  Timing is of fundamental importance in biology and our life. Borrowing ideas
from mechanism, we map our clock signals onto a gear system, in pursuit of
better depiction of a clock signal implemented with chemical reaction networks
(CRNs). On a chassis of gear theory, more quantitative descriptions are offered
for our method. Inspired by gears, our work to synthesize a tunable clock
signal could be divided into two parts. Part I, this paper, mainly focuses on
the implementation of clock signals with three types of duty cycles, namely
$1/2$, $1/N$ ($N > 2$), and $M/N$. Part II devotes itself in addressing
frequency alteration issues of clock signals. \textcolor{black}{Guaranteed by
existing literature, the experimental chassis can be taken care of by DNA
strand displacement reactions, which lay a solid foundation for the physical
implementation of nearly arbitrary CRNs.
