Fabric conveyor belt essentials for engineers
The more I speak to customers about fabric conveyor belts, the more I notice how many different technical terms and part names are used for the same action or part. Although we always manage to understand each other, it’s clear that using a common terminology would be far more efficient.
So here is our Habasit guide to the key fabric conveyor belt terms engineers need to know, plus some important information on nosebars, counter flexion, crowning, accumulation, pre-tension, belt tracking and delamination.
The basic parts of a conveyor:
1. Driving pulley
2. Head or tail pulley (depending on the running direction)
3. Slider bed
4. Carrying roller
5. Sub pulley
6. Deflecting roller (idler)
7. Tension pulley (take-up pulley)
8. Carrying roller (on the return side)
9. Conveyor belt
10. Guiding pulley
|Sign for tension pulley with tension direction||Sign for driving pulley||Sign for running direction|
A nosebar is a knife-edge where a conveyor belt bends over a small radius. It allows smaller items to be transferred between components of a conveyor system. In order to limit the tension that arises in the conveyor belt as a result of bending, the cover angle is reduced to approximately 135˚. This creates the shape of a nose when viewed from the side and upside-down. We talk about a nose roller if the transfer takes place on small rollers mounted next to each other with support points against the deflection. When the belt bends over a fixed plate, it is called a fixed knife.
For more information on nosebars, see: https://habasit.wpengine.com/2018/11/reduce-friction-conveyors-nosebar-knife-edge/
When the underside of a conveyor belt is bent outwards, this is called counter flexion. It’s important for belts to be able to bend in two directions (bending and counter flexion). When guiding profiles are used on the belt, counter flexion increases the minimum required pulley diameter. You can find the counter flexion values set out in the belt product data sheet.
Crowning (cylindrical-conical) on one or more pulleys is used to get a conveyor belt “self-tracking”. For fabric conveyor belts, we recommend a pulley design with a cylindrical part in the center based on 50% of the belt width, with a conical part on both sides ending with smaller diameter on the edges. The height of the crown depends on the pulley diameter, the transverse stiffness of the conveyor belt, and the friction between the belt and the pulley. Our Engineering Guide has tables showing these values.
For more information on crowning, see http://habasit.wpengine.com/2018/07/to-crown-or-not-to-crown-a-lagged-pulley/
If a product is held on a conveyor belt while the belt is running, this is called accumulation. To do this, you need a transport side with low friction compared to the product that is kept in place. Note that for accumulation applications there are joining methods where a preferred running direction is advised to prevent the join from being peeled open.
When installing a conveyor belt, it is important to use sufficient pre-tension. If the pre-tension is too low, you lose grip on the drive after the fabric has been relaxed and it may slip. For a conveyor belt with PET fabric, the pre-tension at installation is at least 0.3% (0.5% for PA fabric), which is usually sufficient for correct operation. If you have a heavy application, it’s easy to calculate the pre-tension required with our Selecalc calculation program. Alternatively, you can tighten the conveyor belt in small steps until it runs non-slip with a full load.
When a conveyor belt is installed and all the pulleys are perpendicular to the running belt, the pre-tension ensures that the crown on the pulleys makes the conveyor belt “self-tracking”. You can influence the belt run and perform a correction using the steering pulley. Automatic conveyors are often used for conveyors with one or more nosebar transfers. In the case of a lateral load, with product sliding on or off the conveyor belt, guiding profiles are often used to absorb the lateral forces and keep the belt in position.
For more information on belt tracking, see: https://habasit.wpengine.com/2017/06/how-to-prevent-unstable-belt-tracking/
Vegetable fats or oils can decrease the adhesion between the different layers of a belt. Over time, these layers can separate from each other, which is called delamination. This effect is accelerated when the sides of the conveyor belt come into contact with the frame of the conveyor and become damaged. Stress due to bending and counter flexion over small transfers also has a negative influence and speeds up this process. Conveyor belts are available for applications involving fats or oils that are resistant to their effects.
Engineering Guide for fabric conveyor belts
For more information, please contact us, or download our free Engineering Guide with more design tips for engineers.