When it comes to sheaves, proper vibration analysis is critical for identifying faults and preventing costly damage to equipment. These vibration analysis tips highlight two common sheave-related issues—eccentricity and misalignment—and presents practical advice for diagnosing them. From understanding the telltale vibration patterns of an eccentric sheave to recognizing the axial movements caused by misalignment, these insights will help you distinguish between faults, minimize downtime, and protect your equipment from premature wear.
How to Diagnose an Eccentric Sheave
Diagnosing an eccentric sheave with vibration analysis is easy to do. But you have to be careful not to mistake it for other common machine faults. For example, unbalance also shows up at 1x in the radial directions. Fortunately, you can always stop the machine, mount a dial indicator and measure the run out of the sheave directly.
As the eccentric sheave rotates around you can see how it wants to move in the radial directions (vertical and horizontal). It makes one complete cycle of movement with each rotation. The vibration forcing frequency is 1x, or the shaft rate, in the radial directions.
The belt tension changes things however…
…and what we measure in real life are higher levels of vibration in the direction of the belt tension. In each rotation of the eccentric sheave, it pulls the other sheave towards it. We will therefore also be able to measure the vibration on both the input and output bearings in the direction of belt tension.
When you look at the animation without the belt you would also expect the phase relationship between the vertical and the horizontal to be 90 degrees. The belt tension changes this too. This is one of the keys to diagnosing this fault correctly. The phase between the direction of belt tension and 90 degrees away from the belt tension will NOT be 90 degrees as expected but closer to 0 or 180 degrees.
As mentioned earlier, an eccentric sheave can be confused with unbalance. Both result in elevated 1x vibration. Although phase analysis might help you distinguish between the two, it is not always reliable. If the vibration levels are excessive and the problem needs to be resolved, the best approach might be to simply shut the machine down and measure the sheave runout with a dial indicator. You might also use a strobe light to “freeze” the sheave. You might actually see it moving in the direction of the belt tension.
How to Diagnose a Misaligned Sheave
Diagnosing a misaligned sheave is easy to do with vibration analysis. Note how the sheave moves in the axial direction, one cycle of vibration per rotation of the shaft (1x). Note the phase relationships. As the top of the sheave moves to the right, the bottom moves to the left – meaning they are 180 degrees out of phase. The part of the sheave closest to the viewer moves out of phase with the part furthest away. This movement doesn’t happen at all when the sheave is aligned as you can see in the sheave on the right side of the video.
If the machine is not running, one could of course place a dial indicator on the axial face of the sheave and rotate it by hand to observe the movement in the axial direction. The benefit of using vibration analysis is that we can detect the defect while the machine is running.
Below you can see the axial movement and phase relationships for a misaligned sheave from a different angle.
If you have ever played with a strobe light to freeze a shaft, you might consider that the axial movement of the sheave might be visible if you “freeze” it with a strobe light. Simply adjust the strobe light to match the rotational rate of the sheave you want to look at.
Misaligned sheave – Why is it important?
Now that you can see how the sheave moves each time it goes around, think about what this is doing to the bearings! In each rotation of the shaft there are now new axial forces on the bearing. These repetitive forces cause fatigue and fatigue will cause your bearings to wear out faster. A 20% increase in load on a bearing cuts its life in half!
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Alan Friedman
Alan, aka the Vibe Guru, has over 30 years of vibration analysis experience, He has trained 1000’s of students around the world up to Category IV. One of the things that makes Alan a great teacher is his ability to teach people where they are at. Whether you are a math challenged millwright, an engineer or a PhD, Alan will challenge you without overwhelming you. If you are interested in condition monitoring you can also check out his book: Audit It. Improve It! Getting the Most from your Vibration Monitoring Program or hire him for an on-site program audit.
A shaft centerline diagram is a plot used in the vibration analysis of journal bearings. It is also called a shaft average centerline diagram. The purpose of this graph is to display the average position of the rotor in the bearing. In short, for different bearing types, there is an expected or “normal” position of the shaft in the bearing. If this position changes, it can indicate a problem.
