Carryback is defined as the material that fails to unload from a conveyor belt, adhering to the belt and typically falling off at some point other than the intended discharge.
It’s one of the main sources of fugitive materials, estimated to account for 85 percent of all conveyor maintenance issues.
Accumulation on moving components from dirty belts can cause premature wear and require frequent cleanup, exposing workers to potential workplace injuries and respiratory diseases.
It can be shown practically and theoretically that a conveyor belt cannot be cleaned 100 percent, because the surface of the belt and the blades are not without imperfections. Still, this doesn’t mean operators shouldn’t take a proactive approach to keep belts clean.
Most industries have gravitated to basic mechanical scraping, with a metal or elastomeric blade for flat rubber or PVC belting as the best combination of effectiveness, ease of maintenance and low belt wear to yield the lowest cost of ownership.
Belt cleaners
Belt cleaning effectiveness varies day to day with changing conditions and the number and type of cleaners applied, as well as the maintenance they receive.
Keeping material in the process is always better than letting it accumulate on components and build up under conveyors. Without effective belt cleaning, as much as 3 percent of all cargo can be lost due to spillage, dust and carryback.
The exposure to hazards and injuries is also reduced when less cleanup is required, saving significant – but seldom considered – indirect costs. The key to consistent cleaning effectiveness is to control the process through proper selection, installation, inspection and maintenance of the belt-cleaning system and establish a safe cleanup routine and schedule.
The use of multiple mechanical scrapers on a belt has been accepted for quite some time as an effective cleaning approach. In most operations, multiple cleaners are required to reduce carryback to a safe, acceptable level while limiting manual cleanup to weekly or even monthly tasks.
Effectiveness versus efficiency
The undulating action of the loaded belt passing over idlers tends to cause fines and moisture to migrate and compact on the surface of a belt.
The amount of carryback that clings to a belt can range from a few grams to a few kilograms per square meter. The level of belt cleaning required is a function of the operational schedule and method of collecting and disposing of the carryback that is cleaned from the belt or dislodged by return idlers and collects outside of the conveyor discharge chute.
When discussing the efficiency of a belt cleaner, it’s meaningless to talk about efficiency without stating the initial level of carryback. When considering the beginning and ending levels of carryback as a measure of improvement, effectiveness is a better term.
Some guidelines do exist. The U.S. Bureau of Mines states that an average of 100 grams per square meter of carryback is a reasonable level of performance for belt cleaning. At this level, a 48-in.-wide belt traveling 2 meters per second and operating 24/7 would create a cleanup workload of about 7 tons per day – a significant labor investment that also increases worker exposure to a moving conveyor and the associated risks.
Carryback level determines the cleanup schedule, but in reality, a typical belt cleaner loses effectiveness over time due to wear, lack of inspection and maintenance. On systems with average or poor maintenance, effectiveness values are generally in the range of 40 to 60 percent, thus a need for multiple cleaners exists.