February 7, 2017


By Tom Shelton

Rotational Centerline

When we align equipment, we are aligning the rotational centerlines of the shafts. We have a stationary machine and a moveable machine. In most cases the stationary machine’s center of rotation is the target and reference for all measurements and corrections of the movable machine.

When reading laser or dial indicator values, the terms of “Positive” or “Negative” are used to describe the position of the movable machine relative to the reference centerline. When we move a machine that is mounted in a horizontal position, such as a pump and motor, we reposition the movable machine by moving in only two planes, vertical and horizontal.

In the vertical plane, positive refers to above the reference centerline, and negative refers to below the centerline. When viewing the horizontal plane, typically from the driver point of view, positive is to the right of the reference centerline and negative to the left of centerline.  This is the case regardless of the Fixturlaser tool you are using.  The graph below could be viewed two ways. It could be viewed as representing the vertical position (as viewed from the side) or the horizontal position (as viewed from above).

If you are using the NXA or XA laser system from Fixturlaser the visuals you see on the results and live screens are an actual representation of the movable machine’s position. Vertical and horizontal angularity and offset are clearly shown as are the feet position.

The pictures below are of the NXA Pro or Ultimate. Position of the movable machine vs. the reference machine will be easy to see because of this accurate depiction illustrated on the interactive user interface. When using any of the Fixturlaser Alignment tools the right/left or high/low vs. +/- reference will be the same as illustrated.

A few things to remember:

  • The positive or negative signs (+/-) for the displayed values refer to position and the value is the distance from the reference centerline at that point.

  • When adjusting the movable machine, while in the live screen, and the foot values are increasing, move the other way!
  • At times, gross misalignment can cause your coupling values to be positive and the foot values to be negative or the other way around. This where the pictorial representation is helpful.

  • When correcting the vertical position in the shim screen, pay attention to the “add” or “remove” shims direction of the arrows.

  • If making live vertical corrections, for machines with adjustable chocks, the coupling and foot values are the position of the moveable machine and the arrows show whether to remove or add shims.



  1. Kevin on November 2, 2017 at 4:13 pm

    What would cause a shaft on an exhaust fan to ware on only one side? We had replaced bearings on this fan and with in a month the bearings had failed, and on one end of the fan shaft the shaft was wore down about an 1/8″ only on one side of the shaft? (not completely around shaft)

  2. Tom Shelton on November 8, 2017 at 5:10 pm

    Kevin, Per our phone conversation.

    The wear on one side is typical of a machine element rotating around a stationary shaft, however his is not the case as this is the outboard
    bearing of the blower mounted to a base. Had the shaft been rotating or slipping in the race the wear would be the full circumference of the shaft.

    At turns out there was a .003 shaft deflection due to a loose fit between the bearing collar and shaft. As the shaft rotated, the bearing race would wobble axially on the shaft causing the wear. As the looseness increases, the movement increases.

    Thanks for your question.