Compressor and Motor Shaft Elevations 1

[FH]

This is about compressor and its motor shaft alignments.
Usually compressor manufacturers put motor and compressor shafts in the same horizontal elevations, is there any specific reason for that?
If the motor's shaft is higher or lower, still its alignment with compressor's shaft can lie in a flat plane. So apparently putting both shafts at the same elevation is not a necessary requirement. Am I right?

 

[electricpete]

By the time you couple it up and run it, you want both shafts in the same vertical plane and same horizontal plane.

There may be some opprotunity to move the motor vertically up after initial installation before coupling during alignment, but that is limited. And building a platform under each foot can be a bad thing.

Sorry if I misunderstood the question.


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(2B)+(2B)' ?

 

[MikeHalloran]

At first thought, it seems like a convenience, but given the tolerances that motor and compressor manufacturers typically hold on bore to flat dimensions like that, only an amateur would just bolt both to a flat plate and not check the alignment, or have a good coupling on hand, or pre-make shims.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

The shafts have to be aligned.

That is the axis of both shafts have to line on the same line.

That is all that is necessary.

 

[MikeHalloran]

True, getting the shafts aligned is all that's necessary. ... if they stay aligned. Let's er, table that issue for now.

One problem with having both shafts at the same nominal height to start is that you could have to shim either part, so you need shims to fit both. Whereas, if they were different nominal heights, you'd only need to shim one, so you prepare one set of shims.

If you're lucky, at least you have one sturdy planar surface to wrestle the components around on, so all the shims can be the same.


Mike Halloran
Pembroke Pines, FL, USA

 

[racookpe1978]

Gears or belts are an immediate source of expense, maintenance trouble, and even greater alignment problems.

If you have anything but a "perfectly aligned" flange-flange connection between the shafts, your customers are going to get another supplier. So nobody builds them any other way.

 

[MikeHalloran]

C-frame motors with nominally guaranteed alignment get less common in larger sizes.
I think we are talking about motors and compressors with flat(ish) bases, aligned for direct drive, which IMHO is not a super great idea absent a radially and angularly compliant coupling.
Agreed that belts and gears can be a maintenance headache. In the common circumstance, they are used between e.g. a two-pole motor and a load that can't be driven that fast, in order to save weight and/or cost over a lower speed motor. One also has to be careful when ordering a motor, because generic motors usually come with bearings that are not rated for any substantial radial load.


Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

The purpose of shaft alignment between the driver and driven element in a turbo-machinery train is so that they are co-linear when in normal operation. It is desirable to have them in a common plane both vertically and horizontally. Design elements are incorporated to make this easier. In most designs, one component is designed to be the “movable” and one component is designed to be the “stationary”. The moveable component in a motor/compressor train will almost always be the motor. In order to accommodate the moves necessary, the motor will be designed to have some nominal amount of adjustable shim under each support foot. We require a 1/8” nominal shim to allow for adjustment up or down.

The alignment is accomplished using laser equipment capable of meeting the required tolerance. In our facility, we would normally expect the alignment to be within 0.002” of the target in both the vertical and horizontal planes. The targets are based on the expected movements of the machines in the running condition. For a typical API centrifugal compressor, the OEM would provide the alignment targets. Even though the compressor is supported by feet at the elevation of the shaft centerline, some amount of growth is unavoidable. Yesterday, we completed the alignment of a 4000 HP, 9000 rpm Coker wet gas compressor. The train consists of a motor, gear increaser and compressor. The targets called for the motor to be set 0.010” higher than the gearbox and 0.006” to the East. This allows for the fact that the gearbox will grow much more in the vertical direction than the motor. The horizontal offset allows for the spread of the gears at full speed and load.

The targets for compressor alignment required that the compressor be set 0.015” higher at the inboard support feet and 0.006” higher at the outboard feet. The compressor was to be set 0.004” to the West. The difference in vertical offset on the two ends of the compressor allowed for the different temperatures primarily driven by the heat of compression. The outboard end was the discharge end which runs much hotter than the inlet end.

For machines with no gearbox, the alignment targets usually only require a vertical offset. Both the motor and compressor would be designed to grow straight up, with very little horizontal movement.

Even with an alignment tolerance of 0.002”, a flexible coupling is needed. It would be impractical to achieve and maintain an alignment precise enough to allow for a rigid flange coupling. Without the flexible coupling, the shaft would fail from high cycle fatigue in bending. Coupling manufacturers will brag about the capability of their flexible coupling to accommodate misalignment. But, no matter what the supposed capability of the coupling, a precise alignment will reduce the loads on the bearings, seals, gears and other components. A precise alignment will result in lower vibration levels and increased reliability.


Johnny Pellin

 

[moltenmetal]

FH's question isn't whether or not the shafts have to be aligned, which is obvious. He's likely got a vee-belt-driven piston or diaphragm compressor and is used to seeing the shafts of the motor and compressor set at the same elevation. This means that either the compressor or the motor is shimmed, or installed on some sort of stand, to achieve this, rather than simply bolting them to a common baseplate with the motor mounted on an alignment/belt tensioning fixture.

I've seen this done too, by some manufacturers. I've also seen plenty of belt-driven equipment where the two shafts are at different elevations.

I'm curious too: is there some physics involved, or does this make alignment of the shafts/sheaves easier, or is this purely an aesthetic thing?

 

[electricpete]

I agree the original question was odd.

Your interpretation sounds reasonable to me, atlhough op should be the one to clarify.

We have plenty of belt-driven fan shafts at different elevation than motor shaft. I'm sure there is no problem with that as long as equipment and support including tensioning arrangement is designed for it.

I can think of one possible reason some designers might prefer the same horizontal level: the belt tensioning device typically moves horizontally. If the belt were 45 degrees from horizontal, then the force applied by belt tensioning screws in horizontal direction must be sqrt(2) higher than if belt were horizontal. I doubt that's a big problem but it's the only thing I can come up with.

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(2B)+(2B)' ?