How to detect imbalance or misalignment in a team that rotates at low speed?

Good evening:

We are monitoring equipment that rotates at low speed (below 20 RPM) and our concern is how to detect that the equipment is unbalanced or misaligned, is there any method in this regard for detection?.
We have disassembled a equipment and found it misaligned and could not be detected in the vibrational monitoring.

Regards.

Original Post

Misalignment can occur at low speeds like 20 RPM but unbalance is a bit unlikely since the forces created vary as the speed squared.  So at low speed, usually low forces exist, at least due to unbalance.  There are exceptions, a bent shaft or a missing component for instance. 

From an instrumentation standpoint, at low speeds like 20 RPM, specialized  instrumentation will likely be required.  A velocity transducer for instance may have very little response, if any, below 240 CPM. 

What type bearings (rolling element or fluid film) support this rotor and what instrumentation have you been using?  

In your case, at such low speed, vibration measurement won't give you reliable unbalance and misalignment diagnose. I would send the rotor to workshop balancing, bring it back and do precise alignment. Normaly, rotor at such low speed doesn't need to be balanced at all.

From your specification sheet...Frequency range: – ±10%: 1,0 to 5 000 Hz – ±3 dB: 0,7 to 10 000 Hz.

This more or less says it all.  The transducer you are using has very little value at 12 RPM.  The portion of the specification reading ±3 dB: 0,7 to 10 000 Hz is the key part as is the curve titled Typical Frequency Response.  +3 dB is a 41% error (reading high) and -3 dB is a 30% error (reading low).  At the low end, the 0.7 Hz or 42 CPM, the device could exhibit these errors.  The device essentially can't detect components in the vibration signature at the typical frequencies of 1X (12 CPM) and 2X (24 CPM). These of course are commonly associated with unbalance and misalignment.

As I said, and "BECAR" reinforced, unbalance would not normally be a factor at this low speed.  It would be desirable to know the actual machine details.  We don't anything except it rotates at 12 RPM.

By the way, your misalignment found.pdf file doesn't open, at least for me.

 

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