Long Allen Keys

================= Screwfix long series: Imperial = 1/16" = 3" - 3/8" = 7" . The metric part of the set are shorter - 1.5mm = 45mm - 10mm = 115mm. Sizes in between in proportion.

Cic.

In message , Kevin Rayner writes

Not being funny, but it depends on the size (i.e. a 6mm one tends to be larger than a 3mm one)

for normal long reach allen keys, somewhere between 4" and 6",

Not disagreeing of course, but from an engineering viewpoint, why is that? I've often wondered as I reach for a longer screwdriver to shift a stubborn screw, to which the same principle applies... From my A-level physics many moons ago I remember that torque (Nm) = force applied (N) x radius (m); so if the radius stays constant, how does torque increase? Isn't it independent of shaft length according to this equation?

David

A question that has erked me for some time. The only answer I could come up with is that we rarely have the sharp end of the driver 100% in the screw slot and given an angle of perhaps 10 degrees from the core of the screw we can impart a significantly greater torque.

Arthur.

They vary with size - like spanners. However, there's only two common sizes. B&Q had the long ones last time I looked in both metric and AF.

I *think* a longer screwdriver usually has a larger handle so you get better purchase on it.

You are right - the length makes no difference in the case of the screwdriver (other than the fact the bigger driver may have a bigger handle as Dave mentioned).

In the case of the Allen key however, I think you may have answered your own question! You need to remember that the key is L shaped, and you can use either the long or short section of the L to apply torque to the bolt. Hence if you use the short part of the L, you are then applying leverage to the long bit - say 4" on a typical key. A long reach key at

Or 2 hands (e.g. yankee driver)