By: Christian A. Smith (Published via Vibrations Magazine)
A gearbox was replaced after being reported for significant gear wear. After it was replaced and ran for about 7 hours, operations reported high vibration and temperature. The Jensen bevel gear style gearbox is driven by a 13 HP vertical motor at ?? rpm and outputs a 3-blade mixer at ?? rpm. The entire mixer assembly is mounted onto the side of a large acid and water mix tank. Movement could be seen with the naked eye, and high temperature felt with nylon gloves.
Initial vibration readings were highest on the motor in the horizontal axis around 0.66 ips-pk which was 126% higher than previous readings. The dominate vibration across the coupling was 29.5 Hz (1,770 CPM) and 59 Hz (3,542 CPM) which was 1x and 2x the input turning speed (see figure 1). A significant amount of low frequency peaks were evident along with a raised noise floor which led to suspicions of flow turbulence. Dual channel phase analysis was conducted across the coupling which indicated close to a 180-degree phase difference; this led to inspecting the coupling for misalignment. Although an alignment was performed before the agitator was reinstalled, it was found to be severely misaligned. Following the Falk Torus Coupling manual, we performed an alignment using a straight edge and feeler gauges to a better tolerance than called for.
After the alignment issue was corrected, the equipment was restarted, and high vibration, temperature, and movement were unfortunately still with us. The equipment was again shutdown and sent out to be inspected. The output shaft and propeller were checked for excessive run out and imbalance - both were within tolerance. The gearbox had excessive backlash and a severely worn bushing despite the short operation of the gearbox; both were corrected, and the equipment returned to service.
With only minor issues found to the equipment, investigations into the process and operation of the machine were performed. It was determined that the agitator had an interlock only allowing start up when the tank was at 50,000 gallons and shut down at 45,000 gallons. Jensen Mixers recommends maintaining a 48-inch level above the agitator at all times which turned out to be above the current interlock. The startup level of 50,000 gallons was about 15 inches too low and the shutdown level of 45,000 gallons was almost 24 inches too low (see figure 2). The interlock level was adjusted up to 62,000 gallons for both startup and shutdown to insure the agitator was never run below optimal tank level (48” above agitator).
After these adjustments were made to the tank level system, the agitator was reinstalled and started up with no issues. Overall vibration was significantly lower around 0.19 ips-pk and the temperature decreased. It was determined that the low tank level (< 48” level) was the source of the problem causing premature gearbox issues such as the worn bushings and gear wear.
About the Author
Christian A. Smith is a vibration specialist with Nelson Electric Motors tasked with developing and improving systems, practices, and programs. He holds a Bachelors of Science in Mechanical Engineering from Lamar University, ISO Category III Vibration Analysis Certification, Level I Infrared Thermography Certification, Optical Gas Imaging Certification, and a Failure Analysis Certification.