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The enigma of screwdriver

Hello all friends:

Recently, the way of screwdriver operation has raised the question in our class. Everyone who has worked with screwdriver knows the screwdriver is made up of a tip and a typical hand comprising cylindrical handle of a size and shape to be held by a human hand, and an axial shaft fixed to the handle. The handle and shaft allow the screwdriver to be positioned and supported and, when rotated, to apply sufficient torque and open screws and bolts. The question is why the longer the axial shaft is, the easier the screws are opened? That is, if you have two kinds of same screwdrivers in all aspect except in the length of their shaft, the longer screwdriver open the screws more easily. While in accordance with torsion law, torque which cause to open screws, is not related to the length. (T=τJ/r)

We want to know what the reason of this matter is and what factor cause to do so?Please contribute in this argument and give your opinion. 

Thanks,

Comments

Wei Hong's picture

Hi Mohsen,

This is not a mechanics of materials question, so to look at the torsion formula simply wouldn't help.

To answer this question, you may need to try a screw driver and see how the torque is actually applied (from your hand).  Although our forearm can apply a pure torsion (try rotating your hand without any load), the axis of this torsion can seldom be aligned to that of a screw driver.  Because of this, the torque applied to a screw driver is the consequence of a complex motion of your hand, wrist, and arm (together forming a crackshaft-like mechanism, try yourself).  I have not done a detailed analysis on the mechanism, but my first impression is that longer screw drivers tend to align better with the torsion axle of your forearm.  The reason may be that when your hand is applying a force (moment), it is quite difficult to control the alignment at the same time.  The alignment may be a consequence of equilibrium.

This could be a good machine (mechanism) design problem.

Wei

Hello Prof. Hong:

Your reply seems to be perfectly reasonable. Now, I make sure this subject is really an enigma, so that I received only one reply. Laughing

Thanks a lot for your help. 

Mohsen

 

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