## 1 Introduction
The blades cut the straw as it passes between the counter blade plates. The faster the chopper shaft rotates, the finer the cut. The blades are subjected to high centrifugal forces as the Claas combine harvester chopper rotates at speeds of up to 3550 min\({}^{-1}\). Therefore, the blade strength reserve factor is an important factor influencing the durability of the blades and the damage that can occur. The higher the coefficient of safety, the longer the blades will last, and foreign objects will cause less damage. The tests were carried out using a numerical simulation (at 2400 min\({}^{-1}\)), which reproduced the performance parameters of a real blade when chopping wheat straw. It was found that this type of blade must have a strength of at least 430 MPa,  
Figure 1: The principal design of a cereal straw chopper: (**a**) 1—adjustable counter blade; 2—chopper shaft; 3—blades; 4—blade attachment axes. (**b**) Straight blade connection to the chopper shaft. and the stresses generated in the structure during operation were 267.1 MPa. Therefore, the coefficient of safety in the simulation was 1.61,..., 1.72, while the minimum recommended coefficient was about 1.5 [12]. Various literature sources indicate that using hardened spring steel 65 Mn to produce blades can significantly reduce their wear [12]. Comparing the arrangement of the chopper blades along the shaft axis with the spiral arrangement also shows advantages. The longitudinal arrangement ensures that the straw mass is chopped evenly, despite the possibility of uneven dropping of the straw on the chopper (e.g., when the combine harvester is tilted), which would be difficult to achieve with a chopper with a spiral blade layout. This increases the likelihood of clogging the straw chopper and deterioration of the chopping quality. In addition, the longitudinal layout reduces the load on the chopper shaft, with a calculated load reduction of \(\sim\)9% compared with the spiral layout [12]. Studies on using straw as an important renewable raw material show that the essential element in the initial stage of processing for use is the chopper blade, which is made of a constructional steel strip, usually with a hole for mounting and two blades. Manufacturers offer different types of straw chopping blades, but the most common are straight, double-bladed blades with serrated or smooth blades. Straightblad, double-ledged blades are not as good as the clippedblade type in terms of chopping quality, but they are cheaper due to the simpler manufacturing process. In addition, curved blades reduce the energy required for the chopping process compared with straight blades. This is due to the different angles of incidence between the curved blade and the straw. The lowest energy demand for cutting is required when the blade of the blade meets the straw at an angle of 20-30\({}^{\circ}\)[8]. This results in a lower load on the combined harvester and lower fuel consumption [13]. From the presented data, it is difficult to explain the three or even 10 times difference in the blade wear rates, unless the study was conducted using blades with very different characteristics (composition and hardness) or a combined harvester with extremely uneven straw mass supplied to the chopper. In the literature, one often finds dependencies between the wear and blade hardness, which are related to the material composition. The presence of harder structures (e.g., WC impurities) results in higher blade wear resistance. Various techniques can be used to increase the hardness, such as laser coating [16]. However, again, it is difficult to explain the different wear of blades when the conditions are similar [14; 15]. It seems that not only material properties play a role in the mechanics of blade wear. The literature review shows that regardless of where grain straw is used (fertil, renewable biofuel, etc. ), the most important guarantee of successful use is compliance with shredding technology requirements, i.e., reliable and efficient operation of the shredding blades.