Design Application

Seals protect in worst environments

New seal designs protect rolling elements in bearings in two contrasting, but equally severe applications. Tom Shelley reports

Bearing seals and shields are vital if rolling element bearings are going to be employed in severe environments; they will protect against contaminants and prolong bearing life by keeping bearings clean and well lubricated.

When choosing the appropriate seal or shield for an application, designers must consider the lubricant, its delivery system and the expected operating conditions.

Two of the worst environments must be in steel rolling mills with all the associated hazards and down on the farm, where mud, water, abrasive materials and a lack of maintenance can all be expected together.

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Bearing seals are generally more effective barriers than bearing shields, but they require more turning effort, generate more heat, and have a lower speed limit.

High loading and extreme shock

Rolls in steel rolling mills are usually supported by four-row, sealed, tapered roller bearings. In service, they have to survive high loading, extreme shock loads and water and scale ingress. Twenty years ago NSK addressed the problems with its Sealed-Clean roll neck bearings.

Now, in a new development, the company has developed Extra Capacity Sealed-Clean roll neck bearings. These are said to deliver up to 34% extra load capacity, higher reliability, longer life under severe lubrication conditions and easier handling. The improved performance is the result of a combination of optimised mechanical design, new bearing materials and patented sealing technology.

In a conventional sealed roll neck bearing, the width of the main seal restricts the length of the rollers which limits the load capacity of the bearing. In the new design, the cage flange is included within the seal space, 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 is said to raise the basic dynamic load rating capacity of the sealed roll neck bearings by up to 34% when compared to 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 said to be NSK’s latest '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 in large equipment such as rolling mills.

Experience shows that under dirty operating conditions, bearings can easily become contaminated with the ingress of debris. The particles get indented on the 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 the company 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.

However best practice requires that contamination should be prevented in the first instance. The new seal designs, both with patents pending, are for main seals and bore seals.

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 the 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 of achieving this is to allow the bore seal, which is not directly exposed to water, to 'breathe'. 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 seal employs surface contact instead of linear contact. Linear contact bore seals demonstrate high seal tightness both during rotation and while stationary. However, the new surface contact bore seal demonstrated high seal tightness while stationary and, imprtantly, adequate breathing capability during rotation.

To confirm the effectiveness further simulations were performed. In these tests the bearings operated across the speed range from 100 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 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 the new bearings offer greater reliability than conventional sealed roll neck bearings.

The bearings are available in sizes from 170 to 509mm bore. Basic dynamic load ratings extend from 1,010 to 5,750kN respectively.

Agricultural demands

SKF has developed a new generation of oil and grease seals for use in agricultural and similar machinery.

Traditional seal designs comprise a radial lip, with or without hydrodynamic features, a garter spring and static seal between the outside diameter and housing.

Mud Block, the name given to the series of seals, has been created by SKF Seals Division at RFT in Villanova d'Asti, Italy. The seals represent an increase in the complexity of sealing technology, compared with a standard oil seal design. The improved protection they provide is the result of the design of their lips.

Often, the failure of a seal in transmission, is due to water and mud ingress at the radial lip. In the Mud Block series, the geometry of the seal design has been altered to provide the optimum protection against water and mud ingress at this point.

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The new design offers a number of standard features throughout the series. These include: a stop bumper between the sleeve and seal, which facilitates assembly and ensures correct position of the axial lip; the outer surfaces of the seal and the sleeve inner diameter are rubberised to provide good corrosion protection; and the inner lips are pre-lubricated with water-resistant transmission grease. Consequently, this offers better reliability during assembly and replacement in the field and eliminates the need to carry out shaft machining.

The seals are tailored to the customer's requirements for assembly. Full rubber outside diameter designs can be provided with or without a flange and the outside diameter can be fabricated a half rubber half metal combination. The inner radial lip can be varied to offer oil or grease sealing, i.e. a standard lip design with garter spring for oil sealing; and several radial lips fabricated in rubber for grease sealing.

Mud blocks can be fabricated in a variety of elastomer materials, depending upon the component applications and operating conditions. The range of materials includes: nitrile, polyacrylate and fluorinated elastomeric compounds. Seals are required on non-driving wheel hubs that usually operate at low speeds, as well as on pinions, which experience high revolutions and high temperatures.

The performance of the Mud Block series is said to have been confirmed in both laboratory testing at RFT in Italy and in field tests carried out in collaboration with key customers.

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