Certain things like bikes, cars, and furniture use hexagonal bolts for fastening. Hex bolts can only be used with the right diameter key and they don’t slip like Phillips and Flatheads. Also, the hexagonal tip keeps bolts from falling so you don’t need a magnet to hold your fasteners. Furthermore, it’s easy to identify which Allen key you need for each fastener, and you can use ballpoint hex keys if you need to work at an angle.
Since the hex bolt design is so practical, why don’t we use this type of fastener for everything? Why don’t we see hex wood screws and hex drywall screws ?
Edit : I’m asking about fasteners in general (like screws, bolts, etc)
In: Engineering
Bolts need a nut. Some things do not have room to put a nut in, other things look ugly with a bunch of nuts hanging off
For fasteners using hex keys, There is the Metric set and there is the SAE set. some of them are really close to each other in terms of measurement but not quite the exact fit so if you use the wrong one it may damage both the fastener and/or the key.
In contrast, square heads have one set of sizes and Phillips has one set of sizes. Hex is more common where the fastening tool is included in assembly such as furniture.
Hex heads and drivers are more expensive, and less practical for some applications. And once they strip, good luck.
> Hex bolts can only be used with the right diameter key
This is literally one reason. Do you have any idea how annoying it would be if every time you needed to unscrew something you had to find the _exact_ right size key to use? Philips head screwdrivers are nearly universal. Barring gigantic size differences, you can use the same Philips screwdriver for a large variety of screw sizes
Sizing matters. Really small screws can’t have a hex. You’d strip that out the first time you over tighten.
Also, you can buy 2 or three sizes of flathead or Phillips head screwdrivers and be good for 95% of screws out there. You have to have the exact size for hex.
All in all, it really depends on the application of the fastener.
From my personal experience with hexagonal nuts and bolts: they suck. They’re good in very specific situations, but if you’re using a drill, you’re more likely to strip them out than you are with cross head.
If they get rusty at all, you can no longer get the bit in to remove them–which can be true of cross screws, sure, but you have MUCH more leeway with those.
It absolutely is not easy to ID what size you need for specific hex screws; I have to patiently do trial and error every time I need to take the scale at work apart to repair it, because we have 20 hex bits and only one of them fits in the hole, whereas a cross screw will accept almost any cross screwdriver.
Plus, there’s metric vs english–do you need a mm, or a fraction of an inch? Have fun digging through ALL your bits to figure it out!
Think about drywall screws for a second. Their goal is to be able to apply force through the paper. If you had a hex-headed bolt, it would tear the paper as the head touched (making it impossible to apply force through it), and you could never get the top of the fastener flush or slightly below the surface of the wall.
There are a lot of applications where having the drive mechanism be internal to the fastener is integral to its function. Think about Allen head fasteners used in tight installations, flat head screws meant to be driven flush, anything where the fastener is visible like switch cover plates.
Edit: typos
Your terminology is incorrect. What you are referring to are socket head cap screws. “Hex bolts” is a common name for fasteners with an external hexagonal head, but the correct name for most of those are hex head cap screws.
But as to the reason for slotted or Phillips screws over socket head cap screws, head size is probably the biggest. The head of socket head fastener is generally larger for a given fastener size than a Phillips. If you are using a flat head or button head socket head screw the head is similar size. However for fasteners smaller than 8mm or 5/16″ the socket size is small enough on these flat head and button head fasteners that they are very prone to stripping.
And then most people have a Phillips or flat blade screwdriver at home, while fewer have a set of hex keys.
For construction fasteners socket heads were fairly common but in the last few years torx seem to have become more common. Torx are even less prone to stripping out as they have more contact area than socket heads. But the smallest sizes are prone to breaking th tip of the driver.
Flat head screws are significantly cheaper to mass produce, so they get chosen by anyone who wants to save a few cents to edge out competition.
Allen cap screws don’t like to be installed by older machines and people with poor torque control, they tend to be over-tightened and break off. Phillips head screws are specially developed to push the bit out when tight enough.
Modern machinery doesn’t really have this issue anymore, but modern engineers are instead designing things to work without fasteners wherever possible, because of how expensive and prone to failure fastener joints are, so there’s little incentive to standardise…
Countersunk screws, like drywall and wood, don’t have much room for a hex socket (depth-wise), so it would be very small and it would be hard to apply sufficient torque without damage. I’ve used countersunk hex machine screws and they are really best for cosmetic applications, not real fastening.
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