Frequent questions from designers and operators of ventilation systems with fans concern the life of fan bearings or their service intervals. It has been found that 50% of all electric motor failures are due to bearing failure.
The question of lifetime can only be answered tentatively in all cases, as a number of factors affect the lifetime of bearings. These include motor speed or number of poles, motor axis position, number of starts versus continuous running, speed control by frequency converters or phase voltage regulators, bearing temperature, additional axial loads in the direction of the motor axis caused by changes in fan flow or position, etc. In general, the following guide values ideally apply.
- Exclusive bearings ideally have a service life of 20,000 to 24,000 hours.
- ball bearings ideally have a life of 30,000 to 40,000 hours.
Factors that shorten life
- high motor speed (e.g., a change from 6 to 2 poles can mean a change from 40,000 to 20,000 hours)
- vertical mounting of the motor or its axis (e.g. horizontal mounting of axial fans)
- extreme changes in bearing temperature or exceeding the permitted temperature
- condensation of water in the equipment (see condensation article), which can occur, for example, in exhaust fans mounted in building envelopes
- large changes in flow rate over time in axial fans or in general in variable air volume ventilation systems (the worst situation is in VAV systems with 3-phase motors and frequency converters, regulated by other variables such as flow rate, temperature, pressure, etc., according to manufacturers' data, lifetime reductions of up to 70% have occurred in some cases)
- axial overloading of bearings which can occur when motors and fans are mounted vertically, at the same time as the operating point in the area of maximum flow or rapid temporal changes in flow
- improper belt tensioning, both under and over tensioned
- the use of phase controllers which can induce high frequency vibrations in magnetic circuits that are transmitted to the bearings
- the use of frequency converters in three-phase motors without electrically isolated bearings, causing microerosion of the ball surface and sliding paths, lubricant degradation and drying out (the phenomenon is caused by micro discharges in the bearings, caused by the short tripping time of the triacs and the non-zero sum of the voltages in the switched three-phase system, the phenomenon can be detected by an oscilloscope connected between the shaft and the grounding point, see the documentation of the bearing manufacturers for details)
- vibration of the system caused by imbalance of the impellers, for example due to dirt, fan operation outside the working area, incorrect operating point for parallel running fans, etc.
In these cases, the reduction in service life can only be estimated very roughly; in any case, it is recommended to carry out periodic checks of the bearings in accordance with the building's operating rules, as well as periodic inspections of the wiring, or connection, alignment and cleaning of the fans.