In a typical papermaking process, water, heat, steam and dust put even the best lubrication products and procedures to the test.
Each section of a modern papermaking machine – consisting of the forming or wire section (often called the “wet” section), the press section, the drying section and the reeler – presents the bearings within it with threatening conditions.
At various stages in the process the threats come from water, heat, steam or dust, and these perils are real even when the lubrication products and processes are the best available for each application.
It is important, then, for maintenance and operations personnel to practice the best procedures and avoid common errors that can jeopardize bearings and cause unplanned downtime, poor product quality or even serious accidents.
Figure 1. Automatic Grease Lubrication
Forming Section
Because the pulp entering the first section of a paper machine – the forming section – is about 99 percent water, the section’s bearings are exposed to large amounts of water as the pulp’s water content gets reduced to about 80 percent. By the time it leaves the section, the paper web is self-supporting.
Both grease and circulating oil lubricate bearings in forming sections. However, grease is more common. It is used in probably 80 to 90 percent of wet sections and helps the seals in keeping water out of the bearings. In fact, in this application grease functions as much as a water barrier as a lubricant.
As a general rule, fill the bearing cavity 100 percent while filling the housing cavity 30 to 50 percent. Most housings are equipped with a labyrinth seal; fill this seal completely with grease as an added sealing measure.
There are three factors to consider when selecting relubrication intervals and quantities. These are lubrication and sealing function and the grease leakage from the housing. From a lubrication point of view, the amount of grease in the bearing should be just sufficient to supply an adequate quantity of base oil to the roller paths, rollers and cages.
From a sealing point of view, it is important that the labyrinth seals are always completely filled with grease. From a leakage point of view, the grease quantity should not be too large. Large quantities of grease generate heat in the bearing, especially at high speeds, when the bearing attempts to pump out most of the grease. After the relubrication has been carried out several times, the free space in the housing will be filled with grease.
Each subsequent relubrication will result in purging of the housing of a similar amount of grease. Therefore, large regreasing quantities will present a risk of leakage onto the wire. The grease selected should be formulated with superior corrosion inhibitors and water washout resistance.
Some mills employ bolt-on shrouds to help keep water out of bearings while others use different options such as contact seals. Still, the real key to avoiding water damage is a lubrication system or routine that keeps water out and purges water that gets in. Circulating oil as well as grease can get the job done if mill personnel effectively use filtration, vacuum dehydration, centrifugal action or some other method to remove water from oil.
In fact, some bearings and speeds call for circulating oil lubrication even in a forming section. When that is the case, regular monitoring of oil for water content is necessary to ensure both the oil’s lubricating qualities and the mill’s effectiveness at water removal.
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1. Suction tube filter 2. Heater 3. Pump (with motor) 4. Duplex filter 5. Cooler (with thermostat) |
6. Pressure control valve |
12. Speed reducers |
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Figure 2. Basic layout of a circulating oil lubrication
system for a paper machine. |
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Press Section
The press section removes additional water from the paper web. In this section the web gets pressed and felt “belts” absorb and take away the water. Lubrication challenges in this section include finding the correct lubrication and lubrication system to handle heavy loading on the rolls.
Upper press rolls in the press section are the most problematic due to the position of the load zone, bearing size and speed. In some cases, it is difficult to obtain greases with the required base oil viscosity and stiffness necessary to prevent roller skidding as the rollers re-enter the load zone.
Because of such conditions, circulating oil is typically used in the press sections of modern paper machines. Removal of water from the oil and rigorous regular oil testing are required. Testing helps ensure the effectiveness of both the water removal and the oil’s lubricating properties. Be aware that circulating oil may not be a panacea in the press sections of high-speed paper machines.
Heat generated in large press roll bearings may mandate large quantities of oil for cooling. Sometimes a controlled inlet oil temperature, achieved using a heat exchanger, will help maintain the oil’s required viscosity at the bearing operating temperature. Each added feature increases the system’s cost. Modern machines with a circulating oil system are typically equipped with a labyrinth seal and an oil flinger, which help prevent both the ingress of contamination and oil leakage.
If the loading or speeds in a press section using circulating oil lead to continuing frequent failures and unplanned downtime, mill personnel may choose to address the problem with SKF NoWear coated bearings. Calculating return-on-investment for the coated bearings should take into account downtime, production losses and quality issues.
Dryer Section
In the dryer section, the paper web’s water content gets reduced from between 50 and 65 percent to 5 to 10 percent. The process involves the use of both drying cylinders heated by steam and moving felts pressed against the surface of the advancing paper web.
Also, paper machines for the production of various products include Yankee cylinders in the dryer section. Several meters in diameter, Yankee cylinders are heated by high-pressure steam that helps dry the paper web as it’s pressed against the cylinder.
In the dryer section, the enemy of bearings and lubricants is both heat and water. Water affecting bearings or lubricants in this section would likely come from an aggressive wash-down during shutdowns.
Because of the high temperatures in modern papermaking machines drying sections, the journals of the bearings in drying and Yankee cylinders are usually insulated. These cylinder bearings can experience temperatures reaching 125°C (257°F). Circulating oil is the best choice for lubrication.
There are, however, potential pitfalls in the design and operation of a circulating oil system for such high temperatures.
