I’m often contacted by engineers asking for assistance with replacing, retrofitting or designing belts for curved conveyors – known as powerturn installations – which are used to change the direction of a conveyor by anything from 30° to 180°.
These complex installations are found in a wide variety of industries, from food to materials handling, distribution and warehousing, and each requires customized fitting and design, as well as highly accurate manufacture.
In simple terms, a curved belt consists of a number of sections joined to form a cone-shaped belt, which is then tensioned around tail pulleys. These pulleys can be conical or cylindrical, depending on the application.
How do you select the best design for a powerturn installation? It starts by being aware of the basic mechanics involved.
1. Belt guidance is essential
Due to the nature of the curved belt, there are strong transverse forces that pull the belts inwards towards the center point of the curve. These forces must be offset by using guidance at the outer edge of the belt, with either a drive chain to do the work, or belt tension using rollers or profiles. The three main types of guidance used at the outer belt edge are:
- A drive chain
- Pairs of roller
· Guidance using a drive chain
In our experience, guidance using a drive chain is simple, effective and optimal in situations where relatively small transfers are needed (for example in the food industry). All the tension is taken on the outer edge of the belt by the chain, with the belt acting as a sleeve.
The chain runs at the outer edge of the installation, with the belt connected to the chain via springs, metal brackets, or most often by plastic loops, so it is both driven and held in the correct position. As the drive takes place via the chain, the belt isn’t friction driven as it would be if there was a driving roller, which means belt running tensions are low, thus optimizing belt service life.
· Guidance using pairs of rollers
This solution uses pairs of rollers mounted on the outer belt edge that run on curved rails to offset the transverse forces. Very high-precision belt and frame fabrication are a must in this case, as well as highly accurate mounting hole placement on the outer edge of the belt. As the belt in this situation is driven by a drive roller, the belt service life will be affected by the higher running tensions. The belt set up and tracking control also need careful monitoring to prevent the belt becoming damaged.
· Guidance using profiles at the belt edge
In this case, an extruded profile is bonded, or often sewn, onto the outer edge of the belt, which is kept in position by the profile running under angled rollers. This method needs extremely accurate positioning of the guiding profile, and there can be relatively high energy losses caused by the flexing of the plastic bead through the guiding pulleys. However, the result is a belt that can be relatively simple to change or replace as setup is minimal once the rollers are relocated against the profile.
In our experience, for distribution warehouses and applications using heavier belts, the latter two types of guidance, where the belt does the work, are more suitable since a heavier and more durable belt can be selected.
2. The choice of belting is now wider than you think
Unlike traditional belts, which are longitudinally flexible but laterally rigid, belts used on curved installations need to be flexible in all directions, since they are subject to bending stress in all directions.
Some engineers believe that the only way to achieve this is to select a belt with a carcass made up of a multifilament weave in both the warp (length) and weft (width) directions, so that the belt is flexible in all orientations. However this is not always necessary. By carefully cutting and joining segments, almost any belt material can be fabricated for a powerturn installation, ensuring a much wider choice of belt options – including blue belts and TPO- or TPU-coated belts for food applications with demanding cleaning requirements.
For example, if the belt required is a single ply belt, using a relatively big pulley diameter (15 mm or 16 mm), it is even possible to use a belt that is made to be laterally rigid, as it will still be flexible enough across the width. We proved this recently with a customer in the UK using a monofilament belt.
There are limitations, however. Monofilament belts cannot be used in applications with a static nosebar at infeed or outfeed, since friction will drive the mono filaments out of the belt.
3. Innovative solutions can solve joint strain
In a recent case, we were asked to help with belt that was failing because of joint strain in a void area between the outfeed roller set up and the chain sprocket. In this case, we designed a reinforced area to stop the belt joints failing.
Wherever possible it is good to cut down on the number of joints used. Not only do multiple joints and panels cost time and money, they can also lead to buckling due natural variations that occur during the fabrication process.
4. Accuracy is essential
Belt fabrication for powerturn installations must be extremely precise during cutting and joining. Whereas in the past curved belts were cut by hand, new CAD technology provides greater accuracy and precision. It has to be said, however, that it is generally not the fabrication process that causes problems, but inaccurate data and measurements.
5. There’s no limit to the size of powerturn installations
It is possible to cut a curved belt from belt material up to 3 m wide. But larger powerturn installations are also possible. Using panels, we have made a 180° powerturn with a belt width of 1900 mm, an outside radius of circa 6 m, and a total circumference of circa 37.5 m.
6. 100% product placement accuracy is impossible on a powerturn
I’m often asked if it’s possible to maintain exact product placement through a powerturn. Unfortunately the answer is: not currently. A line of chocolate bars will never be in exactly the same position at the outfeed as the infeed. The mechanics just don’t allow this. This is simple product movement, not slippage, and it is due to how powerturns are designed and the fact that the true datum point for zero degrees is actually positioned between the return and the conveying belt paths.
Do you have any questions about powerturn installations?
Getting powerturn installations right depends on expertise and experience, and the right partner. If you’d like information about how Habasit can provide exactly the right belting for your powerturn installation needs, please contact me.