Assure machine reliability on rotating equipment

Shaft and housing conformance are critical to electric motor repair.

By Joe Conyers, SKF

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Machinery repairs can measurably improve machine uptime, if critical control points are observed to ensure proper shaft and housing repairs for machines with rolling element bearings. Starting with a typical industrial machine, the electric motor, the repair process will be examined, focusing on shaft and housing conformance.

A typical motor repair process begins when the machine is picked up from the plant site and delivered to the shop, which will have begun recording data, such as digital photos and electrical resistance readings. The shop then cleans and breaks down the machine into separate components. For a typical electric motor, the main parts are the rotor, stator, and end bells, or housings. Shop personnel have collected many other parts, such as housing covers, sealing components, couplings, junction boxes, and fan assemblies, and they’ve tagged and stored these items for further evaluation.

The bearings are removed from the motor rotor. Visual inspections are conducted, looking for changes from the as-new condition. Do the bearing bores (inside diameters) or outside diameters look corroded or polished? What about the bearing side faces? Best practice will also require saving the bearings for a complete damage analysis.

Reference materials

Look to these published works for guidance with your motor repairs.

1. Machinery’s Handbook, by Erik Oberg
2. ANSI/EASA Standard AR100-2010
3. SKF Installation and Maintenance Guide 140-710 Version 3/2007
4. SKF Bearing Handbook for Electric Motors 140-430 Version 7/2011

The shaft and housing seats are examined next, checking for wear or damage. If the mechanic observes polish wear on the shaft seat and then drags a thumbnail across the shaft and feels a step on the shaft, it’s obvious the shaft will have to be replaced or at least repaired. Some seat wear may be acceptable, but any signs of excess wear should be investigated. A good rule is 80% contact between mating surfaces. Fretting corrosion on shaft and housing surfaces is common, and it should be cleaned up. Start with a lint-free rag and solvent, and step up to a mild abrasive, if needed. The objective is to leave as much of the existing material in place as possible. Nicks or burs should be removed with a honing stone. Aggressive polishing or filing might destroy a component that could have been easily repaired.

Once the shaft and housings have been cleaned and inspected, they can be measured for conformance. What is needed for the shop to properly check conformance of shaft and housing seats?

  1. published standards and practices
  2. certified measurement tools
  3. documentation to record as-found and as-repaired measurements
  4. trained and knowledgeable staff

Published standards and practices

The important thing about having shop-wide standards is right in the name — standardization. Without published, readily available tolerance limits, the individual mechanic or machinist is responsible for making a go/no-go decision on behalf of the shop. You could say, “A good machinist should know what to measure.” A good machinist, and many skilled mechanics, knows exactly what and how to measure shafts and housings for component conformance. Shop management needs to publish acceptable shop standards and tolerances, available to everyone on the shop floor responsible for assuring the quality of shafts and housings. The entire range of rolling element bearing shaft and housing tolerances can be found in many locations. Tolerances for fits listed in the tables haven’t changed in decades; the physics of mating one part to another haven’t changed, but the recommended fits have evolved over time.

Good examples of recommended bearing fit tolerance tables are in the ANSI/EASA Standard AR-100-2010. These references could be adopted shop-wide as a controlled document, handed out to all relevant shop personnel, and replaced as needed if standards evolve in future. Some bearing manufacturers have published more current recommended fits.

Best practice documentation should have expected tolerances printed directly on the form used to record measurements. Otherwise, take these steps.

  1. The machine owner might have published tolerances.
  2. Consult the machinery print. Every machine is made to a drawing.
  3. Call the machine maker, who may have reference tolerances archived.
  4. Use accepted standard engineering practices to select the proper fit.

Published recommended fits are excellent guidelines, but only in everyday cases, such as standard ball and cylindrical roller bearings on typical motors. What does the machinist do when he encounters an atypical machine? A good example is a hollow shaft, vertical motor. When the bearings are pressed on to the hollow shaft, it may give more than a solid shaft and require a slightly increased press fit over standard. How much more?

When all else fails and established tolerance cannot be determined, it has to be created. If you’re brave, reference materials have a published methodology for choosing the proper fits for any situation. Once you have made your best guess, it’s strongly recommended that you confirm your guess with a reputable bearing manufacturer’s applications engineering service.

Certified and capable measurement tools

Typical industrial machines have shaft and housing tolerances that begin at ISO grade 5. For a part measuring 30–50 mm, IT5 tolerance grade is 11 microns, or about 0.0004 in. However, the permissible deviations for a cylindrical seating, for example, is IT5/2, in this case 5.5 microns, or about 0.0002 in. Measurement tools must be capable and calibrated to achieve reliable, repeatable measurements. A micrometer certified to an accuracy of 25 microns (~0.001 in.), for example, would not be capable of measuring IT5 tolerance in this size range.

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