| | | Fan & Pump Monitoring The majority of rotary fan and pump systems can be broken down in to three categories; direct coupled, gearbox coupled and belt coupled. Each of these systems will exhibit a unique set of failure modes between the motor and shaft interface. Faults at the blade or driven unit interface are common across the various configurations. Unbalanced Fan or Pump This is likely to be caused by a build up of debris on the blades over time and can put stresses on to other system components. Depending on the amount of imbalance, this generally results in high levels of vibration and can be detected using accelerometers fitted on both the fan/pump bearing and the motor bearing.The typical vibration frequency of the unbalance would be at the shaft rotating frequency. If the problem affects all blades then the frequency would be the shaft rotational speed multiplied by the number of blades. Like all bearings manufacturer, sealed bearings will fail over time. Premature failure can be the result of insufficient or contaminated lubrication, as well as excessive operating temperatures. The failure will cause elevated levels of vibration at the shaft frequency and the vibration spectrum signature will normally be high in shaft frequency harmonics. The bearing will typically run at a higher temperature during the early stages of failure. Fitting a sensor offering both vibration and temperature detection is the ideal.Vibration can also be measured by monitoring the shaft transverse movement with proximity probes, if access to the bearing is difficult. Roller or Ball Bearing Defects As above, the same causes of failure apply, although for roller or ball bearings, manufacturing defects do play a larger role in premature failure mechanisms.Vibration monitoring again is the best method of detection, although the exact signature of failure is dependent not only on the rotational speed but also the bearing race dimensions and configuration. Usually these defects are best detected by using shock transducers mounted directly on the bearing housings. Misalignment This type of problem normally occurs following incorrect assembly of the system components. eg misalignment between the shaft, motor and driven unit.This is normally detected through bearing vibration monitoring, ideally utilising both horizontal and vertical mounted transducers.The typical vibration frequency would be 1, 2, or 3 times the shaft speed depending on the exact nature of the misalignment. Gearbox Coupled Type Apart from the bearing monitoring detailed above, gear tooth faults can also be detected at an early stage, using accelerometers mounted directly on the gearbox casing. The typical vibration frequency of a tooth fault can be detected at the shaft frequency multiplied by the number | | of teeth, as well as harmonics of this. Although in some cases gear hunting problems can generate frequencies 1/10th to 1/20th shaft speed. It is also important on gearbox monitoring to differentiate between tooth faults and shaft/bearing faults. For better diagnostics, a once per shaft revolution pulse is sometimes required. Generates vibration frequencies of 2x, 3x & 4x the belt frequency, normally 2x being the dominant peak and amplitudes are usually erratic. Eccentric Pulleys/Sheaves & Misalignment These problems all cause high levels of vibration at the rotating frequency. If misalignment is the problem, the highest vibration is usually at the fan/pump end and in the axial direction (eccentric vibration is normally in line with the belt). Belt Resonance This is normally detected as a small peak of vibration of regular frequency and amplitude and should only be a problem if the frequency coincides with the motor or fan/pump rotating frequency. Monitoring Solutions A low cost, effective solution, for the protection and monitoring of single or multiple units is to have vibration transducers, industrial temperature sensors and eddy current probes (for shaft vibration, eccentricity or thrust wear) feeding into a local unit with signal conditioning.The Sensonics DN range of products is ideal for this type of application | | |
| | | The DN2601 is capable of independently monitoring two channels of vibration, allowing user configurable alarm windows for the switching of internal contacts to raise a warning or to trip the fan or pump. The outputs can be used to log/view the pump/fan condition from a remote location (either by analogue current/voltage outputs or by a comms link). The DN2604 offers both vibration and temperature monitoring in a single DIN rail mountable unit; this in combination with Sensonics PZAT range of dual sensors (vibration and temperature) forms a very cost effective solution. Speed measurement with Overspeed protection is available in the DN2608 module configuration. | | |