“The selection of the most appropriate material for bearings is one of the most important factors affecting pump reliability,” Thordon Bearings’ Strategic Account Manager, Keith Brand told delegates at Water Expo 2023 in Miami recently.
In his paper Selecting the Right Bearings to Improve Vertical Turbine Pump Reliability, Brand said “…the nature, particularly the abrasiveness, of the pumped media, is a key variable affecting optimal pump bearing selection and lifespan.” The other key variable is application. “Abrasive wear and dry starts are the most common causes of bearing failure,” added Brand.
Taking a deep dive into common oil and water-lubricated bearing materials, such as bronze, carbon graphite, thermoset laminates, Teflon, thermoplastics and elastomers, Brand explained that they all have their pros and cons. Some bearings have better dry-running capability but poor abrasion or chemical resistance, while others may be better suited to high-temperature environments but fall short on mechanical strength. All tend to be more costly in terms of OPEX.
“In some applications the upper bearings are not lubricated upon start up. This could mean 30 seconds or more of high-speed dry bearing operation, completely ruling out certain materials, such as rubber,” said Brand.
“Rubber bearings can burn up from frictional heat created with little to no lubricant and most phenolic bearings cannot handle running dry for more than sixty seconds. Here, oil-lubrication or a water flush is required to lubricate the bearings until process water can lubricate the shaft, but this also adds to the cost and can be detrimental to the environment,” continued Brand.
Brand said some manufacturers have attempted to make pump components more chemical and abrasion resistant by isolating the pump’s oil-lubricated bearings and shafts from process water. “But the biggest drawback to this approach is the additional cost,” he added.
Brand told Water Expo attendees that while there is no single correct answer to bearing selection, understanding pumped mediums and dry startup requirements will narrow down the options. “Ultimately, understanding bearing material selection will keep pumps pumping longer, reduce downtime and maintenance costs, and prevent catastrophic failure,” Brand said.
Brand’s paper has a Thordon material selection guide, showing the grades recommended for VTPs, with direct comparable reference to the key variables such as temperature limits, abrasion resistance, shaft material compatibility, pumping media limitations, dry start capabilities and more.
The whitepaper can be downloaded here.