Guidelines for align an electric motor with a big Gear Box
Guidelines for align an electric motor with a big Gear Box
(OP)
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
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





RE: Guidelines for align an electric motor with a big Gear Box
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
RE: Guidelines for align an electric motor with a big Gear Box
Thanks for the inputs:
Gear Type: Hellicoidal, Bearing Type: Rolling Bearing, Coupling : Direct, Ratio 2.57
Thanks again
Carlos
RE: Guidelines for align an electric motor with a big Gear Box
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
RE: Guidelines for align an electric motor with a big Gear Box
Johnny Pellin
RE: Guidelines for align an electric motor with a big Gear Box
RE: Guidelines for align an electric motor with a big Gear Box
A very nice display of command on this topic (definitely worth a star).
RE: Guidelines for align an electric motor with a big Gear Box
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
RE: Guidelines for align an electric motor with a big Gear Box
ΔL = alfa*L*ΔT
RE: Guidelines for align an electric motor with a big Gear Box
Can you tell me what´s L?
Carlos
RE: Guidelines for align an electric motor with a big Gear Box
RE: Guidelines for align an electric motor with a big Gear Box
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
RE: Guidelines for align an electric motor with a big Gear Box
- 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.
RE: Guidelines for align an electric motor with a big Gear Box
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?
RE: Guidelines for align an electric motor with a big Gear Box
Mike Halloran
Pembroke Pines, FL, USA
RE: Guidelines for align an electric motor with a big Gear Box
Some exampples of this useage where "direct coupled" and "belt-driven" are identified at 2 contrasting choices (no mentiong of rigid coupling):
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(2B)+(2B)' ?
RE: Guidelines for align an electric motor with a big Gear Box
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(2B)+(2B)' ?
RE: Guidelines for align an electric motor with a big Gear Box
Mike Halloran
Pembroke Pines, FL, USA
RE: Guidelines for align an electric motor with a big Gear Box
rmw