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Importance Of Gear


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The Importance Of Gear Assembly Phase


by Jack B. Hamilton

Abstract


This brief case history discusses the uncommon topic of gear assembly phase (GAPF) and its appearance in a gearbox following rebuild. Case was not taken to match mark the meshing gears prior to repairs and the predictable result was the appearance of gear assembly phase frequency (GAPF) vibration & harmonics thereof following repairs. Calculations for this uncommon fault frequency are provided.

Abstract

PREVIEW


“This case history deals with one station on a machining line that had to be removed and rebuilt as a result of a smashup caused by a malfunction of a parts transfer. New bearings and a new shaft were installed, but the gears were not replaced. The repair happened between scheduled vibration readings. Figure 1 shows the vibration readings before and after the station was rebuilt and put back into service. Many new peaks could be seen in the spectra.

Figure 2 shows the gear assembly phase frequencies (GAPF) and harmonics. The reading was taken on the gearbox input. The input speed is 892.5 RPM; the gear has 21 teeth that mesh with a gear with 48 teeth. The common prime factor for both gears is three (21 teeth = 3×7; 48 teeth = 3x2x2x2x2). The gear assembly phase frequency is the gear-mesh frequency divided by 3 (892.5×21 = 18,742.5 GMF, divided by 3 = 6,247.5 GAPF).

When the station was rebuilt, the gear assembly was not marked. When the worn gears were reassembled in the gearbox, the teeth were not meshing with the same teeth as previously.

Figure 3 is a zoom around six times gear assembly phase frequency. Side bands of operating speed of the 21-teeth input gear (892.5 RPM) are present as are side bands at the speed of the idler shaft (390 RPM).

The station has operated with no more increases in amplitude. Until a problem occurs with the machining of the part, the station will continue to produce. The maintenance supervisor was notified. New gears were ordered and will be ready for the next rebuild.”

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