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Conveyor Belt Bottom Cover Failure From Idlers and Pulleys

Author : Yijun Zhang

Company : Conveyor Dynamics Inc.

Abstract

Excessive wear on the belt bottom cover resulting from improper idler arrangement and ceramic pulley lagging can lead to early belt failure and unplanned downtime. This paper discusses several failure modes and related field examples: bottom cover damage at the idler junction due to severe bending; belt cover damage due to high normal and shear stress; accelerated bottom cover wear from misaligned idlers; and cover damage from problematic ceramic pulley lagging tiles. The idler junction bending is analysed using finite element modelling. The Idler Junction Pressure Index (IJPI) is introduced as a design tool to limit idler junction stress during the engineering stage.

Introduction

Belt conveyors are efficient and reliable material handling systems. A rubber belt with synthetic fabric or steel cord carcass is typically the most expensive single component of a conveyor. The belt also plays a dominant role in the ownership cost and reliability of a conveyor1. While conveyor belts with more than 15 years of service life are not unusual, premature belt failure can also be caused by accidents, engineering or maintenance issues. The top cover, carcass and bottom cover are all subject to premature failure or damage. The top cover usually suffers from damage related to transport material or tramp metal. The belt carcass may suffer from steel cord breakage and carcass penetration. The belt bottom cover is not in contact with transport material, instead the bottom cover is in constant rolling contact with idlers and pulleys. As a result, premature bottom cover failure typically starts from the interaction between the belt and the idlers and pulleys. This paper discusses several scenarios of early belt bottom cover failure.

Cover Failure At The Idler Junction

General Analysis

The conveyor belt bends at the junction between idler rolls. The bending creates compression in the belt’s top cover and tension in the bottom cover. The extent of this deformation depends on the idler arrangement, belt properties and belt loading. Figure 1 shows a finite element analysis (FEA) of the deformation in a steel cord belt on three-roll trough (idler set not shown in the figure). The idler junction bending area is along the belt travel direction, symmetric along the belt centre line, and separates the belt’s contact between the centre roll and wing roll.

Deformation in the idler junction area is mainly due to bending. Deformation in the idler contact area is mainly compression from the material and belt load and shear from rolling resistance. The circles in the idler contact area are high pressure points directly under individual steel cords, which relate to a different type of bottom cover failure.

Figure 1. Equivalent (von Mises) strain in the belt bottom cover of a steel cord belt from finite element modeling

High tensile strain in the bottom cover, due to severe bending at the idler junction, can facilitate surface cracks. The surface cracks may develop from short lengths to long stretches and further coalesce to cause deep cover failure. An example of this idler junction failure is illustrated in Figure 2. Some belt manufacturers’ manuals mention this type of failure, but without detailed analysis2.

Figure 2 . Idler junction cover failure

Bending at an idler junction is affected by several factors: trough angle; belt thickness; cross-sectional loading of belt; material surcharge angle; idler spacing; and convex and concave curves. Increased bending is characterised by a smaller radius of the bending curvature, and less bending by a larger radius of the bending curvature. The following is a list of the main factors that influence idler junction bending.

Trough Angle

A steep trough angle like 60° increases the bending, compared to a shallow trough angle like 35°.

Belt Thickness

A thicker belt reduces idler junction bending, as the bending stiffness is augmented.

Material Cross-Sectional Loading and Idler Spacing

The combined weight of loaded material and belt is heavier than an empty belt. The weight of material is typically the major force and increases the idler junction bending. The bending strain in the idler junction is highest when the belt is right on top of idler rolls. The bending relaxes between idler stations. This relaxation is referred as the belt flexure3. By the same token, because idlers provide support to the belt, wide idler spacing increases the idler junction bending.

Material Surcharge Angle

The material surcharge angle affects the idler junction bending. High angles create a heaped pile of material in the trough belt, which increases the material loading pressure at the middle of the centre roll and reduces the loading pressure around the idler junction and vice versa.

Vertical Curves

At vertical convex curves, a component of belt tension is added to the belt weight and material loading in the gravitational direction, pushing the belt against idler rolls, increasing idler junction bending. A concave curve reduces idler junction bending since the vertical component of belt tension is subtracted from the weight load.

