December 10, 2015
Common Precision Shaft Alignment Questions Asked In Our Training Classes
By Tom Shelton
While training mechanics all around the US, I have noticed that many of the questions asked in class are the same regardless of the industry. Fixturlaser tools are very intuitive and easy to use, so this puts the mystery on the alignment process. So here are some answers to some common alignment process related questions that I am asked in class. Many of these topics are covered by other blog post in more detail.
1. “Are the pre-alignment steps really that important?”
Yes they are. The pre-alignment steps are designed to get you close to aligned and to minimize unintended movement. Control of unintended movement is critical to aligning to a tight tolerance. A few causes of unintentional movement can be rusted bases, soft foot, cupped washers, worn bearings, 2”x4” under the belly of the motor (yep), broken pipe hangers, etc.
As you can see the potential for unintentional movement is great. We train that there are four mandatory prealignment steps. “Mandatory” means that they are absolute to a successful precision shaft alignment (with minimal moves). Visit our YouTube Channel at https://www.youtube.com/watch?v=U_04dRQZUD4 to view a video with the recommended steps.
2. “Does the coupling affect the alignment?”
When performing an alignment, the “flexible” coupling can influence the rotational centerlines of both shafts when grossly misaligned. The reason rough alignment is essential is to minimize these coupling influences. In addition, backlash (looseness) in the coupling can cause measurement errors while aligning. Once the machines are aligned, the coupling has little effect on the alignment.
3. “My bases are really bad, can I still align the machines?”
To achieve and maintain precision shaft alignment of rotating machinery (to tolerances of a few thousands or less) requires the machines to be stable. If the bases are in such disrepair that they are flexing or moving, maintaining a precise aligned condition if virtually impossible. The bases should be repaired. To align a machine, the machine must be alignable.
4. “What can I do if there is a lot of vibration from surrounding equipment?”
Fixturlaser Shaft Alignment Systems include functions to negate the effects of vibration while performing an alignment. The most common function being “Sample Time”. The alignment system takes multiple measurements per second. The default sampling time can be increased by the user. Increasing the Sample Time increases the total number of measurements, the vibration spikes are “averaged” out of the results. The NXA and XA include a function to test the surrounding vibration levels and compensate according. The Vibration Filter is another function to negate high vibration levels. Review your owner’s manual or call us to discuss your options.
5. “How clean is clean?”
Dirt and debris under the feet of machine will most likely cause Soft Foot. Any type of compressibility under the feet must be cleaned out. If you do a thorough cleaning job on the base and motor feet, the alignment process will go much smoother.
** Hint- When the hold down bolt holes are threaded directly into a base, the thread area will often pull up slightly creating a ridge around the threaded hole. When cleaning be sure to run a file over the bolt hole in the base.
6. “Should I use a torque wrench when tightening down the hold down bolts on the motor?”
I personally recommend using a torque wrench. When tightening the hold down bolts, a criss/cross pattern in multiple passes is recommended, 3 passes minimum “pulling” the motor down evenly and gradually will help you maintain the alignment while tightening the bolts.
On more flexible frames you may have to make even more passes. I was involved in an alignment that took 11 passes to tighten the hold down bolts and maintain the alignment. If a torque wench is not available or won’t fit in the space one mechanic should tighten the bolts to evenly pull the motor down.
7. “What if my soft foot is showing up on both feet on one end of the motor?”
Do you remember that 2”x4” under the motor that I mentioned earlier? If you have soft foot occurring on multiple feet on one side or end of the motor, something (the coupling or others) is holding your motor up. Soft foot occurs in the diagonal plane. Think of a bar stool, you would never see a bar stool with two short legs on one side not touching the ground without some outside influence. The stool always rocks corner to corner in the diagonal.
8. “If I find a soft foot or feet, where should I shim?”
Right where you found it is the logical answer. The goal to eliminating soft foot is that all feet of the motor are resting on the base with even pressure or weight.
9. “My motor/pump is outside, due to temperature changes, do I need to re-align from winter to summer and again from summer to winter?”
There a lot of factors involved in this. Points to consider would be: the criticality of the machine, the acceptable tolerance of misalignment, and the amount of growth that occurs.
Calculating Thermal Growth requires three points of data. 1. The change in temperature at each foot from offline to running (cold to hot). 2. The coefficient of expansion for material of the machine frame(s). 3. The length of the material experiencing the thermal growth, the distance from the fixed point (the base) to the center of the shaft centerline.
The coefficient of expansion on common materials (cast iron, steel, etc.) is a tiny number. For instance mild steel (.00000645”). The temperature change is equal, so the influential variable is the height of the shaft centerlines. When I have calculated the growth on machines in this situation, thermal growth/shrinkage of a machine is relatively even on both pump and motor if the distance from the base to the shaft centerlines is close to equal on both elements of the machine. In short, I would not be too worried about it.
You can calculate Thermal Growth for yourself by using VibrAlign’s free Therm Align app for smart devices. Here is the link to our website with links to Google Play and Apple’s App Store to download the app: http://acoem.us/tools/mobile-apps/
10. “Why didn’t you bring donuts?”
I have no answer for this, only shame.