Force distribution in modular radius belts
Modular belts make up an important product group within Habasit. The types suitable for curved applications are called radius belts.
In today’s blog, I will discuss some important design aspects which must be considered for this belt range. I would like to start with an explanation about the main forces that act on the belt and the accompanying support.
Radial and traction forces
In case of a straight conveyor, the pulling force will be equally divided along the width of the belt. If the design is done well, there will be no transversal forces. This is quite different on a radius belt. When pulled through a curve, the belt rows on the inner side of the curve will move together causing the belt to collapse. When this happens, the traction force Fs1 (which is equally distributed over the belt width in the straight section) has to shift to the outside of the curve. This shift needs time and has already started before the curve even begins. Only the outermost links of the curve are carrying the tangential force FT . In the picture below, you can see that one reaction force of the tangential force FT is the radial force FRi.
The radial force FRi increases along the curved section. The highest traction force is close to the drive (at the end) since the entire belt weight together with the load must be pulled. Because the strength of a radius belt is always higher in the straight pull than in the radius pull, a conveyor design with a curve at the beginning followed by a long straight track is the recommended design.
Between two opposite curves, radial force FR2 shifts from one side of the belt to the other. In order to allow a smooth shift, a straight distance l1 of minimum 2 x the belt width bo is required.
The precondition for a proper sprocket engagement is a uniform force distribution over the belt width. This can be achieved with a straight length for the straight sections l0 and l2 of 1.5 times the belt width b0. (see picture below).
But what to do if the straight section L2 at the in-feed must be shorter than 1.5 x b0? In this case, instead of sprockets, rollers should be used. Since the belt can no longer be tracked by rollers, a proper guidance with wear strips must be used. However the minimum length should never be shorter than 1 x b0.
To reduce the section before the drive is more difficult. Basically, the roller can also be used here, but only if the drive is positioned as a lower head drive. If you have more detailed questions on this topic, we recommend contacting our local support.
Side guidance through a curve
As described before, there will always be the radial force FRi when a radius belt runs through a curve. To ensure that this force is not causing an issue, a proper belt guidance on the inner as well as on the outer side is required. Habasit offers different solutions like hold-down tabs or side-tabs. A wear strip can also be installed above the belt. The return design should look the same. Below you will find some design proposals.
Summary
The design of a radius belt conveyor is much more complex than for a straight running conveyor. Various forces need to be considered. It is clear that only a few design aspects have been discussed in this blog. More information can be found in our Engineering Guide for Modular Belts. In case of any doubt about the maximum force that is allowed, please use our web based LinkSeleCalc tool. If you have any further questions about the conveyor design, we offer support and advice through Application Engineers who are always just a phone call or e-mail away.
