Rock Products

OCT 2016

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Page 24 of 63 ROCK products • October 2016 • 23 as take-up pulleys. In addition, these bearings commonly have expansion capabilities for high-temperature or outdoor applications. Typically, heavier duty conveyors have a non-expansion bearing placed on the drive end of the pulley and an expansion bearing placed on the driven end. Tapered Roller Bearing Characteristics A tapered roller bearing would be con- sidered the 1-ton truck. Tapered roller bearings use conical rollers that run on conical races. The rollers are wedged between the inner and outer rings and make contact along the full width of the raceway. Tapered roller bearings generally have a significantly higher load rating than ball bearings or spherical roller bearings, and they can handle purely radial, purely axial or combined load- ing. Tapered roller bearings are not self-aligning, though some unitized, mounted bearings are designed to allow for misalignment and expansion. Therefore, tapered roller bearings are best used in the most heavily loaded applications where the shaft can be properly aligned and fixed at installa- tion, such as the drive or tail pulley. Critical Selection Criteria •  The most critical selection criteria for a bearing is the load rating. The load rating for a bearing is typically described by two metrics: static capacity and dynamic capacity. •  The static capacity (C 0 ) of a bearing is the maximum static load the bearing can support before the rollers begin to permanently deform the raceways. The static capacity is primarily used to rate bearings that remain station- ary under load or run at very low speeds (< 10 rpm). Therefore, for most applications, the static capacity will not be a determining factor for bearing selection. • The dynamic capacity (given as C for ball and spherical, C 90 for tapered) of a bearing is the more important char- acteristic but is often misunderstood. The dynamic capacity of a bearing is the load at which 90 percent of a given group of the bearings are expected to meet or exceed 1,000,000 revolutions (90,000,000 revolutions for tapered). The dynamic capacity is used to pre- dict the fatigue life of a bearing and should not be considered the maxi- mum load of the bearing in service. So, rather than the static or dynamic capacity, bearings should be selected primarily by their predicted L 10 life. L 10 is a statistical estimate of fatigue life based on the dynamic capacity of a bearing. The L 10 life of a bearing is the length of time which 90 percent of a given group of bearings can be expected to meet or exceed at a given load and speed and assumes ideal, identical run- ning conditions for each bearing. Increasing either the load on the bear- ing or the speed will increase the rate of fatigue and thus decrease the L 10 life. The L 10 life does not account for lubrication, contamination, vibration, temperature changes, etc.; all of which will reduce the actual life that a given bearing can reach. For many applications, an L 10 life of 30,000 hours is considered the min- imum. However, conveyor designers often require an L 10 of more than 100,000 hours to ensure the value and performance of the selected bearings and reduce the risk of unexpected failure. Though a longer L 10 life may require a more expensive bearing, the initial cost must be weighed against the potential cost of multiple replacements and repeated downtime. Environment Though often overlooked, the bearing's environment is also a crucial factor for bearing selection. For example, two sets of conveyor bearings with iden- tical loads and identical maintenance service can have dramatically different life. Why? One conveyor may be used in a clean, climate-controlled, indoor environment, while the other conveyor may be used outdoors with exposure to temperature changes, contamination and moisture. Although load ratings may justify the same bearing for both applications, the second conveyor requires a bear- ing with a more rugged sealing system due to the application's environment. The type of seal is independent of the type of bearing, meaning that the seal will need to be considered based on the environment after the type of bearing has been selected based on the loads. Of course, the initial cost will increase between "good," "better" and "best," in mechanical design. However, you may find the difference is a very small per- centage of the overall conveyor cost and insignificant when downtime and main- tenance costs are considered. Proper selection of your conveyor's bearings should be the first priority in your operation's preventive main- tenance program. The cost to maintain a conveyor equipped with the correct bearings is far less than one that isn't. Understanding the basics in bearing design will ensure uptime and savings for your operation. Brad Droegmiller is a senior account manager and Michael Dunn is a bear- ing applications engineer for Baldor Electric Co.

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