How does an Undersized Shaft Contribute to a Conveyor Pulley Failure?

March 25, 2019

While conducting a failure analysis study, a conveyor pulley problem received some interesting, and disconcerting, root-cause data. The causative factor didn’t suggest any issues with the pulley mounts or ends caps, nor was the cylindrical cladding on the verge of collapsing. On the contrary, it was the hardened core, the cylinder shaft that was responsible for the conveyor system’s inoperable status. This, unfortunately, is an undersized shaft. 

A Change of Investigative Perspective

Ordinarily, investigator’s look into the operational patterns that form during the day-to-day runnings of a conveyor system. A loading problem maybe bends a shaft. From there, the deflection energies cause mounting misalignment errors to propagate. Troublingly, in the case study mentioned in our opening paragraph, a sleuthing engineer has reversed his point of view after it was found that the equipment wasn’t overloaded. Already, the clues are falling into place. This looks like an undersized shaft is underperforming. 

Explaining Deflection Dynamics

So, what happens when an undersized shaft tries to handle a heavy load? Maybe a revision was made in the design? Maybe financial cutbacks impacted the pulley? Whatever the reason, that shaft is bending because it doesn’t have a wide enough diameter to handle a stated belt load. Now, as it bends, a deflection cycling effect builds. The belt rises and falls at that one site. Initially, thrust loads and axial stresses are going to affect the mounting bearings. The bearings experience fatigue, plus brinelling. The indentations form, the bearings squeak and squeal, and vibrations grow. Next, the deflection stress accumulates at the ends of the cylinder. The strain tugs at the end caps, it pulls at the weld joints and cylinder seams, and the high-cycle energy builds towards a climax. 

Fracture Incidents and Drive Transmission Defects

Performance and reliability issues accumulate, especially when this issue takes place on a heavy-duty conveyor system. Steel shafts are slightly elastic, of course, but properly sized shafts recover instantly after a transient load passes. For that undersized steel rod, it can’t provide enough recovery elasticity to counteract the excess energies. As a result, the stress distribution intensity climbs uncontrollably until that shaft is permanently bent. Fractures are next, with hairlines cracks appearing on pivotal power transmission sections along the conveyor system’s drivetrain. 

In effect, all pulley shafts have a slightly elastic core. So they’re not brittle, but they are fatigue-resistant and durable. As long as the shaft is sized correctly, it’ll express a proportionate quantity of alloy plasticity. Undersized shafts can’t provide this load-counteracting recovery feature, so they bend and deflect. As the defection stress spreads, end caps crack and drivetrains fracture. The deflection energy, caused by the undersized shaft, can’t be distributed, so it becomes mechanical stress, which severely degrades the conveyor system.

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