Everyday across the globe, pump manufacturers are subjected to continuous demands to reduce both the cost and the size of the pumping systems that they produce. Because they are the most widely- used, the main focus of these cost-down demands is centrifugal pumps. Accounting for around 80% of the total UK market, alone, these pumps are the workhorses of industry, providing the users with the widest range of flow, volume capacity, and head.
The problem for any pump manufacturer designing more compact pump systems is that any reduction in size cannot be at the expense of flow. What this means for key pump components such as bearings is that they, in- turn, must become more compact, but at the same time provide higher-load capacity to meet new operating demands. NSK has worked with the global pump industry to meet these requirements and the result is a new range of high-load capacity deep-groove ball bearings for centrifugal pumps. The basic dynamic load rating of these bearings has been improved by 10% approximately, and their fatigue life by around 30%, without any changes to the standard bearing dimensions. These improvements have been achieved via a combination of a new heat treatment process, enlarged ball diameters, and by optimising the interior design of the bearing cages.
The improvements delivered by NSK’s high capacity bearings can be judged by reference to Table 1, which shows the boundary dimensions and basic dynamic load ratings of both standard and NSK’s new high capacity bearings. Taking as an example a 6306 bearing (30mm bore x 72mm od): in its standard configuration this bearing has a basic dynamic load rating of 26700 N and a high-load capacity rating of 29, 800 N: an increase in basic dynamic load rating of approximately 12%. As a result of this, bearing fatigue life is increased by approximately 40% for similar load conditions.
One of the key factors in the longer life of the high-load capacity bearings is NSK’s new UR heat treatment process, which is effective in alleviating the stress concentrations that cause surface- originated flaking on bearing material. Surface-originated flaking tends to occur under contaminated lubricating conditions. Stress concentrations develop immediately around indentations formed by foreign particles. NSK’s UR heat treatment process prevents this from happening, by increasing the amount of retained austenite within the bearing material. As a result, bearing durability is significantly improved in contaminated environments. This improvement is illustrated in Fig1: the graph shows that the life of UR treated bearings has improved by twice that of standard bearings operating under conditions of contaminated lubricant.
Another problem that NSK has addressed with its new pump bearing technology is creep. This is the name given to wear that may occur between the bearing outside surface and housing bore due to low-speed sliding. It is the result of loads becoming excessively high on the driving shaft of a pump. If not addressed, Creep causes excessive shaft vibration, which can trigger abnormal oscillation and lead to impeller damage.
Common measures against creep include an interference fit between bearing and housing. For pumps, however, a tight fit is not practical, as it makes pump assembly more difficult and does not allow for additional loading caused by an increase in the shaft length due to heat generation. A better solution, developed by NSK in its new range of Anti-Creep bearings, is to install O-rings into the two grooves on the bearing outer ring. The frictional force of the O-rings helps to prevent the occurrence of creep.
BMPC Bearings Provide Greater Reliability & Life In Harshest Pumping Applications.
The launch of the high-load capacity bearings follows that of the BMPC series of bearings which are designed to provide greater reliability and longer life in the harshest of centrifugal pump applications, in pulp and paper, wastewater treatment, steel mills, irrigation, mining and many other demanding industries.
The BMPC series exceeds the demanding API 610 (American Petroleum Institute standard 610) which, because of its comprehensive and detailed nature, has become a worldwide industry standard. The standard calls for angular contact ball bearings, with a 40° contact angle, arranged in a back-to-back configuration, and giving a minimum life expectancy of 25,000 hours (3 years) continuous running, or 16,000 hours at maximum radial and axial loads.
The BMPC series surpasses the API 610 specification, by incorporating the strongest machined brass cage in the industry, providing superior reliability in the harshest of applications. BMPC series bearings also achieve longer service life, through smoother running, reduced levels of friction and lower temperature rise: the latter as a result of the better heat dissipation of the brass cage material.
Engineered for reliability, the BMPC Series design is an amalgam of high strength materials technology, ABEC3 (P6) manufacturing tolerances and pump specific axial internal clearances. It delivers higher running accuracy, plus optimum levels of bearing stiffness, shaft positioning accuracy, load sharing and cooler running. In addition, the combination of super finished raceways, lowest inner-ring run-out and highest ball grade, all contribute to reduced levels of ball skidding and, hence, smoother operation.