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Shaft Critical Speed - In Layman Terms

Shaft Critical Speed - In Layman Terms

Shaft Critical Speed - In Layman Terms

I am a chemical engineer and not familiar with the intricacies of rotating equipment. I understand that critical speed is the RPM at which the shaft has a natural frequency and thus experiences high vibration.

I am bidding a project that involves the sizing of a blower-motor system. The customer has specified "the fan wheel and shaft assembly shall be designed to 1.4 critical speed." I am not sure what that means in layman terms. To provide some guidance on what I am wondering: Does this mean that, assuming X=Critical Speed, the normal operating RPM of the shaft must equal 1.4*X? (There are no VFDs)

If this is true, then they want the operating RPM to be above the critical speed? Also then are there no issues during start-up and shut-down?


RE: Shaft Critical Speed - In Layman Terms

You have that backwards. It means that they require the Critical Speed to be > or = 1.4 * running speed. This is to avoid having to traverse a critical speed during startup or shutdown.

RE: Shaft Critical Speed - In Layman Terms

The language isn't clear to me in regards to the operating speed relative to the critical speed. Do they want the critical speed to be 40% above or below the operating speed? I would talk with the customer to get clarification.

The idea is to have sufficient separation between the nominal operating speed and the critical speed, such that high vibration (and associated high forces, stresses, and failure) isn't experienced during normal operation.

If the operating speed is above the critical speed, then the machine will experience higher vibration during start-up and shut-down operation as the rotor passes through the critical speed. The vibration response at resonance is dependent on the damping.

How big of a fan are you talking about? What kind of bearings (rolling element vs journal) does it use?

RE: Shaft Critical Speed - In Layman Terms

Thanks DPay and spciesla. I will clarify with the customer.This fan is moving about 4000 LB/HR of 650F air.

The customer has requested "Static Oil Lubricated Tunnel Block BLO Bearings". On that note, could you please provide any background on what "BLO Bearings" are? I have had a terrible time finding information on the web. What it appears like to me is that "BLO Bearing" was a product line owned by Baldor but purchased by Robinson's Fans. I am unclear if the customer does not know better and is specifying a brand (unknowingly) in their RFP rather than specifying a bearing type.

RE: Shaft Critical Speed - In Layman Terms

Quote (MichaelDeeds)

I am unclear if the customer does not know better and is specifying a brand (unknowingly) in their RFP rather than specifying a bearing type

The customer IS specifying a brand, but that doesn't mean they are doing it unknowingly. Its is extremely common for equipment RFP/RFQs to specify details like this- because the shaft bearings are a part of the system that requires monitoring and/or maintenance by the customer's staff, they are either A) specifying a bearing that experience tells them is superior in this application or B) specifying a bearing that is similar or exactly the same as many other machines already in their possession, which means their maintenance people are already familiar with the tools and methods required to maintain them, or possibly both.

RE: Shaft Critical Speed - In Layman Terms

jgKRI, that is a great response and it very much does make sense to me. Now that I understand they are requesting the brand name 'BLO Bearings', could you explain what the balance of their request means? "Static Oil Lubricating Tunnel Block"?

My best guess is that the bearing system will just have an oil bath and not an aux. oil system with circulating flow. I cannot guess what "Tunnel Block" represents though.

RE: Shaft Critical Speed - In Layman Terms

Quote (MichaelDeeds)

could you explain what the balance of their request means? "Static Oil Lubricating Tunnel Block"?

I would say that 'static oil lubricating tunnel block' is basically a generic description for the BLO series of bearings.

The BLO series is a pair of single-row ball bearings, lubricated by static oil bath, installed in a single housing through which the shaft passes- the 'tunnel block'- with a bearing at each end. It basically replaces the normal pair of pillow block bearings you would use in an application like this with a single part that houses both bearings, keeps them aligned perfectly, and keeps them lubricated.

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