In most cases, the motor has constant rotation speed, so the speed at which it advances on the bolt is also constant. If the bolt is curved, the radius of the

In most cases, the motor has constant rotation speed, so the speed at which it advances on the bolt is also constant. If the bolt is curved, the radius of the motion, relative to the hinge, is constant through the whole range of motion. Constant linear speed + constant radius = constant angular speed. Some folks prefer to "simplify" the design and just make a straight bolt instead. Again, there's constant linear speed. But this time the radius, relative to the hinge, is not constant (you have a triangle, instead of a circular slice). So the angular speed is not constant anymore. You could vary the motor speed to compensate for the straight bolt, but why bother. Curving a threaded piece of thin metal rod is not that hard.