Introduction
This case study describes damage in a ball mill girth gear, detected with the Airius I wireless vibration sensor, using vibration velocity and Enveloping Filter 3 measurements.
The customer Aurubis Bulgaria AD is a leader in the non-ferrous metals business in Bulgaria and a leading copper production plant in southeаstern Europe. The main activity of the company is the production of copper anodes and copper cathodes, as well as byproducts such as sulfuric acid and iron silicate.
The pinion drive gear of the ball mill runs with a slow rotational speed – 187 RPM. Ball mills operate using steel or other balls as grinding media. For the customer, obtaining long pre-warning times for gearbox and pinion bearing failures is essential.
Conclusion and summary
For many years, the customer has used six- and eight-channel versions of the Intellinova Compact online system to monitor the autogenous mill with SPM HD and HD ENV. In 2019, they invested in four Airius wireless vibration sensors with Wi-Fi communication for use on a ball mill. Two of these sensors are installed on the pinion bearings of the ball mill, which is the subject of this case study. The Airius sensors are of the AIR01-10 type, measuring three-axial vibration, temperature, and enveloping, in the 2–1,000 or 10–5,000 Hz frequency range.
The gear damage detected by Airius is best visible in measuring point P430TM01-PN-04 – Pinion bearing – North.
Measurements presented in this case study are collected from January 17th, 2020, to October 7th, 2020. At the beginning of April 2020, the vibration velocity trend nearly doubled in amplitude from 5.5 mm/s to 9.2 mm/s. The velocity remained stable over the next four months with an even trend. From the beginning of August 2020, the velocity started to increase again, and on October 11, 2020, it reached 21,5 mm/s.
In the spectrum, there are dominating gear mesh frequencies (GMF) of the pinion and girth gear with sidebands, matching symptoms of gear damage.
Gear mesh frequency (GMF) at speed 187 RPM: GMF1 = Z1/Z2 = 26/268 = 81,03 Hz
In addition to the readings from standard vibration measurements, from April to May 2020, the system registered increasing bearing noise with the Enveloping technique, with frequencies matching the BPFI (Ball Pass Frequency Inner Race) bearing symptom.
Since damage development was closely monitored from April 2020, the management estimated that the ball mill could operate until the planned annual maintenance stop in November 2020, when they replaced the girth gear. On it, a broken tooth was found.
Application description
Figure 1 3D graphic of a ball mill
The ball mill consists of an electrical motor, a pinion gear, and the mill drum with the girth gear.
Figure 2 Ball mill close-up – Airius sensor positions on the pinion bearings.
Measuring points:
– P430TM01-PN-03 – Pinion bearing South
– P430TM01-PN-04 – Pinion bearing North
Image 1 Ball mill; P430TM01-PN-04 – Pinion bearing North.
Image 2 Ball mill; P430TM01-PN-04 – Pinion bearing North.
System setup
Measuring equipment
2 x AIR01-10 – Frequency range 2-1000 / 10-5000 Hz
Measuring techniques
– EVAM – VEL, ACC in three axes
– Envelope, Filter 3 (500-10,000Hz)
Condmaster setup
Settings used in Condmaster Ruby 2019:
Case description
Measuring point P430TM01-PN-03 – Pinion bearing South
Measurements are taken from January 17th, 2020, to October 15th, 2020.
Image 1 VEL trend graph in three axes – horizontal, vertical, and axial, from January 2020–October 2020
Measuring point P430TM01-PN-04 – Pinion bearing North
Measurements are taken from January 17th, 2020, to October 15th, 2020.
Image 2 VEL trend graph in three axes – Horizontal, Vertical, and Axial, from January 2020 – October 2020.
Image 3 Trend graph, VEL-V (Vel RMS, ACC RMS, ACC Peak-to-peak), from January, 2020–October, 2020.
Image 4 EVAM – Vibration velocity VEL-V spectrum – 07.10.2020.
Image 5 EVAM – Vibration velocity VEL-V spectrum, October 7, 2020 – Z1 Pinion. Gear mesh frequency with sidebands, matching with gear damage symptom.
Image 6 EVAM – Vibration velocity VEL-V time signal, October 7, 2020.
Image 7 Trend graph, Enveloping – Filter 3, from January 2020–October 2020.
Image 8 EVAM – Vibration velocity VEL-V Colored Spectrum Overview.
Image 9 Enveloping – Filter 3 spectrum from April 17, 2020. Matching BPFIM symptom for inner race damage.
Image 10 Enveloping – Filter 3 time signal from April 17, 2020
Image 11 Enveloping – Filter 3, Velocity VEL-V Colored Spectrum Overview.
Appendix
Enclosed are pictures of the girth gear, where a broken tooth was found.
Image 12 The girth gear with a broken tooth.
Image 13 Girth gear with a broken tooth.
Image 14 The broken tooth on the girth gear.
Image 15 The outer race with the damaged tooth
