Motors, Drives

How slow roll runout works in electric motors

Vibration measurement a requirement on API motors.

By Papa Diouf, P.E. and Bryan Oakes, Baldor Electric January 30, 2018

Read the full .pdf of this story "Understanding Slow Roll Runout in Electric Motors" here for more information and images.

Vibration measurement of the radial shaft movement in rotating components is critical to electric motor monitoring and diagnostic testing. However, high shaft runout can lead to inaccurate vibration readings, because so-called slow roll runout, caused by mechanical and electro-magnetic defects in the shaft probe track, is independent of the shaft vibration.

Thus, the vibration measured during operations includes the shaft runout, which could increase or reduce the recorded vibration. If the vibration reading is higher than the true machine vibration, then unnecessary alarm or shut off conditions can be triggered. On the other hand, if the vibration reading is lower than the true machine vibration, then premature failure can occur.

Measurement of slow roll runout is a standard requirement on American Petroleum Institute (API) motors when non-contacting probes are specified. The API 541 covers the minimum requirements for special-purpose, form-wound squirrel-cage induction motors, 500 hp and larger for use in petrochemical applications. Unless otherwise specified, oil film bearings are by default used in API motors.

In this specification, all hydrodynamic bearing motors intended to operate at speed greater or equal to 1200 rpm, shall be equipped or have provision for non-contacting vibration and phase reference probes. When vibration probes are supplied or provision for probes is required, a probe track area must be supplied and treated so that the total combined mechanical and electrical runout does not exceed a certain limit. 

How it’s done

Such testing typically is done using non-contacting proximity probes, such as eddy current proximity probes. The probes measure a varying gap voltage between a shaft and probe tip. Variation is mostly due to vibration, but also reflects the impact of slow roll runout.

Let’s look at some different runout types, measurement methods and instruments used, acceptable levels according to the API standards, contributing factors to high runout levels, and its impact on vibration measurement.

The non-contacting proximity probe used is part of a transducer system that also includes an extension cable and proximitor. As noted, the system measures the gap voltage variation between probe tip and probe track on the rotating element.

This gap continuously changes, mostly due to the shaft vibration, but also reflects any probe track out-of-roundness, concentricity between the probe track and bearing journal, surface defects on the probe track area, shaft misalignment, shaft bending, or variations in the electromagnetic properties of the shaft material near the circumference of the probe track area.

All these non-vibration-dependent changes of the gap between shaft and probe tip define total indicator runout (TIR), or simply, runout. Runout will appear in the vibration readings and can lead to measurement errors. That’s why understanding runout is crucial to rotating machinery monitoring and diagnostics. 

Definitions and details

Slow roll, as defined in API 541 5th Edition section, is a condition in oil-film bearing motors or generators in which the rotor moves between 200 to 300 rpm. At this speed the dynamic effects are minimized. Vibration is almost non-existent. In this state, proximity probe readings should be sensitive to probe track mechanical defects, including out of roundness, or those related to surface finish, lack of concentricity between the bearing journal and track area, non-straight shafts, or electromagnetic defects in the shaft material.

A slow-roll condition can be measured: 1) in the assembled machine; 2) on the rotating assembly positioned in v-blocks on the bearing half shells; or 3) in a lathe. Slow roll runout has two aspects: mechanical and electrical.

Mechanical runout (MRO) is a measure of the shaft cylindrical surface deviation from a perfectly round surface, concentric with the bearing centers. Deviations include: surface out-of-roundness; mechanical defects on the surface, e.g., surface finish or scratches; or lack of concentricity between the surface and the bearing journal centers. Mechanical runout is measured with a dial indicator or a contacting probe.

Electrical runout (ERO) is a measure of shaft surface electrical conductivity and magnetic permeability variation. Non-uniform shaft electro-magnetic properties interfere with the magnetic field of the proximity probe, thus causing a change in the processed signal as a gap voltage variation.

Note that ensuring transducer system operation requires the components to be matched sets. If these components are not matched properly, measured vibration amplitudes will not be accurate. Proximity probes are, as a default, calibrated to AISI 4140 steel. If the steel is significantly different, it could impact measurement accuracy. Proximity probes can be calibrated to other materials if necessary.

How it’s done

To measure slow roll: 

  1. The inductive coil is excited with alternating current which creates an alternating magnetic field.
  2. When a changing magnetic field interacts with a conductive material (such as the shaft), small currents, called eddy currents, are induced in the material.
  3. The eddy currents, in turn, create an opposing magnetic field, resisting to the original magnetic field.
  4. Interaction between the two magnetic fields is dependent on the distance between probe tip and target material. As distance varies, changes in the interaction between the two magnetic fields is converted into voltage output.
  5. Voltage output is then converted into vibration units of displacement in mils or microns.

One common mounting configuration consists of two eddy current proximity probes mounted on the bearing housing and located at 90° apart and at 45° from the vertical shaft centerline.

Probes can be mounted inboard or outboard of the bearing journal depending on motor design. Probes are placed over the shaft in an area specially machined and adjacent to the bearing journal. This shaft area, called the probe track zone, is machined to minimize mechanical and electrical runout. Track zone width is dependent on probe tip size. A minimum width of 1.5 times the diameter of the probe tip is recommended for the track zone. This ensures the induced magnetic field from the probe tip fully penetrates the machined area.

