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Partially threaded screw

  1. Jul 24, 2010 #1
    I would be pleased if someone could explain me how a partially threaded screw (lag screw) exerts compressive force across a fracture for instance. I want to know how this is different from a fully threaded screw, i can be very happy if someone could put all the forces in both systems. Thanks in advance.
  2. jcsd
  3. Jul 25, 2010 #2
    Are you talking about those screws that are only threaded near the tip?

    If that is the case, here is what I think you asked about:

    Say you want to attach a thin plank of wood to a thick piece of wood. You would drill a hole through the plank and to some sufficient depth into the thick wood. If you want the screw to compress them together (like the fracture you mentioned), you want the threads to grip only in the thick wood when it is fully screwn in (i.e. the threads should not cross the interface between the two). The threads will then pull the screw into the thick wood and the head of the screw will pull the plank along with it, thus compressing the two parts together.

    If the screw had been fully threaded, such a compressive force between the two would not arise. The screw would not pull the thin plank towards the thick wood, since the thin plank is stationary while the thin screw moves through it. That would only happen after the head is fully screwn into the plank, but then the screw can no longer be turned without rotating the plank along with it...

    Hope my explanation was understandable...
  4. Jul 29, 2010 #3
    rammbbs is right.

    The threads make the screw move into a material converting rotation into translation
    The head resists the progression into the material.
    If the screw is fully threaded the rate of progress into both materials would be the same, so if there was a between the 2 the gap would never close. When the head engages no further rotation can take place.
    However if the thread is only in the far half of the screw, the the screw can continue to rotate when the head engages, and instead of the near part of the screw engaging the material (no threads here), it just spins in its hole, allowing the screw head to drag the the near material towards the far material, cmaking the gap smaller, and later compressing the two together.

    There are two extensions to this in fracture management. It is possible to make a fully threaded screw compress the two matrerial together...simple make the near hole wider so the threads do not engae it (but obviously not wider than the head.

    The screw can be threaded both ends, but have different pitches. If the far threads have a wider pitch than the near threads, then the far end of the screw will engage the material and travel further into the material than the near threads...resulting in compression as the screw length does not, obviously change.
  5. Jul 30, 2010 #4
    Thanks for your prompt replies. I got completely the point of saiftynet. But i have one part of my original question left. As mentioned by torquil, the threads will be pulling the screw head in. I want to know how that force is generated. Is it some elastic force? In that case, how is it inward? If possible, i want to know the normal reactions of all surfaces and how the entire system is in equilibrium. Thanks in advance.
  6. Jul 30, 2010 #5


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    Exactly, tightening a screw stretches the metal of the screws body - the elastic strength of the body of the screw provides the force.
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