The phenomenon of electrostatic charging has been known since the ancient Greeks. It was observed and described on amber by Thales of Miletus around 550 BC.
Two conditions are necessary for the formation of electrostatic charge. Firstly, there must be a triboelectric effect, i.e. two bodies with different electron affinities must interact mechanically, e.g. rub against each other, whereby electrons are transferred from one body to the other. On the other hand, at least one of the two bodies must be a non-conductor, so that the potential difference formed cannot be equalized by current flow. The characteristic of a non-conductor is given by the material polyurethane.
In a timing belt, the charge can be generated by the belt/pulley meshing, by scrapers and by sliderbeds, see Figure 1. The meshing is a strong “generator” of charge, particularly at higher speeds, due to the frictional meshing of the belt teeth into the pulley teeth gaps. The contact with the conductive pulley does not lead to discharge. Usually, there is a surplus of electrons on the belt, i.e. a negative charge.
In technical processes electrostatic charges have two effects that need to be taken into account:
The opposing electrostatic charge of two bodies leads to forces of attraction. As an example, this causes plastic chips to stick on the powder coating of sanding machines. Plotter manufacturers use the effect to hold paper flat on the machine table. The gecko uses electrostatic charges to be able to move vertically upwards on smooth surfaces, see Figure 2.
If the potential difference due to electrostatic charging exceeds a certain limit, which is called the flashover voltage, a sudden charge equalization occurs in the form of an electric arc.
We are familiar with these discharges from lived experience: Whenever shoe soles and carpet material or pants and sock material are combined in an unhealthy way, charge accumulations form which are discharged into the skin with a small arc when the flashover voltage is exceeded.
Such lightning discharges are also undesirable in technical environments. In PCB production, they can cause damage to sensitive electronic components, see Figure 3. They also pose the risk of igniting a flammable or explosive atmosphere.
There are two ways in which electrostatic charges can be combated. One is to improve the release of electrons. This can be achieved by higher humidity (often used in spinning mills), by blowing with ionized air onto the charged surfaces or by using conductive tension rollers. In the case of timing belt drives, however, the continuous transfer of new electrons through the meshing process is so strong that these measures only have little effect.
More effective is the antistatic treatment of timing belts which improves the mobility of electrons on the belt surface by increasing the electrical conductivity of the belt material in combination with electrically conductive tension members. This prevents local accumulations of electrons, evenly distributes the charges and thereby increases the surface area that can release electrons. It has a very good effect on timing belt drives.
Timing belts become electrically conductive through:
- Addition of conductive particles (conductive carbon black, metallic fibers, carbo-nanotubes): R < 105 Ω, this is referred to as “electrical conductivity”. Caution: This admixture usually leads to increased hardness of the polyurethane!
- Addition of conductive polymer molecules as masterbatch: 105 Ω < R < 109 Ω, this is referred to as “antistatic properties”.
- Application of conductive backings, e.g. a conductive fabric: R < 105 Ω.
The effect of the additives depends upon their concentration. Below a critical concentration Kc, practically no conductivity can be detected - no network formation (percolation) of the conductive particles takes place. When Kc is reached, the conductivity rises sharply to a final value. If the concentration is increased further, the conductivity only changes slightly, but the mechanical properties of the belt (strength and abrasion resistance) decrease significantly. Therefore, the crux of the matter is to find the right concentration and ensure that the conductive particles are evenly distributed throughout the belt.
In recent years, the fear of arc discharges has led to a veritable hunt for ever lower resistance values in the specification of timing belts. In the following chapter, we will show that this does not make technical sense.
BRECO timing belts essentially consist of a load-bearing tension member and the matrix material polyurethane, from which the belt backing and teeth are formed in an extrusion process. Steels and aramids are used as tension member materials. Timing belts 50 AT10 with a length of 2 m were examined on the test bench in order to demonstrate the relationship between the material, the resulting resistance and the electrostatic charge during operation. The speed was 1,000 rpm and the pre-tensioning force was 600 N. Table 1 lists the polyurethanes, their hardness and the tension members used. In the TPU designation, ST means “standard”, AS means “antistatic”. With resistance values in the GΩ range, the standard materials are practically non-conductors. Resistances in the kΩ to MΩ range are achieved with the various AS materials. As expected, the non-conductive aramid tension members always lead to higher total resistance values.
The electrostatic charging was exclusively caused by the teeth meshing during the run. A Keyence SK H050 was used to measure the charge. The measuring spot diameter corresponded to the belt width of 50 mm. The measurements were taken at 21 °C room temperature and 20% relative humidity.
- The operation of a timing belt causes electrostatic charges, which can lead to arc discharges, even without contact to scrapers. Therefore, all concerns of machine manufacturers in this regard are justified.
- In combination with a conductive tension member, any form of antistatic treatment of the TPU leads to charge values “close to zero”, even if the resistance of the belt is between 108 Ω and 109 Ω.
- Aramid tension members should be avoided in charge-critical applications.
Disclaimer: Breco/Brecoflex belts are not recommended for use in flammable or explosive environments.
