Web Exclusive: Innovations in bearing technology

By |  April 18, 2012

By James A. Oliver and Joe Esmoer

At one operation of a global aggregate producer, sand is moved from its source uphill to processing facilities via 11 vertical and 11 horizontal conveyors. Each conveyor came equipped with four open spherical roller bearings in the head and tail pulleys for a total of 88 bearing positions. Along every step of the way, these unsealed, open-type bearings in standard housings with standard seals were continually exposed to contamination from the ingress of sand, dust and grit.

The contaminants damaged bearing housing seals and attacked the bearings, necessitating the replacement of housings every 18 months and bearings every nine months. Unanticipated costs for parts replacement, lost production and manpower were difficult to control and the goal of uninterrupted production became unpredictable.

In search of a remedy, the producer turned to application specialists, who suggested testing a conveyor solution featuring a sealed spherical roller bearing. The two-year test compared the cost and performance of the existing open bearing and the sealed type.

Over the test period, the open bearing failed every nine months, while the sealed spherical roller bearing performed without fail over the entire test period. An added advantage: The sealed spherical roller bearing with relubrication groove (filled with high-performance grease at the factory) required less-frequent lubrication.

The outcome: A switch was made to sealed bearings at all bearing positions and the processor ultimately realized lower overall and more predictable costs; more reliable operation; reduced maintenance time and money; reduced cost for lubricant, lubricant disposal and labor; and an overall boost in productivity.

Cost of downtime

The processor in this case is not alone in coping with the challenges of keeping conveyors up and running in the face of adverse operating conditions. Aggregate and mining operations particularly get hard hit when bearings prematurely fail – often losing an average eight hours of production owing to unplanned downtime and seeing associated costs skyrocket.

SKF recently studied the performance of large conveyors in mining applications to shed light on the root causes of bearing failure and to calculate the subsequent cost of downtime and repairs. Five mining operations were evaluated with a primary focus on head pulley bearings in conveyors with an average of 20 positions.

The study found that while operators universally expressed a preference to coordinate replacement of bearings with the replacement of pulleys every four years, reality usually quashed these plans. Inadequate sealing, relubrication and the ingress of contaminants into a bearing’s cavity were found to be the root causes of premature bearing failures.

Premature bearing failures in bulk conveyors can occur in the head and tail pulleys, the take-up pulley, and the impact idlers. In these locations, the ingress of dirt, sand and other abrasive contaminants into the bearing is virtually impossible to stop unless special measures are taken.

One conventional way to increase mean time between failures (MTBF) is to continuously pump large quantities of grease into the housing to protect the bearing. While re-greasing can extend the MTBF, the initial cost of lubricant is usually high, and costs escalate when labor, disposal and environmental impacts are tallied.

For medium- and large-size conveyor pulleys, trending has turned to spherical roller bearings mounted in a split housing and protected by a contact, labyrinth or auxiliary taconite seal, if space allows. This represents a better alternative than unprotected open bearings, but problems may still occur, even when large quantities of grease are fed to the bearings to purge contamination. Additionally, taconite seals, in particular, can add considerable expense and increased axial length to an assembly, making alignment difficult.

True story

At one operation, grease use was cut by more than 220 lbs. per year at each bearing position by replacing open spherical roller bearings with sealed spherical roller bearings. The investment paid off in a year, while the operational costs associated with each bearing position were halved. This can add up to considerable savings, since the disposal cost of grease can be up to three times higher than the purchase cost.

Sealed spherical roller bearings best suit applications where uptime is critical and where the need for bearing maintenance needs to be capped or reduced. Sealed spherical roller bearings also can mitigate grease leakage, especially important where environmental concerns are an issue. Grease use and disposal is taking on increasing importance and, being a greased-for-life concept, the sealed spherical roller bearing exhibits good environmental attributes. The bearing reduces the escape of potentially polluting grease and cuts the cost of waste grease disposal.

The sealed spherical roller bearing has been designed to provide a reliable bearing solution offering significantly reduced need for maintenance, extended service life, and fewer unplanned breakdowns and production stops – requiring less maintenance and contributing more savings in operating costs and staff time.

A variety of innovations in the design, heat treatment, sealing and lubrication of rolling bearings have carried significant benefits for conveyor applications in aggregate and mining operations. Such innovations remain vital in promoting increased equipment reliability and minimizing the total cost of ownership of mining assets.

James A. Oliver is director of sales support engineering for SKF USA Inc., and Joe Esmoer is manager of Industry Specialists for Mining & Metals for SKF USA Inc. www.skfusa.com

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