Bearings selection: the key to prolonging pump life
08 May 2018
Well designed and manufactured bearings will ensure that a pump works more efficiently, can tolerate higher temperature running and cope with varying load conditions. Perhaps more importantly, bearing selection will also have a marked effect upon the life expectancy of a pump, as Phil Burge, Marketing and Communications Manager at SKF explains.
A process pump is a particularly well-engineered mechanical system that, with minimal intervention and maintenance, will work reliably for very long periods of time. Peak performance is frequently taken for granted where this workhorse of industry is concerned; yet, over time, there will be some deterioration.
A pump's bearings - and more particularly the type of pump motor bearings - will likely have the most impact on its long-term reliability, its energy efficiency and life expectancy.
In regards to the types of bearings that are most suited to pump applications, double row angular contact ball bearings are increasingly being offered for their ability to cut mechanical friction and thus operating temperature. Double row angular contact ball bearings, which correspond in design to two back-to-back single row angular contact ball bearings, take up less axial space and have a high load carrying capacity, especially in the axial direction.
However, single-row angular contact ball bearings (SRACBBs) are a more popular choice for small-to-medium sized pumps, delivering performance, reliability, and uptime benefits while reducing maintenance costs, noise, vibration levels and even lubricant consumption.
SKF can now offer an improved SKF Explorer class SRACBB that contributes to a reduction in the total cost of ownership of pumps and compressors through improvements in reliability and energy efficiency. Featuring a redesigned and stronger brass cage that ensures quieter, smoother running and greater tolerance of shock loads, these 40° contact angle bearings have a 30 percent higher limiting speed compared with that of previous designs.
In addition to the redesigned 40° contact angle, a new contact angle of 25° is also offered, giving the bearing a further 20 percent increase in speed over that of the 40° version, while enabling higher radial loads to be accommodated. In each case, the ball-to-cage contact arrangement allows cooler running and greater temperature stability, while noise and vibration levels can be reduced by 15 percent in both the 40° and 25° versions.
In terms of pump type, for multi-stage centrifugal pumps the new 25° contact angle standard offers an upgrade to the hydrodynamic/floating radial bearings that typically support the predominantly radial loads in these units. For double-suction impellers or closed impeller pumps, the 25° standard provides cooler running, substantially reduced vibration levels and extended service life in applications with high radial loads.
By combining the two bearing angles, the robustness of fluid handling applications or electric motors with vertical shafts can be improved.
The importance of bearing seals
The pump bearing and its seal is an integrated system that performs four crucial tasks: lubricant or liquid retention, contaminant exclusion, fluid separation and confinement of pressure within the pump housing. Seal selection will be influenced by the application and any environmental concerns; for example, seals exposed to relatively constant pressure differentials would be best served by pressure seals (in which the seal cavity is pressurised). Dynamic radial seals are generally the best choice for centrifugal pumps as they create the necessary barrier between surfaces in relative motion.
Sealed variants of the bearing types described above are also available to perform these essential tasks – particularly to prevent contaminants from entering the bearing during installation and subsequent operation. Sealed SKF SRACBBs can be combined with low friction non-contact seals that ensure the same load carrying capacity as open, non-sealed variants. These non-contact seals are suitable for bearing arrangements where, due to limited space or for cost reasons, external seals are not practical. They also save space axially, enabling a more compact machine design. Moreover, sealed units are ready-to-use, being factory-filled under clean conditions with the correct quantity of high quality grease.
A note on lubrication and other maintenance solutions
Bearings and seal selection, of course, are important steps towards ensuring high reliability and long service life; however, in the world of pumps, where more than a third of premature bearing failures occur due to inadequate or incorrect lubrication, getting this vital aspect of maintenance right is critical. Regular lubrication helps to prevent deterioration of the pump’s bearings and other related components, but the process is rather more complicated than simply getting the grease gun out as and when it is deemed necessary to do so.
Many pumps are located in awkward places, and the lubrication process can be arduous and labour intensive as maintenance operatives attempt to access tight, often poorly lit and potentially hazardous sites. As part of the overall design process, therefore, it might be advantageous to install a fully automatic lubrication system that protects both the integrity of bearings while reducing overall operating costs. It might also be worth considering the inclusion of condition monitoring, which will not just flag up abnormal operation, but also indicate when it is appropriate for maintenance intervention before that abnormality results in a costly failure.
Fitting appropriate bearings and seals is certainly a vital aspect of the pump design process, but once a pump is in service it needs the support of solutions and services primarily to ensure reliable, trouble-free operation, but potentially to add years to its life.
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