Fan bearing problem

[Haris777]

We have ACE make vertical fin fan radiator for our refrigeration unit. Thrice, its fan blades have broken and damaged the radiator also. We checked the pulley alignment, belts, bearing intactness, but didnt find any prob. 1st time was due to material failure of fan blade but next two failures have no apparent root cause. Fan RPM are 600, there are two plumber block bearings for main fan shaft, an idler bearing for belt adjustments and one driving pulley connected to Engines crankshaft.
Can anyone suggest, what could be the possibilities for these failures. Vibration data also doesn't seem to be abnormal.

 

[Tmoose]

got pictures?
Installation, new fan, broken fan as a minimum

Did ACE supply the fan? If so What does ACE have to say?

 

[electricpete]

I agree about pictures. Examination of the failure is starting point. Everything else is a guess.

I'll throw out one guess. What about torsional oscillation since this thing is apparently driven by a reciprocating engine (which is different than the normal equipment I monitor). Especially if you have some type of synchronous belt (I'd expect vee belt would provide some torsional damping).

Strobe inspection might be worth at try to see if anything unusual is apparent like flexing of the blades. It probably has to be pretty severe in order to be apparent by eye with strobe, but doesn't hurt to check.
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(2B)+(2B)' ?