Pulley life
November 1, 2007 By: Carol Wasson Pit & QuarryAn inside look at a little-known key to maximum pulley performance.
Maximum pulley life is essential to effective material handling and high-capacity production. After all, you can make do without an idler or two, but, as for pulleys, you need them all. Those who have replaced a failed pulley know that it can take several hours or even a full day of downtime and maintenance labor.
Consequently, it's a task that causes some to procrastinate, leaving a failing pulley in operation, wobbling on one end, soon to cause further damage to other components and to the belt itself. With that said, it's important to understand what factors contribute to pulley stress, and how premature pulley failure can be prevented.
The key to pulley life is directly related to the specification of the pulley hub/bushing type. XT bushing/hub assemblies and QD bushing/hub assemblies are both industry-standard design options offered by most manufacturers.
 End disk bellows from bushing/hub assembly movement. |
According to Superior Industries, a manufacturer of conveyor equipment and components, XT bushing/hub assemblies are proven to reduce pulley failure by minimizing end disk bellow and bushing expansion, which are the major contributors to pulley stress, according to company engineers who have applied finite element analysis (FEA) to examine and compare bushing assembly performance. FEA is a 3D computer simulation technique used in product design, development and engineering analysis. This extensive research indicates that the QD has a failure rate seven times that of the XT design.
As this report will show, the XT design yields more calculable and consistent variables, allowing the specification of the right pulley for the application and ensuring maximum pulley life.
Just the basics
"Most maintenance personnel think the major difference between the QD and the XT assemblies is in the bolt-hole spacing (a three-bolt flange vs. a four-bolt flange), however, the key issue is the angle of the hub and bushing taper. The taper angle on the XT is three times that of the QD," said Jarrod Felton, chief engineer of standard products for Superior Industries, who refers to the accompanying bushing/hub cross-section illustration to further explain the findings.
From a basic standpoint, Felton outlined the way that a pulley bushing/hub system works and how the QD design may affect installation and performance: "The hub is like a big steel donut with an inner diameter that has a taper. The bushing has a mating taper that matches up when driven into the hub, causing a clamping or wedging action. A QD bushing taper is 3 degrees, while an XT bushing taper is 9 degrees, which means that you have to drive the QD bushing in further to get the same clamping force.
 Pulley Stress Contributors |
"If the proper torque setting is not used when installing the QD bushing, it can be driven in much further than it is supposed to go. This causes end disk bellow and bushing expansion, which, when combined, account for 80 percent of the stress placed upon the pulley," he said, referring to the accompanying pie chart showing various pulley stress contributors.
The technical talk
From a more technical standpoint, XT bushings have a 2-in.-per-ft. taper, requiring minimal axial movement on the shaft. This reduces additional stress on the end disk of the pulley. Alternatively, QD bushings have a 3/4-in.-per-ft. taper, so the bushing hubs are wider and longer, allowing more movement, which then creates added stress on the end disk. FEA analysis indicates that the QD bushing assembly bellows 1.5 times further than an XT bushing assembly.
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