Optimising idler roller performance

Researchers Tiago Cousseau and Yusuf Badat from TUNRA Bulk Solids explain how novel testing solutions can address the gaps in standardised testing.

Idler rollers are integral components of belt conveying systems, significantly influencing operational behaviour, power consumption, and overall system suitability.

While a single idler roll is relatively inexpensive, lost labour and downtime due to roll failure can be prohibitively costly, potentially reaching hundreds of thousands of dollars per hour, with catastrophic consequential losses (e.g., damaged belts) running into the millions.

Idler lifespan is notoriously variable, ranging from mere months to several decades, even for seemingly identical idlers operating under similar conditions.

To mitigate premature failures and improve efficiency, design protocols must extend beyond basic standards to address complex, application-specific operational and manufacturing factors.

TUNRA Bulk Solids leverages advanced, non-standardised testing methodologies to overcome critical deficiencies in current industry standards, providing manufacturers with actionable insights for product development and enabling end-users to select components optimised for their specific operational environments.

Key failure mechanisms

Premature idler failure is primarily linked to rolling bearing failure (resulting in noise, seizure, or high temperature) and excessive roller shell wear. Failure mechanisms are often highly dependent on the idler roll type (carrying, impact, or return) and its location (centre or wing).

For instance, analysis of nearly 100,000 idler replacements showed that:

• Overall, rolling bearing failure and roller shell wear are the main reasons for idler replacements.
• Seizure (a form of rolling bearing failure) occurs predominantly on wing idlers. This is often attributed to water and dust contamination as wing idlers are more exposed than centre idlers, which causes grease to lose its lubricity.
• Central idlers failures are typically linked to its higher loads, which decreases rolling bearing life and increases misalignment.

The critical role of manufacturing quality

A significant source of premature rolling bearing failure is related to structural deficiencies and poor manufacturing quality. In particular, bearing failure occurs due to inadequate dimensional and geometrical tolerances of the bearing housing, leading to excessive bearing misalignment.

In Performance-based design: Case study of conveyor belt idlers, numerical simulation and geometrical analysis showed that the total rolling bearing deflection, resulting from the combination of shaft/bearing house deflection and manufacturing form error (coaxiality), caused idlers to operate outside the nine-minute deflection limit set by the bearing manufacturer. This highlights that misalignment is not only dependent on operational load but also on manufacturing tolerances, such as bearing house eccentricity.

Limitations of current industry standards

Idler roll durability and performance standards, such as SANS 1313, ISO 1537, and NBR 6678, as well as guidelines provided in handbooks like CEMA, offer crucial design requirements for load capacity and rotational constraints. However, these resources are often insufficient to address the subtle but critical factors that determine longevity and efficiency in demanding environments:

1. Rolling bearing misalignment: While standards specify maximum allowable bearing deflection limits (e.g., nine to 12 minutes, depending on the standard), they do not provide standardised methods to predict or test the total operating misalignment that includes both shaft/shell deflection and manufacturing inaccuracies (like bearing house eccentricity). The design calculations provided typically focus only on shaft deflection due to load.
2. Grease life and formulation: Theoretical prediction models for bearing life and friction losses do not take into account rolling bearing misalignment or the effect of grease formulation. This is a major gap, as grease life can be the limiting factor over bearing fatigue life, and high-performance greases (e.g., polyurea vs. lithium grease) can extend expected life by significant margins (e.g., 70 per cent longer life observed in a case study due to a higher grease performance factor).
3. Shell wear evaluation: Standard methodologies often rely on pure sliding tests, which are typically inadequate for accurately comparing the wear performance of modern composite idler rolls. Wear is a primary replacement driver for return idler rolls, and non-optimised wear rates can also lead to increased belt wear due to slip.
4. Outdated Sealing Tests: Standard ingress tests, such as the dust resistance test detailed in SANS 1313-3 and DIN 22112-3, are frequently considered too mild to be truly representative of heavily contaminated field environments, resulting in most idlers passing the test regardless of their field performance. Conversely, the standard water ingress test (submerging the roll halfway for 96 hours) is often too harsh, causing many rolls to fail even if they perform adequately in the field.

TUNRA’s advanced durability testing protocols

To overcome these limitations, TUNRA Bulk Solids employs specialised protocols aimed at providing actionable data to help manufacturers quickly improve idler performance and enable end-users to select idlers that better fit their operations.

Misalignment checks under load

TUNRA can perform static misalignment checks under both loading and unloading conditions to accurately determine the relative sources of misalignment, as schematically presented in Figure 1.

• Unloaded checks determine misalignment resulting solely from manufacturing inaccuracies, such as bearing house eccentricity.
• Loaded checks quantify misalignment induced by shaft deflection and shell/bearing house deflection.

