Why does it get harder with time to rotate a screw?

[Wrichik Basu]

A couple of forces act on the screw and create the turning effect. But after sometime it gets harder. So, net torque is decreasing, but why is it decreasing? Frictional force comes into action, but friction doesn't depend on surface area. So, frictional force will remain constant.

So what decreases the net torque of the screw that makes it harder to rotate with time?

 

[A.T.]

but friction doesn't depend on surface area...

...if the total normal force is fixed. But if pressure is fixed, increasing the area will increase the normal force and thus friction.

 

[anorlunda]

Are you asking about a screw in wood. I can imagine that wood fibers have a bit of stiffness. As they are parted, that reduces pressure along the grain. But as the fibers relax with time, the stiffness is reduces and pressure around the circumference is reduced, thus increasing friction. I

s that understandable, I'm finding it hard to describe in words. If it doesn't make sense, I'll try to draw a picture.

 

[Wrichik Basu]

Are you asking about a screw in wood. I can imagine that wood fibers have a bit of stiffness. As they are parted, that reduces pressure along the grain. But as the fibers relax with time, the stiffness is reduces and pressure around the circumference is reduced, thus increasing friction. Is that understandable, I'm finding it hard to describe in words. If it doesn't make sense, I'll try to draw a picture.

Not in wood. Consider a screw driven in a screw socket in a machine, or a screw being driven into a nut that is fixed somewhere.

 

[sophiecentaur]

So, net torque is decreasing, but why is it decreasing?

Sorry but which torque are you referring to? If you screw into wood, the effort required usually increases as the length of thread in the hole increases. On other occasions, the screw can clear out the hole in a dirty or corroded nut and the torque required can get less.
Perhaps you could describe the situation you are referring to.

 

[Khashishi]

You need to overcome the friction to turn the screw. For a constantly turning screw, the torque applied to the screw by the screwdriver equals the torque applied by friction, so if it is harder to turn, then that means the friction is increasing.

Generally speaking, as you screw a screw deeper into some material, there will be more contact between the threads and the material so the friction will increase. But if you are talking about a nut, once the bolt goes all the way through the nut, the amount of contact is constant, so it shouldn't get harder as you screw in more. But real life nuts and bolts are imperfect things, so it could get harder or easier depending on the roughness of the threads or the quality of lubrication or sediment stuck in the threads or imperfect sizing, etc.

Once the screw head or cap starts to press against the part, the friction will increase very rapidly because the upper thread of the screw will press against the lower thread of the socket.

 

[Wrichik Basu]

Sorry but which torque are you referring to? If you screw into wood, the effort required usually increases as the length of thread in the hole increases. On other occasions, the screw can clear out the hole in a dirty or corroded nut and the torque required can get less.
Perhaps you could describe the situation you are referring to.

I was trying to say that when the screw has gone in quite a bit, then it becomes harder to keep the external torque constant. that phrase is a bit ambiguous, I admit, and am sorry.

 

[sophiecentaur]

I was trying to say that when the screw has gone in quite a bit, then it becomes harder to keep the external torque constant. that phrase is a bit ambiguous, I admit, and am sorry.

OK - so it pretty well has to be due to the increased length of screw causing more friction force. Unlike the scenario that's used in introductions to friction, the total normal force increases as more wood is being pushed aside (contact area increases but pressure can be the same) so, even with an 'ideal' coefficient of friction, the friction force should increase.