Belt sag is the vertical sag of the belt between adjacent idlers. Often belt sag doesn’t allow for effective sealing of the belt, which increases dust and spillage.
Belt sag encourages material spillage which increases the risk of trips, slips, falls and entanglement.
Fugitive materials like spillage and dust increase maintenance costs and reduce efficiency, plant safety, and product quality. Fugitive material may bury idlers, conveyor components, or structural supports, requiring costly clean-up labour and replacement parts. Belt sag also increases belt wear and power consumption.
Belt sag can be eliminated by correctly supporting the belt. For an effective, minimum-spillage transfer point, the belt line of travel must be stabilised with proper belt support in the conveyor’s loading zone. A flat, sag-free belt line in the skirted area is essential to successfully sealing the load zone. Ideally, the belting should be kept flat as if it were running over a table that prevented movement in any direction except in the direction the cargo needed to travel. This would eliminate sag and make it easier to seal. To prevent spillage and reduce the escape of dust, eliminate belt sag by ensuring that the stringer support structures are straight and parallel.
Footings must provide a rigid support structure to prevent stringer deflection. The amount of material being loaded, and the level of impact forces, must be considered to prevent excessive deflection under load. Properly spaced stringers tied to rigid footings ensure a good base for the remaining structure. Conveyor Equipment Manufacturers Association (CEMA) provides a valuable resource for construction standards for conveyors and loading zones: “Conveyor Installation Standards for Belt Conveyors Handling Bulk Materials”.
There are a number of techniques and components that can be used, independently or in combination, to control belt sag by improving belt support in the loading zone. They include idlers, belt-support cradles, and impact cradles.
The spacing between the rolling components has a dramatic effect on the idler’s support. Idlers should be placed close enough to support a fully loaded belt preventing the belt from sagging excessively between the rollers. If the belt is allowed too much sag, the load shifts as it is carried up and over each idler and down into the valley between. This shifting of the load increases wear in the belt and power consumption. The sag also encourages material spillage.
Belt cradles can be installed along the line of the conveyor belt, and when replacing idlers with non-rolling components they should be installed between idlers underneath the loading point as far down the conveyor line as is required to stabilise the belt. In loading zones with high levels of materials impact, the belt cradle should be installed downstream from an energy-absorbing impact cradle, to provide maximum belt stability. Skirting effectiveness is increased resulting in reduced transfer point spillage and dust emissions.
An impact cradle is located underneath the belt at the impact zone and is designed to support the belt. This reduces spillage and helps absorb the impact of falling material. Impact beds are also used to decrease the tendency of sharp edges of material piercing the belt and they are useful in stabilising a belt at the loading point, making it easier to seal the belt against spillage.