The importance of conveyor belt tensioning
Published by Lucy Stewardson,
Every bulk material handler has a vested interest in technologies that help to reduce hazards and prevent injuries, given the number of conveyor-related accidents that occur during routine maintenance and cleanup. Seemingly mundane tasks, such as adjusting belt cleaners and removing spillage often require employees to work in close proximity to the moving conveyor, where even accidental contact can result in serious injury in a split second. Spillage can also contribute to the risk of fire by interfering with pulleys and idlers, and by providing potential fuel. Even worse, in confined spaces, airborne particles can create the right ingredients for an explosion.
The buildup of fugitive material can occur with surprising speed. An amount equal to just 4 g/hour (the equivalent of just one packet of sugar) results in the accumulation of about 700 g at the end of the week. With an escape rate of 4 g/min., the accumulation will be more than 45 kg/week - more than 2 tpy. If the spillage amounts to just one shovelful/hour (not an uncommon occurrence in some operations), personnel can expect to have to deal with more than 225 kg of fugitive material every day.
Belt cleaning to reduce carryback
There are a number of belt cleaning technologies available to conveyor operators. However, most designs in use today are blade-type units, which use urethane or a metal-tipped scraper to remove material from the belt’s surface. Typically, these devices require an energy source, such as a spring, a compressed air reservoir, or a twisted elastometric element. This will hold the cleaning edge against the belt. Due to the fact that the blade directly contacts the belt, it is subject to abrasive wear and must be regularly adjusted and periodically replaced in order to maintain effective cleaning performance.
A key factor in the performance of a cleaning system is the ability to maintain the proper force that is required to keep the blade against the belt. Blade-to-belt pressure must be controlled to achieve optimal cleaning with a minimal rate of blade wear. There is often a conception that the harder the cleaner is pressed against the belt, the better it will clean. However, research has shown that there is actually an optimum range of blade pressure, which will most effectively remove carryback material. Increasing tension beyond this range raises blade-to-belt friction, shortening blade life and increasing both belt wear and power consumption, without improving cleaning performance.
Operating a belt cleaner below the optimum pressure range also delivers less effective cleaning and can actually accelerate blade wear. Although it may appear to be in working order from a distance, a belt cleaner lightly touching the belt may appear to be in working order. Yet, in reality, excessive amounts of carryback can be forced between the blade and the belt at high velocity. This passage of material between the belt and blade creates channels of uneven wear on the face of the cleaner. As material continues to pass between the blade and the belt, these channels increase in size, rapidly wearing the blade to a jagged edge.
Even with a properly installed and adjusted cleaner, a common source of blade wear that often goes unnoticed is running the belt empty for long periods of time. Increasing the wear of both the blade and the belt, small particles embedded in the empty belt’s surface can create a sand paper effect. Even though the cargo may be abrasive, it often has moisture in it that serves as a lubricant and coolant.
The cleaner blade being wider than the material flow is another potential source of wear, causing the outside portion of the cleaning blade to hold the centre section of the blade away from the belt. As a result, carryback can flow between the belt and the worn area of the blade, accelerating wear on this centre section. Eventually, the process will create a curved wear pattern – sometimes referred to as a ‘smiley face’ or ‘mooning’.
As urethane cleaner blades wear, the surface area of the blade touching the belt increases. This causes a reduction in blade-to-belt pressure and a corresponding decline in cleaner efficiency. Therefore, most mechanically-tensioned systems require periodic adjustment (re-tensioning) in order to deliver the consistent pressure needed for effective carryback removal.
To overcome the problem of the blade angle changing as the blade wears, a radical-adjusted belt cleaner can be designed with a specially-engineered curved blade, known as Constant Angle Radial Pressure (CARP). With this design, the changes in contact angle and surface area are minimised as the blade wears, helping it to maintain its effectiveness throughout the cleaner’s service life.
New air-powered tensioning systems are automated for precise monitoring and tensioning throughout all stages of blade life, reducing the labour that is typically required to maintain optimum blade pressure and extending the service life of both the belt and the cleaner. Equipped with sensors to confirm that the belt is loaded and running, the devices automatically back the blade away during stoppages, or when the conveyor is running empty. This minimises unnecessary wear to both the belt and the cleaner. The result is consistently correct blade tension, with reduced power demand on startup, all managed without operator intervention. For locations that lack convenient power access, one self-contained design uses the moving conveyor to generate its own electricity, which powers a small air compressor, maintaining optimum blade pressure at all times.
Even the best designed and most efficient mechanical belt cleaning systems require periodic maintenance and/or adjustment, in order to prevent performance deteriorating over time. Helping to prevent damage and ensure efficient cleaning action, the proper tensioning of belt cleaning systems minimises wear of the belt and cleaner blades. Belt cleaners must be engineered for durability and simple maintenance, and conveyors should be designed to enable easy service, including required clearances for access. Service chores that are worker friendly and straightforward are more likely to be performed on a consistent basis.
The use of factory-trained and certified speciality contractors can also help to ensure that belt cleaner maintenance is done properly on an appropriate schedule. Furthermore, experienced service technicians often notice other developing system or component problems that can be avoided if they are addressed before a catastrophic failure occurs. This helps conveyor operators to avoid potential equipment damage and expensive unplanned downtime. By setting the cleaning goal necessary for each individual operation and purchasing a system adequate for those conditions as laid out in the CEMA standards, it is possible to achieve carryback control and yet obtain long life from belt cleaners. The bottom line is that properly-installed and adjusted belt cleaners help to minimise carryback and spillage, reducing risk and overall operating costs.
Read the article online at: https://www.worldcement.com/special-reports/05022019/the-importance-of-conveyor-belt-tensioning/
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