This paper proposes the definition of variable center distance gear, establishes its mathematic model, discusses its mechanical
properties and investigates the corresponding gear trains. Major findings are summarized as follows:
1) Similar to fixed center noncircular gear, the axodes of VCDGs are also noncircular cylinders and can be regarded as pitch
curves. Thus, the generation of VCDGs is the same as that of fixed center noncircular gear.