The dial indicator bar sag describes a bending of the hardware used to support a dial indicator or other part which spans the coupling. The bending action occurs as a result of gravity and cannot be totally eliminated in almost all cases of alignment. Numerous attempts have been made by fixture manufacturers to minimize the amount of sag that occurs; however, none have been successful in “eliminating” it for all alignment situations, only in minimizing it.
Factors that influence how much sag exists include:
- Weight of the dial indicator and other parts that are overhung.
- Height of the supporting fixture required to clear the coupling
- Span of the indicator bar(s)
- Stiffness of the fixture hardware materials
- Specific geometry of the hardware arrangement.
In all cases of alignment, efforts should be made to minimize the amount of sag present. If it is not kept to a minimum, it is often not repeatable and, therefore, introduces different amounts of error. As long as the amount of bar sag is known and is consistent, it can be compensated for during the alignment process.