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The reason why you need friction torque in these applications is quite simple; you do not want the panel or lid to free fall (thanks to the earth gravity) or slam your fingers at the moment of releasing your hand from the lids. So for friction hinge to work, it has to provide enough friction torque to counterbalance the natural torque which is developed by rotating a mass about a pivot axis. In short, it has to provide enough frictional torques to hold the weight of panel or lid. By doing so, when you release your hand the panel or lid will stop right there during the travel of rotation.
How can hinge generate frictional torque? Basically, frictional torque is generated when one object is fixed and is rubbing against with another moving object in rotation. As to how much is the frictional torque generated depending on many factors, like the pressure between two objects, material being used, etc. Long ago as we have learned in the physic class, there are two forms of friction, static friction and kinetic friction. The static friction is always coming first and higher than kinetic friction. The difference between static and kinetic friction has a great influence on how well the friction hinge works and feels. Generally speaking, the higher the difference and lower in friction hinge quality and possibly the cycle life. Since the frictional torque is being generated between two objects rubbing against each other, the wearing on these two objects will occur inevitably. How soon the wearing will decide the cycle life of friction hinge. Simply saying, if the wearing comes too quickly, then the cycle life of friction hinge is shorter.
Now we hope you have a brief concept about friction hinge. Just keep in mind, the friction hinge has three major features: the frictional torque, the difference of static and kinetic frictional torque and the cycle life. We will discuss more about friction hinge applications in another article soon.