MG995 servo motor does not produce the expected torque

[Wawa]

@alija22

But PCA9685 board is designed for 6 volts. what will happen if I use 7.4 v power supply.

Nothing will happen, because power to the servos (the screw terminal) has nothing to do with power to the PCA9685.
Power for the PCA comes from the Arduino (VCC pin of the servo board).
Leo..