Root Cause Analysis of Mechanical Failure in a Pelletizing Machine | Phantom Gen 3
In this case study, we will analyze a problem that has arisen on a pelletizing machine since it began operating in 2021. This machine has consistently experienced premature breakage of the connection joint (also known as the “spider”) in the mechanical jaw coupling that connects the motor and the drive.
To accomplish this task, we employed Erbessd Phantom vibration analysis equipment. This equipment uses triaxial sensors to obtain accurate and reliable measurements of the machine on a periodic basis.
Observed characteristics of the equipment
The object of analysis is a pelletizer used in animal feed production. This system consists of several key components:
- Jaw Coupling with Connecting Joint (Spider): The equipment consists of an electric motor that runs a drive shaft by means of a jaw coupling incorporating a connecting joint or “spider”.
- 12-Channel Drive Pulley: From the drive shaft, the energy is transmitted to a driving pulley that has 12 channels with their respective belts.
- Belts: These belts are responsible for transferring the movement of the 12-channel driving pulley to a driven pulley, which has a cantilevered rotor-type shaft.
- Granulating Mill: The driven pulley is attached to the pelletizing die. This die contains two inner sprockets that compress the flour, moving it from the access hopper into the die holes to produce pellets.
Tasks Performed: Initial Analysis.
Due to the unpredictable nature of the failure, a thorough investigation was carried out to identify the root cause. Over a 10-day period, Erbessd Instruments’ Phantom vibration sensors were used to monitor the machine’s operation. These sensors are perfect for this purpose as they can be placed on the machine and will take data periodically.
The parameters examined included global vibration values, such as velocity and acceleration, as well as high and low frequency bands. Spectra and waveforms at different points and directions on the machine were also analyzed. This approach provided a complete understanding of the machine’s behavior and aided in the identification of the cause of failure.
Technology
The following equipment was used to record and analyze the data:
Achieved results
By comparing the measurement data with the acceptable values established by the standard, an accurate picture of the health of the machine in question has been obtained. ISO 10816-3 provides clear limits and criteria for determining whether vibrations are within safe ranges or indicate potential operating problems.
The results obtained from measurements made with triaxial vibration sensors compared to the values given in the standard are presented below:
| POINT | POINT DESCRIPTION | REGISTERED VALUE | ISO 10816-3 EVALUATION | ISO 10816-3 VALUES |
|---|---|---|---|---|
|
1.- MNDE V |
Motor Non Drive End V |
9.488 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
1.- MNDE H |
Motor Non Drive End H |
8.12 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
1.- MNDE A |
Motor Non Drive End A |
5.048 mm/s |
UNSATISFACTORY |
> 4.5 mm/s & ≥ 7.1 mm/s |
|
2.- MDE V |
Motor Drive End V |
41.54 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
2.- MDE H |
Motor Drive End H |
18.456 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
2.- MDE A |
Motor Drive End A |
38.712 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
3. DDE V |
Driver Drive End V |
18.725 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
3. DDE H |
Driver Drive End H |
21.889 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
3. DDE A |
Driver Drive End A |
13.62 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
4. DNDE V |
Driver Non Drive End V |
10.303 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
4. DNDE H |
Driver Non Drive End H |
15.246 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
|
4. DNDE A |
Driver Non Drive End A |
11.309 mm/s |
UNACCEPTABLE |
> 7.1 mm/s |
Spectrum comparison
By comparing the measurement data with the acceptable values established by the standard, an accurate picture of the health of the machine in question has been obtained. ISO 10816-3 provides clear limits and criteria for determining whether vibrations are within safe ranges or indicate potential operating problems.
The results obtained from measurements made with triaxial vibration sensors compared to the values given in the standard are presented below:
Motor Non Drive End - Vertical Axis
Motor Drive End - Vertical Axis
Driver Drive End - Horizontal Axis
Driver Non Drive End - Horizontal Axis
Results
Motor technical analysis
No obvious mechanical failure pattern is detected. However, in the time waveform (TWF) plots, random impacts of very high amplitude can be seen, which generate excitations at the lower frequencies.
SKF NU322 ECP/C3 DE Motor Bearing:
- There is no harmonic fault present for this bearing.
- The bearing is in state 0.
SKF 6316 VL0241/C3 NDE Motor Bearing:
- There is no harmonic fault present for this bearing.
- The bearing is in state 0.
Driver technical analysis
No obvious mechanical failure pattern is detected. However, in the time waveform (TWF) plots, random impacts of very high amplitude can be seen, which generate excitations at the lower frequencies.
SKF 22226 EK DE Driver Bearing:
- There is no harmonic fault present for this bearing.
- The bearing is in state 0.
SKF 22226 EK NDE Driver Bearing:
- There is no harmonic fault present for this bearing.
- We have observed harmonics of the shaft rotation speed, and their amplitude is low.
- The bearing is in state 0.
Diagnosis
After ruling out any signs of bearing failure and finding no obvious failure pattern, an intriguing hypothesis is put forward: random impacts affecting the drive shaft could be the result of abrupt changes in the rotational speed of the driven pulley. This is because the die sometimes exhibits significant resistance to rotation due to momentary blockage of the processed material.
Generally, the motor has the ability to compensate for this moment of inertia, although in some cases the drive has protected the motor by switching it off due to the excess consumption this could generate. This applies excessive stress to the jaw coupling, resulting in a drastic reduction in the life of the connecting joint of this component, known as the “spider”.
Results
In summary, thanks to the periodic measurements collected by the Phantom sensors, we have determined that no immediate maintenance intervention is required with regard to the bearings.
However, we have identified some areas where we could improve the operating efficiency of the machinery. For example:
- It would be beneficial to consider redesigning or improving the raw material (flour) feed system so that it flows more evenly, avoiding blockages.
- It would also be advisable to review and improve the clamp coupling so that it can withstand the required effort without causing premature wear of the connection joint.
Vibring is a Spain-based company and an authorized distributor of Erbessd Instruments. Specializing in the development of innovative solutions utilizing advanced technology for diagnosing and monitoring the condition of industrial machinery, Vibring achieves this through non-invasive monitoring and analysis of mechanical and electrical variables.
The primary application of these solutions lies in the realm of predictive maintenance, involving the analysis of vibrations and electrical variables. The company boasts specialists certified to ISO standards and possesses extensive experience in machinery diagnostics, serving leading companies in sectors such as food, chemicals, mining, among others.
