# What Defines the Power of a Screw?

I'm in a class that is talking about screws (common everyday type of screws). The question came up with regarding exactly defines the power of a screw?

We're sure that the smaller the space is in between the thread contributes to more clamping power. For the other factor, we're thinking that the length of the screw is also important. Some in the class think that it's the length of the "stick" that is required to go around the screw head (to tighten it) - in other words, the length of the screwdriver. Others speculate that it's the diameter of the screw head.

What is the right answer? Thank you.

jedishrfu
Mentor
By power you mean holding power right?

A screw is a simple machine:

http://en.wikipedia.org/wiki/Simple_machine

and so using it you gain a mechanical advantage ie it takes a small force to screw it in but a large force to pull it out.

CWatters
Homework Helper
Gold Member
Perhaps think about how screws can fail.

For example if you over tighten a screw several things can happen..

1) The screw snaps or fails in shear (twisting) - typically the head breaks off or the shaft fails where it changes from plain to threaded. This is typically due to excess torque. For example if you try driving a screw into a hardwood block a brass screw might fail half way in while a steel screw will be fine. The amount of torque required to drive in the screw depends on (for example) the friction between the threaded part and the material it's going into. A standard trick is to drill a pilot hole and apply some silicon sealer to reduce friction and reduce the torque needed to drive it in. In general a larger diameter screw can withstand higher torque than a smaller one.

or

2) The material that the screw is driven into fails. Instead of the screw going into the material a "tube" of material is pulled out or "stripped". If you have ever assembled a plastic toy you may have discovered that if you overtighten a screw it just starts turning without getting any tighter. For this reason screws with fine/small threads are limited to hard/strong materials (eg metals) whereas coarser threads are used on screws intended for softer materials (eg wood).

We're sure that the smaller the space is in between the thread contributes to more clamping power

I'm not sure what you mean but this but..

A nut which is loose fit on it's bolt would be weaker than a nut which is a good fit on it's bolt.

Mark44
Mentor
1) The screw snaps or fails in shear (twisting) - typically the head breaks off or the shaft fails where it changes from plain to threaded. This is typically due to excess torque. For example if you try driving a screw into a hardwood block a brass screw might fail half way in while a steel screw will be fine. The amount of torque required to drive in the screw depends on (for example) the friction between the threaded part and the material it's going into. A standard trick is to drill a pilot hole and apply some silicon sealer to reduce friction and reduce the torque needed to drive it in. In general a larger diameter screw can withstand higher torque than a smaller one.
On something of a tangent, my dad showed me a trick that is helpful. Load up the threads of a screw by dragging it across a scrap of bar soap before driving the screw. It'll go in more easily.

jedishrfu
Mentor
On something of a tangent, my dad showed me a trick that is helpful. Load up the threads of a screw by dragging it across a scrap of bar soap before driving the screw. It'll go in more easily.

Yeah my Dad taught me that too and the one of using a toothpick if a hole is stripped and you still need to put the screw in place. Poke the toothpick in the hole and break it off and then drive in the screw.

Mark44
Mentor
Yeah my Dad taught me that too and the one of using a toothpick if a hole is stripped and you still need to put the screw in place. Poke the toothpick in the hole and break it off and then drive in the screw.
Or a piece or two of a stick match broken off. The soap works better when it's a little wet, but it also words when it's dry. The advantage of soap over the silicone sealer that CWatters mentioned is that you might not have any of that sealer on hand, but there is usually some soap around.

CWatters