August 1, 2012
While bulk conveyors routinely carry heavy loads for the aggregate industry, they can carry daunting operating challenges along the way, too. As an all-too familiar example, the ingress of dirt, sand, and grit at critical rolling bearing positions, historically, has caused production headaches when contaminants first damage bearing housing seals, then invade the bearings, and, ultimately, subject conveyor systems to unplanned shutdowns costing thousands of dollars and tons of time.
The problem can often be traced to the traditional use of unsealed, open-type bearings with standard housings and seals in a conveyor head and tail pulleys and take-up pulleys (among other locations). These types of bearings and housings offer little, if any, protection from abrasive contaminants, prompting some operations to respond by pumping large quantities of grease continuously into a bearing’s housing as a first line of defense. While this re-greasing may help, somewhat, in the short term, downsides can develop over the initial cost of lubricant and subsequent issues and costs associated with increased labor, need for constant lubricant disposal, and potentially adverse environmental impact.
An increasingly popular alternative is to operate unsealed, open-type bearings mounted in a split housing equipped with contact or auxiliary taconite seals. This offers improvement over minimally protected open bearings in standard housings, but space limitations may be a problem as taconite seals can be relatively costly and may increase the axial length of an assembly. These seals can also pose difficulties in achieving proper alignment.
The encouraging news is that leading manufacturers in the rolling bearing industry have continued to develop more advanced technologies to help keep conveyors up and running.
New generation of bearings
An entirely new generation of higher performance spherical roller bearings has injected new life into bearing reliability and service along the conveyor line. A combination of improved bearing material, unique bearing design, and upgraded manufacturing processes has helped to maximize the effect of lubrication and minimize the effects of wear and contamination, while reducing friction. The result: A proprietary performance class of spherical roller bearings providing higher load-carrying capacity and extended bearing service life.
A new generation of roller bearings has increased reliability along the conveyor line.
These are well suited for the rigors of conveyor applications in the aggregate industry due to an optimized design that benefits from refined internal geometry tolerances and improved surface structures. From the perspective of materials, the bearings are manufactured from high quality, clean, and homogenous bearing steel that undergoes a unique heat-treatment process, increasing the bearing’s wear resistance. The purity of the steel imparts the bearings with strength and durability, and the specialized heat treatment creates a combination of hardness and dimensional stability.
One key to improved performance is the redesigned internal geometry. Using proprietary software, engineers analyzed the interrelationship of the bearing’s rolling elements, raceways, and cage. Then, they devised methods to reduce friction, heat generation, and wear in the bearing, while increasing the ability to withstand heavier radial and/or axial loads.
The bearing’s rolling elements are manufactured to extremely close tolerances to reduce vibration levels and equalize load distribution. Cages are enhanced to improve guiding of the rolling elements and optimize the effect of the lubricant while lowering heat, friction, noise, and vibration levels. The surface topography of the bearing’s rolling elements and raceways enhances the formation of a hydrodynamic lubricant film, maximizing the effects of the lubricant, while reducing heat generation and wear. Ultimately, they minimize environmental impact by reducing lubricant and energy consumption.
Both during testing and in real-world applications, these bearings have been shown to run cooler, smoother, and longer than standard rolling bearings, particularly in heavy load slow-rotation applications, such as bulk conveyors.
Triple protection barrier
One of the technology developments engineered for conveyors — a three barrier solution — has introduced an environmentally friendly and cost-effective bundle of technologies to extend bearing service life and help prevent premature bearing failures without resorting to complicated auxiliary taconite seals and/or large quantities of grease.
This proprietary solution consists of the following interrelated components: Factory-sealed, high-performance spherical roller bearings (the newest generation) pre-packed with specially formulated grease inside the bearing; optimized split pillow block housings with positive TLC-type external seals; and a recommended grease in the housing.
The bearing is lubricated, sealed, and mounted inside the housing, which is sealed and internally greased. This effectively delivers three layers of protection for the bearing during assembly and operation: The integral bearing seals keep the lubricant in and contaminants out of the bearing cavity; the TLC seals in the housing protect against extremely fine contaminants and eliminate any need for expensive taconite seals; and the grease packed into the housing (at installation) on both sides of the sealed bearing completes the protection.
The double-lip sheet-steel-reinforced seal is manufactured either from nitrile rubber or hydrogenated nitrile rubber and specifically engineered to prevent contaminants from entering the bearing’s critical rolling contact area. Depending on operating and ambient conditions, up to 90-percent reduction in grease consumption can be realized using these sealed bearings as part of the three-barrier solution. This can be attributed to the small quantity of grease required to lubricate a sealed spherical bearing inside the sealed housing, compared with relatively large amounts of grease necessary to purge contamination from an open, unsealed bearing.
Automatic lubrication technology
Sometimes, bearings will have to be lubricated periodically and, when they are, problems may arise, especially when they are lubricated manually. Bearings must receive the right lubricant in the right amount at the right time to minimize wear and promote longer service life. Too often, excessive (or not enough) lubrication may be applied; lubricant consumption may increase due to overuse; and maintenance time, energy, and costs can spiral.
Single-point automatic lubricators can be gas driven (left) or electro-mechanical driven (right).
These lubricators can deliver the proper pre-set amount of lubricant for bearings and can prevent too much or too little lubricant being used. Single-point automatic lubrication technology is becoming more widespread as a practical delivery system to prevent the problems typically associated with manual lubrication. (Automatic lubricators also can be incorporated into a three-barrier solution to lubricate a bearing when needed in extreme operating conditions without using excessive amounts of grease.)
Single-point automatic lubricators in service 24/7 can serve as timely and accurate hands-off solutions to deliver the proper pre-set amount of lubricant for bearings. (Some systems have even been tested and approved for use in mines and underground areas, among other applications.) Standard versions usually will be offered either as gas-driven feed types or electro mechanical-driven systems and engineered for reusability. They can especially come in handy at hard-to-access or potentially hazardous lubrication points.
Automatic lubricators (typically sized from 60 milliliters to 250 milliliters, depending on manufacturer and model) offer particularly welcome solutions when equipped with user-friendly features that can make life easier along the line. When evaluating options, users should consider whether the lubricator:
• Provides for flexible and user-adjustable dispense settings over long periods of time to minimize time and effort;
• Integrates transparent lubricant containers to allow for visual inspection of dispense rate, enabling timely refills to be made as needed; and
• Contains high-quality lubricant specially formulated for the application and supplied ready-to-use to avoid potentially detrimental mismatches.
The ramifications of harsh operating environments encountered by conveyors in the aggregate industry have paved the way for new thinking to address — and remedy — age-old problems. The latest innovations in bearing-related technology carry new solutions to help keep lubricant in and contaminants out of critical bearings, increase conveyor uptime, minimize lubricant use, significantly reduce maintenance time and money, and maximize an operation’s overall productivity and profitability.
James A. Oliver is director-sales support engineering for SKF USA Inc. He can be reached at 267-436-6516 or via e-mail at email@example.com.
Joe Esmoer is manager, industry specialists for mining and metals for SKF USA Inc. He can be reached at 520-749-6618 or via e-mail at firstname.lastname@example.org.