October 8, 2015
Do You Have Bar Sag?
By Brad Case
I’m not referring to what happens after you have had too many adult beverages at your favorite watering hole! I’m referring to the bars that holds your rim dial indicator(s) when performing a precision shaft alignment.
Bar sag also called rod sag needs to be compensated for, should it occur, otherwise you are introducing a measuring error.
So what is bar/rod sag? Simply put, it is how much the dial indicator mounting bar deflects when the rim dial indicator is install. How much the bar/rod deflects depends upon the type of bar (solid or tube), the length of the bar and weight of the dial indicator. Here are a couple of screen shots from VibrAlign’s video on Bar/Rod sag to illustrate.
Check out the full 2-1/2 minute video to see how to measure the bar/rod sag by clicking here: Rod Sag Video
Recently, I took a tech support call from a client who had several different individuals on site performing shaft alignments one using dials, the other a laser. In this case both checked the alignment of the same machine. The laser alignment system results and the dial indicator alignment results varied greatly. The reason? Bar sag. When the client had the dial indicator user re-measure, with the bar sag compensated for, the laser and dial indicator alignment results were in agreement.
So, once the bar/rod sag is known how do you compensate for it? Several ways.
If you are using the Fixturlaser Dials Kit, the mounting bars have known sag amounts which are already programmed into the free Fixturlaser Dials App so the aligner doesn’t have to perform a bar sag check.
However, the Dials App does allow you to enter bar sag for other reverse dial indicator kits mounting set ups. To enter bar sag manually open the Dials App, go into tools and uncheck the Fixturlaser Dials Kit box then press the yellow check mark.
After you have entered the dimensions for the machines to be aligned and named it, press the bar sag icon at the bottom of the screen.
Follow the instructions for measuring the bar sag then enter the amount as a positive value for the S and the M dial indicator into the app.
The Dials App handles the sag compensation as is indicated by the sag indicator icon on the results screen.
Not using the Fixturlaser Dials App and performing a “traditional” reverse or rim & face dial indicator alignment? (Measurements taken at 12, 3, 6, and 9 o’clock) No worries. Perform the bar sag check as shown in the video. Position the dial indicators at 12 o’clock, zero the indicator dials then turn the rim indicator(s) dial to the amount of sag measured as a positive number.
To save time and to keep from having to check bar/rod sag for every alignment, make a sag chart for your particular dial kit beforehand.
Should you need to make a dial indicator mounting rig for special applications the best choice for the mounting bars is a tube verses a solid rod. A rod deflects or bends much easier than a tube. The mounting brackets do need to be substantial to minimize deflection.
The example shown on the left is made with square tubing with very little deflection, just a couple of mils on the rim indicator (1.0 mil = .001”). The example on the right made with all thread rod had over 25 mils of deflection on the rim indicator..
Bar/Rod sag has caused a lot of alignment frustration for many aligners, using dial indicators, over the years. However, once known, it can be easily compensated for making the alignment process much quicker with fewer adjustments.
Additional information for the Fixturlaser Dials APP: Click here
Additional information for the Fixturlaser Dials Kit: Click here