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fburgos

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Reply with quote  #1 

Hi,

We have a screw compressor (3600rpm) the compressor unit was changed for a new one, the flanges have rabbet fit, what could be wrong right.

This is a Chinese manufacturer and we have no local support.

As the mechanics couple the new compressor to the old motor notice misalignment, i was called to verify the alignment, laser is too big therefore I use dial indicator (0.05mm sag).

At first TIR radial was 0.21mm and axial 0.23mm/100mm.

We loosen the all the foundation bolts (keeping flange tightened) and found "soft foot" coupling side of compressor and motor, im plannning to fill the gaps with shims and test for "soft foot"

machine.png 

New alignment readings were made, new TIR was Radial 0.29mm and axial 0.07mm/100mm, it’s out of tolerance radial.

For testing I added shims to the motor flange and axial TIR was improved to 0.01mm/100mm. but radial still is 0.23mm now I dont know what shoul I do to correct the radial alignment.



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Danny Harvey

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Reply with quote  #2 
Ferdy,

That looks like quite a challenge as it exists.

My guess would be that the flange is poorly machined.  Fixing it would be a real problem because it might be eccentric and also have depth irregularities that cause axial misalignment.

Can you do away with the flange? Then it becomes a normal alignment with hopefully a new coupling.  Some grout around the base might be necessary too.
Alex

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Reply with quote  #3 
First thing I noticed that could influence your results is your coupling does not have suitable axial gap (if it has any at all). This is the first thing to correct and see if there is some improvement in alignment manouvre. As Danny said, I also have some thoughts about the flange. Does it have to be installed at all? Isn't the base construction strong enough to withstand dynamic forces? From the photo I would say the motor and the compressor are suficiantly fixed without the flange. The other option is to take the flange off, make vertical alignment, mark the shims regarding their positions, take the flange back on and do alignment again, this time on the flange if there is some gap on the flange bolts.
John from PA

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Reply with quote  #4 
If you can turn the compressor by hand, mount the dial indicator on the compressor shaft and do a sweep of the rabbet for TIR.  Then I would do the same on the motor, just to confirm it is OK.  This might shed some light on the corrective actions moving forward.
fburgos

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Reply with quote  #5 
Coupling has a Small gap but faces don't touch.... I think I can move away the coupling.

I was thinking about removing the flange as it seems the problem for the alignment, but then I thought about torsional vibration could this be a bad thing??
John from PA

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Reply with quote  #6 
Quote:
Originally Posted by fburgos
I was thinking about removing the flange as it seems the problem for the alignment, but then I thought about torsional vibration could this be a bad thing??


Removal of a stationary flange would like have minimal effect on the torsional system as long as the motor has effectively a good rigid base.
GaryVibe

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Reply with quote  #7 
First thing I notice as others, the coupling gap seems tight. Is there enough room on the shaft to separate the coupling enough to remove the rubber insert and recheck the radial readings. 
A more drastic step would be replace this type of coupling with a shim pack style that can be separated and serviced without removing the motor. 
And I agree that the flange may be the issue. Checking the "New Component" for radial runout shaft to flange may indicate were the problem is. 

Have you been able to separate the coupling to be able to "sweep" the shafts (one stationary other rotated)? I am not sure I would rely on the outer diameter of the coupling being true to the shaft, from your pictures it looks like you cannot, but just a thought. 
RustyCas

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Reply with quote  #8 
Are we calling the piece in the middle the “flange”? If so, I would think it could be removed and then have the inside diameter of both faces turned on a lathe. Wouldn’t need to remove much, but in this situation all the flange is doing is adding some stiffness to the system, not “locating” the compressor (since it has it’s own support). I think you need the flange given how flimsy the compressor supports look.
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fburgos

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Reply with quote  #9 
Yes Rusty, im calling "flange" to the thing between motor and compressor, have not consider increasing the diameter align as usual when finished just tight the bolts and verify.
arie mol

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Reply with quote  #10 
A stiff connection of motor to compressor through a stiff intermediate piece (motor and compressor 'flanged') is basically a statically indeterminate design. The whole thing supported by two (nde and de) motor feet and two compressor feet (de and nde) will inevitably result in soft foot condition and requires very accurate shimming. However this shimming will be executed in cold condition. In hot condition thermal expansion may create a new soft foot and uncontrolled stress and strain in frame of machines. Over my decades I have seen too many bearings prematurely seized minutes after start-up, because a design is statically indeterminate.
A second issue is the machining of the flanges. The machined plane of the flange and the machined plane of the motor / compressor feet must be exactly perpendicular. If not, then overstress in one or two feet will result in deformation of frame and a fair change of bearing seizure during thermal excursion.
There is basically a operational problem concerning the stationary components. The alignment problems when aligning the motor and compressor shafts are an indication for a stationary problem, not rotational. An improper alignment of stationary components is a serious threat for bearings.
Cheap machines are not always best quality machines[mad]. What I would recommend is to try to shim as accurately as possible in cold condition. Than re-check in hot condition and shim halfway hot and cold.


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