**4. Discussion**

The different types of walnut cracking devices were compared and the results are listed in Table 6 [9,10,12,17,34–36]. It is clear that compared to other types, the extrusion type had a higher working efficiency [22,36]. In addition, the load with multiple contact points was significantly better than a pair of forces and two pairs of forces, showing a larger *SCR* and *WKR*. Studies have suggested that single point or line loads cause uneven forces on the shell, poor crack extension, and damage to the kernels [13]. However, multi-point walnut cracking not only contributes to crack generation and expansion but also reduces the stress value and deformation used to break walnut shells on the condition that the kernel stays whole [8]. Additionally, it is worth pointing out that many walnut cracking devices only focused on the improvement of the *SCR* but ignored the *WKR*. Fortunately, the walnut cracking device designed in this work considered simultaneously a larger *SCR* and *WKR*. Nonetheless, the following two points of the multi-point extrusion type walnut cracking device need to be further enhanced.



**Table 6.** Comparison of different types of walnut cracking devices.

Note: Ph walnut cracking efficiency.

## **5. Conclusions**

The present work described an engineering solution to the walnut cracking problem. A machine for cracking walnuts was designed and manufactured and also evaluated for performance. The response surface test results showed that all the parameters had a significant influence on the three indicators, whereas only the CA had an extremely significant effect on the *SCR*. The obtained regression equation could be used to quantitatively predict the cracking quality under different operating parameters. Taking all the indices into comprehensive consideration, the machine performance was found to be optimum at a CA of 0.47◦ and an RS of 108 r/min. For the 'Wen-185' walnut, the *SCR*, *WKR*, and *Es* were 97.24% against the predicted 98.40%, 92.03% against the predicted 92.94%, and 1.88 kJ/kg against the predicted 1.80 kJ/kg, respectively. The variety adaptability tests showed that the cracking device was well-adapted to the 'Wen185', 'Xinwen-179', and 'Zha-343' varieties. The cracking device had excellent cracking results for walnuts with a shell thickness (h) < 1.2 mm and a gap between the walnut shell and kernel (d) ≥ 1.6 mm, for example, the 'Wen-185', 'Xinwen-179', 'Zha-343' varieties, as well as other walnut varieties with thin shells and a larger gap between the walnut shell and kernel. This paper provides a theoretical reference for improving and optimizing walnut cracking devices' processing parameters.

**Author Contributions:** Conceptualization, H.Z. and Y.Z.; methodology, H.L.; software, H.L.; validation, Y.T.; formal analysis, J.C.; investigation, H.L.; resources, H.Z.; data curation, H.L.; writing— original draft preparation, H.L.; writing—review and editing, H.Z. and Y.Z.; visualization, H.L. and Z.Z.; supervision, H.Z. and Y.Z.; project administration, Y.T.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Chinese Natural Science Foundation (12002229, 31160196), the President's Foundation of Tarim University (TDZKBS202001), the Open Project of the Modern Agricultural Engineering Key Laboratory (TDNG2022101, TDNG2021104), and the Shishi Science and Technology Program (Grant No. 2021ZB01).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors express their thanks to the Chinese Natural Science Foundation (12002229, 31160196), the President's Foundation of Tarim University (TDZKBS202001), the Open Project of the Modern Agricultural Engineering Key Laboratory (TDNG2022101, TDNG2021104), and the Shishi Science and Technology Program (Grant No. 2021ZB01) for their financial support

and to all of the people who assisted with the writing of this paper. The authors are grateful to the anonymous reviewers for their comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