API 541 requires that the slow roll runout be measured during coast-down when the rotor speed is between 200 to 300 rpm. At this speed range, probe-recorded displacement is almost purely runout without any vibration. On non-API motors, slow roll runout can be recorded at approximately between 10% to 15% of the operating speed. Total runout recorded must meet the required limit set forth by the motor specification.

Acceptable levels

Electric motor manufacturers refer to customer specifications to determine the acceptable levels of slow roll runout. API 541 limits the slow roll runout to 30% of the allowable unfiltered vibration peak to peak (1.5 mils), or 0.45 mils for induction motors. This limit applies to an assembled motor.

If the runout limit is not met during manufacturing or initial testing, the motor will be disassembled and the shaft reworked. This process can be time consuming and costly. Usually, to save time, motor manufacturers partially assemble the motor (see Figure 3) and perform a quick test to check slow roll runout and bearing alignment and temperature. If the slow roll is within the limit, then the motor will be finish assembled before the complete testing is started.

For this reason, API 541 standards has set a runout limit on the rotating assembly (rotor and shaft assembled) while supported in V-blocks. With this method, the allowable combined mechanical and electrical runout limit is 25% of the unfiltered allowable vibration limit, peak to peak (1.5 mils), or 0.375 mils. Keeping the runout within 0.375 mils increases the chances of achieving the desired limit with the motor assembled.

However, a rotating assembly can have a very low runout on V-block and yet still exceed limits after motor assembly. Contributing factors include misalignment caused by cocked bearings, non-concentric frame bracket fits, a rotor bent during assembly, damaged probe track area, or other challenges.

Some motor manufacturers go further and self-impose a combined mechanical and electrical runout limit that is much lower (less than 0.25 mils) on the shaft bearing journal and probe areas. This avoids finding issues arising later in the manufacturing process. 

Impact on vibration

In the past, simple arithmetic subtraction was used to compensate vibration levels from slow roll runout. If the vibration amplitude was 1.6 mils peak to peak and the slow roll runout was known, for example, to be 0.45 mils, then (1.6 – 0.45) = 1.15 mils was considered the true vibration.

This is incorrect because both the vibration and slow roll runout are waveforms and cannot simply be added or subtracted without filtering them. The unfiltered vibration contains all the frequency components that are in the incoming signal. When a vibration signal is filtered at a particular frequency at running speed, for example, it is expressed in an amplitude and phase angle that can be described as a vibration vector. As a vector, the filtered vibration at a given frequency, such as 1 or 2 times, can be compensated with the filtered slow roll at the same frequency as a vector addition.

Per API 541, the compensated vibration displacement filtered at running speed frequency (1x) shall not exceed 80% of the unfiltered limit. Compensation is not used in general by motor manufacturers, but can be useful in certain situations. Compensation could also increase the vibration depending on the angular position of the vectors. 

What impacts runout

Mechanical runout is the measure of shaft deviation from a perfectly cylindrical surface. It is mainly impacted by the manufacturing and assembly process and changes over time during motor operation. Improper selection of cutting tools or machining parameters can lead to higher surface roughness. Mechanical damages, such as scratches, scoring, and dings on the bearing journal or probe track will affect the mechanical runout.

Since runout is measured in reference to the bearing journal, a non-concentric probe track to the bearing journal will result in high MRO. It is also impacted by the following: 

  • A straight shaft pressed into a bowed rotor
  • A bent shaft pressed into a straight rotor
  • A misaligned shaft caused by improper fit between the motor frame and the bearing cartridges
  • A sagged or bowed rotor due to thermal instability in the rotor.

Electrical runout is the measure of shaft material non-uniformity. When electrical runout is measured using non-contacting eddy current probes, the interaction between the emitted magnetic field and the induced magnetic field is converted into distance. Any phenomenon that can change the magnetic interaction between the probe tip and the shaft will affect the runout. These include non-uniform material grain structures, non-uniform electro-magnetic properties, or magnetized shaft. Manufacturing of the shaft, whether the result of a forging or hot-rolled steel process, can affect the metallurgical properties of the material and consequently the ERO. 

Final words

Slow roll runout for electric motors and generators is a condition in which the combined electrical and mechanical runout is measured on a rotating shaft at slow speed between 200 and 300 rpm, according to API. Since runout impacts the vibration readings and can lead to measurement errors, it is important to understand its various sources and how to mitigate it.

Monitoring runout level during the manufacturing process helps avoid having to disassemble the machine and return the rotor to the lathe or grinder for rework. Not meeting the slow roll runout limit after the machine is assembled can be costly to both manufacturers and customers.

Senior mechanical design engineer Papa M. Diouf (IEEE Member, 2013) graduated from Purdue University in Indiana with a MSME in 2007. He has been with Baldor Electric since 2006. He is a registered engineer in the State of South Carolina. Manager of the large ac design and mechanical engineering groups,

Bryan K. Oakes (Senior IEEE Member) has been with Baldor Electric since 1989. He is member of the API 541 and API 547 subcommittees, and an author of multiple IEEE papers.

ONLINE extra 

Read the full .pdf of this story "Understanding Slow Roll Runout in Electric Motors" here for more information and images.


Measurement of slow roll runout is a standard requirement on API motors when non-contacting probes are specified.

Electric motor manufacturers refer to customer specifications to determine the acceptable levels of slow roll runout.