For monitoring and diagnostic testing of electric motors, measuring the vibration of radial shaft motion in rotating components is crucial. High levels of roller radial runout can lead to inaccurate vibration readings, but radial runout of what is known as low-speed rollers, caused by mechanical and electromagnetic defects tracked by shaft detectors, is independent of shaft vibration.
Thus, vibrations measured during operation, including axial radial runout, may increase or decrease the recorded vibration. If the vibration reading is higher than the actual machine vibration, it may trigger unnecessary alarms or shutdowns. On the other hand, if the vibration reading is lower than the actual machine vibration, it may cause premature failure.
When selecting non-contact detectors, the measurement of radial runout of low-speed rollers is a standard requirement of the American Petroleum Institute (API) for motors. API 541 covers special-purpose, 500 hp and above molded squirrel-cage induction motors used in petrochemical applications. Unless otherwise specified, sliding bearing oil film is used by default in API motors.
This specification states that all hydrodynamic bearing motors intended to operate at speeds of 1200 rpm or higher should be equipped with or have a non-contact vibration or phase reference detector installed. If a vibration detector is provided or needs to be prepared, the detector tracking area must be provided and addressed to ensure that the combined mechanical and electrical radial runout does not exceed certain limits.
Figure 1: An eddy current, non-contact proximity detector is part of a sensing system that also includes an extension cable and a proximity module. This system measures the gap voltage between the detector probe and the detector track on a rotating component. Image source: Baldor
This type of test is typically performed using a non-contact proximity detector, such as an eddy current proximity detector. The detector measures the change in the gap voltage between the shaft and the detector probe. The change in the measured value is primarily due to vibration, but it also reflects the effect of radial runout of the low-speed roller.
The following section will analyze in detail the measurement methods and instruments used for different types of radial runout, the factors that lead to high levels of radial runout according to the acceptable levels specified by API standards, and their impact on vibration measurement.
A 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 gap voltage between the detector probe and the detector track on a rotating component. This gap voltage changes continuously, primarily due to shaft vibration, but can also reflect any detector out-of-roundness, concentricity between the detector track and the radial bearing, surface defects in the detector track area, shaft misalignment and bending, or changes in the electromagnetic properties of the shaft material near the detector track area.
All these vibration-independent changes in the gap voltage between the shaft and the detector probe define the total indicator reading (TIR), also known as total radial runout. Radial runout is displayed on the vibration reading and can cause measurement errors. This is why understanding radial runout is crucial for the monitoring and diagnosis of rotating machinery.
Definition and Classification
As defined in section 6.3.3.3 of API 541, fifth edition, a low-speed roller is a condition of an oil film bearing motor or generator where the rotor speed is between 200 and 300 rpm. At this speed, dynamic effects are at their lowest level, and vibration is almost non-existent. Under these conditions, the proximity detector readings should be closely related to mechanical defects in the detector track, including out-of-roundness, defects related to surface treatment, lack of concentricity between the detector track and the radial bearing, shaft bending, or electromagnetic defects in the shaft material.
The condition of low-speed rollers can be measured in the following ways: 1. inside an assembled machine; 2. on a rotating assembly placed on a V-block of a bearing half-housing; or 3. on a lathe. Radial runout of low-speed rollers primarily originates from two sources: mechanical and electrical.
Mechanical radial runout (MRO) measures the deviation of a shaft's cylindrical surface from a perfect cylindrical surface, as well as its concentricity with the bearing's centerline. Deviations include: surface out-of-roundness, surface mechanical defects (such as surface finish or scratches), and lack of concentricity between the surface and the bearing's axial center. MRO is measured using a dial indicator or a contact probe.
Electrical radial runout (ERO) measures the deviation between the conductivity and permeability of the shaft surface. The non-uniform electromagnetic properties of the shaft interfere with the magnetic field approaching the detector, thus causing changes in the processed signal as the gap voltage changes.
Please note that for the sensing system to function properly, the components must be matched. If these components are not properly matched, the measured vibration amplitude will be inaccurate. By default, proximity detectors are calibrated using AISI 4140 steel. Significant differences in steel type may affect measurement accuracy. If necessary, proximity detectors can be calibrated using other materials.
Figure 2: The detector can be mounted on the inside or outside of the bearing journal, depending on the motor design, and is mounted on top of the shaft in a specially machined position close to the bearing journal. This section of the shaft, known as the detector track zone, is machined to minimize mechanical and electrical radial runout. A minimum width of 1.5 times the detector probe diameter is recommended for the track zone.
Achieve effective measurement
The radial runout of the low-speed roller can be measured through the following process:
An induction coil is excited by alternating current, which creates an alternating magnetic field.
When a changing magnetic field interacts with a conductive material (such as a shaft), a small current called eddy current is induced within the material.
Conversely, eddy currents create an opposing magnetic field that opposes the original magnetic field.
The interaction between the two magnetic fields depends on the distance between the detector probe and the target material. As the distance changes, the change in the interaction between the two magnetic fields is translated into a voltage output.
The voltage output is then converted into displacement vibration units in mils or micrometers.
A common installation configuration includes two eddy current proximity detectors mounted on a bearing housing, 90 degrees apart, at 45 degrees to either side of the vertical centerline of the shaft.
