What is the difference between shaft displacement and shaft vibration, and is there any directionality?
Thread Content
What is the difference between shaft displacement and shaft vibration? What directions do X-axis displacement and Y-axis displacement correspond to respectively?
Axial displacement generally refers to the axial direction, while shaft vibration refers to vibrations in two directions that are perpendicular to each other radially
Vibration can have three indicators: vibration displacement, vibration velocity, and vibration acceleration. These three indicators all have a direction. Vibration displacement is generally used to reflect the clearance; it is applicable to vibration parameters at lower frequencies, as well as the vibration of hydrodynamic bearings ； Vibration acceleration generally reflects vibration energy and is suitable as an indicator for higher vibration frequencies, such as in bearings. The vibration velocity lies between the two. Generally speaking, the X direction and the Y direction represent vibrations in two mutually perpendicular vibration directions within a plane perpendicular to the axis.
Do X-axis displacement and Y-axis displacement mean the same as X-axis vibration and Y-axis vibration?
Displacement: Nowadays, two sensors are generally used for simultaneous measurement, with the measured value representing the axial movement of the rotor. Vibration: What is generally measured is shaft vibration (which is different from casing vibration, as the measurement locations differ). Shaft vibration is usually measured using two sensors, X and Y; these sensors are installed on the compressor’s casing at angles of 90° to each other, that is, perpendicularly. The values obtained represent the shaft vibration levels in these two directions. It is crucial to pay close attention when installing the sensors – the vibration sensing elements must be aligned with the rotor’s position; otherwise, the measurement results may be inaccurate!
The displacement is in the axial horizontal direction, while the vibration is radial and perpendicular to the axis direction! Shifting by a value results in vibrations of two values perpendicular to the axis; there are two!
Axial displacement and axial vibration are two completely different concepts. Displacement refers to the amount of axial movement, which is generally determined by the total axial clearance of the thrust bearings or by the clearance of the bearings that provide axial positioning, plus the axial clearance of the gland. Axial vibration, on the other hand, is the component of vibration in the axial direction. Additionally, for a component of a shaft system, there can only be axial displacement; there are no X-axis or Y-axis displacements.
Currently, some manufacturers are gradually integrating the axial vibration of compressors into their online monitoring systems, as axial vibration can reveal certain fault conditions!
Why are X and Y axis vibrations set for shaft vibration? What is the relationship between these two types of vibration?
Axial displacement is the DC value output by the eddy current probe; it measures the change in the axial position of the shaft. In practice, only positive values are obtained, and the direction depends on the definition specified in the protection system； Axial vibration is the alternating current output by the eddy current probe; the fundamental frequency amplitude has no directionality, but in diagnosis and dynamic balancing, the direction of the first harmonic is commonly used for evaluation.