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Big Al

Sr. Member
Posts: 102
Reply with quote  #16 
I've just had a cardan shaft removed yesterday. DC motor driving a single reduction gearbox, with the cardan shaft between gearbox and process roll. Roll dimensions approx 900mm dia x 3.5m long, approx 4.5 tonnes. The cardan shaft is about 150mm dia and maybe 1500mm long.

Over a period of 4 days, the online vibration data in the motor and gearbox went from normal to the point where I asked for the machine to be stopped and shaft inspected. Immediate observations were excessive heat in the roll end U/J and obvious corrosion. Today I've inspected the shaft in the workshop and both bearings on opposite ends of the cross-piece had collapsed. Strangely the vibration levels in the roll itself didn't register a large change, probably because the accelerometers are mounted radially and the dominant vibration was axial. I also believe that the sheer mass and rigidity of the mounted roll meant that it was easier for the energy to travel into the plinth mounted motor/gearbox.

The velocity overall value climbed from an average of approx 1-1.5mm/s in the motor and gearbox to between 3-4mm/s. Harmonics of shaft speed increased, with a hi-res spectrum showing up to 20 harmonics. These were mostly <0.5mm/s, with occasional harmonics rising above 1mm/s. There didn't appear to be any pattern to the larger harmonics (3x and 10x). Possibly natural frequencies?

The lube schedule was within spec, but there was definitely evidence of water corrosion around the entire failed joint.

I also found this groovy little video during my research, which explains cardan shaft motion in a simple enough way for the likes of myself to understand.

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