A Comprehensive Guide to the Mechanical Repair of Rotating Machinery

Ensuring the longevity and reliability of industrial rotating machinery hinges on meticulous mechanical repair and adherence to stringent standards. A thorough understanding of the critical components and their required tolerances is paramount for any successful repair. This guide details the essential steps and specifications for the mechanical overhaul of such equipment, covering everything from shafts and bearings to balancing and accessories, reflecting best practices for quality and performance.

Shafts: The Foundation of Rotation

The shaft is a critical component that requires careful inspection for any signs of wear, cracking, scoring, or lack of straightness. Shaft extension dimensions are crucial and must be verified against established standards.

• Diameter Tolerances: Specific diameter tolerances must be maintained for both imperial (NEMA) and metric (IEC) frame size machines.

• Permissible Runout: To ensure straightness and prevent vibration, the permissible runout of the shaft must be checked against the relevant tables for NEMA or IEC machines.

• Keyseat Tolerances: The keyways, or keyseats, must be true and accommodate keys with a tap fit. Tolerances for keyseat width are also specified in distinct NEMA and IEC standards.

Bearings: Enabling Smooth Operation

Bearings are fundamental to reducing friction and supporting loads. They must be inspected for common failure modes such as spalling, contamination, fretting, fluting, and scoring.

• Ball or Roller Bearings: The fit of the bearing in its housing and on the shaft is critical. These fits should be measured and compared to the manufacturer's specifications. Any fits not within tolerance must be restored. Replacement bearings should be equivalent to the originals unless a redesign is approved by the customer. In the absence of manufacturer data, guidelines such as ANSI/ABMA Std. 7 can be used.

• Sleeve Bearings: The fit of a sleeve bearing in its housing and its diametral clearance should be set to the original equipment manufacturer's (OEM) specifications. Caution is advised when using general guideline values for sleeve bearing clearance, as they may not account for complex hydrodynamic and thermal variables. For horizontal machines, bearings must be positioned to prevent end-thrust. Oil rings should be round within 0.015” (0.38 mm) and rotate freely, and seal clearances should be set to OEM specifications.

Lubrication: The Lifeblood of the Machine

Proper lubrication is essential for the longevity of bearings and other moving parts.

• Grease: For grease-lubricated bearings, inlet passages and tubes should be cleaned and filled with a grease that is compatible with the customer's lubricant. In the absence of specific instructions, the grease reservoir should be filled to about one-third of its capacity.

• Oil: Oil lubricants must also be compatible with what the customer uses. A means to indicate the proper oil level, such as a sight gauge, is necessary. Any evidence of oil leaks must be investigated and rectified.

Frame and Housings: Structural Integrity

The machine's frame and bearing housings must be examined for any defects. Any cracks or breaks should be properly repaired, and all fits restored to the manufacturer's specifications. Specific mounting surface tolerances, eccentricity, and face runout are defined for different NEMA and IEC flange-mounted machine types.

Laminated Cores: The Electromagnetic Heart

The stator and rotor laminations are crucial for the machine's magnetic field and should be checked for hot spots, physical damage, or missing components.

• Rotating Elements: The core of the rotating element should be inspected for any looseness on the shaft or spider. The runout of the core's outside diameter should not exceed 5 percent of the average radial air gap or 0.003” (0.08 mm), whichever is smaller.

• Stators: Stator laminations must not be loose in the frame. The stator bore should be true and concentric with the frame's rabbet diameter.

Balancing: For Smooth, Vibration-Free Performance

Dynamic balancing of the rotating element is critical to minimize vibration and extend the machine's life.

• Standards: In the absence of a customer-specified level, balancing should adhere to ISO 21940-11 for rigid rotors and ISO 21940-12 for flexible rotors.

• Balance Grades: A balance quality grade of G2.5 is recommended for machines rated 2500 rpm or slower, while a grade of G1.0 is advised for those rated above 2500 rpm. Adhering to these grades helps ensure the machine meets final vibration limits. Balance weights should be placed so they do not interfere with other components.

Slip Rings and Commutators: Ensuring Power Transfer

These components are vital for conducting electrical current to and from the rotating parts.

• Machining: Both slip rings and commutators should be turned to be concentric with the shaft's bearing journals. The total indicated runout should not exceed 0.0030” for surface speeds up to 5000 ft/min and 0.0015” for speeds greater than that. The surface finish should be between 40 and 60 micro-inches.

• Commutators: For commutators, the mica insulation between the segments should be undercut to a depth of 1 to 1.5 times the slot's width. The edges of the copper bars may require beveling to remove roughness. Both components must have sufficient material stock remaining to ensure proper brush performance.

Brush Assemblies: The Point of Contact

Properly maintained brush assemblies are crucial for machines that use them.

• Brushholders: These should be clean and free of debris. Brush fit should be checked for excessive clearance. During final assembly, brushholders should be set with a clearance of 0.060 inches to 0.125 inches from the commutator or slip rings.

• Brushes: The face of the brush must be seated to make full contact with the commutator or slip ring surface. Brushes worn beyond their useful length should be replaced with the correct size and grade.

• DC Machine Brush Setting: The brush rigging must be positioned so that the brushes are set for "brush neutral," a position that is critical for proper commutation.

Air Gap and Accessories

The air gap between the rotor and stator is a critical performance parameter and should not vary from the average by more than 5% for two-pole machines or 10% for other machines. Finally, all accessories such as capacitors, starting switches, terminal boards, space heaters, and temperature sensors must be tested for proper electrical and mechanical operation and replaced with equivalent components if damaged.

These comprehensive repair practices are in line with the recommended procedures found in the EASA Standard AR100-2020 for the repair of rotating electrical apparatus.