Through study analysis and experience the writer will attempt to continuously rationalize and expand the conventional conveyor technology revealing the link between theory and practical issues and in the process, through a deeper understanding, take the technology beyond its presently perceived limits. This approach has, in the past 20 years, yielded a greater understanding, a broader application of the principles, and even new technologies to the market place.
Experiences/Frustrations with Conveyor Belt Specs:
Why don’t Belt Manufacturers understand what they publish?
At Dos Santos International we are experts in the Sandwich Belt High-Angle Conveyor Technology. The Sandwich belt system works on the principle of hugging bulk material continuously in a sandwich between two smooth surfaced rubber belts. Hugging pressure on the bulk material develops its internal friction, which resists any back sliding tendencies, allowing the material to convey at any high angle up to vertical. The writer rationalized this technology as an expansion of the conventional conveyor technology during the period 1979 thru 1982. This work culminated in the landmark article “The Evolution of Sandwich Belt High-Angle Conveyors” by Dos Santos and Frizzell.
All Dos Santos Sandwich Belt Conveyors start with a troughed bottom belt that receives the bulk material load in a conventional manner. The bottom belt is joined by a top belt which sandwiches the bulk material between. The belt sandwich is then supported along a convex curve of inverted troughing idlers along which the conveying angle is increased up to the ultimate high angle. In the case of the DSI Snake Sandwich System, shown on Figure 1, the profile is made up of alternating convex curves where the inner belt is supported on the convex curve of troughing idlers and the outer belt hugs itself and the conveyed material up against the inner belt according to the relation:
Pr = T/R,
where Pr is a radial load, T is the belt tension and R is the radius of curvature.
A moment is induced at the troughed belt section according to the equation:
M = EI/R,
where M is the moment on the troughed belt section, E is the elastic modulus of the belt in the warp direction, I is the belt section moment of inertia.
Belt stress due to the induced moment is:
Fb = My/I = Ey/R,
where y is the distance from the troughed belt section’s neutral axis.
Since a conveyor belt cannot be subjected to compression, as it will buckle, at a minimum the belt tension must counter the compressive bending stresses, at the inside of the curve. Furthermore, when the belt tension is added to the tensile bending stresses, at the outside of the curve, the combined stress must not exceed the belt’s tension rating.
So, the induced bending stresses are directly related to the belt’s elastic modulus. The lower the elastic modulus the lower the induced bending stresses, permitting tighter convex curves and a more compact transition from the low (conventional) loading angle to the ultimate high angle. Nylon warp fabric belting offers the best solution for tight convex curves.
Indeed, these curvature constraints apply to all convex curves along the DSI Snake Sandwich conveyors. These curvature constraints also apply to convex curves along conventional conveyors though in this case there is typically no great incentive to make such curves tight.
The curvature constraint equations for troughed belt conveyors, based on the basic equations above, are published in the engineering manuals of all major belt manufacturers. The all important Belt (elastic) Modulus is determined for the belt’s long term behavior according to the ISO 9856 Belt Modulus test.
Because of its importance, Dos Santos International always strictly specifies the belt modulus (not to exceed) value. Such specified values are typically comfortably above those already published by the belt manufacturers.
In this light it is frustrating to find, after its manufacture, that the belting we ordered exceeds the specified belt modulus and even more frustrating when the manufacturer claims that they guarantee its performance. Indeed performance is guaranteed to fail in such a case unless measures are taken to compensate for the higher modulus, such as increasing tension to offset the higher induced compressive bending stresses. Higher tension however, may not be possible if such, when combined with the already higher tensile bending stresses, exceed the belt’s tension rating. Indeed DSI design criteria attempts to allow ample margin in case of such mishaps which occur all too often.
Such a mishap, recently, is the source of my frustration and prompted this writing