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Mechanical Inspection of Electric Motors: What Really Matters?

01/13/2021

2020-12-Mechcanical inspection of electric motorsWhen it comes to inspecting electric motors, it’s not just the electrical components that are important. The mechanical aspects of a motor are extremely important to its performance, which is why mechanical inspection of electric motors is so important.

Critical Fits for Electric Motors

Electric motor inspection, when done correctly, always includes a mechanical inspection. As the motor is carefully dismantled, critical fits are measured. These critical fits include …

  • Bearings fits
  • Journal fits
  • Housings
  • Seal fits
  • Keyway
  • Shaft extension size
  • Etc.

For an electric motor to run efficiently and productively, attention must be paid to the often mundane aspects of mechanical fits and tolerances.

Why Critical Fits are Important to Electric Motors

When the dimensions of these fits are not within the correct manufacturer tolerances, things will go wrong with the motor. 

For example, if the ID of a shaft bearing is not within the correct tolerance for the rotor shaft it supports, that shaft can become misaligned and lead to a damaged stator as the rotor begins to rub against it. The bearing may also begin to wear unevenly and will certainly suffer premature failure.

Poor fits can lead to serious vibration issues that can damage other parts of the motor, also. If a mechanical component is no longer fitted within the manufacturer’s set tolerance, that excess space--even if it is very small--allows for displacement that the motor was not designed to handle. That extra displacement means the potential for vibration.

Seal fits that are not within tolerance are also a problem because the result will be a leak. When lubrication is not properly sealed, it can only leak out and affect the friction and temperature of components that are making physical contact. And what may be even worse is that contaminants, including moisture and dirt, can make their way into the lubricant and cause surface damage, increased friction, and higher temperatures. 

Performing Mechanical Inspections on Electric Motors

Now that we’ve established why mechanical inspections are important, we can discuss how these inspections actually take place.

Critical Parameters

The mechanical inspection of a motor includes measuring specific parameters of bearings, shafts, and seals, such as ....

  • Bearing OD (Outer Diameter) to Housing ID (Inner Diameter)
  • Bearing ID to Shaft OD
  • Mechanical Seal Fit Clearance
  • Coupling ID to Shaft OD
  • Runouts

Instruments Used in Mechanical Inspections

Key instruments for performing these types of measurements include …

  • Digital calipers (which are especially useful for measuring lengths but can also be used for inside and outside diameters if necessary)
  • Outside micrometers (for measuring external dimensions)
  • Inside micrometers (for measuring internal dimensions)
  • Dial bore gauge (often used for measuring smaller bearings that have been spun cast and finished)
  • Lathe (used for measuring runout when combined with a dial indicator)

A Note on Calibrating Motor Inspection Instruments

At HECO all the micrometers are calibrated against a calibration standard every time they are pulled out for measurement. This ensures the accuracy and repeatability of the measurements taken with them.

Typical Measurement Process

It’s not enough to take one measurement of a bearing’s diameter and call it done. When measuring sleeve bearings, for example, the ID of the bearing is measured at multiple locations along the length of the bearing using an inside micrometer while the OD is measured at the same locations using an outside micrometer. These measurements are taken at different orientations, resulting in a full set of data points for bearing ID and OD. When data is combined, such as the OD of a shaft and the ID of the sleeve bearing it interacts with, clearances can then be calculated to determine if they are within appropriate tolerances.

Visual Inspection

There is also a visual inspection aspect of performing mechanical inspections. For example, consider sleeve bearings: they often have oil ring keepers or oil rings that require a quick look to evaluate their condition, or a visual check to see if the babbit material is still securely bonded to the interior of the bearing. Technicians also look for signs of surface wear on mechanical components, including scuff marks, gauges, abrasions, and pitting. For insulated bearings, a visual check of the insulation is also key. 

Conclusion

At HECO, any motor you send to us will undergo a thorough mechanical inspection in addition to electrical testing. And all the data we gather during that inspection is carefully documented to inform HECO's troubleshooting process to help us make the best recommendations for how you improve the performance and reliability of your electric motor.

Author: Justin Hatfield at jhatfield@hecoinc.com 

 

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