Cylindrical roller bearings: design choices cut costs and yield greater reliability and longer life

Citation

(2003), "Cylindrical roller bearings: design choices cut costs and yield greater reliability and longer life", Industrial Lubrication and Tribology, Vol. 55 No. 5. https://doi.org/10.1108/ilt.2003.01855ead.005

Cylindrical roller bearings: design choices cut costs and yield greater reliability and longer life

Cylindrical roller bearings: design choices cut costs and yield greater reliability and longer life

Keywords: Bearings, Efficiency, Cost effectiveness

The pressurised, cost-down nature of global manufacturing means that bearings employed in modern machines and equipment must perform well at higher speeds and under higher loads than at any time previously. What is more, they must provide these enhanced levels of performance with market leading levels of reliability. One of the major implications of these requirements for the design engineer is that some applications, once designed using ball bearings, can no longer meet today's demands for L₁₀¹ life or reliability. As a result, cylindrical roller bearings (CRBs) are increasingly being employed as a higher performing alternative (Plate 4).

High kilowatt electric motors are a good example of this substitution process in operation. In many electric motor applications, the drive end bearing is a ball bearing. However, a significant number requires the bearing to carry a radial load that is too high for a ball bearing to yield a satisfactory L₁₀¹ life. This problem can be overcome by substituting a CRB.

Plate 4 Cylindrical roller bearings: design choices cut costs and yield greater reliability and longer life

In many high-speed applications, CRBs are combined with ball bearings. Dimensionally, ball bearings and CRBs are interchangeable. For example, an NU310 cylindrical roller shares the same bore diameter, outside diameter and width as a 6310 ball bearing. Both bearing types have similar limiting speed capabilities, allowing the CRB to operate at maximum speeds virtually identical to the ball bearing. These are important considerations when redesigning a gearbox or other device to carry a higher radial load, as bearing life can be optimised without losing machine performance.

Another consideration in the present highly competitive global markets is cost reduction. The substantial developments in materials, mechanical design, lubrication and sealing that have occurred in bearing design over the last decade mean that, in many applications, bearings can be downsized to reduce costs – size and weight – without affecting machine performance.

A typical example of this initiative in operation is the replacement of pairs of angular contact bearings, with single CRBs in motorised machine tool spindles.

NSK is in the vanguard of the movement to achieve cost reduction through downsizing with two new designs of CRB, the EW and EM series. Both of these products provide an ideal, higher performance, substitute for ball bearings in applications where increased performance demands mean that the latter cannot offer either the life or the levels of reliability required.

The EW and EM designs permit downsizing due to an increased complement of larger rollers and unique high strength cages. In the case of the EW series, these design improvements result in a radial load capacity exceeding that of conventional CRB bearings between 17 and 20 per cent, without, depending on the design, any reduction in axial load capacity. This results in an improvement in theoretical operating life, by as much as 70 per cent.

The EW cage material is a new patent pending steel, which has been developed by NSK. The exceptional strength that this steel brings to the cage not only enables more rollers to be accommodated, but also provides more reliability from the bearings in applications where cage strength is the limiting factor.

The results obtained with the EM series design are equally impressive: a 30 per cent higher load capacity than conventional riveted brass cage bearings of similar size, plus higher strength (4-5 times stronger), higher rigidity, lower running temperature, quieter operation and longer operating life; courtesy of the optimised one-piece cage design.

In both EW and EM series bearings, the efficiency of the cage design is such that the bearings achieve lower running temperatures, despite their increased roller complement. Improved roller guide control from a special pocket profile reduces harmful skewing of the rollers to a minimum, leading to lower temperatures and improved operating life.

The EM cage design also incorporates large corner radii for relieving stress concentrations in the cage and facilitates improved flow of both grease and oil lubricants within the bearings.

In addition to meeting user demands for improved load capacity and life, the EW and EM series bearings address another major concern in industrial design: noise. Here again, the unique cage design of the two bearing types is pivotal. It ensures that the cage is so well balanced that vibration within the bearings is substantially reduced, resulting in quieter running. Just how quiet has been determined in tests. Against competitive designs, the EW units exhibited noise levels from 3 to 7 dB(A) lower, providing a real advantage for designers who need to reduce the noise emissions from their machines.

The many operating advantages provided by the EW and EM series bearings mean that they are suitable for use in demanding applications in materials handling, quarrying, machine tools, steel processing and power transmission – typically large gearboxes and high kilowatt electric motors.

For further information, contact: Michelle Reed, Marketing Communications Dept, NSK UK Ltd, Northern Road, Newark, Notts. NG24 2JF UK. Tel: +44 1636 - 605123.