Pneumatic cylinder and mechanical apparatus using the same

A pneumatic cylinder, comprising:

• a cylinder body formed with a cylinder chamber therein, and having an exterior that is disposed with at least one inlet-outlet passage and at least one pressure-relief opening, wherein the least one inlet-outlet passage is connected to the cylinder chamber;
• a piston assembly contained within the cylinder chamber;
• a connecting rod connected to the piston assembly and protruding out from the cylinder body;
• and
• at least one pressure-relief valve disposed in the at least one pressure-relief opening and having two ends that are connected to an outside of the pneumatic cylinder and the cylinder chamber respectively, wherein when a gas pressure of the cylinder chamber is greater than a threshold value of the at least one pressure-relief valve, the at least one pressure-relief valve works to allow the cylinder chamber to connect to the outside for pressure relief, wherein the exterior of the cylinder body comprises a first side surface and three second side surfaces corresponding to each other, the at least one inlet-outlet passage is two respectively disposed on an upper portion and a lower portion of the first side surface of the cylinder body, each of the second side surfaces has a lower portion, and the at least one pressure-relief opening is three respectively disposed on the lower portions of the second side surfaces of the cylinder body.