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Orbit plots and centerline diagrams an introduction

Vibration Experts

Orbit plots and centerline diagrams are used when analyzing vibration in journal bearings using data from proximity probes. The centerline plot shows where the shaft is located in the bearing. It can be used to see if there is misalignment or excessive preload. The centerline diagram can also be used to detect wear in the journal. This is indicated by the shaft sitting lower in the bearing than it used to because the babbitt in the bottom has worn down.

Orbit plots describe the dynamic movement of the shaft in the bearing. Because two proximity probes (mounted 90 degrees from each other in the radial direction) and a keyphasor are used to take the reading, one can determine where the shaft is located in the bearing as it revolves around and vibrates. Proximity probes measure peak to peak displacement or the distance between the probe tip and the shaft.

Direct orbits contain all of the vibration frequencies. Filtered orbits only display the movement of the shaft at particular frequencies such as 1x, 2x 1/2 x, 0,4x  etc. Because the common faults in journal bearings mostly appear at these particular frequencies, filtered orbits are often analyzed. Common faults in journal bearings include unbalance, misalignment or preloads, looseness or rubs and instabilities such as oil whirl or oil whip.

The Keyphasor(TM) is a third proximity probe mounted in a different axial location on the shaft. When pointed at a key or keyway, the probe will receive a pulsation or quick change in displacement when the keyway passes by the probe. This then provides a once per revolution signal that can be used for order tracking (i.e. filtering the orbit to various frequencies) and as a phase reference.

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In the video below, Jason Tranter of the Mobius Institute offers an introduction to orbit plots and centerline diagrams.

Alan Friedman is a senior instructor at the Mobius Institute as well as the Founder / CEO of Zenco. He is available for on-site training, hands on training and vibration analysis program audits. You may use the form below to contact me.