# Most efficient torque transfer

## Main Question or Discussion Point

I'm wondering what the most efficient way to transfer torque is. I'm not talking about which shape can handle the most torque but rather how can I transfer 100% of the torque. Lets take screw bits for example. It seems to me that the simple slotted or philips head transfer 100% of the torque because the force of rotation generated by the torque is applied completely perpendicular to the surface and hence all the force (and torque) is transfered. Compare this to for example an allen or a torx where the force is not perpendicular and hence some force gets waisted. Is this correct?

If yes I have the following issue. I have the following two metal surfaces sticking out as in the images attached which I am trying to rotate (with e.g. a screwdriver). In the circular one, the torque (generated by the screwdriver) generates the largest force in the F1 direction. However, that's not possible given the shape of it so it can only apply force in the F2 direction, which is the projection of F1 onto the perpendicular direction of the flat surface. On the flat surface sticking out, the torque generates maximum force perpendicular to the surface and hence this would transfer 100% of the torque. (the exact same wound he true for any flat surfaces. such as a philips.) Am I correct here? Thank you very much for your time!

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Actually, the more I think about it the more I'm coming to the conclusion that "if there is no slip, 100% of torque is transfered", the internal forces just vary with the different shapes but ultimately all torque is transfered. Anyway, some help is always welcome =).

CWatters
Homework Helper
Gold Member
Correct.

Consider also what happens if the screw it turning at a constant angular velocity (which could be zero). If the screw doesn't accelerate then the forces acting on it sum to zero. eg At constant angular velocity the torque applied by the driver exactly matches that due to friction between the screw and the material it's in.

Thanks for the reply! But lets say that there is a little slip due to the 2 parts not fitting perfectly, and maybe some material deformation (the object I'm using to turn the heads is made out of ABS plastic). Could the shape of the knob matter in this situation? That is, would it be easier for me to turn the flat head rather than the rounded head (and would it be noticeable?) or is the reply again, the forces are just better distributed when using the flat head but the torque applied is identical?

CWatters
Homework Helper
Gold Member
I think it's important to separate Physics from Ergonomics. The torque required to undo a screw is down to the friction between the screw and the material it's in. That's a physics problem. How easy it is for a human to undo might be down to the design of the tool used and that is more to do with ergonomics than physics.

I've used screwdrivers on screws with damaged heads. They are harder to undo but that's because it's harder to keep the screwdriver from slipping and damaging the head even further. That usually means applying a lot of force in the downward direction. In theory applying that force doesn't require you to expend much energy because work = force * distance and the distance is small or zero in the downward direction. However human beings are inefficient living creatures. Unlike a book shelf we consume a lot of energy just holding up a book.

In general the friction holding the screw in the material and hence the torque required to undo is independent of the screw head design (unless it includes something like a star washer designed to make it difficult to vibrate loose). If the head deforms some of the energy you apply goes into deforming the head but again the torque required to overcome friction between the screw and the material it's in doesn't normally change.

The design of the head slot (Slot, cross/Philips, star/Torx etc) allows more torque to be applied without damaging the head but it doesn't in itself increase the torque applied. Something like a breaker bar (a long bar used to remove car wheel nuts) also makes it easier for humans to apply the required torque but at the end of the day we still have to provide that torque. The actual torque required doesn't change.