January 11, 2010
The Conveyor Challenge: Rolling Steady
Conveyors may appear to be a straightforward application, but when it comes to achieving consistently reliable performance, they are anything but predictable. Bearings are among the most mission-critical factors in conveyor performance, requiring careful consideration of operating load and speed for the type of bearings selected for a particular conveyor application.
The Solution: The Right Stuff
The key to a continuously running conveyor starts with the recognition that it is an interdependent system, requiring the right products working together for the situation. Timken engineers specify highly reliable products in key head and tail pulley positions. The choice between a spherical roller bearing or a more cost-effective ball-bearing housed unit should be made based on load and speed conditions, keeping in mind that the right bearing will likely save the greater cost of downtime in the long run.
Since many conveyor operations are exposed to harsh conditions, incorporating a quality seal is almost as important as having the right bearing, and upgrades often are worth the investment. Operators who want to increase conveyor dependability should consider adding an automatic lubrication system to provide a continuous supply of grease.
The Screen Challenge
Vibrating screens among the most demanding applications for antifriction roller bearings. Operating conditions result in high radial loads and high rotational speeds.
In addition, screens generate a lot of vibration on the bearing’s cage and rollers, further increasing operating temperatures. Complicating the matter, operators commonly run screens in inhospitable environments, including extreme temperatures and with heavy contamination exposures.
The Screen Solution
Timken offers a complete line of spherical roller bearings, including the 23 and 33 series. While the 23 series is more common, the 33 series bearings offer higher load capacity, as they maintain the same bearing bore and outside diameter.
Key to the spherical roller bearing design is the one-piece fully machined brass cage. The cage pocket wraps around the roller, providing excellent roller guidance as the rollers rotate in and out of the load zone. The roller end and cage pockets are engineered to promote lubrication flow and reduce operating temperatures.
Timken also offers a line of cylindrical roller bearings (CRB) with a rugged one-piece machined brass cage, designed for the unique conditions in vibrating screens. CRBs provide increased load-carrying capacity and lower temperature generation than traditional spherical roller bearing designs. Additionally, CRBs can have specially designed roller profiles to minimize stress across the roller contact and provide optimal performance.
A bearing lubricant must perform three basic functions:
1. Reduce friction and wear by separating adjacent surfaces and limiting metal-to-metal contact.
2. Transfer heat from the bearing surfaces.
3. Protect the bearing surfaces from corrosion and dirt contamination.
Proper lubrication and generated film thickness in the raceway and roller-end contacts is vital to lifetime bearing performance, and ultimately the operation of the vibrating screen.
Because improper lubrication and/or abrasive wear will almost certainly reduce the life of the bearings and the screen, selecting the right lubricant and lubrication system are essential to uptime.
For both grease and oil systems, the viscosity of the base oil is one of the most important considerations in selecting the proper lubricant.
Vibrating screens create operating conditions in which bearings have to perform at high temperatures. The load zone of a screen bearing can be 30ºF (17ºC) higher than the sump temperature and 40ºF (22ºC) higher than the housing temperature.
Consider why the bearing’s operating temperature matters: It is critical because it affects lubricant viscosity and lubricant life. As a general rule, a lubricant should maintain a minimum viscosity of 105 SUS (21.8 cSt) at the bearing operating temperature for adequate lubrication.
With the improved analytical tools that exist today, Timken’s engineers are able to suggest the required viscosity based on the load, speed and temperature conditions of a particular screen.
The most common types of lubrication systems used by vibrating screen manufacturers are manual grease, automatic grease feed, oil splash and circulating oil systems. Oil mist and drip feed systems are also used, but to a lesser extent.
With appropriate product considerations in place, it is also important to incorporate condition monitoring to keep conveyors running smoothly. Vibration analysis provides valuable data to help operators optimize their performance of their systems and avoid costly downtime. The service detects potential problems in equipment, allowing operators to address them before they cause a breakdown.
Timken points to an example from a pipeline transportation and energy storage company that was troubled by inconsistencies in its operation, involving large conveyors for seaward vessels loaded with mineral and ore products.
After meeting with the customer, Timken dispatched one of its own field experts to conduct in-depth vibration analysis on the bearings throughout the entire system.
“The findings showed two imminent failures on the first trip and two more on the second trip,” said Vern Couch, senior field engineer. “So far, we’ve caught five damaged bearings that were close to causing catastrophic failures – damage that another service provider wasn’t able to find.”
After being replaced, the damaged bearings – which were manufactured by a Timken competitor – were immediately sent to Timken for damage analysis.
The damage analysis prompted adjustments to the customer’s bearing settings and maintenance procedures to avoid similar problems in the future, and helped to save the company substantial costs, thanks to downtime averted.
About The Timken Company
The Timken Company (NYSE: TKR, http://www.timken.com) keeps the world turning, with innovative friction management and power transmission products and services, enabling our customers’ machinery to perform more efficiently and reliably. With sales of $5.7 billion in 2008 and operations in 26 countries, Timken is Where You Turn™ for better performance.