A significant percentage of downtime and unplanned maintenance costs can be attributed to the misalignment of rotating machinery. Machine vibration, bearing damage, premature seal wear, and coupling damage are all examples of issues pumps experience if improperly aligned at installation. Furthermore, misaligned pumps will be less efficient, and mounts or pump/motor casings can break over time.
The common causes of misalignment include:
1. Shipping and handling - while a pump and motor set is usually pre-aligned at the factory, issues can occur during the shipping and delivery process.
2. Incorrectly mounted base plate - the base plate needs to be mounted on a suitable foundation without distorting or becoming stressed.
3. Installation issues - despite clear installation instructions, it is easy to misalign the motor shaft and pump shaft centreline at an angle (called angular misalignment). You can also misalign the centreline horizontally or vertically. Pipe stress can be a factor that contributes to misalignment at installation.
4. Soft foot - warped, damaged or poorly mounted base plates, poorly selected shims or loose anchor bolts can all contribute to a condition known as soft foot. When this occurs, it is very challenging to get the rotating machines properly aligned. Even if everything looks good while the machinery is shut down, operating loads, including motor torque and thermal stresses can throw things off when the pump begins to run. For this reason, it is important to check the condition of the base plate, the anchor bolts and the pump and motor casings before attempting to align the rotating machinery. Modern laser measurement devices can help detect foot problems by measuring small changes in position of the pump and motor when they begin to run.
Naturally, whenever either the pump or the motor have been re-positioned, it is necessary to re-check the alignment.
There are 3 common tools and methods used to align pumps. Here are the pros and cons of each:
Straight Edge
This method requires placing a straight edge on the pump and motor coupling, then making a visual check to see if the components are aligned.
Pros
Quick and easy to perform
Cons
Highly inaccurate. Today's high-performance machinery and expectations for up time require more accurate measurement.
Reverse Dial Indicator
This method uses two indicators that check for radial run-out and angularity. The readings are then plugged into mathematical formulas to calculate shim requirements for proper alignment.
Pros
Very accurate, far more accurate than the straight edge method
Cons
Time consuming, cumbersome
Requires a high level of technical skill
Reading errors
Laser Alignment
This method uses lasers to determine the relative shaft positions. The computer then makes recommendations for adjustments.
Pros
The most accurate method available (to .0001)
Once familiar with it, operator can align pump/motor quickly
Cons
Cost - the most expensive of the methods available (however these initial costs are offset by extending the life of the pump)
At first, somewhat more complicated to set up
Correcting Misalignment
Correcting misalignment requires re-positioning the pump or the motor so that the centreline of the shafts line up properly. As pumps are usually attached to piping systems, it is usually easier to move the motor. Small adjustments to the motor position in the horizontal plane are usually accomplished by loosening the motor anchor bolts, shifting the motor sideways, then re-tightening the bolts. Vertical adjustments are usually made by adjusting levelling screws or by adding or removing shims under the motor’s feet.
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