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Lubrication is critical to meeting increased demands of machinery
Proper lubrication of moving parts, especially gears and bearings, is critical to performance as demands on paper machinery increase. Lubrication serves to carry away heat, reduce wear and friction, m...
May 1, 2006 By Pulp & Paper Canada
Proper lubrication of moving parts, especially gears and bearings, is critical to performance as demands on paper machinery increase. Lubrication serves to carry away heat, reduce wear and friction, minimize debris problems, and protect bearing surfaces from corrosion. For instance:
* wire section bearings require continuous greasing to stop water ingress,
* press section bearings demand high quality greases and oils due to the loads encountered; and
* drying cylinders require lubricants that can withstand temperatures in excess of 300F.
Increased production often can result in higher operating temperatures, faster speeds and heavier loads. Just as bearings have improved in design, proper lubrication selections and practices must go hand-in-hand to achieve maximum performance.
Too often, companies struggle with ongoing maintenance because of improper lubrication practices. For instance, insufficient lubrication leads to additional friction and increased temperatures, which can cause premature bearing damage. Excess lubrication can generate additional heat, causing the lubrication to break down and lead to potential unexpected and premature bearing damage.
Recognizing early warning signs of lubrication problems, such as increased operating temperatures, lubrication leaks, and unusual noise and vibration can help prevent bearing damage. With inadequate lubrication, temperature gauges often indicate the bearing is running hot and it may even become noisy from the resultant metal-to-metal contact. When bearings are overfilled, the lubricant will break down and darken in colour because of extreme temperatures.
When lubrication failure occurs, it can result in several types of roller bearing damage, including ribs and roller end scoring from metal-to-metal contact. In extreme cases, inadequate lubrication causes the rollers to skew, slide sideways and provoke total bearing lock-up. Recognizing the types of damage caused by inadequate lubrication helps to pinpoint and correct the problem.
Options in lubrication
Just as important as acknowledging proper lubrication levels, is selecting the correct type of lubricant for a given application. The most common mediums are oil and grease. Each type serves to meet specific application needs, so making the proper bearing selection requires a clear understanding of the demands of the application.
Paper machine oils used in bearing applications are either synthetic oils or mineral. Mineral oil is derived from refining crude petroleum and can contain unstable compounds such as nitrogen, oxygen or sulfur, which can affect service life. Synthetic oils are impurity-free and are designed for use in special circumstances where normal petroleum products cannot cope with temperatures. In most cases, additives are used in the oils to resist oxidation, reduce foaming and improve lubricity.
Although lubricating oils are uniform in many characteristics, knowing and understanding the viscosity of oil is critical in determining its operating limits. Viscosity is the measure of the flowability of a liquid at a definite temperature. The faster the flow, the lower the viscosity, and vice versa. The oil viscosity must be high enough to provide a continuous film, but not too high to generate excessive heat.
The flexibility of oils operates better than grease in higher temperature applications because it circulates more freely, resulting in the dissipation of heat. It allows contaminants to be flushed or filtered from the bearing and can allow the bearing to attain higher speeds. Oil levels also are more easily controlled and easier to drain and refill, yet they can be tricky to seal in some situations.
On the other hand, grease, a precise combination of oil, thickeners and additives, is more easily confined to the bearing. Grease acts like a sponge to retain and release oil, but it also performs like a seal against moisture, dirt and other contaminants. With no oil levels to maintain, it requires less frequent lubricating and its thickness makes leakage less likely to occur.
Similar to oil, grease too has its limits for upper and lower operating temperatures. These limits change per the type and amount of thickening agents and the base oil type and viscosity used in making the grease. Operating speeds also need to be considered. Greases typically are limited to a speed of about 2500 feet/minute (rib speed) because of heat generation.
Most importantly, what works for one application may not work for another. Each lubricant offers advantages specific to use and operational demands. The selection of lubrication requires careful expert review of these demands to ensure peak performance.
Almost half of bearing failures are due to improper lubrication. In addition to improper fill levels, other factors can contribute to reduced bearing life and performance, including the wrong lubricant or improper maintenance cycles. Experience has shown that the application of small amounts of lubricant more frequently results in greater bearing life, compared to a large dose over a longer time period. Proper lubrication is an essential component of effective preventative maintenance.
Knowing how to properly deliver and apply the lubricant is important to the performance of that equipment. For oil, there are a variety of methods available. Choosing an oil system is highly dependent on the desired speed of the application. The more heat that can be removed from the system the higher the attainable speed.
