November 19, 2011
To Turn or Not to Turn, That is the Question!
By Brad Case
Bowl mills are used to pulverize coal prior to being blown into coal fired boilers. They are very large machines with heavy rotating components usually driven by an internal planetary gear set in the bottom of the mill. A typical steam generation power plant will have multiple bowl mills for each boiler in the facility.
During a Fixturlaser XA training class at a coal-fired power plant a discussion developed between two mechanics as to whether their bowl mill shafts could be rotated during an alignment. One mechanic was convinced the mass of the bowl mill would necessitate the coupling being disassembled and the alignment performed with one non-rotating shaft. The other mechanic felt that, though difficult to turn, the bowl mill alignment could be performed coupled up thus saving time, gathering more accurate alignment data as both shafts are rotated and eliminate the extra work required when performing an uncoupled alignment. Either way, it’s a matter of choice for the mechanic using the XA.
As luck would have it a 700 HP, 1200 RPM electric motor had just been replaced on one of their bowl mills and needed to be aligned to the mill. Initial inspection of the motor installation revealed a slight bit of concrete to have been broken off from the left rear and right front corners of the foundation under the outer edges of the motor base. There were no shims under the motor feet. The gear coupling was already assembled so 0.050” thick shims were placed under all motor feet as a starting point. The obvious soft foot check revealed the left rear and right front corners of the motor feet to have 0.025” of angular soft foot, on the outboard side of the motor feet, which was corrected accordingly. The final soft correction took an additional 0.005” under the right foot of the motor.
The Fixturlaser XA was mounted, dimensions entered, and measurements were taken. A 48” pipe wrench was used to turn the shafts. The measurements were taken near 9, 12, and 3 o’clock. The dual inclinometers of the XA keep track of the position of the sensors, when taking measurements, so precise positioning of the sensors is not required, speeding up the alignment process on large hard to turn machines.
The bowl mill operates at 1200 RPM, with an allowable angular misalignment of 1.0 mil/1” and an allowable offset misalignment of 6.0 mils, (1.0 mil = 0.001”). The first set of alignment measurements were taken with initial results of:
The mechanics completed a Verti-Zontal alignment by: 1) loosening the hold down bolts; 2) adding +155mil shims to the motor front feet and +220 mil shims to the rear feet to correct the vertical misalignment; 3) correcting the horizontal misalignment to within tolerance as indicated by the XA’s live screen; 4) tightening the motor feet hold down bolts using a cross torque pattern in 3 passes.
A second set of alignment measurements were taken with final results of:
Time from initial to final results, 33 minutes! The alignment was completed with one Verti-Zontal adjustment and the final coupling values where just shy of 3600 RPM alignment tolerances! Could this alignment have been completed with the coupling disassembled and by only turning one shaft? Absolutely! However as is shown, whenever possible, it is to the aligner’s benefit to perform an alignment with the coupling assembled so both shafts can be turned even when the machines are “difficult” to rotate. Faster alignments with less work! Turn baby turn!
What is the allowable misalignment (in deg) of the spherical roller bearing in the planetary gear of a bowl mill?
You would need to contact your bowl mill manufacture or the bearing manufacture for this data .
Hi , Ive been asked to align a compressor to a large motor ….The coupling is yet to be installed but I cannot move either shaft . I have only one Sliding magnetic bracket and the (non precision)coupling halves must stay on. Also they are 2.5 meters apart. (large coupling)…I cant get repeatable data by sliding brackets around both coupling halves and recording….Its just not reliable !There is a bit of shaft exposed at the rear of both couplings but no shoulder to my mag bracket on..So can I conduct a Non Rotating (Laser)Alignment with this situation ?
Andrew, when moving the brackets vs rotating the shafts the sensors will “see” any runout that is present resulting in nonrepeatable readings as you are experiencing. This issue is magnified when both shafts are not rotated and is not an advisable method of shaft alignment.
Is there any possibility to rotate the motor with some sort of turning gear? Turning one shaft will help minimize nonrepeatability.
Lining up the laser sensors (heads) at each measuring position is very important as well. (Maintaining the same relationship at each position). If you are using a current generation Fixtulaser System match the inclinometer values for the M and S Sensor within 0.2 of degree at each measurement point before registering the measurement.
Another option may be to adapt the precision rotating turrets from either the Fixturlaser OL2R or Cardan Bracket set. If you could mount the turrets at the coupling/shaft centerline the precision rotating turrets would eliminate the runout issue. Mounting the precision turrets may require drilling and tapping a mounting hole if the coupling/shaft center is not already drilled and tapped.
To check into this option, you can contact your local Fixturlaser Distributor if outside the USA by going to http://www.fixturlaser.com for a worldwide distributor list
If in the USA go to http://www.vibradev.wpengine.com to find your local sales representative.