February 21, 2012
Who Decides Your Shaft Alignment Tolerances?
By Patrick Lawrence
When you opt to use your own shaft alignment tolerances instead of using the industry standards, how do you decide what to use? The subject of shaft alignment tolerances has been touched on before but a couple interesting discussions have taken place recently during training classes.
The Fixturlaser XAs and GOs allow you the option to input your own tolerances instead of using the industry standards. That’s a great convenience–but we need to understand what these tolerances mean. Some folks confuse tolerance for target. The target is our end result–what our goal is–how we want the machines positioned at ambient. The tolerance is the permitted variation around the target. Generally speaking, the faster the equipment operates, the tighter the alignment tolerances–the closer to zero variation we want. Below are two examples of how tolerances can be misinterpreted.
The first example is from an ambitious XA Pro customer last week who might have his expectations set a bit high. Now I know I just said ‘closer to zero’ but both the angular and offset tolerance values are set to 0.0. This means no deviation–no acceptable window around our target. Is this practical? (That’s not a rhetorical question–I want some feedback!!)
This lead to a great discussion on the zone of good alignment and how we can actually use that variation to our advantage while making corrections.
This next set of tolerances was seen on a customer’s XA Ultimate a couple weeks ago. When asked how those numbers were determined, they said that’s what the pump OEM had recommended. For a 3600 rpm application, this just isn’t acceptable. Compare the highlighted values to the 3600 rpm values: we know we need to get the angle at least 5X better and the offset at least 4X better.
Why such a big difference? Have you, our humble readers, experienced something similar from your pump suppliers? If you are using an outside service provider, are you specifying the alignment targets and tolerances?
One could almost argue it doesn’t really matter since precision shaft alignments can usually be completed very quickly with the right shaft alignment tool and training. We see most of them completed well within spec–or even a tighter one without much additional effort. But when seeking information about a specialty piece of equipment or if the alignment has to be documented correctly, we should know how to make an informed decision that’s in the best interest of equipment reliability.
Case in point, the same XA Ultimate customer decided to use the 3600 rpm tolerance on the XA and not the one from the OEM. It just made sense to them to go with a more accepted alignment tolerance and leave it the best condition possible. It certainly doesn’t harm the equipment to use a tighter tolerance. How did it go? This is what was texted to me after they completed the job.