Sealed clean roll neck bearings deliver up to 34 per cent extra load capacity and longer life under severe lubrication conditions

Citation

Dodd, A. (2002), "Sealed clean roll neck bearings deliver up to 34 per cent extra load capacity and longer life under severe lubrication conditions", Anti-Corrosion Methods and Materials, Vol. 49 No. 2. https://doi.org/10.1108/acmm.2002.12849baf.001

Sealed clean roll neck bearings deliver up to 34 per cent extra load capacity and longer life under severe lubrication conditions

Keywords: Bearings, Life cycles

Four-row sealed tapered roller bearings used on roll necks of rolling mills are required to achieve longer life amid severe environmental conditions such as high operating load, extreme shock loads and the penetration of water and scale into the bearing. New seal technology, developed by NSK, is enabling longer service life and improved reliability in addition to higher load capacity over conventional sealed roll neck bearings.

Introduction

Four-row, sealed tapered roller bearings are used primarily to support rolls in the rolling mills of steelworks (Plate 1). 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 per cent extra load capacity, higher reliability, longer life under severe lubrication conditions and easier handling (Figure 1).

Plate 1 NSK's four-row, Extra Capacity Sealed-Clean™ taper roller bearing

Improved performance is synergy of material, mechanical design & sealing

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.

Figure 1 Features of Extra-Capacity Bearing

Greater number of larger rollers increases dynamic load capacity

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(Figure 2). 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.

Figure 2 Work roll chock of a cold rolling mill

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 (Plate 2). The cage diameter is also increased to facilitate more and larger rollers without reducing cage strength (Figure 3) in performance terms the optimised design raises the basic dynamic load rating capacity of the sealed roll neck bearings by up to 34 per cent when compared to NSK's conventional roll neck bearings. This equates to as much as 2.7 times longer life.

Plate 2 Inclusion of the cage flange within the seal space allows for longer rollers and therefore increased capacity

Figure 3 Comparison of conventional and Extra-Capacity Bearing designs

New Super-TF steel is key to avoiding failures resulting from contaminated or inadequate lubrication

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 (Figure 4), two of the major causes of premature bearing failure experienced in large equipment such as rolling mills.

Figure 4 Field data on the life of Super-TF material when used in roll neck bearings for cold 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 indentation 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.

Unique "breathing" seal reduces negative pressure & water entry

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 failing 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 (Figure 5).

Figure 5 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 700rpm, 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.

Wide range of sizes and load capacities

The new Extra Capacity bearings are available in a wide range of sizes from 170mm bore to 50gmm bore. Basic dynamic load ratings for these units extend from 1010kN to 5,750kN respectively.

Full details and features of the range are contained within a new publication, copies of which are available via NSKs Freephone service on +(44) 500 2327464. Callers should quote the catalogue reference Cat No E1255, and also the title of the publication: Extra Capacity Sealed Clean Roll Neck Bearings.

Details available from: NSK UK Ltd, Tel: +44 (0)115 936 6600.

Andrew DoddNSK European Technology Centre