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When I look at the types of screw drives, I see... a lot.

History probably explains why so many existed, but I fail to understand why some still exist today.

For example, compare Double Hex with Spline .
It seems obviously clear to me that the Double Hex has a higher chance of wearing out or slipping, and it seems to have no advantage whatsoever.

Or for example, compare the usual Hex Socket with the Robertson .
Again, the Hex Socket appears to have a far higher chance of eventually wearing out or slipping, and again, no apparent advantage.

So why do these keep being used? Do they really all have at least one physical advantage over the others? Or is it a cost thing, or is it something else?

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    $\begingroup$ If you think a square is better than a hex, then why wouldn't you go further and ask why there aren't more triangle heads? $\endgroup$
    – Olin Lathrop
    Commented Jan 19, 2017 at 12:08
  • $\begingroup$ @OlinLathrop: Lots of (not-very-great) reasons... one is that unlike the other two a triangle is not both vertically and horizontally (and 180-degree) symmetric, which is both aesthetically less pleasing and (I imagine) probably also inconvenient in some cases. Another one is that I imagine the screwdrivers themselves might be more prone to wearing out or breaking if they get so pointy,on their edges (I'm assuming there is a mechanical reason why they their body tends to assume the same shape as their head in at least the hex case, and that it would still be a reason for triangular). etc. $\endgroup$
    – user541686
    Commented Jan 19, 2017 at 12:14
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    $\begingroup$ @OlinLathrop: Yet another reason was that I didn't try to list everything, so I was bound to miss something... $\endgroup$
    – user541686
    Commented Jan 19, 2017 at 12:15
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    $\begingroup$ It isn't always a technical decision. Sometimes a unique fastener is chosen in order to restrict access to people who possess the matching tool. Then the problem becomes one of controlling access to the tools. $\endgroup$
    – Dave Tweed
    Commented Jan 19, 2017 at 12:59
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    $\begingroup$ "It seems obviously clear to me that the Double Hex has a higher chance of wearing out or slipping" - in some applications that is a clear advantage, if for safety reasons you should always use a new fastener (e.g. because of intentional plastic deformation of the material when tightening it to the correct torque). $\endgroup$
    – alephzero
    Commented Jan 19, 2017 at 17:10

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If you read the Wikipedia page you've linked to, it gives the reasoning behind the deveolpment of many of the types of screw drive. Some ARE better than others, just not at the same thing. There are design tradeoffs to be made when selecting fasteners. For example, some applications may require that a unique tool be used to limit access. (e.g. Apple's use of the pentalobe screw in the iPhone or the penta screw commonly used on municipal water systems) For other applications, it may be desirable to have a commonly available tool such as a phillips screwdriver. For other applications, it may be necessary to be able to drive a lot of screws easily and "camming out" may be an issue...the list of choices goes on and on.

Also, don't underestimate the power of cost. Different heads will have a higher or lower cost of manufacture and thus a higher or lower cost to produce. The difference between a fastener that costs 0.03 USD vs. a fastener that costs 0.035 USD may not seem like much, but if you're designing something that uses 12 of them and has a production volume in the millions of units, that adds up.

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  • $\begingroup$ Well, I did read the article, and indeed, I understood that some are better than others in some way. The problem is the article doesn't suggest that all are better than others in some way. For example, for the double hex, it only says that it allows more torque than a six-point, but that hardly implies there's nothing else that would do the same job better. And as for limiting access or cost, it's hardly obvious that you need >25 different kinds of screws (let alone size variety) for those purposes. It honestly seems to me like you're not sure either, and are just guessing... $\endgroup$
    – user541686
    Commented Jan 19, 2017 at 19:40
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    $\begingroup$ @Mehrdad I have spent years as an engineer in product development and have spent a good part of that time designing joints which use any variety of screws and/or bolts. You're right in that there doesn't need to be so many different varieties of fastener drives with similar functionality, but also keep in mind that some of them (Phillips) were patented and many of those similar were attempts to circumvent those patents. $\endgroup$
    – DLS3141
    Commented Jan 19, 2017 at 20:04
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    $\begingroup$ Honestly, your question is a lot like, "Why are there so many different cars? I mean these two cars are designed for 4 passengers, I see no advantage to this car and it seems less reliable." $\endgroup$
    – DLS3141
    Commented Jan 19, 2017 at 20:13
  • $\begingroup$ Regarding the first comment: I see, patents are worth adding into the answer. Regarding your last comment: well, everything visible about a car matters to consumers. I'm not sure how much screw appearances matter though. $\endgroup$
    – user541686
    Commented Jan 19, 2017 at 20:33
  • $\begingroup$ Appearances are only one of many things that matter when it comes to cars. And yes, appearances do matter when it comes to fasteners. $\endgroup$
    – DLS3141
    Commented Jan 19, 2017 at 20:45

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