The key to motor monitoring and diagnostic testing - how to accurately measure the radial runout of a low speed roller
For motor monitoring and diagnostic testing, it is critical to measure the vibration of the radial axis motion in a rotating component. High levels of roll radial runout can result in inaccurate vibration readings, but the radial runout called the low speed roll caused by the mechanical and electromagnetic defects tracked by the shaft detector is independent of the shaft vibration.
Thus, vibrations measured during operation, including shaft radial runout, may increase or decrease the recorded vibration. If the vibration reading is higher than the actual machine vibration, an unnecessary alarm or shutdown condition may be triggered. On the other hand, if the vibration reading is lower than the actual machine vibration, then the failure may occur prematurely.
When selecting a non-contact detector, the measurement of the radial runout of the low speed roller becomes a standard requirement of the American Petroleum Institute (API) for the motor. The API541 standard covers special-purpose, 500-horsepower and above molded-squirrel cage induction motors for use in the petrochemical applications industry. The plain bearing oil film is used by default in API motors unless otherwise specified.
It is stated in this specification that all hydrodynamic bearing motors intended to operate at speeds greater than or equal to 1200 rpm should be equipped with or fitted with non-contact vibration or phase reference detectors. If a vibration detector is provided or the detector needs to be prepared, the detector tracking area must be provided and processed so that the total mechanical and electrical radial runout combination does not exceed a certain limit.
This type of testing is typically accomplished by using a non-contact proximity detector, such as an eddy current approaching detector. The detector measures the change in the gap voltage between the shaft and the detector probe. The change in measured value is mainly due to vibration, but it also reflects the influence of the radial runout of the low speed roller.
Below we will analyze in detail the measurement methods and instruments used by some different types of radial runouts, according to the acceptable levels specified by the API standards, the factors affecting the high level of radial runout, and its impact on vibration measurements.
The non-contact proximity detector is part of a sensing system that also includes an extension cable and a proximity module. The system measures the change in the gap voltage between the detector probe of the rotating component and the detector track. This gap voltage is constantly changing, mainly because of the vibration of the shaft, but it can also reflect the roundness of any detector, the concentricity between the detector trajectory and the radial bearing, the surface defect of the detector trajectory area, the misalignment of the shaft and A change in the electromagnetic properties of the shaft material that is bent, or near the area of the detector track.
The change in the gap voltage between all of these vibration-independent axes and the detector probe defines the total indicator reading (TIR), or also the total radial runout. The radial runout is shown on the vibration reading and can cause measurement errors. This is why understanding radial runout is critical to the monitoring and diagnosis of rotating machinery.
Previous: Definition And Classification