Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
Don't ever assume that a journal housing will never have to be replaced and that all that will need to be replaced is the sleeve insert. Many mechanics fall into this bad habit due to inattentiveness. Sometimes it is not a bad habit. It is due to Production demanding the equipment back as soon as possible. Putting a good sleeve bearing insert into a bad journal housing is kind of like spitting into the wind. It is going to come back at you in a way you won't like.
Most companies don't routinely stock bearing housings in their warehouse. It is too expensive to do this anymore. So most of the time the routine is to simply replace the sleeve bearing insert and everyone becomes insensitized to whatever might be going on with the housing. Unless the housing is visibly broken into pieces most of the time it never gets a closer inspection. There was an occasion where the sleeve bearing insert was replaced twice within a short period of time. The failure frequency on this bearing had been measured in years and now it was measured in months. As the saying goes "three times the charm" someone finally decided to look a little closer at what they were putting their good bearing inserts into, namely the journal housing. The journal housing is shown in the picture to the left. The damaged sleeve bearing insert was removed.
It is pretty obvious in the picture, in the lower left, that there is severe fretting damage on the lower bearing seat. The fretting damage was probably aggravated by two things, increasing degradation due to age, and looseness brought about by spalling of the material. The picture to the lower right shows a closer view of the fret damage. Notice in the lower right hand corner of the picture where it is circled that there is not fret damage. Using the simple rule for stress being inversely proportional to the surface area at a given load it is easy to see that as the fretting damage diminishes the amount of surface area that can carry the load the stress increases.
|The situation spirals out of control because as the stress increases it does not increase uniformly. The stress along the edges of the damaged areas, which are recessed, is a lot higher than the stress further away from the damaged areas. The pictures below illustrate this concept a little better. In the enhanced picture to the lower right the computer found the edges in the picture to the lower left. There's a lot of missing area now isn't there! Try calculating the stress for some of those interior portions that are literally surrounded by missing material. The problem went away after the journal housing was replaced. No small wonder.
|Fretting damage is usually caused by looseness. Well how does one get looseness if the sleeve insert is always torqued up tightly to the correct pressure? It's called old age. Sooner or later that material IS going to fatigue. Once that happens the torque on the sleeve insert can, and will, relax enough until there is looseness. Other signs of looseness, besides fretting damage, can be missing babbitt on the sleeve bearing inserts. Some damage is shown in the pictures below. The picture to the lower right is a close up of the top circled region in the picture to the lower left. When the bearing insert is loose within the journal housing it bounces and so does the oil film. The oil film collapses more times than one would care to imagine. There's a lot of impacting force between the shaft and the babbitt and the babbitt along the edges is particularly susceptible to this type of damage.
|If you see the beginnings of this type of damage you still have some time, but not a lot of time. Go ahead and put the new bearing insert into the damaged journal housing, but you better be ordering a new journal housing and planning on replacing it during the next production outage.
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