Collision safety is very important when it comes to human-robot collaboration. However, most of the industrial robots are equipped with large capacity motors for high performance, which on the contrary, increases risk of injuries caused by collision between humans and robots. For this reason, collaboration with common industrial robots has been restricted unless the robots are isolated from workers. To address this problem, in this study we propose a safe collaborative robot arm equipped with the counterbalance mechanism (CBM) and sensorless collision detection method. Furthermore, the sensorless collision detection scheme includes the reaction scheme in case of collisions without the use of any expensive sensors. The performance of the proposed CBM and collision detection method was evaluated based on simulations and experiments, respectively. The simulation and experimental results show that it is possible to operate the robot even with only small capacity motors while maintaining its performance, and collision can be reliably detected without any extra sensors for any type of robot manipulator.