Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Vibration Case Study No. 64 - 75 HP Triple Reduction Gearbox Bad High-Speed Shaft Bearings

Vibration Trend Graph - Warning Punch Through

The vibration level on a tank agitator was too high. The vibration spectrum indicated that the bearings on the high speed input shaft were the cause of the problem, or were they?

This agitator drive had a 3:1 reduction ratio. The input shaft turned at a very high speed. Historically, the high speed input shaft on all agitators of this style took a beating. In most cases the bearings on this shaft would fail long before the agitator had reached its useful life.

The vibration level on this agitator had been monitored for several years. On May 22nd, the trend finally punched through the "warning" limit that was established from an historical database average. The warning level represented the maximum deviation that this particular agitator experienced above its average level. The unit was scheduled for an in-field replacement of the high speed shaft bearings only. The unit was not removed and replaced with a rebuilt spare. This would have cost several thousand dollars more, and a prohibitive amount of downtime. On June 14th, vibration readings were taken after the repair. The vibration level had significantly dropped, but had the root cause of the problem been resolved?

Vibration Trend - High Speed Input Shaft - Outboard Bearing In the course of analyzing the possible defects that would cause the vibration level to increase, the spectrum was closely scrutinized. The waveform didn't show any significant impacting, so the problem was more subtle. Looking closely at the spectrum around 70 orders (LOWER LEFT), there are distinct harmonics at running speed of the input shaft (highlighted by squares on the top of the frequency peaks). This was an indication that there was something loose. In all likelihood, it had to do with the two high speed input shaft bearings (BELOW RIGHT).

Vibration Spectrum Zoom @ 70 orders

High Speed Shaft Inboard Brng

Upon removal of the bearings, a closer inspection of the inner races revealed fretting damage ( LOWER LEFT - inboard bearing, LOWER RIGHT -outboard bearing). For a close-up of the fretting damage on the outboard bearing, click on the inner race.
High Speed Input Shaft Bearing - Inboard Inner RaceHigh Speed Input Shaft Bearing - Outboard Inner Race
High Speed Shaft Bearings - Outer Races Closer inspection of the outer races revealed fretting damage on the inboard bearing, and slipping on the outboard bearing. Notice on the outboard bearing how the surface is so shiny that it reflects the camera lamp light. Also look closer and notice how the surface becomes dull and doesn't reflect the lamp light. This demarcation point located where the bearing was sticking out of the housing area. The outer race on the outboard bearing had been turning in the housing. That is why it appears so polished.

The dull area on the outboard bearing makes for a good comparison. Bearings that have a proper fit between housing and raceway, will have that dull appearance when removed.

Vibration Spectrum & Waveform on High Speed Input Shaft - After Repair

So ... did the repair correct the problem, or do we still have a problem? If we still have a problem, when would it likely surface?

The spectrum and waveform to the left, show a significant drop in levels. So does the vibration trend (LOWER LEFT). Also missing are the distinct harmonics of running speed (i.e. of the high speed input shaft). Therefore, replacing the bearings did help.

However, the mechanics did not do anything about the fretting damage. The housing and shaft are probably worn a little larger than desirable. The outer race of the inboard bearing should have been "loctited" into the housing (note, there is special "loctite" made for just such circumstances that will keep the outer race from fretting.) The best repair would have included replacing the high speed input shaft.

Vibration Trend - Predicted Future Failure The vibration trend chart to the left shows the next anticipated date when the vibration level will punch through the "warning" level. This is based upon extrapolating the last 1200 days worth of historical data. The predicted failure date is June 2001. At this time, it will be recommended to replace the high speed input shaft along with the bearings, and "loctite" the outer race of the inboard bearing.

Stay tuned ...


All Pictures and Text Copyright © 2000 - 2016 Contact Mr. Adler Adler Engineering LLC of Wyoming USA

Great care has been taken in the compilation of this article. However, no warranty, expressed or implied, including without limitation, warranties of merchantability or fitness for a particular purpose, are given in connection with this article or any article archived on this website. Although this information is believed to be accurate by the author, the author cannot guarantee favorable results will be obtained from the use of this article alone. This article is intended for use by persons at their sole discretion and risk. Since the conditions of product or material use are outside of the author's control, the author assumes no liability or obligation in connection with any use of this information. The author is not liable for special, indirect or consequential damages resulting from the use of this material.

No part of this article or any article archived in this website, or any part thereof, may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, without the prior written permission of the copyright holder R. H. Adler. Nothing contained in this article or any article archived in this website shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters of patent, copyright, or trademark, and nothing contained in this article or any article archived in this website, shall be construed as a defense against any alleged infringement of letters of patent, copyright, or trademark, or as a defense against any liability for such infringement.