[net.physics] Screwdriver Mystery

3484ajr (04/12/83)

A quote from the Better Homes And Gardens Handyman's Book  :  "To
gain power without tip jumping from slot, use longest screwdriver
you can."  My own  experience  in  using  two  screwdrivers  with
identical  handles  and  sockets  with  different  shaft  lengths
supports this quotation. Why?

ix222 (04/13/83)

Re:  longer screwdrivers work better

I belive that the extra torque comes from the extra
actual twistedness of the blade shaft of the longer
screwdriver sorta like a heavy book hung from a string
can help you break it.  (remember?)  try your test again,
backing out an easy screw and applying the heave gradually.
you should lose the advantage of the longer handle.  ain't
physics good to us?

steve

smh (04/13/83)

As the force exerted increases, the tendency of your hand to
move increases.  Any such deviation parallel to the screw slot
could be expected to reduce the mesh between driver and slot.
Clearly, the angle of deviation for *small* hand motions
(the ones of interest) is approximately linearly proportional
to shaft length.

leichter (04/13/83)

You know, I never thought about it before - but I've known for years that you
should use a long screwdriver when trying to drive a screw hard.  Here's a
simple theory:

It's difficult - impossible, in fact - to avoid placing some sideways force
on the screwdriver handle as you use it.  This causes the screwdriver to
rotate about its point of support, i.e. the point at which it touches the
screw head.  If it rotates too far, some side or the other of the blade will
slip out of the slot in the head and the screwdriver will slip.  Now, for
the same lateral movement of the screwdriver handle, the induced movement of
the head is smaller for a longer screwdriver (since the motion induced is a
rotation, and the angle the screwdriver moves through is smaller when a fixed
lateral movement occurs further from the point about which rotation takes
place).
							-- Jerry
						decvax!yale-comix!leichter
							leichter@yale

BTW:  Psychologically, it FEELS as if you can actually turn harder when the
handle is long.  I think this is just a matter of automatic reactions trying
to keep the screwdriver under control, and causing you to loosen up when it
starts getting out of control.		-- J

Schauble.HIS_Guest@MIT-MULTICS (04/14/83)

The screwdriver will jump out of the slot when the ANGLE it makes with
the screw exceeds a critical value. Your hand has an uncertainty in
positioning the handle of the screwdriver. The longer the shaft, the
greater this uncertainty (in inches) can be before you exceed the
critical angle. This allows you more freedom to move your hand about as
you apply force.

          Paul

jonab (04/14/83)

Reply-To: jonab@sdcvax.uucp (Jonathan Biggar)
Organization: System Development Corp. (A Burroughs Company)
References: <houxq.360> <sdccsu3.498>

	My guess as to why a longer screwdriver will not
	jump out of the slot as easily is that the longer
	the shaft of the screwdriver, the less a lateral motion
	of the head will affect the angle that the tip makes
	with the slot.  This has the effect of steadying the
	unsteady hand, and keeping the screwdriver more in line
	with the screw.

				Jon Biggar ...!decvax!trw-unix!sdcrdcf!jonab

gmark (04/16/83)

In reference to long-bladed screw-drivers, there are other points to consider.
If the screw is very tight due to hard wood, a small pilot hole, etc., where
the tightness is constant, a smoothing out of the torque applied by twisting
of the blade may make it easier for the user to apply the force (grip, etc.).
However, if the tightness is due to some seizing effect, such as wood sap, rust
on a metal screw (especially in sheet metal, or against a washer), a better
"breaker" effect would be derived from a shorter, more rigid blade delivering
a greater instantaneous, moment of force, or impact effect (like an air hammer).
I thought the first statement referred to the blade's slipping out of the slot.
THEN the longer blade would not only maintain more constant pressure against
the sides of the slot, but the longer blade would offer less mechanical
advantage to the twisting effect of the hand of the user NOT in the desired
rotational plane.  In other words, your other hand (not on the handle, but the
blade) would be better able to hold the blade in the slot.  A good test would
be to use a long-shafted socket driver that would not have any tendency to
slip off the screw in the first place.
  
WHEW!  More bubble-gum for the mind.