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.
Cold Alignment Data
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.
Hot Alignment Data
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.
Target Values as Angle and Offset
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.
6 Comments
Hi,
Normally the face readings does not vary in cold and hot conditions except for uneven inboard and outboard bearings tempratures.
Br
Hamidreza
Thank you Hamidreza for your reply. I think the real value of this blog can be greatly improved if we could get discussions going. Does anyone else have a thought on this?….Look for a follow up post soon .
[…] you are new here, you might want to subscribe to the RSS feed for updates on this topic.In my post “How Does Calculating Your Own Alignment Targets Work?”, I discussed using a hot check to determine proper alignment targets. Thermal growth is not rocket […]
As David points out in his post today, it’s all about the temperature distribution. Overhung pumps operate in a similar manner–one end can get hotter than the other. For that matter, TEFC motors can do the same thing–the fan end can be cooler by design.
[…] to subscribe to the RSS feed for updates on this topic.A couple of weeks ago we calculated cold alignment targets from field data. We ended up with angles and offsets but the class wanted to go through the math […]
Being in an industry where large motors are common (thousands of hp) stopping and then starting for a hot alignment check is not very feasible due to safety and performance reasons. Thus we need to get thermal growth right the first time. Calculating the TG values are what is recommended prior to each alignment job due to changes in temperature. The only time TG numbers are consistant is during extended outages but during brief shut downs due to failures set numbers are never correct. Calculating TG based upon the conditions of the support structure and equipment at the time of alignment is a must for accurate alignments. Having pre-determined TG values is a waste of time for most applications where large induction motors are involved. You can get away with preset numbers on small NEMA frame motors and random wound motors (say 50 hp or less) but not on large ones. A rule of thumb I use is if the shim plane to bearing center line is less than 6″ preset numbers may be used with delta Ts less than 70F.