eli5: Why are there different types of screwnail heads? Does it really affect the strength or integrity of the screw if the head is a Phillips, Robinson or standard?



eli5: Why are there different types of screwnail heads? Does it really affect the strength or integrity of the screw if the head is a Phillips, Robinson or standard?

In: Engineering

Yes, there are differences in torque, wear, and failure modes. Slots provide a lot of torque, but can slip out and harm the product. Phillips cam out under torque, so they are less likely to slip. Torx and other fancy styles have higher torque without cam out, but they can wear more quickly. Exotic fasteners serve to make it harder for users to get inside and mess things up, the options are endless.

Reasons for different screws include (but is not limited to)

1. Patents
2. Traditions and culture
3. Limiting home repairs/ right to repair
4. Practicality (some screws are better suited for some tasks)

It has to do with driving the screw. Flathead is easy to make. Simply cut a slot in the head. It’s can be a pain to line the driver up with it though and it slips out and strips the head easily. Philips improves on the lining up but it cams out of the hole easily, also tending to strip the head in the process. This was an advantage in early use when power drivers didn’t have effective clutches; it made it harder to overdrive. Robertson (what I think you were referring to when you said Robinson) solved many of these problems but wasn’t adopted widely due to the way he went about licensing his patents (or so I’ve been told). Torx (or star drive as some people call it) is a further improvement on ease of lining up the driver and preventing the head from slipping and .stripping the head.

Other specialty heads exist, some trying to improve the drivability, some for security reasons. None of those designs effect how the screw performs in terms of holding or strength. There are screw designs that do that (see GRK fasteners as an example, their screws have quite a bit of thought and engineering put into them).

Editted for typos and to add a point or two.

Standard slip all the time and are the worse choice. Phillips are better because the cross design makes the screwdriver self-center.

Robinson are the best because the square hole is so superior. Self- centering, and the square shape means you can torque the shit out of the screws without the screwdriver slipping.

Robinson never really caught on outside of Canada though.

It does not affect the strength or integrity of the screw. The advantages is purely in the price of the screws and when installing the screw. A Robertson screw have the advantage over slotted screws that it have four surfaces instead of two so it can take twice the amount of force before deforming, and it is also self centering so that the screwdriver is always in the center where you get more torque. So you can apply much more torque to a Robertson screw then to a slotted screw of the same dimension. Phillips screws on the other hand is designed to prevent overtorquing. If you apply too much force to a Phillips head the screwdriver will push out instead of deforming the head of the screw or stretching the threads. This makes it much harder to damage the screw when fastening it then with the Robertson screw.

Modern screws use hex drives or torx. These are both six sided which allows you to apply even more torque then with the Robinson screw. The torx do allow you to use more force then a hex drive but is slightly more expensive to manufacture. Hex drives can also be driven at an angle with the correct tool.

So if you compare screws with older style heads to the ones with the newer style heads the older styles tends to use a larger head. This is because you need the larger head to get enough torque without damaging the screw. Whereas a newer style head can withstand a lot more torque with a smaller head. But once the screw is fastened there is no difference between the different styles.

Edit: Robinson -> Robertson

Flatheads are easier to make, and in theory, if you have a deep slot on the screw and the perfectly correct screwdriver bit, you can apply more torque without slipping.

In practice however, people don’t use perfectly correct screwdriver bits so they end up just stripping the surface. Also, more torque isn’t always a good thing, it can cause the screwdriver or the screw to break.

That’s why the Philips head (and other similar designs) were invented. Even if you don’t get perfect alignment or use the perfectly correct screwdriver bit, if you press down while screwing, you’ll catch the screw holes eventually. You also can’t overtighten because the screwdriver bit will cam out.