January 13, 2012
How Does Calculating Your Own Alignment Targets Work?
By David Zdrojewski
A recent training class offered us an opportunity to check both the hot and cold alignment conditions on a blower. It was a great time to calculate our own thermal offset values from field data and compare them to the OEM recommendation. So how do we go about doing this? Why would we even want to do this?
Calculating alignment targets can be accomplished a couple ways–see Checking Your Thermal Targets and Thermal Growth… for examples. We were using the Fixturlaser GO Pro, so we took the chance to calculate the shaft alignment targets ourselves. It’s a simple calculation–we just needed a ‘hot’ alignment point and a ‘cold’ or ambient alignment point. These two points tell us how the machine moves when put in service. It’s like finding it’s trajectory once launched into operation.
A 1200hp, 3600 rpm two-stage blower was our equipment set to monitor. Since the blower was already out of service, the first order of business was to collect the cold, or ambient, condition.
This was our first data point. You’ll notice we weren’t overly concerned with getting everything perfect. The offsets were just outside our tolerances and that would be just fine for the next step. We removed our gear, replaced the coupling guard and started it up. Time for it to get good and hot.
A couple hours later we went back to get our second point. It is important to get the ‘hot’ data as quickly as possible. As the equipment cools down and moves back to its cold position, we lose accuracy. Again, we’re just interested in where it landed–the final angles and offsets.
Now it’s time for a little math. We have a starting point and an ending point. We assume the equipment will take this same path every time so we can set our equipment in the opposite direction and have it grow back into alignment. To get this position, we take our cold angles and offsets and subtract the hot angles and offsets.
Let’s work through the math starting with the vertical shaft alignment information. Again, it’s the cold data minus the hot data. Starting with the angles: (-0.1 mils/in) – (+0.1 mils/in) = -0.2 mils/in. Do not forget to include whether it is positive or negative. Now the offsets: (+2.9 mils) – (+4.7 mils) = -1.6 mils.
Now the horizontal shaft alignment information. We can see that the horizontal angles did not change from cold to hot. It moved to the side evenly. (-0.3 mils/in) – (-0.3 mils/in) = 0.0 mil/in. For the offset: (-5.2 mils) – (-6.2 mils) = 1.0 mils.
These are our target values for the final alignment. By setting it a little low and to the right when we align it at ambient conditions, it will grow up and to the left and be in perfect alignment at operating conditions.