Idler rolls, assembled in sets, play a vital role in belt conveyors designed for the transportation of diverse bulk materials. Their impact on the conveyed material’s behaviour, power consumption, and overall conveyor suitability is comparable to that of the conveyor belt itself.

1. Introduction

While various idlers have been developed to perform adequately in different installations, determining the most appropriate one remains a challenging task.

The primary function of an idler roll is to provide load support with minimal resistance to movement, particularly addressing radial loading from the belt and materials across specific idler spacing. This role encompasses absorbing impact at the loading point and managing belt tension in curved sections, which can be challenging to quantify. 

Modern conveyor design methods have enhanced our comprehension and control, providing clearer insights into idler implications.

Assemblies of idler rolls are strategically designed to influence the belt profile, often forming a trough or tube. It’s essential to acknowledge that the trough’s shape, consistency, and flexibility significantly impact idler roll behaviour. Troughing rolls, positioned at an angle to the vertical as wing rolls in a trough and aligned with the belt direction, generate axial load to offer guidance or tracking forces. Aspects like mounting method, alignment, and self-cleaning are critical considerations in idler frame design. External interaction with the conveyor system extends beyond supporting a normal load, encompassing longitudinal load, power transfer, and operational behaviour.

Photo 2 : Rollers (Idlers)

 

It’s noteworthy that all main longitudinal resistances in the conveyor are transferred through the idler rolls. Therefore, an awareness of how idlers affect the entire conveyor system and interact with other components is essential for making informed and optimal selections.

2. Idler (Roller) Roll Design

2.1 Construction

Idler rolls play a crucial role in providing radial support against gravity and belt tension, particularly in curved belts. They facilitate belt bending and material cross-section, allowing longitudinal movement with a gradual increase in belt tension. Minimizing torsional resistance and aligning idler roll life with conveyor life are primary design goals.

The design of idler rolls is fundamentally simple, comprising a rotating outer cylindrical surface and bearings mounted on a stationary shaft. Various components, such as shafts, bearing positions, and outer cylinder materials, impact the conveyor differently. The bearing set design, featuring embedded antifriction rolling elements on a stationary shaft, significantly influences idler performance.

2.2 Idler Rotating Resistance

Torsional resistance to rotation at the bearing set contributes to idler power loss. This resistance results from rolling element sliding, lubricant shear, and seal drag, which varies widely with design.

2.3 Bearing Rolling Resistance

Bearings in bulk material handling conveyors are precision machined, hardened, and designed for pure rolling with minimal deformation. Lubrication with grease, subject to shear and churning, affects speed and temperature-dependent resistance to movement.

2.4 Assembly Considerations

  • Bearing sets, including shaft attachment to the idler frame and a rotating housing, are imperfect in aligning rolling elements under load. 
  • Misalignment causes additional sliding, friction, noise, and wear, impacting contact stress and fatigue life. 
  • Different bearing types require distinct axial support and assembly methods.

2.5 Grease Contribution to Rotating Resistance

Grease uniquely contributes to idler power performance, resisting shear strain with varying stiffness and shear strength. Understanding the impact of grease on conveyor power requires characterization for speed and temperature ranges.

2.6 Rotation of Bearing Seals

Seals protect precision bearing elements from contaminants, with their resistance to rotation impacting power. Seal effectiveness varies, and different functional concepts are employed to isolate bearings from the external environment.

3. Idler (Roller) Life

Life of the Idler is directly dependent on the life of its primary components, bearings and seals.

3.1 Bearings Life

The lifespan of bearings is fundamentally constrained by the contact stresses occurring between the rolling element and the races they traverse. The design of the bearing set must account for various potential scenarios that may arise.

 

Photo 2: Idler Bearing

Bearing manufacturers provide dynamic load ratings or capacity information for incorporation into this formula, along with a static load rating serving as an allowance for occasional overloads that might result in minor damage, accelerating fatigue failure. Additionally, the lifespan of lubricants acts as a constraint on the statistically projected cycle life and requires consideration either as a primary limit or through the practice of re-lubrication.

The size of a bearing significantly impacts cost, availability, and economies of scale. However, the selection process may be influenced by design philosophy, potentially leading to the choice of a heavier bearing than strictly necessary. 

Moreover, a diverse array of additional bearing loads may arise, influencing the actual stress in the race. These loads can substantially impact the precision of the straightforward radial calculation and should be taken into account in both bearing sizing and friction assessments.

3.2 Seal Life

Great costs are frequently incurred for the inclusion of seals between rotating and stationary components. These seals serve to isolate the bearings from external contaminants, ensuring their proper functioning. In demanding applications, high-performance seals are typically employed, although they may require more power.

The nature of seals and their appropriateness varies. In many cases, idlers feature either a stationary or rotating cover over the outer shaft, extending to or beyond the bearing cavity diameter. The rotating design, characterized by a smaller diameter, results in less open area. However, any material that enters through this design becomes trapped between the bearing and the cover.

4. Idler (Roller) Selection

Idlers cannot be evaluated independently. They are integral components that interact with various other elements, leading to a diverse range of conveyor performances. The overall conveyor is considered a system with inherent balances and compromises among its components.

  • Operating Categories: Key operating categories, including nominal capacity, idler life, performance assessment, and rotating behaviour, significantly impact overall conveyor design and cost. Identifying and focusing on these categories to clarify their effects on initial and future costs is essential.
  • Key Operating Parameters: Crucial factors for specifying idlers include expected roll load, life expectations, required roll drag performance, and allowable roll run out and imbalance. It is recommended to convey clear information on these parameters to optimize with belt speed and idler spacing.
  • Considerations for Conveyor Designers: Conveyor designers are encouraged to provide detailed information on expected roll load, life expectations, required drag performance, and allowable run out and imbalance. 

A comprehensive approach to idler selection, taking into account various factors and engaging closely with manufacturers is required.

5. Conclusion

Procuring idlers for optimal value presents several challenges, as outlined earlier. Having a clear comprehension of the idler’s objectives and the diverse factors affecting their task is undoubtedly advantageous in the procurement process. A targeted product specification contributes by defining a uniform and suitable set of expectations tailored to the specific application. Similarly, awareness of potential issues and accumulated experiences enhances the overall quality of the final purchasing decision.

6. What is Novelty Steel Offering?

Novelty Structures supplies belt conveyor rollers along with other components for various applications.

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