What is k1% kadm and breaking load?
k1% is a central concept in the terminology of both conveyor and drive belts and figures often in our technical documentation. k1% is EU-standardized and the industry's way of expressing the modulus of elasticity of fully synthetic transmission belts and conveyor belts. It indicates the force in Newton required to elongate a 1 mm wide strip to 1% elongation.
In a little more everyday language this means: If you hold a 1 cm wide and 1 m long strip of the belt at one end and hang a weight on the other, the k1% value indicates how many kilograms the weight need to have in order to elongate the strip by 1 cm. There are two different values for k1%: static k1%static and relaxed k1% a.r.i. (after running in). The static value applies to a previously completely unloaded product and is also included in the product code.
Examples: FAB-5E, NHM 10EKBV where "5" and "10" is the desired k1% static value for these products during the product development.
k1% must not be confused with the maximum admissible load kadm, which is something else entirely. Some products may not, for example be tensioned by more than 1%. For others, we can go up to 3% or even more. If you want to compare different conveyor belt strengths based on the number in the type code or table value for k1% you should therefore be careful not to draw any conclusions until you have determined the table value for admissible load kadm to compare with!
Both k1% a.r.i. and kadm are used as parameters in our technical formulas to determine the recommended initial load and maximum load.
Breaking strength is another concept that may be relevant for steel and other metals. For synthetic materials the breaking strength is not relevant as this is highly dependent on a variety of variables. Synthetic materials are time dependant, that is, how quickly it is tensioned to break. The most significant factor for a synthetic material service life is fatigue or alternating stress. This implies kadm is far lower than the breaking load since it is defined by the theoretical infinite fatigue strength.
Relaxation and structural elongation, time influenced
As previously mentioned, synthetic materials cannot be used in the same way as steel since the fibres are time dependent in a way that metals are not.
Stressing e.g. Polyester, Polyamide (Nylon®) and Aramid (Kevlar®), you will get different elongation depending on for how long you are loading the material. Only after more than a week, the elongation comes to an end: The material has finished its relaxation. In many cases it is a question of about 50% relaxation meaning that after one week in operation the fibres have elongated to twice their initial value. However, most of the relaxation occurs within the first few hours.
Fabrics are composed of braided threads where the lengthwise (warp) yarns are waved. This waveform changes when the fabric is subjected to load so that the threads become straighter and the fabric elongates. After a prolonged stress the elongation becomes permanent, we've then got a structural elongation of the fabric.
In practical use, this means that it takes about a week for a transmission or conveyor belt to be run in completely, but that most of the adaption occurs after half a day. Tensioning a conveyor belt "until it draws" is thus not recommended. Instead, follow our recommendations to tension to at least 0.3% to 0.5% (depending on the material).
The shaft load we indicate when we calculate e.g. a transmission is after running in (relaxation), which in turn means that the shaft load becomes nearly twice as high at the moment of installation. Therefore check that the plant can stand this load. If not, the belt needs to be tensioned in 2 - 3 steps.