Guidelines for align an electric motor with a big Gear Box 3

[petronila]

Dear Friends

We are going to couple a 1500 HP -4 Pole- sleeve bearing electric motor with one Hellicoidal gear box, is direct coupling, the problem is the process start with in no load for 30 minutes and later take total load so is not a continuos duty.

Wich considerations we have to take account for a good alignment including termal expand and all this? The shaft diameter is 4.5 inches.

Thanks and regards

Carlos

 

[JJPellin]

By most standards, that is not a very large gearbox. The gearbox manufacturer should have provided tables with expected thermal growth and horizontal movement. I did not understand if this is a gear increaser or a reducer. I did not understand if it was single helical or double helical. You did not specify if this was a sleeve bearing or roller bearing arrangement. If you do not have the manufacturer's projected thermal growth, then I would calculate the growth. Since gearboxes tend to be foot mounted and warm all the way to the base plate, they typically grow much more than an electric motor. The motor will probably grow vertically less than 0.005”. The motor manufacturer should be able to provide a projected thermal growth of the motor.

I would measure from the base plate to the shaft centerline. I would calculate the thermal growth from ambient temperature up to running temperature (about 150 °F for many of my comparable gearboxes). I would measure the center to center distance between the pinion and bull-gear and do the same calculation. The top of the gearbox will get wider when it heats up. In addition, the gears will tend to spread apart forcing the two shafts to the outer side of the bearing clearance. I would expect the drive gear to move to the outside by one half of the calculated horizontal thermal growth plus about one half of the driven gear bearing clearance. Some will tell you that the gearbox should be doweled under the pinion and that all or most of the movement will be in the direction of the bull-gear. Direct measurement of gearbox movement has convinced me that this is not correct. The dowel pins position the gearbox, but the hold-down bolts keep it in this position.

When rolling the alignment, it is better if you can turn the shafts from the motor side in the direction or normal rotation. If this is a gear reducer with a small pinion, you have to be especially careful of this. If the rotation for turning the alignment tools lifts the pinion up in its clearance, it is difficult to predict where it will end up. Most gearboxes of this sort are down-mesh. If this is an up-mesh box, then I would consider rotating it in the direction opposite of normal rotation so that you are always forcing the pinion down to the bottom of the bearing. You can try it both ways to see which one gives better repeatability. If this is a gear reducer with sleeve bearings on the pinion, it is preferred to perform the alignment uncoupled. The weight of the coupling and the stiffness of the coupling can lift the smaller pinion out of the bottom of the bearing clearances and give a false result.

If you have to run the unit unloaded, then the motor and the gearbox will not get up to full temperature. You will be running with a poorer alignment during this time. But for a motor and gearbox this size, I would not expect any problem. You should circulate the oil system with the oil heated up to at least 100 °F prior to startup to minimize this initial misalignment. You may also want to reduce the thermal offsets slightly to reduce the cold misalignment. I usually round these values down slightly so that I pass through the ideal alignment during heat-up and end up a little bit on the other side at normal running conditions.

If you could describe the gear type, bearing type, coupling type and whether this is an increaser or reducer, you may get more usefully comments.

Johnny Pellin

 

[petronila]

Hi Mr.Pellin

Thanks for the inputs:

Gear Type: Hellicoidal, Bearing Type: Rolling Bearing, Coupling : Direct, Ratio 2.57

Thanks again

Carlos

 

[petronila]

dear Mr.Pellin

I would like to give you best information

The motor is mounted in sleeve bearing. The gear Box is double helical and is a reducer. The gear box bearings are roller bearings. Both , motor and gearbox are foot mounted.

The Reducer is 2.36 ratio.

Best vregards and thanks

Carlos

 

[JJPellin]

Thank you for the additional details and answers to my questions. I can make a few additional comments. Double helical gears tend to be less sensitive to direction of rotation when rolling the alignment as compared to single helical. Since there are roller bearings in the gearbox, it will also be less sensitive to direction of rotation (down-mesh versus up-mesh). In my last reply, I incorrectly stated to plan for the gear to move one half of the calculated horizontal growth plus one half of the driven gear bearing clearance. This should have said drive gear bearing clearance. But, since you have roller bearings, that part is irrelevant. The bearing clearance is essentially zero. I am still uncertain about the coupling. You state that it is direct coupled. I interpret that to mean a rigid coupling. But that would not normally be an option to couple a sleeve bearing motor to a roller bearing pinion. I assume you have some sort of flexible coupling. The exact type is probably not as important as the distance between flex planes. The longer the coupling is from flex plane to flex plane, the less sensitive it will be to operating with some cold misalignment. The worst case would be a close coupled arrangement such as a grid coupling with limited end float. We have a number of motors that originally had this arrangement between the motor (3000 to 4000 HP) and the gearbox (ours are typically double-helical increasers with sleeve bearings). It was common to have vertical offsets across these couplings of 0.010” to 0.015” and horizontal offsets of 0.003” to 0.005”. I don’t recall every having a problem with high vibration or coupling failure as a result of running cold (unloaded). Because of the sensitivity of those old close-coupled grid couplings, we have converted all of them to non-lubricated disk-pack couplings. Using a disk pack coupling with inverted hubs allows you to move the two flex planes much further apart. There is nice coupling of this configuration made by Rexnord under the brand name Thomas (Series 54 RD). This also eliminates the need for lubrication maintenance.

