A constant torque compliant mechanism (CM) generates an output torque that keeps invariant in a large range of input rotation. Because of the constant torque feature and the merits of CMs, they are used in automobile, aerospace, medical, healthcare, timing, gardening, and other devices. A common problem in the current constant torque CMs is their preloading range that is a certain starting range of the input rotation. In the preloading range, the output torque of a constant torque CM does not have the desired constant torque. It increases from zero to a value. The preloading range usually accounts for one-third of the entire input rotation range, which severally weakens the performance of constant torque CMs. In this paper, the preloading problem is eradicated by using precompressed beams as building blocks for constant torque CMs. It is challenging to synthesize constant torque CMs composed of precompressed beams because of the integrated force, torque, and deflection characteristics. The synthesis of constant torque CMs is systemized as parameter optimization of the composed precompressed beams. The presented synthesis method is demonstrated by synthesizing constant torque CMs with different numbers of precompressed beams and validated by experimental results.

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