Avoid Shaft Deflections: Know the Right Outer and Shaft Diameter for Your Conveyor Pulley

March 10, 2019

Shaft deflection challenges can occupy a conveyor system designer’s thoughts for protracted periods of time. Think about their role below a load conveying ribbon. The pully cylinder and its cladding do assume important functions here, of course, but it’s the shaft that performs as the mechanical backbone. If it deflects, deviates from the norm, then the pulley end discs experience stress. 

Diameter Undersizing: What’s The Problem?

The problem is the pulley backbone. Even though the shaft alloy is hard and durable, it’s bending because the rod is too thin. And the issue is having an effect on system performance. As the bowing effect increases, perhaps because the conveyor system is transporting a heavy load, the drum end caps experience stress. The shaft bends, it pulls those two end discs inwards, and they deform slightly. As a result, those welded end sections experience fatigue. 

Gathering System Impacting Variables 

Know the sum of the loads propagating through the flexible ribbon. First on that list, there’s the product loads, which are created by the travelling objects on the belt. Then there are belt tension stresses, plus the weight of the pulley. All of these burdens strike pulleys axially, and they will overwhelm its core if it’s not equipped to deal with those stresses. True, a ductile alloy can “spring back,” but this attribute becomes sorely tested as those loads persist. We need a solution, not just a hope-and-pray attitude that relies on a modulus of elasticity. We need those loading factors to slot into a shaft deflection formula. 

Creating Elegant Shaft Diameter Solutions

A deficiency in outer diameter pulley size can be avoided, with engineering mathematics providing the solution. Likewise, shaft diameter selection problems become non-issues when we use this numbers-based approach. A known alloy is sourced first, a material that’s hard but slightly ductile. Next, the formula, which is defined in Appendix A of the ANSI/CEMA B105.1-2003 guidelines is used as a design tool. The belt material and its elastic attributes slot into the formula. Then there’s the loading factors and belt tensions to add to the equations. Essentially, system profiles and pulley dimensions determine the formula variables, then the calculation ascertains the diameter of the outer pulley and the shaft. 

It’s a two-part issue. Mechanically speaking, the disc is bookended by two different power transmission segments. On the rim, the pulley outer diameter exerts stress. In the centre of that disc, the shaft receives those stresses and defects. If we’re to stop this phenomenon and eliminate disc fatigue, we must understand those stresses and have the tools (engineering formulas) to counteract their effects. Once that’s done, correct diameter pulley cores handle that load without experiencing any deflection.

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