This work presents a new spatial mechanism for three-dimensional body guidance. The moving platform of this mechanism is supported by a C-S leg and three S-S legs. Driving unit is the cylindrical joint and has two input parameters. The strategy for synthesizing the C-S leg is proposed and at most eight positions of the spherical joint can be prescribed, while at most seven positions can be prescribed in designing each S-S leg. This CS-3SS mechanism can thus be synthesized by prescribing at most seven precision poses. For this multi-loop spatial mechanism, both noticeable works that are the analysis of configurations and strategy for evaluating branch defects are carried out. The mechanism by giving two inputs has zero degrees of freedom and is analogous to a spherical kinematic chain with five links. At most eight configurations can be obtained and the criterion of double configurations is derived successfully. These results are based on to develop the strategy for evaluating branch defects. This strategy has three stages which are calculating the values of criteria, checking properties of other branches and final verification. Two numerical examples are presented to illustrate the design, the evaluation of defects, and the performance of the proposed mechanism.
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