The biggest decision is determining which tolerances to align.
In the alignment training classes that we teach, questions arise about alignment tolerances. We often ask, “What are your company’s alignment tolerances?” Here are the most common responses:
- We don’t have any.
- As close as we can get it.
- When we get a smiley face!
- When the coupling turns green.
- Whatever the equipment manufacturer tells us to do.
The purpose of this article is to shed some light on the topic of alignment tolerances.
Which “Alignment” Are We Talking About?
The alignment in this article is typically referred to as coupling alignment. In reality, coupling alignment is not about aligning couplings at all. It is about aligning shaft centerlines. The shaft centerline of a moveable machine—typically a motor—is aligned to the shaft centerline of a machine that generally is not moved, such as a pump.
What Is Shaft Centerline Alignment?
Shaft centerline alignment is the positioning of the rotational centers of two or more shafts such that they are co-linear when the machines operate under normal conditions. There are two types of misalignment, offset and angular (see Figure 2).
When shafts are aligned, the offset and the angular must be aligned. The alignment must be on two planes—vertical and horizontal.
|Figure 1. Shaft Centerline Alignment|
|With an offset misalignment, the shafts of the two machines being aligned may be parallel, but not in the same plane. When properly aligned, the shafts are co-linear, which means that any point on either shaft lies in a straight line with any other point on either shaft.||With angular misalignment, the two shaft centerlines being aligned intersect, or form an angle relative to each other. The coupling faces will be at the same angle of misalignment as the shafts, unless they are bored incorrectly. |
|Figure 2. Two types of misalignment|
The goal is to align shaft centerlines. The main reasons for aligning shaft centerlines are:
- Increased reliability of the equipment being aligned
- Decreased wear on the bearings, seals, gears, couplings and other components that make up the machines being aligned
- Reduced vibration levels
- Decreased energy consumption (typically a small amount, but sufficient misalignment can cause increased energy consumption)
- For warranty purposes, because machine manufacturers specify it
The definitions of alignment and misalignment and why we align are straightforward. However, tolerances are like opinions—everybody has them.
All laser manufacturers and coupling manufacturers have determined their alignment tolerances. Most alignment training organizations and engineering firms have them, as well. They are all different.
AGMA has very specific tolerances on gearing. AFBMA has specific tolerances on bearing sizes. NEMA has specific designations for motors. ASTM, ISO, API—pick an acronym, they all have specifications on machinery. Why isn’t there a governing body for alignment?
Simply stated, no industry standard on alignment exists because too many variables are involved. Some of these variables are: