Impact Idler Case Study: Protecting a High-Drop Limestone Conveyor

A loading point is one of the hardest-working sections of a belt conveyor. Large pieces of material fall onto a small area of the belt. As a result, the belt, rollers, bearings and support frame must absorb repeated shock.

This case study explains how a limestone processing operation in South Africa upgraded a high-drop transfer point with a heavy-duty impact idler solution.

The existing conveyor had frequent belt damage, failed rollers, material spillage and frame deformation. Standard steel rollers could support the belt during normal transport. However, they could not absorb the strong impact created by large limestone pieces falling from a height of 3.2 meters.

To solve the problem, the loading zone was fitted with rubber ring impact idler rollers, reinforced shafts, protected bearings and stronger support frames. The spacing between the idler sets was also reduced.

After 14 months of operation, the upgraded system showed clear improvements in belt protection, roller reliability and loading-zone stability.

Impact Idler Project Overview

The project involved a limestone crushing and bulk material handling facility in the Free State region of South Africa. The conveyor moved crushed limestone from a transfer chute to the next processing stage.

The main project conditions included:

• Industry: hard rock mining and quarry processing
• Material: crushed limestone
• Material density: approximately 1.6 t/m³
• Maximum lump size: 250 mm
• Conveyor belt width: 1,600 mm
• Belt speed: 3.2 m/s
• Material drop height: 3.2 meters
• Environment: heavy dust and continuous vibration
• Main components: rubber ring impact rollers and reinforced impact idler sets
• Design reference: CEMA E and ISO 5048 loading requirements

The customer’s company name remains confidential. However, the working conditions and project data are retained to show how the solution was developed.

Impact Idler Roller Challenge at the Transfer Point

The transfer chute created a concentrated loading force. Crushed limestone fell directly onto the center of the conveyor belt.

Because some pieces reached 250 mm in size, each drop created a strong local impact. The force passed through the belt and into the rollers below.

The original conveyor section used standard steel troughing rollers. These rollers provided basic belt support, but they had little cushioning capacity.

Over time, the loading zone developed several related problems.

Belt punctures and cover damage

Sharp limestone edges repeatedly struck the same section of the belt. The top cover developed deep cuts and punctures. In more serious areas, the impact also affected the internal belt plies.

Bent shafts and seized rollers

Rigid steel roller shells transferred much of the shock directly to the shafts and bearings. Repeated impact caused shaft movement, bearing damage and roller seizure.

Belt sag between idler sets

The original idler spacing left unsupported areas below the chute. When heavy material landed on the belt, the belt moved downward between the rollers.

Material leakage around the skirt

Belt sag created gaps between the belt and the skirt rubber. Fine limestone escaped through these gaps and collected around the conveyor structure.

Frame fatigue

The support frames received repeated shock loads. Over time, this increased the risk of bent brackets, damaged welds and poor roller alignment.

The customer needed more than a stronger roller. The complete loading-zone support system had to be reviewed.

Impact Idler Belt Conveyor Failure Analysis

Before selecting a new impact idler belt conveyor arrangement, the loading conditions were studied as one complete system.

The main factors included material mass, lump size, drop height, belt width, belt speed, roller spacing and the strength of the existing frame.

The review identified three main causes of the failures.

First, the standard rollers could not absorb enough impact energy. Their steel surfaces provided strength but very little elastic movement.

Second, the space between the idler sets allowed the belt to sag. Therefore, even if one roller remained in good condition, the complete loading zone did not provide continuous support.

Third, the bearings and shafts received too much direct shock. Once a bearing became damaged, the roller stopped turning. The moving belt then passed over a fixed steel surface, which created heat and additional wear.

The solution needed to:

• Absorb repeated material impact
• Support the belt across the complete loading zone
• Protect the bearings from shock and dust
• Reduce belt movement below the skirt
• Fit the existing conveyor structure
• Withstand continuous industrial operation
• Simplify future roller replacement

Conveyor Impact Idler Solution Design

The final conveyor impact idler solution combined rubber cushioning, structural reinforcement and closer support spacing.

Instead of relying on one design feature, the system used several parts that worked together:

• High-elasticity rubber impact rings
• Thick steel roller tubes
• Reinforced solid shafts
• Heavy-duty bearings
• Strong bearing housings
• Multi-stage labyrinth seals
• Water-resistant grease
• Reinforced impact idler frames
• Reduced spacing below the transfer chute

This combined design allowed the impact rings to absorb the first shock. Meanwhile, the roller shell, shaft and frame supported the remaining load.

The closer spacing also created a more continuous support surface below the conveyor belt.

Impact Idler Rubber Ring Construction

The upgraded rollers used separate rubber rings mounted around a thick steel tube.

When limestone struck the belt, the rubber rings compressed. This movement helped absorb and spread the impact before it reached the internal roller parts.

Separate rings were selected instead of one rigid steel contact surface because they offered several benefits:

• Better impact absorption
• Lower peak force on the belt
• Less vibration
• Reduced direct shock on the bearings
• Local compression under large material pieces
• Added protection for the steel shell
• Quieter contact at the loading point

Rubber quality was important. The material needed enough flexibility to cushion the impact. However, it also needed enough hardness to hold its shape under the loaded belt.