Factors in making a dryer section’s circulating oil system successful include oil flow rates, steam temperatures and sealing. High bearing operating temperatures make the oil susceptible to oxidation and carbonization. Carbonized oil can clog oil lines, restricting flow and resulting in an inadequate volume of oil to the bearings.
Carbon buildup in the dryer lubrication system can cause either the inlet or the drain lines to become clogged, resulting in shortened bearing life. The monitoring of oil flow helps guard against this situation. In some machines, labyrinths are built into the bearing journals. These too can fill with oil deposits, making the sealing system ineffective.
When the labyrinths become clogged, diverted oil can migrate down the shaft and come into contact with the paper product, affecting its quality.
To compensate for restricted lines, clogged drain holes and labyrinths, mill personnel will often decrease oil flow or decrease oil baseline viscosity. For example, a bearing requiring two liters of oil per minute may be reduced to less than one liter per minute. Reducing oil flow will ultimately reduce a bearing’s service life. Oil with a lower viscosity will flow more easily through restricted lines, but may be inadequate for the application’s lubricating needs.
A secret to success in supplying oil to bearings in the dryer section is to provide the right high-quality oil to the bearings at the right rate and viscosity. Contrary to conventional wisdom, oil in this section is not intended to cool the bearings. Oil can be supplied too cool as well as too hot.
The right viscosity is ensured by the oil being in the right temperature range (50°C to 60°C/122°F to 140°F is common) when it enters the bearing. Monitoring the outlet temperature is also important. The oil must maintain its lubricating qualities throughout its contact with the bearings.
In difficult dryer-section applications, the oil supplied to the bearings must be in first-class condition, with water content below 500 parts per million. This calls for filtration, vacuum dehydration, centrifugal action or another method to effectively remove water from the oil. In addition, mill personnel should regularly test for water content, lubricating qualities and particles that signal bearing or sealing wear.
To detect water in oil, mill personnel might consider installing an analytic device on site, especially if they cannot contract with a laboratory that provides quick turnarounds. If results are not returned from the laboratory in a timely manner, it may be too late to correct a water-removal problem and save the bearings.
Figure 3. A technician performs a lubricant test.
Such testing can screen oil for water content,
lubricating qualities and particles that
signal machine wear.
Reeler
Paper machines produce a continuous paper web, typically 8 to 10 meters wide. Eventually, the web must be slit and made into smaller rolls. First, however, a reeler at the paper machine’s discharge end creates a huge spool of paper as wide as the web and weighing as much as 100 tons. When this first roll of paper reaches the desired diameter, reeling is continued on a new spool.
The principal bearings in the reeling operation are those on the reel drum, which feeds the paper web onto the spools, and those on the reel spools, onto which the paper web gets rolled. Compared to other paper machine sections, the reeler is a good environment for bearings. It is dry and about 25°C (77°F). Still, the bearings must be protected from paper dust.
Reeler bearings are commonly lubricated with grease. Seals and regular applications of grease will keep dust out of the bearings, but maintenance crews should ensure that neither manual greasing techniques nor automatic lubricators purge old grease onto the finished product.
Like other parts of paper machines, the latest reelers are designed to operate faster than ever (approximately 1,500 meters per second), and in these cases circulating oils are sometimes used for lubrication. Here, filtration is the answer to problems caused by dust and regular testing will be needed to monitor filtration effectiveness. Visual inspection will also help keep filters from becoming clogged and stopping oil circulation.
Shutdowns and Washdowns
Paper machine shutdowns for maintenance and repairs are inevitable and necessary. If there is water in a bearing’s lubricant at shutdown, it may begin to corrode the bearing’s internal components. In grease-lubricated bearings, start a shutdown by purging the bearings to remove any water-laden grease. In bearings using circulating oil, good water-removal systems and regular monitoring for water content will minimize water problems at shutdown.
In the best paper mills, shutdowns and good housekeeping indicate that machines get washed down regularly. Washdowns put even more water into sections where water is already prevalent (such as in the forming section) and increase the amount of water in sections with inherently less water (such as the dryer section). Typically, reelers are not washed down but may be cleaned with compressed air during shutdowns.
Precautions must be taken before, during and after washdowns to ensure that water does not get into the lubricant and decrease bearing service life. It is best to regrease bearings just before shutdown. During shutdown, free water in the lubricant, especially water mixed with cleaning chemicals, will rapidly start a corrosion process on bearing surfaces.
Additional relubrication may be carried out during a shutdown in order to push the water-contaminated grease out from the labyrinth seals. Before washdowns, check bearing seals to determine whether they require replacement due to excessive wear.
Checking seals for wear is an integral part of any effective preventive maintenance program. During wash downs, avoid high-pressure spraying at the sealing gaps, because that can force water into the bearings.
Final Comments
Following the preventive and proactive measures outlined here will help paper mill maintenance and management personnel ensure that machines produce at the rates and in the grades they were commissioned to supply.
Moisture and heat contamination are uniform and significant through the machine and paper-making exercise. Adequate oil circulation is critical to assure proper fluid film thickness, to assure contaminants are removed from machine-lubricated contact surfaces.
Lubricant temperature control and an acceptably high range (145°F to 160°F) will help settle high-density particle and drive-off moisture. Other means to remove moisture should be examined for effectiveness and financial value.
