The full rotatability of a linkage refers to a linkage in which the input may complete a continuous and smooth rotation without the possibility of encountering a dead center position. Full rotatability identification is a problem generally encountered among the mobility problems that may include branch (assembly mode or circuit), sub-branch (singularity-free) identification, range of motion, and order of motion in linkage analysis and synthesis. In a complex linkage, the input rotatability of each branch may be different while the Watt six-bar linkages may be special. This paper presents a unified and analytical method for the full rotatability identification of Watt six-bar linkages regardless of the choice of input joints or reference link or joint type. The branch of a Watt without dead center positions has full rotatability. Using discriminant method and the concept of joint rotation space (JRS), the full rotatability of a Watt linkage can be easily identified. The proposed method is general and conceptually straightforward. It can be applied for all linkage inversions. Examples of Watt linkage and a six-bar linkage with prismatic joints are employed to illustrate the proposed method.

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