Reliability Engineering Snapshot TM

Sleeve bearings, such as the one shown to the left, are about as reliable a bearing as you can get. What makes them reliable is their simplicity of design. There are no moving parts within the bearing itself (except for the oil rings). As opposed to ball bearings that have many moving parts that can fail, the sleeve bearing relys only upon a film of oil between the rotating shaft and the sleeve bearing itself. The bearing surface is smooth, and is made of a rather soft cast material known as babbitt. There are many different kinds of babbitt. The material can be composed of varying amounts of lead and tin. In general, as long as you keep the surface speed of the shaft below 3,000 fpm you'll have no problems.

The principle of operation with regard to how the shaft gets lubricated is simple. There are two oversized rings placed upon the shaft. The shaft rotation pulls the oil ring causing it to rotate. The rotation of the ring causes it to pick up oil in the bearing housing reservoir. The oil spills off onto the shaft, and lubricates it. The oil is then distributed throughout the bearing by means of oil pressure relief grooves (see the pictures). As long as there is enough oil in the housing so that the oil rings can dip into the oil, pick it up, and then distribute it onto the top of the shaft, there isn't a problem. The system will work flawlessly.

However, the very reliable sleeve bearing has one Achilles heel. Lower the oil level in the bearing housing such that the oil rings cannot pick up any oil, and the bearing will fail. Such a case happened to this bearing.

HOW IT HAPPENED

This system had a circulating oil supply. The oil was delivered to each bearing where it dropped into the bearing housing. The oil rings were simply there in case there was a loss in power to the circulating oil system. In theory, if there was a power loss, the oil rings would pick up oil in the housing and distribute the oil, thus preventing a bearing failure. However, somewhere in history someone enlarged the oil return line hole in the bearing housing so that oil could get out quicker. The only problem with this modification was that the bottom of the oil return hole was at the very bottom of the housing. When the power was lost to the circulating pumps, the oil drained completely out of the bearing housing. The oil rings could not pick up any oil because there wasn't enough oil left in the housing to pick up.

In the pictures that follow you can see how the babbit rolled into the oil pressure relief grooves. This made the situation worse because there was no way to distribute the scant remaining oil to the interior portions of the bearing.

Vibration Trend Chart

This bearing lost its oil several times in the course of time between 180 and 220 days on the trend chart. We first caught the problem at around 200 days. The momentary drop in vibration level, before it took a turn for the worse and increased to its highest level, is where we increased the amount of oil going to the bearing. Unknown to us at the time, we were artificially forcing oil through the restricted oil pressure relief grooves that were clogged with rolled babbitt.

Replacing the bearing dropped the vibration level to below the warning limit.

Vibration Signature After Forcing More Oil to the Bearing

This is the vibration signature on the bad bearing that had the rolled babbit in it. Additional oil pressure reduced the bearing vibration, but the signature content indicated that the bearing was already damaged and needed replacement. The multiple spikes are at rotating speed and indicate looseness within the bearing. The giveaway to the permanent damage was the inconspicuous hump out at approximately 80 orders. This hump was never there. It indicated a boundary lubrication condition between the shaft and babbitt. A boundary lubrication condition is where the shaft actually contacts the high asperities of the babbitt material, knocking them off or moving them.