Pulp and Paper Canada

BEARINGS: Bearing down on design and preventive maintenance

November 1, 2002  By Pulp & Paper Canada

The Timken CompanyPlants recognize savings in time and money with bearing repairHow would you describe the expression “run of the mill?” The usual. Average. Nothing special.If you are involved in the …

The Timken Company

Plants recognize savings in time and money with bearing repair


How would you describe the expression “run of the mill?” The usual. Average. Nothing special.

If you are involved in the everyday operations of a plant, then knowing the mill is running smoothly is nothing to take for granted. In fact, in today’s complex operations, it takes on a whole new meaning – one that revolves around accuracy, throughput, uptime and performance. The impression of carefree operation definitely will be short-lived if any bearings become damaged.

In the past, a damaged bearing would likely become a boat anchor rather than be repaired component. Delays would typically be caused as a new bearing is ordered. The damaged bearing would be scrapped, making a bad situation worse by needlessly wasting an expensive resource that still retains value. Realizing that “two wrongs don’t make a right,” bearing reconditioning programs continue to become an increasingly viable alternative to buying new replacement bearings.

This modern day change in rationale continues to fuel the growing popularity of reconditioning services. The Timken Company, for instance, has been reconditioning bearings for more than 55 years, but only marketing this service for the past twelve. Bearings can often be returned to original specifications, including the same warranty, for less time and money than purchasing a new bearing.

Reconditioning can save 50 to 90 percent of the cost of purchasing a replacement bearing. As any plant manager will tell you, a plant’s bottom line is not determined solely by the price of its components. Even more importantly than the price of the bearing itself, reconditioning caters to the largest concern of any plant manager – plant downtime.

Categories of Bearing Repair

When you detect bearing damage, the proper remedy can be assessed only when the cause of that problem and extent of the damage are determined. Environmental issues, such as ingress of contamination and water into the bearing chamber, are the primary culprits for a bearing’s early demise. But, there are also many other hazards that can dramatically shorten a bearing’s life. Typically, when damage is assessed, the degree of bearing reconditioning needed is identified as one of the following:

Type I repair consists of bearing inspection and minor trouble-spot repair. This level is used to assess the bearing’s true condition, including thorough cleaning, polishing, resetting of internal clearances and spot-grinding any small surface breakouts or spalls to prevent further damage.

Type II repair involves major bearing repairs and may include regrinding of the races, spacers, cone bore and cup outside diameter (O.D.) components that are not repairable are replaced at this level.

There are hundreds of ways a bearing can start to break down, not all of which are repairable. Spalls that are very large or run across the entire race, very deep line or water etching, excessive scoring on large ribs or cracked race components are conditions that are just not economical and sometimes cannot be reconditioned.

If a bearing can be repaired, it should be sent to experienced experts who have an intricate knowledge of the process and product. Reconditioning practices should be applied only when absolutely necessary. A bearing can be reground a limited number of times and still meet specifications, so unnecessary repair can shorten a bearing’s life.

In the final analysis, bearing repair and reconditioning programs are gaining in popularity. Whether it’s spherical, cylindrical, or tapered, mill maintenance operators will want to consider taking advantage of this cost-effective solution.

BEARINGS: Spherical roller bearing for felt rolls

As an integral part of the wet end of a paper machine, felt roll bearings are subject to the ingress of corrosive contaminates during operation as well as wash down procedures. Contaminates reduce bearing life to months, yet the anticipated theoretical fatigue life is years for the applied loads and speeds. The reduction of bearing service life stems from the ingress of contaminates through the existing housing seals.

To combat the ingress of contaminates in the field, felt roll bearings must be frequently lubricated to purge contaminates. In many cases, lubrication programs are also consolidating lubricants that are not suitable for felt roll applications. The end result is reduced bearing life and overall performance.

To increase bearing life, reliability and performance, contaminate must be kept out of the bearing. Better sealing is required while modification to existing roll and housing components must be kept to a minimum. To achieve this goal, an optimized spherical roller bearing with integral seals has been developed by NTN. The bearing incorporates two robust contact seals, which complement any existing sealing currently utilized in the housing. Avoiding the ingress of contaminate prevents accelerated wear, limits lubricant deterioration, reduces lubricant requirements and hence extends lubricant intervals. The bearing package provides a specifically selected premium lubricant pre-filled at NTN’s manufacturing factory guaranteeing lubricant cleanliness, quantity and quality at installation.

When installation environments are also frequently less than ideal, a factory pre-lubricated sealed spherical roller bearing has proven extremely convenient during assembly. As these bearings are designed as a direct drop in replacement to the common ISO 223xx series bearings found in felt roll applications, they are completely compatible with existing roll designs and no modifications are necessary to the surrounding components.

Marcus Wickert, P.Eng., manager of engineering, NTN Bearing Corporation of Canada.

BEARINGS: SKF – Eliminating smearing in soft calendar bearings.

Paper mills often experience problems in the calendar section where the paper is glazed between rolls. The bearings in these rolls need to operate in an environment of varying loads, ranging from virtually zero to full load. During loading, rolling elements in the bearing are forced to accelerate quickly which can give rise to smearing problems. Smearing is a serious form of wear and will lead to failure. Consequently the bearings have to be replaced, which involves substantial costs for the bearings, maintenance personnel and machine downtime.

SKF has provided extended bearing life in harsh operating conditions by utilizing new surface technology provided by the NoWear solution.

In one mill, the standard bearings used before were replaced about once a year. Using the new technology from SKF, they have now been able to run the machine more than three years without any bearing problems. Since the calendar rolls are taken out for regrinding every third year the bearings now outlive the rolls. One result of this is that scheduled stops can be cut in three. Another important aspect is that have not been any unplanned stops since the new bearings were implemented. There is a large potential for savings here, as the cost of lost production per breakdown was in excess of $7500 per hour.

The operating conditions of this machine are:

Design Speed1300 m/min

Trim width 7.2m

Paper GradeNewsprint

Normal load stack 1:100 – 120 kN/m, C/P * 30

Normal load stack 2:30 – 40 kN/m, C/P * 100

These bearings are made of steel with the rolling elements, and raceways if necessary, being coated with a new type of diamond-like carbon with a thickness of several microns. The coating utilizes a physical deposition process that has been optimized for bearings and other components used in difficult operating conditions.

The coating is especially adapted to bearings and prevents failure where conventional bearings cannot – applications suffering from wear, poor lubrication or other difficult conditions. The coating can successfully be applied in a wide range of bearing applications running in demanding environments. For a coating to function properly in a bearing, it has to withstand rolling contact f
atigue, create little friction and prevent adhesive wear.

The characteristics are:

Hardness1200 HV

Friction coefficient (dry)0,1 – 0,2

Thickness~ 1 m

Max Temperature+350 C

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