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It happened recently that I broke the head off two screws, once by using an electric drill and once by tightening the screw by hand. For example, it can happen that the pilot hole in the wood is too small and I reach the maximum torque.

I am sure that the maximum torque which a screw can withstand before the head breaks off depends on the material and on the shaft diameter (without the threads obviously).

How can I calculate or where do I find a table with the torque admissible? I would like values from 3 mm shaft up to 8 mm shaft and a relationship between material strength (which I can look up myself) and calculated or listed values.

This is not about knowing the torque I need to apply to secure something, I'm interested in values which should not be reached to avoid damage.

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    If it happens more often than you would like would suggest on making sure pilot hole is right size and maybe using wax or soap on the threads. The odd screw may be defective or driven into more dense material(knot hole).
    – crip659
    Commented Oct 25, 2021 at 10:45
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    This isn't a mathematical problem, especially since cheap imported screws won't give you numbers in the first place. It's a matter of experience and learning the feel of what works.
    – isherwood
    Commented Oct 25, 2021 at 12:45

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Even if you have the spec sheet from the screw manufacturer on how much torque the screws can take during driving (even if you do find a spec sheet such as that one for a screw I'm familiar with, it may not list a torque value), remember that wood is a natural product and its strength and resistance to the screw turning will vary from piece to piece. Any two pieces of wood, even of the same species and from the same tree can have different strength characteristics.

Knowing that you could use 100 ft/lb to drive a screw into one piece of oak (random, made up internet number), does not mean that 100 ft/lb of torque will be safe to use to drive a screw into a spot at the other end of the same piece of oak, much less a different one.

I've experienced this myself any number of times. For example driving screws to hold down my pressure treated 5/4" SPF decking material into PT 2x joists below. Some screws would drive so quickly that they would almost over drive without the impact driver ever getting to "impact mode", while others would trigger "impact mode" almost immediately, and would take a long time and a lot of effort to drive.

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  • I think you misunderstood the question: I'm asking for the maximum torque a screw can withstand before the shaft or the head breaks, therefore it is independent from the wood: with some woods maybe I need to drive almost all of the screw before the friction is enough to break the head, with some other woods maybe I only drive half of it and then the friction is already so high that the shaft breaks. It's not related to the material of the wood, only to the material and diameter of the screw.
    – FarO
    Commented Oct 25, 2021 at 15:20
  • As noted in the answer, @FarO, even if you do find a spec sheet ... it may not list a torque value. I'm not certain you'll find the info you're after. You'd probably have to directly contact the manufacturer to ask. Even if you know the precise torque the screw can handle, unless you're driving every screw with a properly calibrated torque wrench, what good is that info going to provide to you? If you're regularly hitting the torque limit and not getting your screws to drive, then you're not drilling the right size pilot hole. You should find that info just about anywhere online.
    – FreeMan
    Commented Oct 25, 2021 at 15:25
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It depends on the size and the strength of the screw material. Most notable is stainless steel , 304 and 316 can be strengthened only by cold work. This is done but not nearly as effective as quench and temper of regular steels. When putting SS in my deck , I twisted off a few heads before realizing that a pilot hole and a clearance hole were both necessary for long screws. Brass is the same story. Today the bulk of steel screws are quenched and tempered, often with gas carburizing. I have many old wood screws that were not hardened but have relatively thick shanks for strength. Like everything else, the quality of the heat-treatment may not be as good as possible resulting in brittle screws; typically the head fractures. You may try larger diameters, pilot and clearance holes, different brands ; avoid any impact as that exaggerates the brittleness.

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  • Thanks for that pointer. I've always thought that some SS screws were more brittle than steel; now I have a bit more in the way of keywords to search on.
    – Aloysius Defenestrate
    Commented Oct 26, 2021 at 0:10
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It depends on the screw, I sprained my wrist once driving a socket-head #14-12 screw into hardwood with a drill.

Unless you have a torque driver and a datasheet from the maker of the screw the best approach is probably to drive some test screws into some scrap wood and use them to experiment with different clutch settings.

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  • Indeed I didn't think about this: I can take some sample screws, drive them in a very hard wood with different settings, and then find out at which setting they break. Or for any task where I need x screws, get x+1 or x+2 screws and test the first ones for strength. So far this is the answer which more closely answers my question. If you complete the answer with my comment I can accept it.
    – FarO
    Commented Oct 25, 2021 at 15:22
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    How'd you sprain your wrist? (genuinely curious)
    – DMoore
    Commented Oct 25, 2021 at 16:37
  • it was a drill (one of the bigger drills), noit a drill-driver it had a one-speed motor. socket heads don't torque out, so when the screw jammed all the torque was on my wrist, also I had a weird grip on the handle that made it hard to let go of the trigger,
    – Jasen
    Commented Oct 25, 2021 at 18:45

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