vibration levels with spring isolators 4

[executives]

I know that there are several standards for acceptance levels of vibration for equipment rigidly supported. However,if a piece of equipment is supported by spring isolators, are there any acceptance levels on record. I would think that the equipment itself would have higher amplitudes of vibration is it is supported by springs.

Any help out there?

Thanks

 

[rob768]

Yes, when equipment is resiliently mounted, on rubbers or steel springs, it will deflect more than when it is mounted on a rigid foundation. The idea is that the springs isolate the vibrations coming from the equipment (or isolate the equipment from external, usually adjacent) sources of vibration or noise.
Different rules apply for rigidly compared to resiliently mounted equipment.

 

[Tmoose]

NEMA and ISO have guidelines/standards that differentiate between mounting methods. Something like 20-50% higher on resilient mounts, with moderately strict definitions for resilient mounting. Usually a single "overall" vibration amplitude, or 1X (rotating frequency) vibration as I recall.

 

[yeos]

You did not specify what equipment and which industry it is being applied in. However, generally for rotating equipment in the oil & gas installations (offshore), the spring isolators must have an isolation efficiency of at least 90%.

You must also ensure that the vibration level transmitted to the foundation/structure does not exceed 4.5mm/s RMS (velocity).

Other factors to consider is the deflection limit (amplitude) of the springs, ie. 1 inch or 2 inch deflection etc. Be careful when you design the location of the spring isolators. Try as much as possible to locate the isolators evenly, based on equal mass distribution of the equipment ie. each mount should have equal load sitting on it.

Try to check with the vibration isolation specialist eg. korfund isolators of the US.

Hope this may help.

 

[electricpete]

To follow up on Tmoose' comment.

For integral horsepower motors, NEMA MG-1 allows 0.12 ips (pk/0) when tested rigidly mounted and 0.15 ips when tested resiliently mounted. That 0.03 ips is a difference of 20 to 25% depending on which number you compare it to.

This applies for factory testing of a motor. It's not intended to apply for installed equipment although most users are happy if their equipment comes close to the factory limit.


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[rob768]

For generator sets, the difference is as much as 4.5 mm/s for rigid supports, and som 20 mm/s for resiliently mounted sets.