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Assure machine reliability on rotating equipment

Feb. 25, 2014
Shaft and housing conformance are critical to electric motor repair.

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.

For machinery shafts and housings, a cylindrical shape is not only being measured for overall size (dimensional tolerance), but also for shape (form tolerance). Measure and record at least four points every 45° circumferentially and in two planes perpendicular to the shaft to document the roundness and taper of the seats. For larger components, add an additional measurement plane for each 50 mm (2 in.) seat width.

Check the measurements to see if they are within the required tolerance. Remember that a typical micrometer or bore gauge measures a diameter, not a radius, and therefore the tolerance is twice the allowable deviation. If these diameters are measured within acceptable tolerances, a dial indicator measuring total indicated runout in the same plane would still show a deviation of no more than half the tolerance. Any deviation outside tolerance will require a repair. For tapered shafts, special tools (sine bars) and drawings are required to assure conformance.

Shafts can be repaired in a number of ways, but make sure your shop has approved, documented repair processes, with qualified personnel. Housings can be most easily repaired with commercially available pre-made sleeves. The housing to be repaired is bored to accept the sleeve; the sleeve is inserted and bored to the proper housing tolerance. In either case, the repaired seats must be re-measured, and the data recorded and checked to ensure conformance.

A few shops are still measuring bearings prior to installation, which is no longer considered a best practice. Prior to the 1960s, bearings had so much variation that shops would measure each bearing and repair the shaft to the individual bearing. Modern bearings from world-class manufacturers use Six Sigma statistical process control techniques to reduce variation and outliers. Rely on established, authorized distributors to prevent poor-quality third-tier and even counterfeit bearings from entering your supply channel.

Documentation to record as-found and as-repaired measurements

Minimal documentation of standard shaft and housing conformance should include:

  • four-point, two-plane measurement of shaft and housing seats
  • shaft and housing shoulder runout
  • total indicated runout at critical shaft points (for example, coupling seats, bearing seats, seal seats)
  • 80% contact at bearing shaft and housing seats
  • surface roughness within tolerance
  • maximum shoulder fillet radii not exceeded.
Joe Conyers is senior consulting engineer, training and development, at SKF Service Division, Reliability Maintenance Institute. Contact him at [email protected]. SKF certifies electric motor repair shops. A list of certified shops can be found at www.skf.com/rebuilder.

Shoulder runout is critical. The bearing side faces make direct contact with the machine shaft and housing shoulders. Excessive shoulder runout may lead to excessive vibration and higher bearing operating temperatures.

Trained and knowledgeable staff

Having published standards, proper tooling and good documentation are worthless if your staff isn’t properly trained to interpret the measured results.

Can your staff properly interpret your published shop standards? Do they understand why a certain measurement is required? How does the measurement add to quality? Can they properly evaluate the measured results, especially if a non-conformance is found? When is a repair required? When are measured values out of tolerance? Detailed, published procedures and documented standards will reduce misinterpretation of measured results.

An acute focus on quality of your repairs starts with robust published standards and procedures, capable measurement devices, detailed quality documentation, and superior training. Not only will these efforts reduce scrap, loss, and rework during repairs, but repaired machines will leave the shop with the highest possible reliability.

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