February 1, 2018
X, Y, Z Axis. What do they stand for?
By Tom Shelton
Everything must have perspective, a point of view, to be communicated. To communicate the three spatial dimensions, we use the X,Y, Z coordinates. These denote height, width and depth. In referring to machinery we use the same X,Y, Z denotations, but we give them different values or meanings. To make it even more interesting, there are no specific rules governing the meaning leading to confusion when attempting to communicate in these planes.
Figure 1, This is the “Hand Rule” used by the electrical trades to denote Thrust, Current and Field.
Figure 2, This is the “Right Hand Rule” used to denote the 3 axes.
Although the Right-Hand Rule shows the three axes, the spatial aspect is still not known in relation to a machine. This reference is used in the Machine Tool Trades. It is taught that the Z axis is referencing the vertical axis which is the center of rotation referred to as the axial plane.
Figure 3 This denotes the rotational axis of a CNC machine. This does not however denote a moniker of X, Y or Z. The direction of the thumb is referencing axial plane.
Knowing your axis reference is critical to understanding the information that is being conveyed. Your industry or trade may dictate the directional meaning. Figures 1-3 illustrates that knowing what the reference is, is like a key to a secret language known only by a select group. If you don’t have the key you may not understand the message.
Vibration analysis and precision shaft alignment are no different. They each have their own set of “rules” regarding the X, Y, Z axis that are task and/or tool specific.
Using a machine analyzer, such as the Fixturlaser Smart Machine Checker, the axis reference changes based on the position of the tri-axial sensor (accelerometer). The tri-axial sensor measures vibration levels in all three axes at once. This allows for many sensor position options. The only rule is that you must tell the display unit how reference mark on the sensor is positioned relative to the 3 axes by simply selecting the correct picture on the Smart Machine Checker display screen.
Figure 4, Note the sensor position reference in the top right corner. In this position, the X is Axial, Y is Radial Horizontal and the Z is Radial Vertical.
Figure 5, In this one the X is again Axial, but the Z and Y have reversed. Notice the sensor position reference in the top right.
When performing precision shaft alignment on horizontal rotating machinery, the Z axis refers to the rotational axis or rotational centerlines. The Y axis refers to vertical position (perpendicular to the base) and the X axis refers to the horizontal position (parallel to the base). As seen in Figure 6a below.
When aligning vertical rotating machinery, the Z axis is also the center of rotation, however X and Y are used to reference the two axial planes 90 degrees from each other as in Figure 6b.
Figure 6a & 6b
Another question that surfaces frequently during a horizontal shaft alignment is regarding positive and negative alignment values of the movable machine in relation to the stationary machine or reference shaft. Once again perspective or point of view is the key. When discussing the vertical position of the movable machine we typically say positive is up and negative down. +Y or -Y in Figure 6a above. Another way to say it may be positive is away from the base and negative is toward the base.
In the horizontal plane the positive and negative position (left or right) of the movable machine is again qualified by the point of view. When viewed from the back of the movable machine negative is to the left and positive is to the right of the reference shaft. -X and +X in Figure 6a above. This is the point of view when using Fixturlaser Shaft Alignment Systems.
Regardless of what trade or industry you are communicating (working) in, it is necessary that you know what plane is being referenced by each of the 3 dimensions to be able to correctly utilize the information you have gathered before taking corrective actions. Using incorrect references may lead to incorrect diagnoses of a condition or unclear communication of a problem or design between mechanics. Be sure to understand the reference before acting upon information regardless of the source.