This methodology separates the sources of misalignment, providing clear direction for improvement, either setting tighter machining tolerances or altering the structural design. This helps ensure the idler selected by the end-user will operate within acceptable misalignment limits established by bearing manufacturers.

Enhanced sealing and lubricant performance testing

To replicate the aggressive contamination encountered in mining and bulk handling, TUNRA has updated its sealing assessment capabilities:

• Inclined dust ingress test: This modified test simulates a heavily loaded wing idler (up to 6 kN) with one end enclosed in a dust chamber for 96 hours (Figure 2). The increased load simulates field conditions that lead to gap opening in the sealing package. This is far more realistic than the mild standard test. Post-test analysis may include using techniques like Inductively Coupled Plasma (ICP) or X-ray fluorescence (XRF) spectroscopy to quantify the concentration of wear metals and contaminants that successfully breached the sealing system and entered the rolling bearing grease.
• Water ingress assessment: In addition to the standard submerged water ingress test, TUNRA can perform tests at varying water ingress levels to pinpoint the exact contamination paths (e.g., housing interface (L1), lip seal (L2), or shaft interface (L3)), as shown in Figure 3. After testing, idlers are stored for 10 days to assess grease performance under contamination. Techniques like Fourier-transform infrared spectroscopy are used to check for water absorption and oxidation, and rheometry assesses changes in grease consistency, both related to grease life.

Shell wear and durability

Instead of relying on pure sliding tests which represents wear of seized idler rolls, TUNRA supports methods where the idler roll rotates to compare the wear characteristics of different composite materials, offering a more relevant assessment for predicting shell wear under normal operating conditions.

Minimising friction losses

Idler design directly influences conveying system energy efficiency. Power losses are dictated by two primary factors associate with idler rolls: indentation rolling resistance (IRR), controlled mainly by the idler’s outer diameter and belt bottom cover viscoelastic properties, and rim drag losses, determined by the lubricant, rolling bearing, and sealing system.

Standard predictive models often fail to accurately estimate friction losses. For example, current models showed differences up to 80 per cent for rolling bearings and up to 600 per cent for labyrinth seals compared to measured experimental data, highlighting the necessity of dedicated testing.

TUNRA addresses efficiency measurement through two specialised rigs:

IRR Rig

The IRR test facility evaluates the indentation rolling resistance as a function of key parameters, including idler roller diameter, load, belt speed, sag ratio, and temperature.

• Impact of diameter: Experiments have rigorously confirmed that increasing the idler diameter provides significant efficiency benefits, primarily by reducing contact stresses and lowering IRR. Increasing the diameter from 152.4mm to 400mm resulted in IRR savings of approximately 50 per cent under typical operating conditions.

Rim drag test facility

TUNRA operates a specialised rig designed to measure idler rotating resistance (rim drag force), allowing for the individual component testing and performance breakdown required for optimisation.

• Component breakdown testing: The rig allows for measuring the frictional resistance contribution of components separately. The entire idler is tested, followed by testing with the labyrinth seal removed (or the grease from the labyrinth seal), and finally with lip seals removed. This process yields individual friction losses for rolling bearings, labyrinth seals, and lip seals. Additionally, different lubricants for both rolling bearing and labyrinth seal can be evaluated for their energy savings capacity.
• Rolling bearing and lubricant optimisation: Larger diameter idlers operate at lower rotational speeds for a given belt speed, leading to lower rolling bearing friction torque, which is further reduced due to the larger radius of the idler. Testing allows for the selection of lubricating grease with base oil viscosity that ensures low friction losses while maintaining high durability (e.g., targeting a viscosity ratio k ≈ 2).
• Labyrinth seal analysis: Rim drag losses are highly dependent on the grease properties within the seals. Testing validates the use of non-Newtonian models to predict friction losses more accurately than standard Newtonian models. This helps optimise grease selection (low flow index and low limiting shear stress) and seal geometry.

Conclusion

Current idler performance standards often provide minimum acceptance criteria but lack the depth required for true performance optimisation in modern, high-volume conveying systems.

Critical factors like rolling bearing misalignment due to manufacturing eccentricity and shell deflection, the influence of grease formulation on component life, and the need for rigorous, real-world wear and ingress testing are inadequately addressed by specifications such as SANS 1313, ISO 1537, NBR 6678, and the CEMA handbook.

By offering non-standardised, novel testing solutions, including enhanced sealing assessment (inclined dust ingress, grease performance under water contamination), detailed misalignment analysis, and comprehensive efficiency testing (IRR rig for diameter effects, Rim drag rig for component breakdown), TUNRA Bulk Solids enables:

• Manufacturers to quickly pinpoint design flaws and refine machining tolerances for superior durability and minimum friction losses.
• End-users to test and select idler components (bearings, seals, lubricants, and diameters) scientifically proven to deliver the highest efficiency (up to 55 per cent energy savings observed with larger diameters) and longest life for their specific operating conditions.