Vibrating screens are perhaps the most punished of all manufacturing equipment. Used to sort and separate rock in mining and quarry operations, the screen systems are continually pounded by rocks, infiltrated by dirt and grit, and subjected to tremendous loads and forces. Operating speeds are typically high, lubrication runs the risk of contamination from the pervasive dust and debris, and premature wear of key components is always a possibility.
To avert costly downtime and expensive repairs, many operators play it safe by over-maintaining equipment and replacing bearings more often than necessary. But, even in these cases, failures can occur suddenly and catastrophically, causing damage to shafts and other components. Equipment owners face continual high-maintenance and downtime costs, while equipment manufacturers are forced to deal with high warranty costs and customer dissatisfaction.
One solution to keeping the power running in vibrating equipment systems is to combine machinery fault detection technology and high performance bearings in a single, integrated system.
Using this kind of system, a machine operator can monitor specific machine and process faults, bearing condition, bearing temperature and lubrication condition, and take corrective action before there is a catastrophic and costly failure.
Fault detection systems can monitor on a continuous or periodic basis and alert the operator to a problem if a process or machinery fault occurs. Machinery and process faults include loose mesh, "bottoming out," overload, loose members and other faults. One component of a good fault detection system is a sensor specifically developed for a vibrating screen. The sensor’s electronics should be encapsulated in epoxy to make it compact and rugged — an ideal choice for the harsh operating conditions of vibrating equipment.
Signal data can be sourced from the sensor for analysis in two ways: A BNC unit and an MCM unit. Though each approach offers a different level of analysis, each offers the capability to evaluate machine condition through enveloped acceleration technology, which measures vibration to evaluate machine and bearing condition and bearing housing temperature.
The basic BNC unit is designed to be used with a portable data analyzer for periodic equipment monitoring. The MCM unit can provide the user with more advanced continuous monitoring capabilities, including the ability to set vibration limits and connect to a Programmable Logic Controller (PLC) or Distributed Control System (DCS). By enabling early detection of telltale machinery faults, fault detection technology helps avoid catastrophic failures, allowing maintenance to be scheduled at optimum intervals, rather than under emergency conditions.
In addition to monitoring vibration, fault detection technology can also enable users to monitor and trend operating temperatures. Using the vibration and temperature signal data, ineffective lubrication, gear failures and out-of-service condition can be detected.
Another aspect of a flexible system is a top-notch bearings system. Specifically, these bearings should be designed for superior performance under the harsh conditions of vibrating equipment. Bearings in flexible systems for vibrating applications should feature hardened steel cages and a hardened steel guide ring centered by the outer ring. These types of bearings have greater internal radial clearance than conventional bearings.
A good bearing system should also use a PTFE (polytetrafluorethylene)-coated bore and toroidal bearings to eliminate the need for costly heat treatments, coatings or sleeves that are meant to prevent fretting damage caused by a loose fit on the shaft.
A bearing with a PTFE-coated bore eliminates the metal-to-metal contact between the shaft and the bearing that causes fretting corrosion. This reduces the friction between the bore of the bearing and the screen shaft, and allows the shaft to expand without adding internal forces to the bearing system.
At the same time, a toroidal bearing is designed to be used as the non-locating bearing in applications where there is a fixed and a floating bearing. This toroidal bearing can be mounted on the shaft of a vibrating screen with an interference fit, and still safely accommodate the axial expansion internally so that the inner ring does not move relative to the shaft. Fretting corrosion is eliminated, as is the possibility of internal forces acting on the system’s two bearings.
Design engineers can use a good bearing system to increase the output rating of their screens and extend the existing bearing arrangement’s service life. In some applications, screen manufacturers may be able to use a smaller bearing to gain the performance normally expected from a larger, conventional bearing ?Â± opening doors to new designs that are lighter, faster and require less power.
Bob Cassells works on the Engineering Hotline at SKF Canada. You can reach him at 1-800-268-4161 or e-mail him at firstname.lastname@example.org