Case Study Analysis Of Two Stage Planetary Gearbox Vibration

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by Ken Singleton

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ABSTRACT

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This paper describes in great detail the vibration analysis of a ring gear defect within a two stage planetary gearbox. General operation and layout of planetary gearboxes are described. Calculations of all gear speeds, gearmesh frequencies & bearing defect frequencies are explained in detail. Also explained is a little known forcing frequency called the planet passing or planet pass frequency that is unique to planetary gearboxes. A combination of spectral, waveform, high-frequency Peakvue data and auto correlation are used as analysis tools to solve this problem.

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PREVIEW

“Background:

Gearboxes used in underground coal mining are of compact design. A typical two stage planetary gearbox, 800 HP, 40.173:1 Ratio with 1800 RPM input is shown in Figure 1. The unit was received by a repair facility for rebuild following failure from an overload incident. It was reported that the bearings were replaced and that one bearing had broken into many fragments. Following repairs a no load spin test of the gearbox was performed as a check for bearing faults, Figure 2. There was an audible impacting type noise from the input planetary section.

Analysis:

During the spin test, vibration data was measured using an accelerometer with a rare earth magnetic mount. Initial inspection of the data indicated impacting and ringing of natural frequencies of the gearbox, Figure 3. The impacts measured a 221.87 mSec period or 4.507 Hz ~ 704.6 CPM. The FFT of the time domain data showed harmonics of 704.6 CPM, and indication of excitation of several natural frequencies of the gearbox. Before a determination of the source of the vibration could be made, an understanding of the gearbox design was required as well as calculation of the excitation frequencies. Based on the information provided by the drawing shown in Figure 1, several calculations were made to obtain the shaft speeds, bearing fault frequencies, and gear meshing frequencies. Epicyclic gear boxes derive their name from the epicyclodial curves that the planet gears produce during rotation. There are three general types of epicyclic arrangements: 1) planetary which consists of a stationary ring gear combined with a rotating sun gear and moving planet carrier, 2) star configuration which consists of a stationary planet carrier coupled with a rotating sun gear, and 3) solar gear that has a fixed sun gear combined with a moving ring gear and planet carrier. The planetary arrangement is most common and is shown by the schematic in Figure 4. The subject gearbox had the planetary arrangement for the 1s1 and 2nd stages. Input was from the sun with three planets supported by a carrier revolving about the sun pinion and the ring gear fixed.

Gearbox Inspection:

With the likely problem area in the gearbox identified, the gearbox was dissembled for inspection. A small fragment of the disintegrated bearing race was found imbedded in the unloaded side of one tooth of the ring gear. The bearing fragment was removed, the damaged tooth was dressed, the gearbox reassembled and spin tested again.”

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