What are k1%, kadm, and peripheral force? One can feel they are reading a new language when trying to determine the significance of these words, but in today’s blog I will translate their meaning and discuss the implications of each one.
k1% is a central concept in the terminology for both conveyor and power transmission belts, and is often found in our technical information. It is a way to express the module of elasticity for synthetic transmission and conveyor belts. The k1% value indicates the tensile force (in Newton) per unit of belt width (in mm) which is required to elongate it by 1%. The k1% value is an indication of the force/elongation relation (or stress/strain relation) of a belt (force needed to elongate a belt by 1%).
It also permits to say by what percentage (%) the belt has to be tensioned elastically to achieve a specific tensile force. There are different k1% values: k1% static, k1% relaxed and k1% a.r.i (after running in).
The static value is part of the technical specifications and the k1% static is measured immediately after the belt sample has been stretched, without considering the relaxation. Because the correlation of force and elongation of synthetic material is influenced by time (Relaxation), it is important to know the k1% value after the relaxation. Both k1% static and relaxed are valid measures for conveyor belts and machine tapes. Power transmission with flat belts is a highly dynamical process. High loads and high speeds create high-frequency loading/unloading cycles between the tight and the slack side. This causes complex stresses in the belt. For accurate calculation of power transmission flat belts it is therefore indispensable to know the exact correlation of force and elongation after the relaxation, after the running-in time period respectively. The value is called “k1% after running-in” (k1% a.r.i). It applies also to folder-gluer belts.
The k1% value is not an indication of the belt strength. The belt strength is defined as “Admissible force (kadm)” in case of conveyor belts and as “Peripheral force” in case of power transmission belts.
Kadm defines the admissible force per unit of width [N/mm], which can be applied on a fabric belt at standard climate conditions (23 °C / 73 °F, 50% relative humidity), made endless with the master joint.
The nominal peripheral force indicates the power transmission capability of the endless belt. Expressed as a force per unit of width [N/mm] which can be transmitted by a 1 mm wide belt with an arc of contact of 180°C and a defined coefficient of friction without exceeding its maximum admissible elongation. These admissible tensile force shall not be exceeded.
The traction layer will of course influences these admissible strength values. The different material we are using e.g., Polyamide, Polyester and Aramid have different strength (E-modulus) and elongation. Where a polyamide could stretch an Aramid fiber can almost not elongate. We turn that in our advantage to create belts which will be forgiving (high shock load for polyamide based belts) or very stable (high accuracy printing blanket for aramid belt). This is what we call creep in our business.
The weaving is also of importance as the use of different pattern will influence on the remaining elongation and on the elastic elongation (elongation of the weave under tension). The elastic elongation is the reason why we always ask to tension a conveyor belt with a minimum value of 0.3% for polyester belt or 0.5% for polyamide belts.
We hope this blog has helped clarify this potentially complex topic. Please do not hesitate to contact us with further questions!