Martin Engineering’s R. Todd Swinderman outlines how proper belt support systems can prevent costly problems.
For a conveyor to control dust and spillage, the design engineer must do whatever is practical to keep the belt’s line of travel consistent.
While there are many factors that influence the belt’s running line both inside and outside the loading zone, a key ingredient is proper support.
Modern belt support systems are designed to maintain a smooth belt path with minimal bumping or disruption to settled cargo, avoiding gaps in the loading zone and minimising belt sag along the system’s length.
New designs reduce maintenance costs and improve safety by providing easier access to components and reducing the labour required for routine maintenance.
Building proper belt support
Before any support components (cradles, idlers etc.) are installed, the structure on which all conveyor equipment is attached – the “stringer” – must be precisely aligned and the footings secure.
Stringer alignment using laser surveying is the preferred method. The Conveyor Equipment Manufacturers Association (CEMA) publication Appendix D provides installation tolerances. It is imperative that footings provide rigid support to prevent movement or deflection. Engineers should account for the amount of material being loaded and the force of the impact to prevent excessive belt deflection under the load.
As noted in Martin Engineering’s Foundations industry reference book, the belt’s line of travel must be stabilised with proper support in the loading zone to achieve an effective, minimum-spillage conveyor.
With a true belt line, support components can better maintain a sealed environment. According to CEMA there are several techniques and components that control belt sag by improving belt support in the loading zone. They include idlers, belt support cradles, and impact cradles.
Idler type and rating
Idlers shape and support the belt and cargo, while minimising the power needed to transport the materials.
They are classified by roller diameter, service type, operating condition, belt load, and belt speed, and then rated for load-carrying capacity and calculated bearing life.
CEMA uses a two-character code that indicates the idler classification and implied load rating, with a letter code followed by the idler diameter in inches, resulting in classes from B4 to F8.
Troughed carrying idlers typically consist of three rolls: one horizontal roll in the centre with inclined (or wing) rolls on either side. These support and shape the belt into a specific trough angle for the conveyance of cargo
Impact idlers typically have a similar load rating to standard idlers, with the same shafts and bearings.
One disadvantage is that each idler supports the belt only at the top of the roller, so the rounded shape and deflective rubber will allow the belt to oscillate or sag away slightly from the ideal flat profile.
Picking idlers have a shallow trough with a wide base, giving larger raw material a wider area on which to settle.
Without that wide base, large chunks would drift to one side of the belt or the other, causing misalignment.
Offset idlers place the centre roller slightly in front of the wing rollers. By reducing the overall height of the idler set, this configuration is well-suited to cramped spaces, such as mines that require heavy-duty support but lack space.
Belt sag
Belt sag, when viewed from the side of the transfer point, is the vertical deflection of the belt from a straight line across the top of the two adjacent idlers.
Even a millimetre of sag is enough to permit fines to become entrapped, leading to abrasive wear. The result is typically high-temperature friction and belt damage, requiring unscheduled downtime to fix.
Idler spacing and power consumption
Sagging causes the load to shift as it is carried up and over each idler and down into the valley between, increasing belt wear and power consumption while contributing to fugitive dust and spillage. This complicated mathematical relationship has been published. CEMA has produced tables of recommended idler spacing for applications outside the loading zone.
Idler maintenance
When idlers seize or sustain damage, the unprotected underside of the belt can be damaged as well.
Extreme temperatures from the friction may cause combustible cargo or the belt to ignite, carrying the fire the entire length of the belt within seconds.
In the past, an idler set could be difficult to access and change, often requiring the worker to reach in past the belt plane. To address this issue, track-mounted idlers are designed as an in-line set on the same track assembly, engineered to be easily extracted in separate sections for maintenance outside of the conveyor plane.
This enables faster, easier maintenance in a safe environment, reducing labour and downtime.
Impact cradles in the loading zone
Impact cradles continuously support the belt edge, eliminating belt sag between the idlers.
Some bulk-handling operations drop material from a great height. Repeated high-magnitude impacts without proper support can damage the belt.
An example would be a 45.7cm lump of sandstone dropping around 2m. This lump will hit the belt with the same energy as a fully loaded refrigerator falling around 3m.
To minimise these negative effects, impact cradles are constructed of a steel support framework with individual impact-absorbing bars made from durable elastomeric materials that combine a slick top surface with one or more sponge-like secondary layers. Some models allow the bumpers to be flipped to double the service life.
Support cradles
Instead of using rolling idlers following the impact cradle, support cradles use low-friction bars to support the belt profile.
Support cradles perform two functions: controlling belt sag in the load zone to reduce spillage and providing a slick surface on which the belt can ride. Other benefits of using cradles under the transfer point include fewer moving parts and no lubrication is required.
Cradle maintenance
Some cradles are engineered to slide away from the stringer. Much like pulling out a drawer to a filing cabinet, a single worker removes the locking pin in a one-tool procedure and simply slides the cradle into the open-access area to perform maintenance outside of the belt plane.
This creates a safer work environment, improves access for higher quality maintenance, reduces the amount of labour needed and minimises the amount of downtime required for the procedure.
Conclusion
By implementing proper belt support systems, operators can prevent costly problems caused by fugitive material.
It is better to design a system that incorporates the slightly higher power consumption required to prevent spillage, rather than suffer the much higher power consumption and greater consequences of fugitive material. The costs of installing and operating belt-support systems represent an investment in efficiency.