As Habasit UK’s Technical Manager for fabric conveyor belts, I’m often contacted by customers when they experience problems with product slippage on their incline and decline conveying applications. The results of slippage, including product damage and waste, line stoppages, and reduced production, demand an immediate solution.
Fabric conveyor belts, both with and without coatings, are the cost-effective workhorses of a wide variety of industries. But in this blog I want to focus on two particular sectors:
- Dry food processing of biscuits, cookies and crackers
- Distribution centers dealing with boxes and crates
Incline and decline conveying applications
In many cases, it turns out that the slippage problems began after the belt became soiled, or there was a change in the specifications of the products conveyed. For example:
- Food processing: the type of biscuits conveyed was altered, meaning that the product was more fatty or oily, or there was now starch or sugar dusting landing on the belt, which compromised the friction of the belt. An often overlooked cause of slippage and loss of position can also be subtle vibrations caused by eccentric rollers, in particular support rollers, or rollers with product build-up on the surface.
- Distribution: the boxes on the belts now had less content and/or a lower weight (not so unusual nowadays), or the packaging surface of the boxes was different, e.g. more waxy, which contaminated the surface of the grip-top structure and reduced friction.
Having established what is causing the difficulties, if it is clear that simply replacing the old belt with an identical new one is not the answer, the next step is to find the most cost-effective way of tackling the trouble.
What’s your angle?
The first thing to look at is the gradient or incline of the belt, to see if it can be altered. A reduction in the angle could well solve the problem without further measures.
It’s actually very simple to determine the maximum possible angle of inclination δmax to convey product X with belt Y. Fix a sample belt (1) on a fixed support, such as a wooden board (2), and place on it one or more of the goods to be conveyed (3). Now incline the support until the goods start to slide. Measure the gradient height hT just before sliding starts, and divide this height by the length of support lT. The formula for the maximum possible angle of inclination is:
δmax ≤ arc sin____
Dry food applications
We usually advise a gradient of 12º to 14º at maximum for biscuit conveyance. There are cases where steeper inclines of 15º or 16º work, usually with woven fabric belts without a coating, as the surface has a better grip. This is because the peaks and troughs in the weave provide a physical grip characteristic, and allow any debris to fall into the troughs. However, if the product specification changes and the belt surface becomes more saturated or coated with oil or fats, or there is a loss of friction, the gradient may need to change, or a belt with a coating or special surface, or even flights, may be required.
Similarly, while boxes can be carried on inclines of up to 30º to 35º, if the box footprint remains the same but the weight decreases, the center of gravity of the box, and its grip on the belt will change, causing slippage. Here again, a change in the gradient of the belt can help, although very often, a new belt surface and accessories like flights, profiles or cleats will also be required.
New belt coatings, surface structure, flights and cleats
In cases where adjusting the gradient of the conveyor is ineffective or impossible, it’s time to look at changing the belt coating or surface, and/or using accessories such as transverse profiles, which can be welded onto a belt using the same compound as the cover.
Food processing: Since cleats or profiles can often be unsuitable for food processing applications due to hygiene and conveyor design restrictions, in these situations, coatings and surface structures offer an excellent way to provide easy cleaning, good release, and reliable positioning without slippage.
There are three common coating types used for food processing (others are available):
- Thermoplastic polyurethane TPU, which offers good grip, is very soft, and has high flexibility, excellent abrasion and wear resistance, plus resistance to edible fats and oils
- Thermoplastic olefin TPO, which is used in Habasit’s Cleanline range, and delivers excellent chemical resistance and outstanding product release
- Silicone, which provides the best grip, with excellent release properties, high-temperature resistance, and good hydrolysis and chemical resistance. Silicone is less abrasion resistant, however, and also the most expensive.
A wide range of special surface structures, from smooth to waffle structures, is also available to support reliable conveying. Even subtle embossing can help to grip the underside of a cracker and prevent slippage. Illustrations of surface structures are accessible on our website.
Distribution: While cleats and profiles can be used to help manage product slippage, many belt surfaces featuring higher grip are available, for example with sawtooth or grip top peaks. PVC covers are also often used in these cases to grab the underside of heavy boxes. Lighter-weight boxes may need a grooved belt with more surface based on a soft compound. There are many options available. Illustrations of surface structures are accessible on our website.