Johnny Pellin

 

[Artisi]

Johnny - excellent informative reply, gets a star from me.

 

[ione]

JJPellin,
A very nice display of command on this topic (definitely worth a star).

 

[petronila]

Thaks for the additional comments,

I think the only information we will need is a good formula for calculate the termal grow, can you please provide, thanks and regards

Carlos

 

[ione]

For thermal growth ?L due to temperature change ?T and for steel you can use a typical expansion coefficient alfa = 6.8*10^-6 (inch/inch/°F).

?L = alfa*L*?T

 

[petronila]

Thanks Ione

Can you tell me what´s L?

Carlos

 

[ione]

L is the reference dimension of the item subjected to thermal growth.

 

[tbuelna]

petronila,

With a single mesh of double helical gears mounted in cylindrical roller bearings, a small amount of axial displacement due to thermal expansion should not present a problem for the gearbox. Since the gearing has no unbalanced axial force and the bearing system does not require an axial constraint.

The bigger worry is any degree of radial or angular misalignment between the motor shaft and gearbox input shaft, with a rigid connection. With that rigid 4.5 inch motor shaft, even small radial misalignments will produce excessive loads on the gearbox roller bearings, which normally have very small radial clearances and are very stiff structurally. Likewise, angular shaft misalignments will cause the gearbox bearing rollers to edge load, leading to severely reduced fatigue life.

As JJPellin recommends, some sort of flex coupling would be a good idea.

Hope that helps.
Terry

 

[Tmoose]

The original question was all installation alignment related, as if all the parts already ordered, but there are a few items I'd want to confirm are in the project folder.
- the gearbox manufacturer's comments about how heavy the flexible coupling hubs and spacer/spool piece can be.
- whether a torsional analysis was done using the selected coupling's torsional sitffness.
- Are there provisions (jacking bolts) to move the motor or gearbox axially to adjust for magnetic center
- IS the coupling a "limited end float" type which is usually required with sleeve bearing motors.
- Are there line items in the alignment procedure describing the motor manufacturer's method of indicating magnetic center, and a step to confirm the motor's magnetic center.

 

[Tmoose]

On Sept 22 you posted about "going to couple" a motor and gearbox.
On September 23 you posted inquiring about some vibration analysis results.

Is this the same machine, and was it run for the first time after the Sept 22 post?

What kind of coupling does it have?

 

[MikeHalloran]

He's started at least two other threads, apparently about the same unit, and all anyone has been able to extract about the coupling is 'direct'. I wonder if it's one of those deals where the motor shaft plugs into a recess in the end of the gearbox input shaft?



Mike Halloran
Pembroke Pines, FL, USA

 

[electricpete]

I have always heard the term "direct coupled" to mean simply that the shafts were coupled together (by rigid or flexible coupling) without the use of a belt or chain. Certainly the OP can clarify, but I can't phathom why anyone would assume this is is anyting other than a flexible coupling.

Some exampples of this useage where "direct coupled" and "belt-driven" are identified at 2 contrasting choices (no mentiong of rigid coupling):

"Drive motors can be direct-coupled to the gearbox or belt-driven."

12.52.3 General-Purpose Design A and B Direct-Coupled Squirrel-Cage Induction Motors
General-purpose Design A and B (TS shaft for motors above the 250 frame size) squirrel-cage induction
motors for the ratings specified in Table 12-6 and horsepower per frame assignments per Part 13 shall be
capable of operating mechanically constructed so as to be capable of operating continuously at the rated
load at speeds not less than the speed indicated in Table 12-6 when directly coupled. Those motors for which this speed is greater than the synchronous speed at 60 Hz shall be capable of withstanding
overspeeds, not to exceed 2 minutes, of 10 percent above the speed indicated in Table 12-6without
mechanical damage. For motors where the speed in Table 12-6 is equal to synchronous speed at 60 Hz,
the overspeed limits in 12.52.1 shall apply, assuming the motor is not energized when the overspeed
occurs. Table 12-6 does not apply to motors used in belted applications. For belted applications consult the motor manufacturer.

http://www.acepumps.com/en/index.php?/site/products/C6/Belt/

Ace belt driven pumps are available in a variety of models which can be belt driven or direct coupled to a power source.

It is recommended that belt designed motors utilizing roller bearings be used on belt
loads only. Direct coupled installations should take advantage of shorter shaft
motors with bearings designed for these applications.

The same care and attention taken in alignment of direct coupled applications should be used for belt drive installations.

=====================================
(2B)+(2B)' ?

 

[electricpete]

Correction: "phathom" should've been "fathom". Don't know where phatom came form... must have been a phantom.

=====================================
(2B)+(2B)' ?

 

[MikeHalloran]

Pete, he's been asked 'what kind of coupling?', like six times, and keeps responding with the word 'direct', only. I hope your explanation and example kickstarts him into further elucidation.



Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

Sometimes there is a language problem where in other languages there aren't the same synonyms to describe it otherwise.

rmw