December 24, 2015
The Language of Alignment
By Brad Case
Rim & Face, Angle & Offset, Parallel & Angular Deviation, Parallel Offset, thousands of inch and mils! What does it all mean? Actually it’s pretty much the same thing, though not exactly.
I’ve been a VibrAlign Technical Trainer since 2008 with training classes in a variety of industries and with students previously using other methods to measure and correct misalignment. What I have noticed in that time is that much of the confusion for mechanics learning a new laser alignment system or dial indicator methodology is simply a misunderstanding of the alignment terms and what they mean.
Whether maintenance technicians are using Rim & Face or Reverse Dial Indicator Sets, or any of the laser shaft alignment systems they are all striving for the same outcome. Which is to have the rotational shaft centerlines of the driver and driven machine(s) aligned (collinear) in the vertical and horizontal planes when the machines are operating.
Even though we are aligning rotational shaft centerlines we typically look at the vertical and horizontal coupling values to determine if machines are aligned. So let’s examine some common alignment terms.
Is the actual radial position (or distance) between the two rotational shaft centerlines. Offset misalignment is typically displayed at the center of the coupling where the power is being transmitted from the driver to driven machine. It is expressed as a single number or value in thousandths of an inch or mils, .001” = 1.0 mil.
Other possible terms to describe offset misalignment are rim, rim reading, parallel offset, parallel deviation, and radial position. They all mean the same except for “Rim”. A Rim Dial Indicator reading is not a direct correlation to the actual offset.
A Rim Dial Indicator measures on the outside diameter (OD) of a coupling hub or shaft. When set to zero and rotated 180 degrees the Rim Dial Indicator shows TIR (Total Indicator Reading) which is 2x the actual offset misalignment.
For instance if the offset misalignment is 15 mils between the shaft centerlines the OD of the coupling hubs are also offset 15 mils at both the top AND bottom (or side to side) so a Rim Dial Indicator reads the 15 mil offset twice for a TIR of 30 mils. It is important to understand this, especially if the aligner is using a laser system and an Offset Tolerance is given to him or her as a Rim TIR value. As a laser alignment system shows the ACTUAL offset misalignment, the aligner must convert the RIM TIR to the real offset value for the tolerance to be correct.
Offset Misalignment = ½ TIR of the Rim Reading
Is the slope relationship of the two rotational shaft centerlines. The slope has a positive value if the offset values are more positive at the outboard feet of the movable machine. Angular misalignment is expressed as rise over run in mils per inch, (mils/1”).
Other possible terms to describe angular misalignment are face, face reading, and angular deviation. Angular deviation may mean the same depending on the context it is used. A Face Dial Indicator reading is not a direct correlation to the angular misalignment.
A Face Dial Indicator reading displays the gap difference (top to bottom or side to side) at the diameter at which the face indicator is measuring, typically near the coupling OD.
Angles cannot be expressed by distance alone (the gap) and must be “qualified” by the OD. For instance a 5.0 mil gap on a 10” diameter is not the same angular misalignment as a 5.0 mil gap on a 20” diameter. To determine the actual angular misalignment divide the gap difference by the diameter at which the gap is measured.
5.0 mil gap ÷ 10” diameter = 0.5 mils/1” angular misalignment
5.0 mil gap ÷ 20” diameter = 0.25 mils/1” angular misalignment
As you can see a 5.0 mil gap on the two different coupling diameters is not the same Angular Misalignment.
We have seen instances where alignment technicians, at XYZ Company, are aligning to the same gap or face tolerance for all machines but are not qualifying the gap by the OD. Machines with smaller diameter couplings are being left with more angular misalignment (out of tolerance) than machines with larger diameter couplings. The benefit of expressing angular misalignment as a slope in mils/1” is the coupling diameter no longer matters. Machines operating at the same RPM are aligned to the same angular tolerance.
So it is important for the aligner to understand the differences and when given a “gap or face” tolerance confirm the gap tolerance is qualified by the correct OD whether using a laser system or dial indicators.
For aligners having to “converse” in several different shaft alignment methodologies understanding the language of alignment can simplify the “conversation”.
For more information on Angular and Offset Misalignment go to: http://acoem.us/resources/concepts/