Introduction Vertical
roller mills (VRM), a.k.a. raw mill or roller mill, are mainly used to grind cement clinker, slag, raw coal, and other raw materials into fine powder during cement production.
In May 2015, when Vietnamese cement manufacturer Hoang Mai replaced a gear assisted by the gear supplier, an SPM online system was installed to monitor vertical mill gearbox condition.
Conclusion and summary
SPM HD readings detected a problem on the bearing cage on the output pinion shaft in December, 2015. With a pre-warning time of four months, the plant scheduled a shutdown for bearing replacement in May, 2016.
When the bearing was removed from the pinion shaft, it was found that parts of the bearing cage were broken (see below).
Application description
An twelve-channel Intellinova Compact system with seven shock pulse transducers type 44000 and four vibration transducers were installed to monitor the motor and two-stage planetary gear and bearing. The Condmaster Ruby 2016 software with the SPM HD Expert and FFT symptom modules was used for setting up the measurement assignment.
The Graphical Overview screenshot below shows the equipment with transducer locations.
Inner gear
Four shock pulse transducers and one vibration transducer were installed inside the gearbox.
External gear casing
– 1x shock pulse transducer is installed on the input pinion shaft bearing casing
– 1x vibration transducer is installed on the input pinion shaft bearing casing (axial)
– 1x vibration transducer is installed on the upper stage planet casing (horizontal)
Motor
– 2x shock pulse transducers installed on the motor DE/NDE bearing casing
– 1x vibration transducer installed on the motor DE casing (horizontal)
– 1x proximity rpm sensor installed on the motor DE
System setup
The measuring assignment setup in Condmaster Ruby:
Other setup in Condmaster
Two sets of measurement assignments were set up in Condmaster:
– One set of the measurement assignments is triggered with rpm for order tracking and TSA function.
– One set of the measurement assignment uses fixed rpm without order tracking and TSA function.
All bearing and gear information was obtained during the gear replacement in early May 2015.
Case descriptions
The case is taken from a fixed-rpm assignment because the rpm sensor stopped working after three months due to a loose contact. However, since the vertical mill runs at a stable and constant rpm, the bearing symptoms could still be seen.
The SPM HD trend for ‘Income shaft bearing-out Pt 5 pinion shaft bearing’ triggered the first alarm in December, 2015. The trend continued to indicate an abnormality and deteriorated further in April, 2016. The plant engineer ordered bearing replacement in April 2016, after which the shock pulse trend dropped back to normal.
An early FTF symptom match for the bearing cage can be seen in the SPM HD spectrum in December, 2015.
Spectrum, time signal, and circular plot on 27th April, 2016, showing damaged rolling elements entering and leaving the loaded zone, thus causing the characteristic spectrum pattern.
FTF symptom match for bearing cage showed matches of FTF at 0.432 orders and 2x FTF harmonic at orders well below 1 rpm.
The below Coloured Spectrum Overview shows the full course of the FTF symptom, starting from December, 2015 through May, 2016. Pre-warning time about four months.
Economic justification
1 day production downtime estimated USD 30,000 (production 500 tons per day).
Unplanned shutdown
10 days waiting time for new bearing delivery (providing FAG and SKF have the bearing in their global stock warehouse)
10 days x USD 30,000 production loss = USD 300,000
Including replacement and alignment
Unplanned shutdown estimate 10 days waiting for bearing and manpower arrangement.
Planned shutdown
1 day x USD 30,000 = USD 30,000
Total saving for planned shutdown = USD 270,000
