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In a paper machine drying section the lubrication system has been the cause of many bearing failures over the years. There are five major factors that influence bearing failures on drying cylinder bearings which continue to happen in the industry today. Highlighted below are the key factors and the solutions to minimize these failures. It is vitally important to understand each one.
The main cause of oxidation comes from the oil being exposed to high temperatures. As it passes through the bearing its lubricating temperatures can be as high as 120°C depending on the machine speed and steam inlet temperature. Temperatures rarely get the chance to cool much as there is a permanent oil bath level, maintained by the height of the bearing oil outlet pipe, to prevent catastrophic bearing failures if the oil flow is blocked for any reason.
It is important to know that the oil flow to each bearing or internal gears is set at the correct flow rate, depending on which bearing, or gear is being lubricated. This is controlled through rotameters which are set at a predetermined rate of flow, normally between 300ml to 1.8 litres per minute depending on machine type and bearing position.
If these rotameters become blocked and restrict the oil flow the bearing temperature increases as the oil flow through the bearing is too low which causes the oil to oxidise which then creates deposits on the bearing side faces. These carbon deposits build up and eventually break off due to vibration and go through the bearing rotating rollers and cause damage which leads ultimately to premature bearing failures. Carbon deposits also collect on the bearing outer covers which causes the same problems.
It is also important to control the temperature of the oil going to the bearings and gears via the oil cooler and the delivery temperature should be around 54°C depending on machine design. This is usually on start up after machine maintenance shut down.
There tends to be a desire to get the machine up and running to production speed as soon as possible, forcing the steam into the drying section at a faster rate than normal which can lead to inner race cracking of certain bearings that may not cope with the expansion due to rapid temperature increase.
Water contamination of the lubricant comes from two sources. The first being leaking rotary steam joints where the heating steam leaks into the bearing housings and contaminates the oil at a rapid rate. The only solution is to replace the rotary joints which can only be carried out during machine shut down.
The fitting of a grease nipple on each steam rotary joint housing can be quite successful in preventing carbon seal damage. It is important to note that a grease containing solid additives like molybdenum disulphide should be used in this application.
The second source of water contamination is through condensation inside the oil reservoir as the oil returning from the bearings can be as high as 80°C. Generally if there is a leak in the oil cooler, during operation the oil pressure is much greater than the cooling water and therefore does not pose a problem with contaminating the oil.
The important factors to note regarding the oil reservoir are the following.
The reservoir breather is positioned in the correct place, on the top at a minimum height of 500mm above the tank top, with the correct capacity for the size of the oil pump’s volume flow.
A vapour extraction blower should be fitted to the top of the reservoir on the oil return side to remove all the oil vapour, so it does not condense into water.
The baffle plate between the oil return side and the pump suction side should be of a height that allows time for the water contamination and any heavy dirt to settle at the bottom and the air and foam to dissipate.
One of the most effective ways to remove water from the oil in the reservoir is to fit an off-line centrifuge so it circulates the oil to and from the return side. It is important to ensure that the centrifuge temperature is kept between 82°C and 85°C with the correct size gravity disc fitted relevant to the density of the oil.
The oil reservoir should be drained daily to check for free water. If this is not carried out the free water will accumulate and eventually be picked up by the circulating pump suction.
Water contamination will increase the lubricants rate of oxidation and shorten the life of the oil. It will also increase corrosion of the bearings and control valves and shorten the life of the oil filters.
Dirt contamination and water are the most common causes of bearing wear, creating problems in different ways.
Water increases the oil’s viscosity and when the water particles enter the bearing load zone, the particles increase dramatically in temperature becoming tiny bubbles of steam which then impinge the bearing surfaces which cause pitting and hence further wear and metal fatigue.
Dirt, on the other hand, enters the bearings causing wear at a rapid rate depending on contamination volume. The wear that takes place on the rolling elements and bearing cages will cause the bearing vibration to climb excessively and the rate of wear will increase rapidly depending on machine speeds.
Foaming in paper machine oil can create a major problem regarding bearing and pump wear. When air is entrapped in the oil and enters the bearing load zone, the air bubbles explode due to high pressure and increased temperature which impinge on the bearing surfaces. This foam will also create cavitation on the circulating pumps which will create pump wear and accelerate the foaming problem.
To help prevent this it is advisable to have roll cloth filters fitted on the oil return side. This will help with wear debris collection and assist in dissipating the foam in the oil.
Most roll filter types have an indicator when the filter is becoming clogged. This can also assist when investigating what contamination is returning to the oil reservoir.
The quality of the paper machine lubricant is just as important as the maintenance of lubricant. As a guide there are some lubricant test results to guide your choice of lubricant.
First is oxidation stability, which is a measure of the lubricant’s ability to resist oxidation when exposed to high temperatures. The RPVOT ASTM D2272 test is a guide to the oil’s remaining useful life, as a percentage of the new oil. It is measured in time and the longer the result in minutes the better the lubricant is.
The second test is the TOST test ASTM D943 which is a measurement of the oxidation stability and the useful life of a lubricant in the presence of water. This is a measurement of the acid number and the result is measured in hours. The longer the test takes to achieve the maximum acid number in hours the better the lubricant.
Water separability is the measure of a lubricant’s ability to separate from water using test method. The faster the water and oil separate the better performance will be achieved in operation which is critical for paper machine oils. The test results are in minutes and the shorter the period for the oil and water to separate the better the performance.
The four-ball wear scar test, DIN 51350-3, is carried out by applying a fixed load across three rotating balls and one stationary steel ball for one hour to establish a lubricants ability to resist wear. The diameter of the scar is measured on conclusion of the test. The smaller the diameter of the scar is directly proportional to the lubricants ability to resist wear.
It is important to choose a superior quality lubricant for paper mill dryer bearing lubrication using the tests results above as a guide. One other important aspect is to ensure that all paper machine oils are manufactured using a group II base oil or higher.
In summary, you want your lubricants to have a high RPVOT (ASTM D2272) and TOST (ASTM D943) score, both measured in time; a short time when scoring Water Separability, (ASTM D1401), and low score 4-ball test, (DIN 51350-3).