Oil bath is a static oil level for low and moderate speed applications.
Drip feed uses controllable sight feed oilers in moderate speed applications.
Wick feed uses a wick in conjunction with a reservoir of oil.
Oil-splash allows the splash from the gear teeth to sufficiently lubricate the bearings and is used for more specific applications, such as gearboxes.
Circulation pumps assure a proper and constant supply of lubricant to the bearings and are often used for low to medium speed, heavy-duty applications.
Oil mist is used for continuous, high-speed applications and offers a continuous spray of pressurized air and oil to the bearings.
Oil-jet lubrication, similar to circulation, may be used for high speeds and heavy loads but requires large drains to prevent the accumulation of oil after passage through the bearings.
Although greases meet different operating demands than oils, they are equally important lubricants and require equally unique, but different methods of delivery. For grease, manual delivery is a viable option, but bearings can easily be over or under filled. When applying the grease, it should be forced between the balls or rollers until the available space inside the bearing is filled completely. Any excess grease should be wiped away. The space on each side of the bearing in the housing should be not more than half filled.
If a grease gun is used, pressure must be regulated so not to damage any seals or other parts of the machinery. Grease delivery generally follows the 1/3 rule. This means that the bearing will expel or purge all grease in excess of about 1/3 of the volume available. This is especially important for bearings with two shields or seals since excess grease cannot get out during operation. Bearings without the shields or seals will expel any excess grease into the free housing space outside of the bearing during operation, which is why the housing space should never be filled completely.
Other delivery options include single-point lubricators and centralized systems. Single-point lubricators provide the option of using several different greases simultaneously on a piece of machinery to compensate for different lubricant requirements precisely where and when needed. Centralized systems offer precise lubrication delivery and intervals, but proper care must be
taken to be sure that the ability to pump the grease is not compromised.
Selecting, delivering, reapplying and maintaining superior lubrication requires following manufacturer’s guidelines and recommendations for each application. By continuing to learn about lubrication and practising a regular maintenance schedule, bearing failure from lubrication problems can be avoided. When lubrication is correctly applied and maintained, it increases bearing life, overall machine productivity, and can reduce costly maintenance repairs. For additional information on proper lubrication, contact The Timken Company at 330-438-3000 or visit www.timken.com. P&PC
David A. Pierman is the product manager, Lubricants and Lubrication Delivery Systems at the Timken Company.
Never underestimate the importance of Lubrication
Lubrication is more than simply an additive to a machine. Instead, it is an integral component of that machine and as such should be closely monitored to assure superior quality. Essential to all the moving parts, especially bearings, lubrication serves to carry away heat and protect bearing surfaces from corrosion and wear.
Too often, customers experience unnecessary maintenance and aggravation because of improper lubrication. Too little lubrication causes additional friction and heat, which can cause premature bearing damage. Excess lubrication, at high speeds, can generate additional heat from churning, which causes the lubrication to break down thermally, leading to eventual bearing damage.
Problems caused by inadequate lubrication have many forms, including roller end scoring from metal-to-metal contact and excessive wear on the rollers and raceways. Excessive heat generation can cause cone large rib face scoring or “welding” damage or cone large rib face deformation. In extreme cases, inadequate lubrication causes rollers to skew, slide sideways and lead to total bearing lock-up.
Thankfully, there are signs that indicate when bearing lubrication is inadequate or in excess, and if detected soon enough can be corrected before catastrophic damage occurs.With inadequate lubrication, gauges and operating temperatures will show that the bearing is running hot and it will become noisy from metal-to-metal contact. When lubrication is in excess, it can break down because of increased operating temperatures and become a dark brownish-black colour.
The good news is that with proper application and a regular maintenance schedule, bearing damage from lubrication problems can be avoided. Here are some tips to ensure proper bearing lubrication:
* Follow set guidelines, established by the equipment builder, for each application
* When applying grease, force the grease between the rollers and cage to ensure it is packed completely full.
* Pay attention to when a bearing needs to be re-lubricated
* Watch equipment gauges for early indications of a problem, such as unusually high temperatures
* Listen for noise or unusual vibration
* Watch for lubrication leaks
* Proper maintenance and handling practices are critical. Failure to follow installation and maintenance instructions can result in equipment failure, creating a risk of serious bodily harm.
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