Idler Configuration

In an inline idler arrangement, where all rolls are on the same plane, there is a gap between adjacent idler rolls. If the gap is wide enough and the belt’s idler junction bending is severe enough, the belt may contact the idler edge. Figure 3 shows an example of a belt contacting the idler edge. When this scenario occurs, the idler edge acts like a knife edge, where the contact stress between belt and idler edge is very high. This leads to an excessive rate of cover wear. Well designed and manufactured inline idler sets should have an idler gap of less than 10 mm to avoid potential contact with the belt.

An improved design is the offset idler arrangement, where the rolls are staggered to create an overlap at the idler junction. This eliminates any potential contact between belt and idler edge. A large diameter idler roll reduces the contact stress between belt and idler, and the idler junction bending is reduced. For the same belt width, a five- roll trough arrangement reduces the idler junction bending, compared to a deep trough three-roll arrangement, as there are additional idler junctions that reduce the bending angle at each idler junction.

Figure 3. Belt contacting idler edge

The length of an idler roll can affect the idler junction bending. For a typical three-or five-roll trough, the idler length is equal. Non-equal roll lengths are often implemented in overland conveyors4, one reason being to change the load distribution between the wing roll and the centre roll to a more optimal level. The material cross-sectional loading capacity is also affected by the length of the idler roll. If the centre roll length is decreased, the idler junction bending increases.

Idler Junction Pressure Index

Conveyor Dynamics Inc. (CDI) developed the Idler Junction Pressure Index (IJPI) as a quantitative tool to indicate the idler junction bending stress. The IJPI allows the designer to explore various conveyor and belt parameters like idler spacing, diameter, roll lengths, belt speed, belt thickness and etc., to reach an optimal design while maintaining allowable idler junction bending. Higher IJPI values mean more bending deformation. As a general design criterion, the IJPI should not exceed 1.0~1.2 range. Figure 4Figure shows a finite element model of the idler junction bending in a 1200 mm wide, 13.5 mm thick belt on a 35° degree trough. The material loading has been adjusted to achieve four different IJPI values, from low to high. It is obvious that the strain and idler junction bending increases with a higher IJPI value from 0.61–1.59. IJPI = 1 can be viewed as a relatively conservative limit. When the IJPI exceeds 1.2, the belt bending becomes severe. At IJPI = 1.59, the belt starts to contact the idler edge.

Figure 4. Finite element modeling of belt deformation with Idler Junction Pressure Index (IJPI). Top graphs: cut plane of a belt near the idler junction area. Bottom graphs: the belt bottom cover (looking from beneath the belt bottom cover)

To illustrate the relationship of the IJPI to conveyor and belt parameters, Figure 5 shows different IJPI values on the same 1200 mm wide belt, at 70% CEMA loading and 5 m/s belt speed, but with varying idler spacing, belt thickness, trough angle and tonnage. Some weight related factors, like idler spacing and tonnage, show a linear relation to the IJPI. Some bending related factors, like belt thickness and trough angle, show a non- linear relation to the IJPI. For example, the IJPI increases beyond a linear relationship with reduced belt thickness.

Figure 5. Idler Junction Pressure Index (IJPI) vs idler spacing, belt thickness, trough angle and tonnage. Belt width is 1200 mm in all cases 13,14

The design limit on the IJPI is derived from studying belt behaviour and service life on running conveyors. A high IJPI should be mitigated by design changes like reducing idler spacing, increasing belt thickness or increasing belt speed. Figure 6 displays the IJPI of selected overland conveyor projects that CDI has worked on. Major system details of these conveyors are listed in Table 1.

Figure 6. Project examples of Idler Junction Pressure Index

Table 1. Selected system details for IJPI calculation
Project NameIJP ITonnage (T/H)Belt Speed (m/s)Belt Width (mm)Belt Thick ness (mm)Trough Angle Idler Spacing (m)Convey or Length
(km)
Zisco0. 33500 Iron Ore4.57501425°515.6
Yandi0. 844000 Iron Ore5.5120019452.54
Impumelelo1. 122400 Coal6.512001745°4.527
Curragh1. 172500 Coal7.512001645°520
Ingwe1. 491800 Coal510501445°4.58.9