February 17, 2012
Keeping a belt conveyor running for long, predictable periods of time is essential to high productivity when processing sand, gravel, crushed stone, or other minerals. Here are some insights on how to keep them running efficiently to maximize uptime.
The primary points of failure in belt conveyors are often the idler, tension, drive, and head pulleys. When one of them has a bearing problem, the belt stops moving — even though the bearings are often not the root cause of the failure. Proper diagnosis of the problem can help increase bearing life and extend overall uptime.
Load sources and types
The main loads on a pulley bearing come from the belt pull, the weight of the pulley, the belt, and the payload. Payloads have been increasing in recent years due to improvements in belt design and the use of stronger materials. There may also be a small thrust load originating from the belt, riding against the lateral belt guide rollers. In the case of a belt track problem, the common solution is to mount fixed bearings on both ends of the tail pulley. This results in additional thrust loads on both bearings. Thermal expansion of the shaft further increases the thrust loads.
Extending bearing life
The harsh, contaminated environments in quarries, mines, and construction sites once meant shortened bearing life was difficult to prevent. Products on the market now have reversed that trend. For example, some new steel bearings resist spalling caused by damage-inducing contaminants, increasing the bearing’s life several times and reducing catastrophic failures, as well as overall maintenance costs.
High-performance spherical roller bearings are designed to reduce slip and minimize flaking between the bearing raceway and rolling element. Molded oil bearings can reduce the number of lubrication failures and eliminate the need to re-lubricate bearings.
Split housings — available in cast iron, ductile iron, and cast steel with a variety of seal options for contaminated environments — can also help resist wear and improve bearing performance. Small housings have two mounting-bolt slots or four bolt slots as an option. The slotted-hole design allows for easier alignment of the pulley. Additional features include an easy-to-read temperature strip mounted to the pillow blocks to alert users of potential failure due to overheating. Blocks also contain strategically located predrilled and tapped holes to allow for remote monitoring.
Tips for mounting bearings
• Do not unpack bearings until you are prepared to mount them. Ensure your work area, hands, and tools are as clean as possible. Have clean gloves, rags, and hand cleaner available to avoid getting contaminants inside the bearing.
• Before mounting, inspect the pulley journals and measure their diameters.
• Measure the journal dimensions at four points circumferentially and three points axially.
• The circumferential measurements will determine roundness, while comparing axial measurements along the same plane will determine taper. Averaging all of the measurements together will determine size. As a rule, the tolerance of roundness and taper is half the journal tolerance.
• If any of these dimensions are out of spec, the journal (pulley) should be replaced. The journals at the seal position, likewise, need to be in good condition. Check for wear or damage that is preventing adequate contact or creating too much clearance for a labyrinth seal. These circumstances also require replacement of the pulley. If the journals on both ends of the pulleys pass inspection, record this information on a journal inspection chart.
• Straight bore bearings also require an interference fit to the journal. The safest and easiest method of mounting this type of bearing is to use an induction heater or, as an alternative, an oil bath heater. Make sure the oil is free of sludge and contaminants. Never heat the bearing more than 250 degrees Farenheit. Never use a torch to heat the bearing.
• Avoid pushing against the bearing case or rollers during installation.
• Make sure packaging-type seals are fully saturated with oil before fitting. Lubricate the lips of oil seals to prevent damage and make them easier to install.
• Pack the labyrinth grooves with grease and make sure they are in proper housing grooves.
• Take measurements to ensure the bearings are correctly located on the journal — this will prevent an abnormal thrust load on the bearings.
Information for this article courtesy of NSK Corp.
For more information, visit www.nskamericas.com.