Focus on Alignment: Why alignment checks must be repeatable
By John Lambert
When it comes to machinery alignment, all measurement should be repeatable and reproducible, as well as to a tolerance. The importance of tolerances, what they are and why we use them are well known. ...
By John Lambert
When it comes to machinery alignment, all measurement should be repeatable and reproducible, as well as to a tolerance. The importance of tolerances, what they are and why we use them are well known. If we are using the manufacturers recommended tolerance and it’s 0.002 or 0.010 at the coupling, whatever it is, when you reach that tolerance you can stop. You don’t need to do any more adjusting. Just as importantly, you now have an opportunity to record that information–a quantifiable number–in the machine’s history file. Then you can use it as the standard when you are verifying alignment during regular PM checks.
If you are doing a flatness measurement on the bed of a CNC machine, you can map out and mark the X and Y axes, take and record the measurement, and then reproduce the same measurement the next day and the next. You can have an outside contractor take the same measurement and if they cannot reproduce the same result, you can ask “why?” Is the bed moving? Why is it different?
I know of one company that uses a laser system with a spindle program to measure the spindles in tooling machines. Because they have their own measuring equipment, the staff can take readings at different times of the day. They say they see differences in the measurements because of temperature changes through the day. Things do move.
One strange story I heard was about a machine shop in Northern Ontario that was built next to a rail line. When the train passed by, they had to stop machining. In an even stranger case in Sweden, a machine shop that was built on a dock had the same sort of problem–when a ship docked, their machines moved. These examples show that unless you are able to reproduce the same measurement value time after time using different people to take the measurement, there is a problem.
If you are using a dial indicator system to align two shafts, you can zero the dial at the 12 o’clock (zero) position. If you rotate the shaft 360, the dial should again read zero. It should repeat. If it doesn’t, you have a problem. Maybe the dial setup is loose or you have a bent shaft or soft foot. Whatever the reason (maybe your ship came in), you must stop and ask why, then find and correct the problem before moving on.
At some companies, I have been told that the laser systems don’t repeat the same readings, although they have had them calibrated. I then suggest that it may be time to replace the system, as you can only have something repaired so many times. The laser they bought 10 or 15 years ago can be replaced today for less money than the original cost, and it will be simpler to use. Another option, if they cannot trust the readings of their laser, is to put it in the box and pull out a dial set. Just make sure it repeats the readings.
Offshore oil rig pump alignment
Here is an example that demonstrates what I’m talking about. My summer was made a little more interesting when I had my first helicopter ride to an offshore oil rig off the coast of Nova Scotia to do a chain and shaft alignment.
The company was very aware of the delicate environment in which it operates. It has set its own standards of zero tolerance for pollution control. Absolutely nothing goes over the side.
This policy creates a problem with the cuttings they produce when drilling. In the past, in places like the North Sea, these cuttings were deposited over the side, creating an environmental problem. The process they now use is to ship the cuttings by barge for safe disposal on shore, but this is very expensive. That’s why a new process is being tried. This involves pumping the cuttings back under the seabed into an open well, literally back where they came from. To do this they use a high pressure pump to force it back down.
This pump is chain driven. The motor, because of limited space, is mounted above the pump. It drives a lay shaft through a flexible coupling. One sprocket is mounted on the lay shaft and the chain passes down through a guard to the other sprocket on the pump.
Although this is not a unique design, it did pose a problem. The chain is a five-strand, 1-in. pitch #80. The sprockets are both 18-in. dia, secured by a flanged taper lock bushing. The problem wasn’t with the chain or sprocket but with the chain casing. Although the casing was strong enough, made from 1/2-in. plate, it was also the oil bath and the sole support for the lay shaft. To complicate matters more, it was also bolted to the pump casing.
As you can imagine, you need parallel and angular adjustment when aligning such a chain. If both shafts are mounted on the same plate, it doesn’t give you any adjustment for the angular. Also, there is very little room at the sides of the casing and the openings could be in better locations.
Faced with having to cut out some additional openings to perform an alignment, they called for advice. I introduced them to the Belt Alignment Transmission Tool, which is designed for this purpose. This battery operated laser device was mounted on magnetic feet on one of the sprockets. It emits a straight-line laser beam (not a dot) parallel to the sprocket. By placing three magnetic targets on the other sprocket, you could visibly see the laser hitting the targets at the same time. The next process was to correct the positions of the equipment so that the beam struck all three targets in the same position.
The beam is so thin that you could quite easily use a scale and measure the distance it is out. To correct the parallel misalignment, we were able to move the pump sprocket 7/16-in. forward on the pump shaft. For the angular, we had to loosen the holding bolts on the chain casing and move it over 1/8 in. with a hydraulic jack.
Now to test the alignment, I removed the laser and rotated the sprocket 180, reattached the laser and checked that the measurement repeated. If it hadn’t, it might have been because the sprocket was skewed when the taper lock was tightened, or the shaft might have been bent. I also checked if I could reproduce the same alignment by removing the laser and targets and reversing them so that the beam was shooting in the opposite direction.
Next it was to be a simple matter of aligning the motor to the lay shaft using the platform’s own Easy-Laser alignment system, or so we thought. Unfortunately, we ran into extreme soft foot problems with the motor. We had to remove the motor to repair the feet. As it turned out, the biggest part of the job was not the chain drive–as we had expected–but rather the motor alignment due to soft foot. However, now we know that if this piece of critical equipment does break down for some reason, it won’t be due to any misalignment or poor installation practices.
John Lambert of Benchmark Maintenance Services Inc., Toronto, can be reached by e-mail at email@example.com or by telephone at (905) 509-6522.