End-of-line Bearing Quality Monitoring on Compact Ventilators - A Case Study

1.6 Test Rig & Instrumentation

In the market segment of decentralized domestic ventilation, compact blowers and axial fans are utilized. The fan acoustic performances are thereby a very important sale argument. Indeed, these fans are likely to be installed in bedrooms and disturb the customer’s sleep. When running at very low rotational speed (so called “sleep mode”), airborne noise is negligible whereas structure borne noise is very important. Intern preliminary studies have shown that damaged ball bearings can enhance the fan acoustic emissions up to 8-9 dB(A). A bearing monitoring in End-of-Line is therefore of greatest importance.
The present case study will be divided into the following steps:
Problem description: end-of-line testing has to cope with different constraints. Some are technical (background noise, hardware), others are economical (production cycle time). The full framework will be presented.
Technical main solution: bearing monitoring is carried out by means of vibro-acoustic measurements envelope analysis with peak mode, inspired by commercial codes. This method will be briefly introduced and its choice will be justified. The method’s advantages and drawbacks as well as its practical requirements and possible conflicts with the aforementioned constraints will be scrutinized.
Practical implementation: this part will be the presentation core. A step-by-step description of the implementation will be presented in order to present the difficulties the authors had to cope with. The main technical points include sensor positioning and fan rotation rate correction. Indeed, the fan driving electronics do not allow a very stable rotation rate, which though is one of the key points in order to get a monitoring with good quality. In order to compensate rotation rate fluctuations, order tracking had to be implemented. Several techniques were tested. A last point deals with automatic decision and quality management. On one hand, operator influence shall be reduced to its minimum. On the other hand, the process quality has to be ensured, which is often evaluated with repetition tests. This represents a great challenge, since peak mode analysis does not allow repetitive results.
Interest of the present communication: End-of-line Testing and condition monitoring do share the same methods and face the same challenges when trying to extract information from vibroacoustic measurements.