Joint mechanism and robot arm mechanism

A robot's joint mechanism comprising:

• a base;
• an arm;
• a first shaft rotatably supporting the arm with respect to the base;
• and
• a balancer mechanism for generating an auxiliary torque that counteracts a load torque around a rotational center line of the arm applied to the first shaft by a weight of the arm, wherein
• the balancer mechanism includes:
• a balancer body including a casing rotatably supported by the base, a piston rod inserted into the casing, and a traction mechanism for pulling the piston rod;
• and
• a transmission member for converting a traction force generated by the traction mechanism into the auxiliary torque and transmitting the auxiliary torque to the arm, and
• the transmission member is pivotally supported with respect to the base by a second shaft,
• the second shaft is configured as a separate body spaced apart from the first shaft,
• the transmission member is engaged with an end of the piston rod at a first position separated by a first distance from a rotational center line of the second shaft, and engaged with the arm at a second position separated by a second distance from a rotational center line of the transmission member,
• wherein the rotational center line of the transmission member coincides with the rotational center line of the arm.