In the industrial field, assembling parts using robot arm requires force information. Some robot arms with torque sensors are commercially available, but they are difficult to use for industrial applications due to durability problems. In addition, mounting a torque sensor on the robot arm increases manufacturing costs and complicates the structure. In this study, we apply Sliding Perturbation Observer (SPO) to 7-axis robot manipulator and estimate reaction force without using torque sensor in simulation. From the estimated perturbation, it can be determined whether the end effector of the robot manipulator will crash part of the assembly.