In this paper an investigation is made to evaluate the experimental performance of misaligned cylindrical and three-lobe journal bearings. A three-bearing double-disc rotor system with a test bearing in the middle and driven by a variable speed DC motor through an electromagnetic coupling is used. Various parameters which have been studied are friction through coastdown analysis, unbalance response and film thickness. The system damping also has been evaluated. The film thickness decreases, friction increases and system damping increases as the bearing misalignment increases. While the first, third and fourth harmonics vibration response is found to be almost steady with a increasing degree of misalignment, the second harmonics vibration response tends to decrease slightly. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

An Experimental Investigation on the Misalignment Effects in Journal Bearings

An experimental investigation of tribological effects on coastdown phenomena in horizontal rotating machinery

Experimental studies of coastdown time

Rotating equipment under the action of dynamic forces, are prone to defects, such as misalignment, unbalance, change of rotor slope, skewed bearings, etc. These defects, if ignored for prolonged periods can cause sudden outages that may have serious consequences. Therefore, application of an appropriate condition monitoring technique is desirable to assess the health of the equipment and plan its maintenance. In this paper, monitoring of coast-down time (CDT) is undertaken to meet the objective. The CDT is the time elapsed between the instant the power is switched off to the rotor system till it comes to rest. The work demonstrates that, the CDT does detect the defects of the shaft assembly. Experiments were conducted on a specifically fabricated rig. The results revealed that the speed decay pattern followed a second order fitting of percentage speed reduction as a function of time. Defect identification parameter (DIP) is defined, which is a ratio of the polynomial coefficients of the first and the second order terms. The DIP values were found to correlate uniquely with the unbalance and the radial off-set defects in the shaft assembly.

Coast-Down Time Monitoring for Defect Detection in Rotating Equipment

Mechanical malfunctions such as, rotor unbalance and shaft misalignment are the most common causes of vibration in rotating machineries. Vibration is the most widely used parameter to monitor and asses the machine health condition. In this work, the Coast Down Time (CDT), which is an indicator of faults, is used to assess the condition of the rotating machine as a condition monitoring parameter. CDT is the total time taken by the system to dissipate the momentum acquired during sustained operation. Extensive experiments were conducted on Forward Curved Centrifugal Blower Test Rig at selected cutoff speeds for several combinations of combined horizontal and vertical parallel misalignment, combined parallel and angular misalignment, as well as for various unbalance conditions. As mechanical faults increase, a drastic decrease in CDT is found and this is represented as CDT reduction percentage. A specific correlation between the CDT reduction percentage, level of mechanical faults, and rotational cutoff speeds is observed. The results are analyzed and compared with vibration analysis for potential use of CDT as one of the condition monitoring parameter.

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Condition Monitoring of Forward Curved Centrifugal Blower Using Coast Down Time Analysis

A technique is presented to assess the mechanical condition of vertical axis electric motors equipped with grease-lubricated ball bearings by comparing observed and predicted rotor coastdown times. A theoretical expression for calculating the coastdown time required, as input, a relationship between rotor torque and rotational speed. Torque values derived from existing theory in the literature proved unsatisfactory. Experiments were conducted on four electric motors with varying bearing size and preload, blade configuration and grease lubricant properties to refine the input criteria. Substitution of these empirically derived torque values into the expression then gave acceptable results. Deviation of actual coastdown times from those predicted indicates a probable mechanical problem and need for investigation. Presented at the 33rd Annual Meeting in Dearborn, Michigan, April 17–20, 1978

T. L. Daugherty

Papers

Coastdown Time as a Mechanical Condition Indicator for Vertical Axis Motors with Grease-Lubricated Ball Bearings