Rotors and Shafts

The shaft and gear teeth of a pinion should be of an integral design. The gear should be a forging with the gear shrunk on a keyless shaft for critical service with a pitch line velocity over 20,000 feet per minute Under 20,000 feet per minute, the gear may be "rimmed," that is, design-welded in place.

The gear teeth should be heat-treated for proper strength and through hardened. Alternatively, surface hardening by carburizing or nitriding is used. Flame and induction surface hardening are also alternative hardening methods but normally are used less than carburizing or nitriding. For new gears, through hardening is preferred, using the surface hardening for later upgrades. Requirements for strength and hardness must result in an adequate durability and tooth stress as determined by API 613. The surface finish of the teeth must be 20 microinches Arithmetic Average Roughness Height (Ra) or better.

Gear elements with adequate hardness and surface finish have a greater resistance to initial scoring, a destructive phenomena found in meshing teeth.

For adequate operation, there are many features that should be incorporated in the rotors. They include: (1) good surface finish, (2) modified tooth forms, (3) high AGMA quality number, (4) hunting tooth design, (5) no resonant responses, (6) high damping journal bearings, (7) good contact pattern between teeth, and (8) good rotor balance. Other factors to lie considered for quiet gear operation are proper diametral pitch, pressure angle, helix angle, overlap ratio, backlash, and minimum apex runout. (For the meaning of these terms, refer to API and AGMA standards.) API 613 has a requirement for maximum apex run-out under subject of "Axial stability of the meshing pair."

Figure 8-21. Three-lobe journal bearing. (Courtesy of Turbocare, A Division of Demag Delaval Turbomachinery Corp., Houston facility)

gas purge connection. Appropriate shoulders or shaft fingers should be immediately inside the seal to aid in the prevention of oil leaks,

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