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Predicting Bearing Failures
Why are vibration based inspections important?
Many of the mechanical defects that occur on printing presses are observable. For these we can use many techniques including preventive maintenance inspections, print problem analysis, and other periodic visual and measurement inspections. Others, however, are often unseen until the failure occurs or the press has already experienced significant production related problems including slowdowns and poor print quality. Two of the more common types of hidden mechanical failures are bad bearings and damaged gears. Of the two, failed bearings are the most common. There are also specific electrical defects that are hidden and can lead to drive instabilities and motor failures. For now let’s look at bearings. Bearings, particularly printing press bearings are high precision assemblies with very close internal tolerances. Printing cylinder bearings in particular, have very close tolerances and littel room for damage before print quality problems occur. In fact, the quality problems will often occur long before the eventual catastrophic failure.
The internal damage to the bearing creates forces that produce very unique vibration at the bearing housing. By measuring and analyzing the movement the type of damage can be determined and often the severity of the damage can be accurately estimated. There are four general areas for bearing failure: looseness that can be internally or in its mounting or housing fit, damage to the surfaces of the races on which the rolling elements bear the load, and damage to the rolling element assembly itself. These are listed in increasing order of potential for catastrophic failure.
Looseness can often be detected by simple lift and endplay checks while the press is down. However, the tight tolerances of the printing cylinders may show very little movement from these checks even while the bearing has worn excessively. It is also unique to the process of preloading the bearings that these can come to have excessive looseness with out to the bearing load surfaces. The preload setting mechanism can often change creating a looseness. It is often a good idea to check and reset preload after six to twelve months on some bearings. While this is a good idea, it is seldom done due to the press time needed and the specific expertise required to do so. This is most important for tapered roller bearing assemblies, less so for spherical and cylindrical bearings.
Vibration testing can often be used to detect looseness, but it is more useful for detecting the wear or damage to the rolling surfaces. Bearing are unique in that the frequencies generated by failure mechanisms are generally not harmonically related to the rotation of the press. This means that the vibration will be distinct from gaps, imbalance and other rotation defects. This is a very valuable condition for the vibration technician.
These defects are often detectable very early in the failure cycle. In many cases, evidence of damage may be measured two or more years before the evidence of near catastrophic failure will occur. That does not mean that repairs should be put off. Internal damage to the bearings will begin to affect print quality long before the bearing fails mechanically.
So, what is a a catastrophic failure? It in one in which the bearing not longer performs even the basic function of permitting the rotation of the cylinder or roller on which it is mounted. It is total loss of the use of the component. Bearings can be “bad” and have irreparable damage to the rolling surfaces long before this catastrophic failure occurs. If the failing bearings can be detected, we can prevent two serious problem. We first can prevent the inevitable deterioration of print quality or press control that occurs as the failure progresses. This can save may hours of frustration and loss of productivity. Second, by preventing the catastrophic failure we prevent the total loss of production and damage to related components.
By: Joe Strader; www.printingpresshealth.com