Flat, square, Philips and a dozen more, what are all those screw heads for? I understand why there would be different sizes; sometimes you want small screws and sometimes you want more solid screws, sometimes you don’t need to apply much torque to make sure they’re tight and sometimes you do.
But why not just one type of screw head? I would have exptected standardisation to have done its wonders by now. What gives? Is the flat head optimal for some applications while the square is best for others? Is the hardware store mafia behind it all?
patents back in the days (actually the Canadian “robinson” type screwhead (square) has an interesting history)
sales jobs (convincing people that your model is better)
specialized applications with unusual screw heads to make removal by unauthorized people difficult
They evolved as the technology improved to allow new styles. The slotted screw and driver requires a lot less machining than the Phillips, which was in turn less complicated than the square drive.
If I had my way, it would all be square or Torx. But this is America, we don’t do standardization here, we do tradition. For instance, all electrical outlets sold today have these combination screw heads that will work with square, Phillips and slotted, but every single cover plate is slotted only. Great idea, use the worst, most slippage prone head on the plate the public sees. Guns, expensive devices with fine finishes all seem to be slotted - and not just any slotted, but “hollow ground” ones with fluted sides.
some were designed for robustness or ease of installation. Flat screws generally suck because the driver blade slips out of the slot. So we got Philips, which doesn’t do that, but is not good for higher torques because the driver will “cam” out of the screw head. So we got Torx, which doesn’t cam out, but over time the splines of the driver can fatigue and break off. Which led to Torx Plus, which have more robust splines, and so on.
In some cases, a new screw head was developed to make it more difficult for people to use, e.g. Tri-wing, security Torx, and so on.
Which is actually by design. At the time there weren’t good torque limiters, and assembly line workers would often over- or under-tighten fasteners. Phillips drivers are great for screwing something in the first time, but lousy for unscrewing, especially repeatedly.
Security is a bit of a misnomer, since anybody can buy the driver heads at any hardware store. I have a set in my truck, in fact. The day I retire I’m going to dis-assemble all the stalls in the Faculty Mens Room and leave them stacked in the middle of the floor. Just because.
The tradeoff between small vs tough is related to screw size, not screw headtype. I said I understand why there are different sizes since size is relevant to the small vs tough tradeoff. A small screw size is better if you want to keep things light but don’t need it to be tough. A large screw size is better if you want it tough but don’t need it to be light.
Screw type is not relevant to the small vs tough tradeoff. Using a flat head is less tough than a square but it doesn’t make it smaller/lighter.
To all other commenters:
So, legacy and security are the main reasons? Can screw head types be patented or are they so basic a concept that they can’t be?
One of the main reasons is automated manufacturing. Security is mostly a secondary consideration.
Philips head screws have a cross shaped head so that they will self center on automated machines that drive in screws. This self centering happens to help when you are driving the screw in with a manual screwdriver, but that wasn’t why they were invented. They were invented to help with automation.
Torx head screws were designed not to cam out. They were designed to be used with torque limiting screwdrivers in automated assembly lines, where they worked better than older screwdrivers that relied on the Philips head camming out to limit the torque.
Square (Robertson), hex, pozidriv, spline drive, and Bristol screws were all designed to improve automated manufacturing.
This isn’t to say that no screws were designed for security reasons, because plenty were. Tri-wing, security torx, one way screws, and the new pentalobe screws used by Apple were all designed for security reasons.
MichaelEmouse, you answered your own question, but when I pointed it out, you didn’t look closely. Here’s what you said:
To expand, and please refer to the link in Post #14.
“Sometimes you don’t need to apply much torque…” [Phillips, because they cam out] “…and sometimes you do.” [Torx, because they prevent camming out and you can apply an exact amount of torque.]
Cosmetic considerations are also pertinent some places.
So why quote: “sometimes you want small screws and sometimes you want more solid screws” as part of the answer?
You quoting back “sometimes you don’t need to apply much torque to make sure they’re tight and sometimes you do” was supposed to be the equivalent of the Cecil’s and engineer’s answers and explain as much as they did in terms of slipping and camming out? Indeed, to get those answers out of you simply quoting back what I had said would take some very close looking.
But it was better to be glib than to actually provide an answer with enough detail to be useful.
Every time I go to buy ink for my printer, and I’m confronted with literally hundreds of different cartridges, the same two thoughts go through my mind:[ol][li]“Why the heck can’t they decide on a just handful of good designs? Why the ridiculous proliferation?” – and then:[/li][*]“Get used to it. This is really no different than when I would hunt down my particular typewriter ribbon when I was young.”[/ol]
Like many applications of slotted (flathead) screws, that’s for appearance. It’s the “cleanest” finish look, particularly if you turn them all vertical.
When I become emperor one of my edicts will be the inclusion of matching drivers with every box of screws. The driver has to fit exactly. Prison sentences for any CEO that doesn’t comply.