April 15, 2010
Pre-alignment Steps Save Money
By Stan Riddle
There are many types of tools with which we can perform an accurate shaft alignment. For most of us, laser alignment is considered the most accurate and effective. But shaft alignment can be done with reverse dial indicator sets, or dial indicators and chain-type brackets, or a straightedge, feeler gauge, and an outside caliper. I’ve even done it with a carpenter’s level, a stack of shims, and a flashlight. Some of these methods are much faster and more accurate than others, but all of them can be used to achieve a reasonable degree of shaft alignment.
I want us to look at some easy things that can usually be done in 15 minutes or so, and can make shaft alignment faster, and more effective, regardless of the method employed.
THE KEY TO GOOD ALIGNMENT IS GOOD PREPARATION
DON’T MAKE ASSUMPTIONS
Just as you should never assume that proper lockout/tag out has been implemented, never assume that the equipment is properly aligned, even if you did it the last time. Structural and thermal changes can occur in the machine components, their piping or ductwork, and the machine bases, which can cause changes in alignment over time. And there is a possibility that it was not aligned correctly the previous time.
If the machine is new, don’t assume it was properly aligned by the installers. And never assume that a machine came from the factory aligned. Even if it was truly aligned when it left the plant, it can change during the ride to your facility, and while being installed.
KNOW WHAT YOUR OBJECTIVE IS
All too often, shaft alignment is based on the coupling manufacturer’s recommendations. These recommendations are not for alignment of the shafts—they are the maximum misalignment allowed by the coupling manufacturer. They do not take into account the excessive forces placed upon the component’s shafts, bearings, and seals, which often lead to premature component failure.
Ask the maintenance supervisor or the plant engineer what the component alignment tolerances are. If applicable, use the component manufacturer’s recommendations for alignment. If none are available, consult with the engineering department, or your alignment tool representative.
In addition, there are thermal and dynamic forces which can act upon the components being aligned. Thermal growth values and dynamic forces should be determined, and compensated for, before precision alignment can be achieved.
Once the machine has been properly locked and tagged, and all sources of energy have been controlled, remove the coupling guard, and make the following visual inspections.
- Are the hold down bolts tight on both the stationary, and the moveable component?
- Are the couplings mounted correctly?
- Are the set screws tight?
- Does the axial spacing between the coupling flanges appear to be correct?
- Can the shafts be rotated?
- Is there excessive backlash noticed in the coupling? Is it due to wear? Should the coupling insert be replaced before alignment is performed?
- Does pipe strain exist in the system, especially near the components to be aligned? Perform a quick visual inspection of the piping system to determine if pipe hangers or other supports are installed correctly.
- Is the machine base structurally sound? Do you notice large cracks in the grout or broken welds in the base?
- Is the machine base mounted solidly to the floor or support structure?
- Have jacking bolts been installed on the machine? Make sure they are not touching the component feet before performing aligning.
HAVE YOUR TOOLS IN PLACE
Have the correct hand tools, shims, and measuring devices in place before you begin the alignment process. This will make the shaft alignment faster. It often makes it more accurate as well, since you can usually complete the process without minimal interruption.
PRE-ALIGNMENT CHECK FOR SOFT FOOT
Once the machine has been properly locked out, before loosening any foot bolts, do a quick soft foot check of the feet of both the stationary and the moveable components. Using a 0.005” shim or feeler gauge, try to insert the shim under the component feet. Measure for soft foot at three corners of each foot, if three corners are accessible.
If the shim will go halfway to the bolt, under any corner, make a quick note of which foot, or feet, are “soft”. A more thorough soft foot check will be done next.
ROUGH CORRECTION OF SOFT FOOT
Loosen all foot bolts on the moveable machine. Using a pry bay, apply slight lifting force under the foot, and measure for soft foot with a shim, or feeler gauge. Add shim as needed to correct for soft foot. Then continue on to the other feet on the moveable machine, until a 0.002-0.003” shim or feeler gauge cannot be inserted under the foot.
Be mindful of an angular soft foot. If a moveable machine foot has an angular soft foot, you may have to cut a partial shim or shims to correct for angular soft foot.
This process is often overlooked by mechanics, but it can save time in achieving precision alignment, and help to reduce gross errors in the alignment process.
Using a straightedge, check the tops of the coupling flanges for parallelism. If a gap exists while checking for parallelism, measure the gap with a shim, or feeler gauge. Insert the measured amount of shim under each of the moveable machine to bring the components into rough vertical alignment. Repeat the procedure on the sides of the coupling flanges to correct for rough horizontal alignment. Be sure to tighten the feet of the moveable machine back down before beginning alignment.
After completing the rough alignment process, you are now ready to begin alignment.
–Stan Riddle, VibrAlign
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