A shaft centerline diagram is a plot used in the vibration analysis of journal bearings. It is also called a shaft average centerline diagram. The purpose of this graph is to display the average position of the rotor in the bearing. In short, for different bearing types, there is an expected or “normal” position of the shaft in the bearing. If this position changes, it can indicate a problem.
The present study is aimed at establishing the vibration standards for precision machine tools. The machine tools are first segregated and then their vibration data are analyzed for determining the normal vibration level and damage factors (DF). After refining and fixing the vibration standards obtained, they can be used to assess the machinery health.
The present study is aimed at establishing the vibration standards for precision machine tools. The machine tools are first segregated and then their vibration data are analyzed for determining the normal vibration level and damage factors (DF). After refining and fixing the vibration standards obtained, they can be used to assess the machinery health.
Accept that you will sometimes be wrong and do whatever it takes to find out the “as found” condition. This is often difficult to accomplish but it is well worth the effort and in the best circumstances this should be implemented as a normal procedure.
Accept that you will sometimes be wrong and do whatever it takes to find out the “as found” condition. This is often difficult to accomplish but it is well worth the effort and in the best circumstances this should be implemented as a normal procedure.
When troubleshooting or using vibration in a Predictive Maintenance program comparing identical machines to each other is often the best method to diagnose a problem – especially if you suspect a problem exists in one or more of the machines. When vibration data is collected on a bearing housing using an accelerometer, one is not only measuring vibration forces, but also the effect of the structure on these forces.
When troubleshooting or using vibration in a Predictive Maintenance program comparing identical machines to each other is often the best method to diagnose a problem – especially if you suspect a problem exists in one or more of the machines. When vibration data is collected on a bearing housing using an accelerometer, one is not only measuring vibration forces, but also the effect of the structure on these forces.
Condition monitoring (CM) is not a life-extending activity. Life-extending activities are things such as lubrication, alignment, balancing and operating procedures. It’s very important to keep this very basic fact clear in all communications within your plant; otherwise, too little importance may be placed on the planning and scheduling of corrective work orders originated in CM.
Condition monitoring (CM) is not a life-extending activity. Life-extending activities are things such as lubrication, alignment, balancing and operating procedures. It’s very important to keep this very basic fact clear in all communications within your plant; otherwise, too little importance may be placed on the planning and scheduling of corrective work orders originated in CM.
During Academy of Infrared Training courses, we see many students who have purchased an infrared camera that is not suited for their intended use. These students have put their trust in a camera salesperson, and, many times, that salesperson was solely interested making a sale. Later, these students are disappointed to find out that the camera is not truly suited for their application. This article aims to help the camera buyer understand what basic camera specifications mean, and also help them determine what type of camera and options are suitable for their application. If you note your answers to questions throughout this article, by the end you should have a custom list of applicable specifications and options.
During Academy of Infrared Training courses, we see many students who have purchased an infrared camera that is not suited for their intended use. These students have put their trust in a camera salesperson, and, many times, that salesperson was solely interested making a sale. Later, these students are disappointed to find out that the camera is not truly suited for their application. This article aims to help the camera buyer understand what basic camera specifications mean, and also help them determine what type of camera and options are suitable for their application. If you note your answers to questions throughout this article, by the end you should have a custom list of applicable specifications and options.
There are four primary components to precision grease lubrication for bearings: lubricant selection, application method, the volume of lubricant to be delivered, and the frequency with which it is applied. There are, of course, many different methods for specifying these values, and opinions can vary significantly as to which approach is best. Due to the variability of operating conditions and machine design, it can be very difficult to be truly precise without introducing the "condition-based" component to the formula.
There are four primary components to precision grease lubrication for bearings: lubricant selection, application method, the volume of lubricant to be delivered, and the frequency with which it is applied. There are, of course, many different methods for specifying these values, and opinions can vary significantly as to which approach is best. Due to the variability of operating conditions and machine design, it can be very difficult to be truly precise without introducing the "condition-based" component to the formula.
