November 27, 2012

# Defining Angularity in Shaft Alignment

By Stan Riddle

There are only two types of measurements for shaft misalignment – offset, which may be called parallel or rim, and angularity, which is sometimes called slope, or face. Offset (parallel or rim) is easy to understand. One shaft is simply higher, lower, or to the left or right of the other one.

But angularity can sometimes be confusing.

It means the shaft centerlines intersect at an angle. With dial indicators, we measure it as a gap difference between the top & bottom and/or the side to side of the coupling. If for instance, a 6″ coupling has a gap of 12 mils across it (top to bottom) that is its slope.

It’s important to note that, in this example, a slope of 0.012″ across a 6 inch coupling face results in an angularity of 0.012″/6 inches, or 0.002″/inch (2 mil per inch). Since the coupling faces and the shaft centerlines are perpendicular, exactly 90 degrees, the slope change across the coupling faces is the same as the slope change along the lengths of the shaft centerlines. In other words, if the coupling faces have a slope of 2 mil per inch, the shafts also have a slope of 2 mil per inch.

This simply means the slope from the coupling center to the movable machine feet slope at 2 mil per inch. If I measure from the coupling center to the inboard feet, say 18 inches, then the inboard feet need 36 mils of shim (2 mil per inch x 18″). If it’s an additional 24 inches to the outboard feet (42 inches total), I need 84 mils of shim at the outboard feet.

This will correct the vertical angularity quite nicely. Horizontal angularity corrections work the same way. We still have the offset to correct, but that is easy to measure once the angularity is gone. Most quality laser alignment measuring tools will show both angularity and offset simultaneously.

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A tender floated for a Laser Shaft Alignment specifies the Axial Accuracy as <0.02mm for a max coupling diameter of 850mm and a max Rotor shaft diameter of 1150mm.

a) In a cold condition the Rotor Shaft needs to be positioned radially 1mm lower than the Gearbox Stub Shaft.

b) If the length of the Rotor Shaft is 18 meters then consequently the back of the rotor shaft could be 0.02mm x 18 = 0.36mm higher than the front at the coupling.

c) However I observe that shaft alignment manufacturers so not specify the ANGULAR or AXIAL accuracy of the systems in mm per meter. They are silent on this aspect and only speak about RESOLUTION – Is this correct?

If I understand your statements correctly:

a) I assume this offset target of 1mm lower is due to thermal changes or float.

b) This angularity should close up when operating conditions are reached.

c) As to resolution vs. angularity, laser tools simply measure when one or two beams hit a detector. As measurements are taken, the position of the beam’s contact on the detector changes linearly (up, down, left, or right, depending on the system). These linear changes, along with some fairly simple trigonometry, calculate the angles and offsets.

Most laser tools these days are capable of measuring with accuracy in microns. So, most lasers are capable of performing the measurement to the accuracy needed.

I work in inches, so if I read your statement correctly, the coupling is 850mm (33.465″), and the angularity is 0.02 mm (0.0008″). That’s an extremely minute angle.