## 5 Conclusions  
This work investigated a new passive grinding method. Different from the conventional active grinding method, passive grinding has no motor to drive the wheel to rotate. It relies solely on grinding pressure load, wheel deflection angle and relative motion to produce the grinding behavior. As a new technology, this grinding method has the advantages of high speed and efficiency when applied to rail preventive maintenance. However, the current research on rail grinding is all about active grinding and there is no research on the methods of passive grinding. In order to study the passive grinding behavior, a passive grinding simulation machine which can measure the grinding force and temperature was developed, and it proved that the tester can be used for the test of passive grinding. Under the condition of control of grinding speed, grinding time and grinding wheel deflection angle, the passive grinding tests of different grinding pressure loads were carried out on the rail surface, and the grinding results were tested and analyzed. The main conclusions from this work are as follows.  
1. The increase in passive grinding pressure load will improve the grinding efficiency but will reduce the service life of the grinding wheel and increase the residual stress on the rail surface. Too low grinding load cannot completely remove defects on the rail surface, and excessive grinding load will cause rail wear and lead to rail maintenance quality degradation. According to the calculation of the experimental results, the passive grinding effect is best when the grinding load is in the range of 2000-2400 N under actual working conditions.
2. Through the analysis of grinding force in passive grinding, it is found that the grinding depth will deepen with the increase of grinding pressure load, which will enhance the grinding force. For every 40 N increase in load on a single grinding wheel in the test, the grinding force increases by about 10 N.
3. The maximum surface temperature of the rail was below 500 \({}^{\circ}\)C after the grinding wheel continued to passively grind the rail for 30 s in the test. Due to the characteristicsof high speed, passive grinding does not make it easy to burn the rail surface at suitable grinding pressure loads.
4. The hollow spherical melt debris accounts for the majority of grinding debris and contains a large amount of oxygen, which is the main component of the spark produced by grinding. The distinction in debris morphology and composition reflects the influence of grinding pressure load on the passive grinding process.  
P.-Z.L.: Validation, Methodology, Visualization, Investigation, Data Curation, Writing--original draft. W.-J.Z. and J.P.: Resources, Funding acquisition, Project administration. X.-D.S.: Formal analysis. F.-R.X.: Conceptualization, Supervision, review and editing. All the authors confirm that the manuscript has been read and approved, and the order of authors listed in the manuscript has been approved by all of the authors. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Science and Technology Department of Henan Province, grant number No. 181200212000.  
The study did not involve humans or animals.  
Not applicable. The study did not involve humans.  
The datasets used or analysed during the current study are available from the corresponding author on reasonable request.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.