Table 5.

Factors and codes of quadratic regression orthogonal rotating test.

Numerical variable		Code variable levels
	Symbol	-2	-1	0	1	2
MM rolling friction factor	X1	0.1	0.2	0.3	0.4	0.5
MM static friction factor	X2	0.3	0.45	0.6	0.75	0.9
JKR surface energy/(J/m2)	X3	0.2	0.35	0.5	0.65	0.8