The detector can be mounted on the inside or outside of the bearing journal, depending on the motor design. The detector is mounted above the shaft in a specially machined position close to the bearing journal. This section of the shaft, known as the detector track zone, is machined to minimize mechanical and electrical radial runout. The width of the track zone depends on the size of the detector probe. A minimum width of 1.5 times the diameter of the detector probe is recommended. This ensures that the magnetic field induced by the detector probe fully covers the machined area.
API 541 requires that the radial runout of the low-speed roller be measured during the run-flat test phase when the rotor speed is between 200 and 300 rpm. Within this speed range, the offset recorded by the detector is almost pure radial runout without any vibration. On non-API motors, the radial runout of the low-speed roller can be recorded at approximately 10% to 15% of the operating speed. The total radial runout recorded must meet the limits specified in the motor datasheet.
acceptable level
Motor manufacturers determine the acceptable level of radial runout for low-speed rollers based on customer requirements. API 541 limits the radial runout of low-speed rollers to 30% of the permissible peak-to-peak value of unfiltered vibration (1.5 mils), or 0.45 mils for induction motors. These limits are suitable for an already assembled motor.
If the radial runout limit is not met during manufacturing or initial testing, the motor needs to be disassembled and the shaft reworked. This process is both time-consuming and costly. Typically, to save time, motor manufacturers partially assemble the motor (see Figure 3) and then perform a quick test to check the radial runout of the low-speed roller, bearing alignment, and temperature. If the radial runout of the low-speed roller is within the limit, the motor can be assembled before full testing begins.
In light of this, the API 541 standard sets radial runout limits for rotating assemblies (assembled rotor and shaft) supported by V-blocks. In this way, the permissible mechanical and electrical radial runout limit is 25% of the permissible vibration limit (1.5 mils) for unfiltered waves. Keeping the radial runout within 0.375 mils increases the likelihood of controlling the assembled motor to the desired limits.
However, radial runout measured on a rotating assembly on a V-block may be low, but may still exceed limits after the motor is assembled. Causes include misalignment due to warped bearings, rib liners not being properly centered, rotor bending during installation, damage to the detector track area, or other issues.
Some motor manufacturers have higher requirements and set their own limits for mechanical and electrical radial runout, which are much lower (below 0.25 mils) in the bearing shaft diameter and detector area. This avoids problems in subsequent manufacturing processes.
Figure 3: The API 541 standard has set limits for radial runout of rotating assemblies (assembled rotor and shaft) supported on V-blocks.
The impact of vibration
In the past, simple mathematical subtraction was used to compensate for the vibration level under low-speed roller radial runout. If the vibration amplitude between peaks is 1.6 mils, and the low-speed roller radial runout is also known, for example, to be 0.45 mils, then (1.6-0.45)=1.15 mils is considered to be the true vibration.
This is actually incorrect because both vibration and low-speed roller radial runout have waveforms and cannot be simply added or subtracted before filtering. Unfiltered vibration contains all the frequency components of the input signal. At operating speed, if a vibration signal is filtered through a specific frequency—for example, expressed by amplitude and phase angle—it can be described as a vibration vector. As a vector, the filtered vibration at a given frequency (e.g., 1x or 2x) can be compensated for using a filtered low-speed roller at the same frequency through vector addition.
According to API 541, the filtered and compensated vibration displacement at the operating speed frequency should not exceed 80% of the limit for the unfiltered wave. Generally, motor manufacturers do not use compensation, although it can be useful in certain situations. Depending on the angular position of the vector, compensation may also increase vibration.
Factors affecting radial runout
Radial runout of a machine is the deviation between the measuring shaft and a perfectly cylindrical surface. It is primarily affected by manufacturing and assembly processes, as well as changes that occur over time during motor operation. Improper selection of cutting tools or machining parameters can lead to higher surface roughness. Mechanical damage occurring on bearing journals or detector tracks, such as scratches, nicks, and bends, can all affect the radial runout of a machine.
Since the measurement of radial runout is performed with reference to the bearing journal, if the probe's trajectory is not concentric with the bearing journal, it will result in high maintenance, repair, and overhaul costs. It is also affected by the following factors:
A straight shaft was pressed into a bent rotor;
The bent shaft was pressed into a straight rotor;
Misalignment caused by improper fixing between the motor frame and the bearing bushing;
Thermal instability within the rotor causes it to dent or bend.
Electrical radial runout is a measurement of the material inhomogeneity of a shaft. When using a non-contact eddy current detector to measure electrical radial runout, the interaction between the emitted and induced magnetic fields is converted into distance. Any phenomenon that alters the magnetic interaction between the detector probe and the shaft will affect the radial runout. This includes inhomogeneities in the material's texture, electromagnetic properties, or whether the shaft is magnetized. Whether the shaft is manufactured using forging or hot rolling processes, the machining of the shaft can affect the metallic properties of the material, thus influencing the electrical radial runout.
According to the API definition, low-speed roller radial runout of electric motors and generators is a measurement of radial runout on a rotating shaft at a low speed of 200 to 300 rpm, combining electromechanical and mechanical methods. Since radial runout affects vibration readings and can lead to measurement errors, it is important to understand its various influencing factors and how to eliminate them.
Monitoring radial runout levels during manufacturing helps avoid equipment disassembly and rework of the rotor on machine tools or grinding mills. Failure to meet the low-speed roller radial runout limits after machine assembly can be costly for both manufacturers and customers.
