I regularly hear from customers whose conveyor belt has shrunk during use, usually within a few weeks of commissioning. They contact me for advice on how to solve this issue. When a belt shrinks it can lead to unplanned production stops, and once the line breaks down, it affects everything: your production and throughput, the quality and shelf life of your product, and even your employees’ time and productivity.
What are the signs that a belt has shrunk?
When you relax a conveyor belt that has shrunk, it will often roll up or look bent (banana-shaped). On occasion, you may not be immediately aware of shrinkage, and only notice it when the motor power required increases, or even switches off due to overheating. This is the result of too much tension in the conveyor belt, which creates extra friction, particularly in applications with a fixed nosebar, or with small rollers that bend too far and get stuck.
When shrinkage is uniform across the belt’s width the deflection of the rollers increases due to the extra tension, and as a consequence, this can cause problems with tracking the conveyor belt. The increase in tension can also rise to such an extent that the bearings are overloaded and break down.
When shrinkage is localized to one or both edges, the belt will often pouch in the middle, causing product misplacement as well as compromised tracking control due to the irregular tension across the belt width.
Why do conveyor belts shrink?
Shrinkage is often caused by external influences that affect the fabric running side. To fully understand this, we first need to look at how standard conveyor belt fabric works.
The transverse width (weft) of the belt often has thick, single polyester threads (monofilament) that keep the belt flat. In the longitudinal direction (warp), there are bundled, thin polyester threads (multifilament). These keep the belt flexible and offer good tensile strength and dimensional stability, so the belt can run over small diameters. In the image, you can see that the multifilament is designed in a sine shape, and this sine shape is an important factor in shrinking.
When abrasive material like sand, flour, sugar, crumbs or salt is pushed into the fabric through the underside (see the red areas in the image below), the multifilament (yellow in the image) is pushed up as the red contamination builds up around the monofilament. The sine shape becomes higher and the distance between the monofilaments becomes smaller, with the result that the belt shrinks. The same effect occurs during cleaning of the belt with water and chemicals. When substances like salt or sugar (an ingredient in many cleaning agents) dissolve and penetrate the fabric during cleaning, these solids remain in the fabric after the water evaporates.
Another cause of shrinkage, which occurs especially where natural fibers like cotton are used, is the absorption of moisture or oil into the multifilament warp.The bundle increases in volume (becoming thicker), which also makes the sine shape higher (shown blue in the image). Once again, the distance between the monofilaments becomes smaller.
Because shrinkage is caused by a change to the structure of the fabric (the sine shape) by external elements, this form of shrinkage is also called “mechanical shrinkage”.
Which industries and applications see the most belt shrinkage?
- Biscuit, cookie, confectionary and bakery: Because of the use of flour, sugar, fat and oil
- Agro-industry: On infeed lines when products like beetroots and carrots come from the field before the dirt and sand have been washed off
- Horticulture: When transporting flower boxes with potting soil and sand
- Production environments where wet sanitation is used, such as on meat and poultry lines
What can be done to limit or eliminate conveyor belt shrinkage?
- Use a belt with an impregnated fabric (especially where moisture causes the problem). However, if contamination builds up under the belt and damages the impregnation, the effects of mechanical shrinkage can only be delayed, not totally eliminated
- Reduce contamination, e.g. by using belt scrapers or sealed edges
- Ensure that there are no sharp unfinished edges on the conveyor framework or on the back of nosebar and live roller assemblies that can cause wear to the belt running side fabric
- If possible, use a belt type with a closed coating on the underside (especially where dirt or salt are involved), e.g. Habasit’s FNB-12EVCQ-W1
- Select a belt with low shrinkage properties, e.g. Habasit’s FNB-5EZCH-P1. This belt is made from knitted fabric and does not have a sine shape. You can use these belt types mainly on short conveyors with lightweight products.
Other solutions can be a fully extruded monolithic belt or plastic modular belts. Each product has its own pros and cons, which is why we have to know the application, environmental influences and technical requirements so we can help you select the best belt for your needs.
Habasit Premium TPU belts with low-shrinkage fabric and special impregnation
In my experience, most problems with belt shrinkage are easily fixed either by ensuring the framework has no sharp unfinished edges and the back of any nosebars and live roller assemblies is finished with a radius and not a cut edge, or by changing to a different belt type. In the bakery industry, for example, switching to Habasit Premium TPU belts, which combine low-shrinkage fabric with special impregnation, has delivered excellent results.
Feel free to contact me at any time for expert advice on your particular needs, or to arrange for an application engineer to visit your site.