multiples.
The settings for slower gears are then limited by the length of the relatively stable speed (approx. 20-sec max.). The drawback of that is reduced accuracy and a reduced Symptom Enhancement Factor (SEF), but readings are still good.
Case description
After the installation of the online system in September 2019, slightly increasing vibrations were ob- served on the motor in the axial direction. The vibration spectrum was dominated by the 1st GMF symptom, which was somehow transmitted to the engine from the gearbox. Other measured param- eters were in the expected values comparable with other mixing lines.
For this reason, we recommend that an inspection be carried out concerning:
- the axial setting (clearance) of the coupling
- the geometry of shafts
Due to the very limited time available, only a quick check was performed, which did not reveal any problems.
From July 2020, we first noticed an increase in the HD ENV Filter 3 parameter with a symptom of bearing damage (BPFO, BSF). Two months later, starting in September 2020, there was a visible in- crease in the SPM HD technology also, with the same spectral symptoms. This is not typical; in the vast majority of bearing damage cases, the SPM HD technique is the first to react with the earliest possible warning, and only then do the envelope values increase.
Soon thereafter, vibrations with the 1st GMF symptom (especially the higher harmonics) began to increase, and it was clear from this that the problem not only damaged the relatively easily replacea- ble bearing, but also the gear teeth.
Image 7 The condition trends for the various measuring techniques started to show increasing values at different points in time, with HD ENV Filter 3 indicating the earliest signs of a potential problem, about two months before the SPM HD trend started to increase.
Image 8 Colored Spectrum Overview, HD ENV Filter 3 with BPFO symptom.
Image 9 Colored Spectrum Overview, SPM HD with BPFO symptom.
Image 10 Colored Spectrum Overview, HD ENV Filter 3 with 1st GMF (4th harmonic).
The planned bearing replacement in February 2021 revealed damage on the outer race and rolling elements with clear signs of electric current passage and imprints on the gear teeth.
Image 11 The damaged bearing.
Image 12 Damaged gear teeth.
Image 13 Damaged gear teeth.
During disassembly, the main reason for the problem was revealed: incorrect mounting of the cou- pling with mechanical and electrical contact, leading to the motor shaft voltage being grounded via the gearbox.
Economic justification
A one-hour shutdown of one mixing line represents a loss of €500.
The planned repair of the gearbox bearing takes about three days, which is €36,000.
However, an incident that would require the unplanned delivery of new spare parts for the gearbox is many times longer; the estimated delivery time of new gears is at least three months.
Other costs (personnel and material), on the other hand, are negligible.
