|Larry J. Goldbeck
One of the most common problems in an under-performing conveyor system is a belt that’s inappropriately matched to the structure, affecting both system efficiency and belt life. While the urge may be strong to specify a “stock” belt for cost reasons, understanding a few basic principles of belt/structure compatibility is essential to achieving optimum system performance. Without it, even a new material handling system can be doomed to inefficiency and lost production.
The rated strength of the belt is expressed as PIW, an abbreviation for Pounds per Inch of Width. This rating is based on the type of ply material, number of plies, and if it is a steel cable belt, the size of the cables. The higher the rated tension of the belt, the more critical the compatibility of the belt and structure becomes.
For example, a 3-ply belt may have each ply rated at 110 PIW, which translates to a 330 PIW belt. This is the maximum rated tension at which the belt can be operated without damage. Exceeding the rated tension of the belt will likely cause breakage, excessive stretch, splice failure, and belt cupping. Factors that affect the rated tension are belt length, width, material, angle of incline, and parasitic drags, such as the size and quantity of rolling components, belt cleaners, and length of transition sealing systems.
The minimum bend radius of a conveyor belt is determined by the number of plies, whether it is steel or fabric reinforced, the ply material, rated tension of the belt, and thickness of the top and bottom covers. Bending the belt over a radius that is too small can cause damage such as ply separation or top cover cracking. Installing a thicker belt on a system that was designed for a thinner one may require larger pulleys to prevent damage to the conveyor belt. This is a common mistake that often occurs if there is some type of belt damage apparent, as the tendency is to place a thicker belt on the system to make it last longer.
Conveyor belts have the ability to be formed into a trough by idlers, which allows the belt to carry more material. However, all belts also have a maximum trough angle, determined by the type of carcass, its thickness, width, and tension rating. Exceeding the trough angle of a belt can cause it to permanently deform into a cupped position, which makes sealing and cleaning difficult, and tracking virtually impossible. Exceeding the belt troughing capability can also damage the top and bottom covers or the carcass in the idler junction area.
To form a trough, a belt is pushed into a cup shape by the troughing idlers, but the belt must travel across the terminal pulleys (head and tail) in a flat position. As the belt leaves the tail pulley, the outer edges are stretched more than the center as it travels around the idlers. If this stretch occurs too quickly, the carcass, top cover, and bottom cover may be damaged.
The top and bottom rubber cover of the belt serves to protect the carcass, and is expressed as a ratio. There is a tendency for belt manufacturers to make the top cover thicker for longer life, while making the bottom cover thinner to reduce costs. However, as rubber ages, it also shrinks. If the top cover is much thicker than the bottom cover, it will shrink more than the bottom cover, curling the edges of the belt upward (cupping). If cupping occurs, an analysis must be performed to determine the true cause(s).
A conveyor belt must be compatible with the system on which it is installed. Specifying a belt without understanding the important characteristics can reduce efficiency and belt life, and it’s recommended that all parameters of the belt and structure be fully considered prior to selection. A complete review of the system is a good way to ensure that the belt used on the system is the right choice. If the belt supplier cannot provide all of the properties discussed here, then you should probably find a more knowledgeable supplier.
Larry J. Goldbeck is manager of conveyor technology at Martin Engineering (Neponset, IL). He is co-author of the Foundations I, II, III, and IV books on belt conveyors, creator and presenter of the Foundations workshops, and holder of three patents on conveyor components.