Improved four-row, sealed tapered roller bearings

NSK's new Extra Capacity Sealed-Clean Roll Neck Bearings deliver up to 34% extra load capacity, higher reliability, longer life under severe lubrication conditions and easier handling

Four-row, sealed tapered roller bearings are used primarily to support rolls in the rolling mills of steelworks.

They are required to achieve longer life while experiencing the adverse conditions of high loading, extreme shock loads and water and scale ingress.

Of all these conditions water and scale ingress is the one capable of reducing bearing life the most, due to its extremely harmful effect on lubricating conditions.

Twenty years ago NSK was the first bearing manufacturer to address this problem with its Sealed-Clean Roll Neck Bearings.

Now, in a new development, NSK has gone a stage further and developed Extra Capacity Sealed-Clean Roll Neck Bearings.

The new Extra Capacity design meets the needs of an industry where increasing downward cost pressures mean that failures in the continuous production cycle are extremely costly.

It delivers up to 34% extra load capacity, higher reliability, longer life under severe lubrication conditions and easier handling.

The improved performance of the new Extra Capacity bearings is the result of a synergy of optimised mechanical design, new bearing material, and patented sealing technology.

The first of these elements, optimised mechanical design, has been employed to achieve higher load capacity, one of the most important factors influencing bearing life.

In the course of achieving this objective NSK has had to address several limiting factors imposed by the design of rolling mills.

Historically, the design restricts three critical bearing dimensions: the outside diameter, which obviously must be less than the total roll diameter and allow space for the chock; the bore diameter, which must be large to ensure the strength of the roll neck; and the bearing assembly width.

In a conventional sealed roll neck bearing the width of the main seal restricts the length of the rollers and, therefore, limits the load capacity of the bearing.

However, in NSK's new design the cage flange is included within the seal space of the Extra-Capacity Bearing, allowing for longer rollers.

The cage diameter is also increased to facilitate more and larger rollers without reducing cage strength.

In performance terms the optimised design raises the basic dynamic load rating capacity of the sealed roll neck bearings by up to 34% when compared to NSK's conventional roll neck bearings.

This equates to as much as 2.7 times longer life.

The second key element in the improved performance of the Extra Capacity roll neck bearings is NSK's patented Super-TF bearing steel.

This is the product of research that addressed the problem of bearing life in conditions of contaminated or inadequate lubrication, two of the major causes of premature bearing failure experienced in large equipment such as rolling mills.

Experience shows that under dirty operating conditions bearings can easily become contaminated with ingress of debris.

The debris particles are then indented on the bearing raceways causing stress concentrations at the identation edges.

In addition, if there is inadequate lubrication, metal surfaces come into contact and small "peeling" damage occurs, leading to stress concentrations around the contact areas.

The result in both cases is that cracks initiate from the surface of the material at the stress concentration points, leading to an accelerated rate of fatigue damage and, ultimately, shorter bearing life.

Research by NSK has revealed that bearing life in conditions of contaminated or boundary lubrication can be improved by changing the microstructure of a bearing steel in order to enhance its ability to relax stress concentrations on the edges of indentations.

This microstructure, with a high volume of retained austenite and a high hardness, produces a longer life even under conditions where there is an insufficient oil film and surface damage, such as peeling, easily occurs.

In practice more effective results are obtained not when either retained austenite, or hardness is increased alone, but only when both properties are increased together, as is the case with Super-TF steel.

In comparison with bearings made of ordinary carburised steel, Super-TF types have shown in tests to offer up to 10 times the life with contaminated lubrication and up to twice the life where the lubricant is clean.

Where the problem is related to insufficient lubrication, the life of bearings made from Super-TF is still impressive, at around 5.5 times that of ordinary carburised steel.

Besides extending service life under conditions of boundary or contaminated lubrication, Super-TF steel also offers increased resistance to wear and seizure as a result of the dispersion of a large number of fine carbides and nitrides in the bearing material.

In comparative tests with bearings made of conventional materials, such as SUJ2 steel, the Super-TF material structure has shown less than one third the rate of wear and a 40% increase in seizure resistance.

The role of Super-TF steel in extending the life of bearings subject to contaminated lubrication is unique.

In a perfect application, however, the contamination should be denied ingress in the first instance.

This truism has been adopted by NSK in the seal designs for the Extra Capacity roll neck bearings.

The new designs (both with patents pending) are for main seals and bore seals in the bearings.

The design of the main seal and bore seal means that they are easy to install and remove.

Moreover, with the outside diameter of the main seal reduced, more space is provided to accommodate the lifting jig that is required when fitting the bearings.

The relationship between the main seal and bore seal is essentially a complementary one, due to a unique condition, which affects the bore seal.

Negative pressure that develops inside Sealed-Clean Roll Neck Extra-Capacity Bearings can cause entry of water through the main seals.

The configuration of the bore seal greatly affects this pressure and hence the ingress of water.

Tighter bore seals cause greater negative pressures, resulting in more water entry through the main seals.

Suppressing the build-up of negative pressure inside the bearing is essential for preventing the entry of water.

The most effective method for achieving this is to allow the bore seal (which is not directly exposed to water) to breathe along with the fluctuating pressure caused by rising and falling bearing speeds and the resulting changes in bearing temperature.

At the same time, the bore seal's original functions - preventing the entry of water and contaminants from the neck side - have to be maintained.

It is precisely this balance between breathing and sealing that NSK has achieved with the development of its new bore seal.

The bore seal employs surface contact instead of linear contact.

The effectiveness of this configuration was demonstrated in initial testing which featured both seal types.

The linear contact bore seal demonstrated high seal tightness both during rotation and while stationary.

However, the new surface contact bore seal demonstrated high seal tightness similar to that of the linear contact seal while stationary, but, importantly, showed adequate breathing capability during rotation.

To confirm the effectiveness of the new bore seal in actual bearings, further simulation tests were conducted.

In these tests the bearings operated across the speed range from 100rpm to 700 rpm, with a constant radial load of 294kN, while being showered with water at a rate of approximately 100L/min.

The conventional bore seal showed increasing negative pressure over time while the new bore seal for Extra-Capacity Bearings suppressed negative pressure throughout the tests.

An additional benefit was that the water content of the grease after the tests was substantially reduced with the new bore seal.

These results clearly indicate that Extra-Capacity Bearings offer greater reliability than conventional sealed roll neck bearings.

The new Extra Capacity bearings are available in a wide range of sizes from 170mm bore to 509mm bore.

Basic dynamic load ratings for these units extend from 1010kN to 5,750kN respectively.

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