## 4 Conclusions  
The aim of this article was to assess the wear of forced-in joint elements after the repeated assembly and disassembly and wear tests which simulated, in a simplified way, the rail vehicle wheel set operation conditions. The importance of certain forced-in joints from the safety point of view was emphasised. In the case of rail vehicle wheelsets, which were repeatedly referred to in this article, wear that is undetected on time can lead to a dangerous incident on a railway track. Forced-in joints operating under rotational bending conditions are subjected to a complex state of stresses resulting from both the assembly process itself and from operation conditions (wear tests), which is one of the factors conducive to the development of wear. The test results presented in the article, along with the mechanism of wear development, can be used as a basis for developing relevant procedures. In the future, these will serve to eliminate the premature wear of the forced-in joint elements and allow the device's failure-free operation period to be extended. The following conclusions can be drawn from the test results:  
* The roughness of the shaft axle seat surface changes with each fatigue cycle. That roughness is the lowest at the beginning of fatigue tests before the joint is assembled. The highest roughness occurs after the second fatigue cycle;
* The kind and nature of the wear of the forced-in joint elements depends on the number of fatigue cycles and joint assembly/disassembly operations;
* Damage resulting from the joint assembly process, such as scratches or chipping of the material, as well as wear resulting from operation under rotary bending conditions, can be noted on the surface of the shafts being examined;
* All shaft surfaces show signs of fretting wear, which is located on the axle seat circumference close to the joint edges in the form of a ring of varying width;
* Fretting comprises plasticised or cracked material build-ups, surface abrasion, and pits;
* Due to the engineering notch, the shafts would crack after 3.6 \(\times\) 106 fatigue cycles at the point of section change;
* Subsequent research into the wear of push fit joints should focus on looking for processes or design methods that limit the development of wear in the joint elements.  
One of the proposed methods to limit the development of wear in a push-on, repeatedly disassembled joint may be the use of PVD coatings or the use of a reinforcing treatment for finishing the surface layer.  
Conceptualization, S.K., D.B., J.D. and A.D.; methodology, S.K., D.B., J.D. and A.D.; software, S.K., D.B., J.D. and A.D.; validation S.K., D.B., J.D. and A.D.; formal analysis, S.K., D.B., J.D. and A.D.; investigation, S.K., D.B., J.D. and A.D.; S.K., D.B., J.D. and A.D.; data curation, S.K., D.B., J.D. and A.D.; writing--original draft preparation, S.K., D.B., J.D. and A.D.; writing--review and editing, S.K., D.B., J.D. and A.D.; visualization, S.K., D.B., J.D. and A.D.; supervision, S.K., D.B., J.D. and A.D.; project administration, S.K., D.B., J.D. and A.D. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.