Large Machine Resonance base Repair

[testtech]

This follows the thread "large machine resonance".

Here is where we stand: The base of the machine appears to be resonant because it is not firmly mounted on its concrete and grout base.

At first glance, this could be easily corrected by simply bolting the base in more locations. However, the concern is that if the grouting is not uniformly flat, the addition of more bolts might end up distorting the gearbox case, leading to possible gear or bearing damage.

How can quality of the grout application be determined without removing the entire gearbox for inspection?

 

[GregLocock]

I'm not too sure how practical this is in your case, but you can measure the quality of the foundation, locally at each mounting point, by measuring its point impedance, with an impact hammer and an accelerometer.

Cheers

Greg Locock

 

[testtech]

Greg:
A good suggestion. However, I suspect it will reveal low impedance at the points where the ODS study showed greatest movement. SO the question remains--if the grout application is faulty, is there any way to add rigidity to the structure (such as adding bolts through the base) that will not distort the gearbox case and will avoid lifting the entire gearbox up to repour the grout?

 

[GregLocock]

Well, that's basically structural engineering. If $$$ are unlimited then drill into the concrete slab in all practical directions, and grout iron bars into place. Probably worth FE modelling that first to check that you'll get the required improvement.

But I have a suspicion that it will be more difficult than that. Oh, longer, thicker, bolts would always help, of course.







Cheers

Greg Locock

 

[fredt]

There is a misconception that cement based grout between the concrete base and the machine base provides an adequately stiff support for the machine. This is generally not the case and to be safe grout should be regarded as largly cosmetic and the machine should be supported on steel packers. I think that your problem probably lies in this area.

I would clean out the grout and re-level the machine by ensuring that there are steel packers and shims extending from the concrete foundation to the machine base. These should be close on either side of each holding down bolt and must be shimmed so that the packer stack at every location is under pressure when the machine is level (otherwise you get a condition known as a soft foot). For added security and despite my comment above I would then apply a good quality, non-shrinking (epoxy) grout to finish the job.

When there is a problem with stiffness that is not related to the above issue it is more likely to be associated with the stiffeness of the base flange on the machine. On a fabricated base there should be stiffening gussets close either side of each HD bolt.

 

[vgarzani]

If a quick and dirty model of stiffness is AE/L, then for grout, even if 'E' is quite small, 'A' surely is huge compared to the anchorage. At my plant we have made big improvements reducing base motion by carefully applying an epoxy grout under existing base plates and skid frames, where ODS and impact response has shown base movement associated with resonance. You have a notion of the loading on the base from inspection of the ODS and you confirm opportunities for improvement with sounding hammer. You have to be careful to provide supply and vent ports for the pumped epoxy and clearly you need to minimize distortion which will occur and expect to repeat machinery alignment after the epoxy is cured.

I think one of the poster child applications for this in power plants is the GE stator cooling pump skid. Two 2-pole motors on large shared baseplates with double suction overhung pumps. Not only was 1X near natural frequencies, but so was twice line freq. for these notoriously susceptable motors. Other applications included IR air compressors, Condensate Booster pumps, and Control Rod MG sets.