If the rings were too soft, they could deform under continuous pressure. If they were too hard, they would transfer more force to the roller and frame.

Therefore, the ring material was selected according to the material size, drop height and working load.

Impact Idler Roller Shaft and Bearing Design

The shaft and bearing assembly had to support both normal belt load and repeated dynamic impact.

For this project, the impact idler roller used a 40 mm solid steel shaft. The larger shaft helped control bending under high loading forces.

Heavy-duty deep-groove ball bearings with C3 internal clearance were also selected. This bearing design provided space for normal thermal movement during continuous operation.

The bearing housings were reinforced and securely connected to the roller tube. This helped maintain bearing alignment when the roller received sudden loads.

The complete structure was designed to reduce:

• Shaft deflection
• Bearing race damage
• Housing movement
• Uneven roller rotation
• Early bearing seizure
• Heat caused by poor alignment

A strong outer shell alone would not have solved the original problem. The internal parts also needed to match the impact duty.

Impact Idler Sealing for a Dusty Limestone Plant

Fine limestone dust was present throughout the transfer area. Therefore, bearing protection was another important part of the design.

Dust can enter a poorly sealed roller and mix with bearing grease. The mixture then becomes abrasive. As a result, bearing friction increases and the roller may eventually stop turning.

The project rollers used a multi-stage labyrinth sealing structure. The seal created a longer and more difficult path between the outside environment and the bearing chamber.

In addition, water-resistant synthetic grease was used inside the bearing area.

The sealing and lubrication system helped:

• Limit dust entry
• Reduce moisture exposure
• Retain grease
• Protect the bearing surfaces
• Maintain smoother rotation
• Reduce the risk of premature seizure

A seal must protect the bearing without creating too much resistance. Therefore, dimensional control during production and assembly remained important.

Impact Idler Frame Reinforcement

The impact idler frame carried the rollers and transferred the working load to the conveyor structure.

In the original system, repeated shock contributed to frame fatigue and deformation. Therefore, replacing the rollers alone would not have provided a complete solution.

The upgraded frames were designed to:

• Hold the rollers at the required trough angle
• Maintain stable shaft positions
• Resist repeated vertical loading
• Reduce unwanted movement
• Fit the existing conveyor mounting points
• Support closer idler spacing
• Allow easier roller replacement

Frame geometry was also important. If the side and center rollers were not positioned correctly, the belt would not receive even support.

Before manufacturing, critical dimensions included frame width, mounting-hole position, roller length, shaft-end design, trough angle and installation height.

By coordinating the roller and frame dimensions, the project avoided field modifications during installation.

Impact Carrying Idler Arrangement

An impact carrying idler supports the loaded belt at the receiving point. It must carry the normal material weight while also absorbing shock from falling material.

For this project, the impact idlers were arranged below the transfer chute, where the limestone first contacted the belt.

The arrangement provided three main functions.

First, it maintained the required belt profile. This helped keep material near the belt center.

Second, it reduced belt deflection under the chute. Better support helped the skirt rubber maintain closer contact with the belt.

Third, it spread the loading force across several idler sets instead of allowing one set to receive most of the impact.

This approach protected both the belt and the conveyor structure.

Impact Troughing Idler Support

An impact troughing idler uses angled side rollers and a center roller to form the loaded belt into a trough.

The trough shape helps contain material and reduce side spillage. However, the trough angle must match the nearby conveyor sections.

If the impact section uses the wrong angle, the belt may experience uneven stress as it enters or leaves the loading zone.

The project therefore coordinated:

• Roller face lengths
• Center and wing roller positions
• Trough angle
• Frame height
• Belt width
• Loading direction
• Nearby carrying idlers
• Skirt board position

Correct geometry helped the impact section support the belt without creating a sudden change in belt shape.

Closer Idler Spacing Below the Loading Chute

One of the most important changes was the idler spacing.

The original spacing was about 1,000 mm. This left long unsupported belt sections below the transfer chute.

For the upgraded loading zone, the spacing was reduced to approximately 300 mm.

The closer arrangement created a more continuous cushioning area. Therefore, the belt had less room to sag when large limestone pieces landed on it.

This change helped:

• Keep the belt closer to the skirt rubber
• Spread impact across more idler sets
• Reduce local belt deformation
• Improve material containment
• Limit stress on individual rollers
• Support a more stable belt profile

Closer spacing should not be applied without checking the conveyor design. The correct distance depends on drop height, material weight, lump size, belt strength and frame capacity.

Installation and Commissioning

Before installation, the maintenance team checked the loading-zone structure and removed damaged components.

The main installation steps included:

• Confirming frame mounting dimensions
• Checking conveyor stringer alignment
• Inspecting the belt and skirt system
• Removing seized and damaged rollers
• Installing the reinforced frames
• Fitting the rubber ring impact rollers
• Setting the reduced idler spacing
• Checking trough alignment
• Tightening all frame connections
• Rotating each roller by hand
• Running the conveyor without material
• Testing the system under normal load

The conveyor was first observed without material. This allowed the team to check roller rotation, belt contact and tracking.

Material