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

A couple of forces act on the machine 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?

• What do you mean by a screw? A machine screw into a single nut? A wood screw penetrating deeper into a block of wood? – BowlOfRed Jan 22 '18 at 7:40
• A machine screw into a single nut – Amar30657 Jan 22 '18 at 7:50
• A lubricated screw on my grandfather's old lathe is very easy to adust, a century after it was first assembled. – Whit3rd Jan 22 '18 at 8:27

## 3 Answers

When you said that friction does not depend on the surface,well, you learnt this while studying the motion of blocks on a plane. Usually the blocks are frictioning because of their Weight, as such a bigger face exerts small Pressure on the Surface as compared to a smaller one. Since P is W over S, friction stays constant.

A screw entering a hole is a different case, as every delta L of it brings in its own contribution.

Related: there will be friction screwing at zero gravity, too.

• "A screw entering a hole is a different case, as every delta L of it brings in its own contribution." maybe so, but why? can you flesh this out a bit more? – pentane Jan 22 '18 at 12:47
• @pentane. Depending on the microscopical picture you might have to destroy micro and nano obstacles or overcome adhesives forces. Or a mix of the two. Also the material around may exer a P, that results in increasing F as more surface go inside . Your question is more on why friction do exist. – Alchimista Jan 22 '18 at 12:59
• @pentane. Imagine you are to glue together two glass slides. You want to reposition them in the middle of the operation. To slide or remove a 2 sqcm will require double the F to do the same on 1 sqcm. – Alchimista Jan 22 '18 at 13:07

A guess:

1) rust , i.e.oxidation next to the contact area as some air will be there, increasing friction.

2) joining of lattices, as in metalic bonding

That friction increases is evident since solvents (WD40) help loosening stiff nuts .

If you run a nut down the length of a rusted screw, the nut progressively scrapes off the rust and gets it jammed into the (very small) clearance space present between the threads on the nut and the screw. once that clearance has been filled with rust particles, the friction force climbs to a very high value- high enough, in fact, to twist the screw apart if you persist in turning it after it locks up.

as Anna V points out, lubrication will help, but so will unscrewing the nut and clearing off the rust before proceeding further.