Objective  To develop an automatic grading machine of litchi.

Method  An automatic monomeric delivery device was designed and a delivery experiment was performed. The influences of different working and structural parameters on the stuck frequency and cavity rate of litchi were analyzed. Regression and combined models were established. Influencing factors were optimized comprehensively.

Result  Both stuck frequency and cavity rate increased with the rotation speed of big turnable, the cycle length of small turnable and the feeding volume increasing. Both stuck frequency and cavity rate increased firstly and then decreased with the number of holes of big turnable increasing. The cavity rate also increased with the offset coefficient of amplitude of small turnable increasing. The major factor influencing the stuck frequency and cavity rate was the feeding volume, followed by the rotation speed of big turnable, then the offset coefficient of amplitude of small turnable. The optimial parameter combination from the combined regression model was better than those from the orthogonal experiment and the independent regression model. The optimial parameters of the delivery device were as follows: the rotation speed of big turnable was 7.35 r·min^–1, the rotation amplitude of small turnable was 14.30 r·min^–1, the cycle length of small turnable was 16.93 s, the offset coefficient of amplitude of small turnable was 0.33, the rotation motion of small turnable changed linearly, the feeding volume was 1.31 kg and the number of holes of big turnable was 10. Based on this combination, the stuck frequency was 0 and the cavity rate was 6%.

Conclusion  The results can provide references for the designation of automatic grading and monomeric delivery device of litchi.